The order of the talks may still change a little bit within single days.
Sunday, March 18, 2012
Arrival: Transportation Sunday afternoon from Bratislava/Vienna airports/railway stations
Get-Together Reception in Smolenice Castle
Welcome address, Peter Skyba, workshop chairman
Monday, 19 March, 2012
9:00 - 11:00 Session I: Fluctuations and heat transport in nanostructures
chairman Tero Heikkilä
- 9:00 – 9:30 Jordan Horowitz (Universidad Complutense, Madrid): Thermodynamics with feedback: Extracting work from information
Feedback can harness thermal fluctuations by converting them into work. However, there is a limit: the second law of thermodynamics for feedback limits the maximum extractable work to the information gained through feedback. In this talk, I review recent progress detailing how this information is converted into work. In particular, I prove and analyze the second law for feedback utilizing an extension of the theoretical framework developed in response to the fluctuation theorems. As a consequence, we find that thermodynamic processes that most efficiently convert information into work are feedback reversible -- they are indistinguishable from their time reverse. This naturally leads to a recipe for designing optimal thermodynamic engines with feedback, which I illustrate with an N-particle Szilard engine.
- 9:30 – 10:00 Jukka Pekola (Aalto University, Helsinki): Work, heat and fluctuation relations in single-electron transport - test of the Jarzynski equality
The physical foundations of commonplace concepts such as information, work and heat can be studied in detail in small systems having just few degrees of freedom. Characteristic of such systems is that the magnitude of thermal fluctuations of energy and coordinate variables can be significant compared to their mean behavior. Fluctuation relations have been developed relatively recently to describe these phenomena. They have been successfully applied in the interpretation of, e.g., experiments performed on individual complex biomolecules. However, a "textbook" example of nontrivial nonequilibrium thermal fluctuations measured in a well-characterized system has been missing. Here we demonstrate experimental readout of the distribution of dissipated energy in a metallic single-electron box that we subject to an external non-adiabatic gate drive at sub-kelvin temperatures . In contrast to previous experimental work, the total heat dissipated in an electron tunneling event can be directly determined from the timing of the tunneling event with respect to the external drive . Also, being a lithographically defined electronic system, the experimental gate protocol can be repeated accurately and indefinitely without degrading the sample, allowing us to recover the heat distribution with a dynamic range of more than three orders of magnitude. The present work shows that nonequilibrium thermodynamics can be studied in solid state systems where realization of quantum coherence effects and an engineered non-Gaussian environment is experimentally feasible.
 O.-P. Saira et al., in preparation.
 D. V. Averin and J. P. Pekola, EPL 96, 67004 (2011).
- 10:00 – 10:30 Markus Büttiker (University of Geneva): Current from hot spots
To demonstrate that the relative stability of non-equilibrium states cannot be found from local criteria, Rolf Landauer showed that hot spots in locations of phase space that might be only rarely visited can be decisive. Later, the author and van Kampen, investigated the noise induced transport generated by hot spots in systems with overdamped Brownian motion dynamics. In these systems the particle is in an inversion symmetric potential and only the location of the hot spots breaks the symmetry.
We next examine hot spots in electrical conductors which are Coulomb coupled. Coulomb coupled conductors permit separate directions of the heat and current flux. We investigate the minimal conditions needed to generate directed current flow for a system of two quantum dot conductors in which both energy and charge states are quantized. In quantum dots energy to current conversion can be optimal with one electron transferred for every heat quantum given up by the hot conductor. We discuss the efficiency at maximum power for Coulomb blockaded quantum dots and chaotic cavities with quantum point contacts.
- 10:30 - 11:00 Teun Klapwijk (Delft University of Technology): Evanescent states and nonequilbrium in driven superconducting nanowiresby N. Vercruyssen, T.G.H. Verhagen, M.G. Flokstra, J. Pekola, and T.M. Klapwijk
We study the non-linear response of current-transport in a superconducting diffusive nanowire between normal reservoirs. We demonstrate theoretically and experimentally the existence of two different superconducting states, labeled global and bimodal, and appearing when the wire is driven out of equilibrium by an applied bias. The microscopic properties of the different states are obtained using two probe measurements of the wire, and measurements of the local density of states with tunneling probes. The analysis is performed within the framework of the quasiclassical kinetic equations for diffusive superconductors.
11:00 – 11:30 Coffee
11:30 - 13:00 Session II: Quantum entanglement and correlations in electronic systems
chairman Pertti Hakonen
- 11:30 – 12:00 Per Delsing (Chalmers University): Microwave photons in superconducting circuits; Dynamical Casimir effect and photon routing
We have been able to observe the Dynamical Casimir Effect (DCE) in a superconducting circuit consisting of a coplanar transmission line with a tunable electrical length . The rate of change of the electrical length can be made very fast (a substantial fraction of the speed of light) by modulating the inductance of a superconducting quantum interference device at high frequencies (~10 GHz). In addition to observing the creation of real photons, we detect two-mode squeezing in the emitted radiation, which is a signature of the quantum character of the generation process. This phenomenon was predicted 40 years ago and has not been observed until now.
We also study an artificial 3 level atom in the form of a transmon qubit coupled to superconducting 1D transmission line . Strong interaction between the artificial atom and photons is revealed in the reflection of propagating microwaves and substantial extinction (99.6%) of the transmission has been observed. A strong control pulse, at the frequency corresponding to the transition between the two upper states, is used to route a weak probe tone at lower transition frequency. The maximum on-off ratio is 99% with a rise and fall time on the order of 10 ns. This fast controllable router provides a fundamental building block for quantum optics on chip.
1. C.M. Wilson et al. Nature, 479, 376 (2011).
2. I.-C. Hoi et al. Physical Review Letters, 107, 073601 (2011)
- 12:00 – 12:30 Sorin Paraoanu (Aalto University, Helsinki) Dynamical Casimir effect in a Josephson metamaterial
Vacuum modes confined into an electromagnetic cavity give rise to an attractive interaction between the opposite walls. When the distance between the walls is changed non-adiabatically, virtual vacuum modes are turned into real particles, i.e. photons are generated out of the vacuum. These are known as the static and dynamical Casimir effect, respectively. Here we demonstrate the dynamical Casimir effect using a Josephson metamaterial embedded in a microwave cavity at 5.4 GHz. We achieve the non-adiabatic change in the effective length of the cavity by flux-modulation of the SQUID-based metamaterial, which results in a few percent variation in the velocity of light. We show that energy-correlated photons are generated from the ground state of the cavity and that their power spectra display a bimodal frequency distribution. These results are in excellent agreement with theoretical predictions, all the way to the regime where classical parametric effects cannot be of consequence.
 P. Lähteenmäki et. al., arXiv:1111.5608.
- 12:30 – 13:00 Christian Lang (ETH Zürich): Observation of Photon Blockade in Circuit QED using Correlation Function Measurements
Creating a train of single photons and monitoring its propagation and interaction is challenging in most physical systems, as photons generally interact very weakly with other systems. However, when confining microwave frequency photons in a transmission line resonator, effective photon-photon interactions can be mediated by qubits embedded in the resonator. Here, we observe the phenomenon of photon blockade through second-order correlation function measurements. The experiments clearly demonstrate antibunching in a continuously pumped source of single microwave photons. We also investigate resonance fluorescence and Rayleigh scattering in Mollow-triplet-like spectra . Additionally we discuss a novel method of measuring correlation functions in the microwave frequency domain using microwave beam splitters, linear amplifiers, mixers, and voltage detectors [2, 3].
References:  C. Lang et al., Phys. Rev. Lett. 106, 243601 (2011).  M. P. da Silva et al., Phys. Rev. A 82, 043804 (2010).  D. Bozyigit et al., Nat. Phys. 7, 154 (2011).
13:00 – 14:30 Lunch
14:30 - 16:00 Session III: Electron pumping
chairman Dominik Zumbuhl
- 14:30 – 15:00 Alexander Zorin (PTB, Braunschweig): Dynamics of single Cooper-pairs in superconducting nanowires enabling quantum phase slips
We propose and realize a transistor-like superconducting circuit including two serial segments of a quantum-phase-slip nanowire with a capacitive gate in between. This circuit was made of amorphous NbSi film (with thickness of 10 nm, width 18 nm, and length of several micrometers), embedded in a network of Cr microresistors ensuring high external impedance and, therefore, as predicted by Mooij and Nazarov, a coherent transfer of single Cooper pairs. In our circuit this transfer of the charge was effectively controlled by the voltage applied to the gate. The obtained experimental results and the model of this single-Cooper-pair transistor will be discussed.
- 15:00 – 15:30 Yuri Pashkin (NEC Nano Electronics Research Laboratories, Tsukuba): Coherent and incoherent charge pumping with Coulomb blockage devices
Charge pumps are promising candidates for the creation of quantized electric current sources and redefinition of the unit of ampere. In my talk I will describe two types of charge pumping devices that were studied in collaboration between NEC Research Laboratory in Japan and Low Temperature Laboratory of Aalto University in Finland. The first device is based on a hybrid single-electron transistor which is capable of transferring single electrons one by one in a controllable fashion. When the device is properly biased and under the external drive, its transport characteristics exhibit well-defined plateaus providing the basis for the current standard. Existing challenges for the creation of the electric current standard will be discussed. The second device is a superconducting single-electron transistor in which coherent Cooper pair current is produced by the non-adiabatic pulsing technique. Depending on the composite pulse shape, both forward and backward Cooper pair pumping can be realized with respect to the bias direction. Quantum coherence is revealed though the periodic dependence of the measured current on magnetic field.
- 15:30 – 16:00 Masaya Kataoka (National Physical Laboratory): Driving single-electron pumps with shaped waveforms towards quantum current standard
Single-electron pumps produce a current by periodically transferring one electron between source and drain reservoirs. When this process is error-free, the current becomes exactly quantised I = ef, where e is the elementary charge and f is the frequency of the electron transfer process. Such devices can be used as a quantum current standard, and a way to realise the redefined SI ampere based on the exact value of e. Over two decades, many schemes of electron pump have been studied, however, the combination of current level (~ 1 nA) and accuracy (~ 1 part in 108) that is required for a quantum current standard has not been realised. Here, we present our recent experimental results on so-called tunable-barrier semiconductor quantum dot single-electron pumps, achieving the current quantisation accuracy of 1 part in 106 at 150 pA level . This type of pump has been reported to operate at relatively high frequency . It has also been found that perpendicular magnetic fields drastically improve the quantisation accuracy [3,4]. Nevertheless, the quantisation accuracy achieved prior to this work was limited to 15 part in 106 at 50 pA level , and increasing the frequency above a few hundreds MHz degraded the current quantisation plateaux. We have overcome this upper frequency limit by driving the pump with a shaped waveform. The important part of the pump process that suffers from high-speed operation is considered to be when the oscillating potential traps electrons. By slowing down this part of the cycle but speeding up the parts that are not important, we find that the quantisation accuracy can be improved by a few orders of magnitude when operated at a high repetition frequency of ~ 1 GHz. We compare the value of the quantised plateau with a reference current, produced using a standard 1 GΩ resistor and a voltmeter traceable to quantum resistance and voltage standards, to the accuracy of 1 part in 106.
 S. P. Giblin et al., arXiv:cond-mat/1201.2533 (2012).
 M. D. Blumenthal et al., Nature Physics 3, 343 (2007).
 S. J. Wright et al., Phys. Rev. B 78, 233311 (2008).
 J. D. Fletcher et al., arXiv:cond-mat/1107.4560 (2011).
 S. P. Giblin et al., New J. Phys 12, 073013 (2010).
16:00 – 16:30 Coffee
16:30 - 18:30 Session IV: Amplifiers and detectors
chairman Mika Sillanpää
- 16:30 – 17:00 Frank Deppe, (WMI Munich): Reconstruction a propagating squeezed microwaves using the dual-path method
Due to the advent of the field of circuit quantum electrodynamics (QED), nonclassical micowave light can be generated in superconducting circuits using recipies similar to those in quantum optical cavity QED. However, as soon as such quantum microwaves are fed into transmission lines and are to be be reconstructed in this propagating state, the simple analogy to quantum optics fails. The reason is that the "microwave" concept of linear, but noisy amplification is fundamentally different from the "optical" concept of single photon detection. The dual-path state reconstruction method was proposed to overcome the amplifier noise using cross-correlation measurements and signal recovery techniques. We apply this method to reconstruct the Wigner function of a squeezed state produced by a Josephson parametric amplifier and compare this result to other techniques for the detection of squeezing applied to the same device.
- 17:00 – 17:30 Emmanuel Flurin (ENS Paris): Three-wave mixing of microwave photons with Josephson junctions and resonators
Three-wave mixing devices, i.e. non-linear circuits converting power among three microwave signals, are key elements of analog information processing in the microwave domain. However, they are based on dissipative components such as semiconductor diodes, or SIS tunnel junctions biased near the superconducting gap. The loss of signal limits their operation and also introduces noise above the minimum required by quantum mechanics. By using Josephson junctions and superconducting resonators, we’ve been able to build such a quantum device.
In its amplification mode, this device can be used as a preamplifier in many experiments where a few photon wide microwave signals must be measured by commercial instruments, which are sensitive to powers several orders of magnitude larger. The price to pay for this amplification is the necessary degradation of information. Indeed, an extension of the no-cloning theorem imposes that a linear phase preserving amplifier adds at least half a quantum of noise to the signal. By amplifying the finite frequency noise emitted by an NIN tunnel junction, we demonstrated that the total measurement chain adds less than 2.3 quanta to this limit thanks to our device. The operating principle of this device is based on Josephson parametric amplification. I will explain this mechanism and present results which illustrate that it can generate quantum correlated pairs of spatially separated photons with different wavelengths known as two-mode squeezed states. This experiment exploits the tunability of our device and the ease-of-use inherent to micro-wave circuits to perform a quantum optics experiment demonstrating this entanglement.
- 17:30 – 18:00 Andrea Vinante (Universiteit Leiden): High sensitivity SQUID-detection of ultrasoft mechanical resonators and application to Magnetic Resonance Force Microscopy
Magnetic Resonance Force Microscopy (MRFM) is a nanoscale 3D imaging technique based on magnetic coupling between spins in a sample and a ferromagnetic tip on a mechanical resonator. High sensitivity MRFM requires lowest possible thermo-mechanical force noise, at subattonewton level, which can be achieved by ultrasoft resonators at very low temperature. We have recently performed the first demonstration of MRFM at temperatures lower than 100 mK, by detecting surface paramagnetic defects in silicon-silicon dioxide interface. In our experiment the force sensor is an ultrasoft silicon cantilever, and we detect the cantilever motion by means of a superconducting pick-up coil coupled to a Superconducting Quantum Interference Device (SQUID). This scheme has proven to be fully compatible with operation at millikelvin temperature. I will also describe a recent experiment where we have achieved a large improvement of the detection sensitivity by directly coupling the cantilever to an optimized SQUID magnetometer. We demonstrate the combination of low displacement and force noise by feedback-cooling the cantilever fundamental mode to about 160 uK.
- 18:00 – 18:30 Juha Hassel (VTT Technical Research Center of Finland): Near-quantum-limited single Josephson junction microwave amplifier
We propose and characterize theoretically and experimentally microwave amplifier based on single Josephson junction operating at microwave frequencies. We introduce a circuit solution based on frequency-selective damping of a Josephson junction enabling direct utilisation of its negative dynamic resistance to produce microwave gain while stability is preserved. Experimentally, we find near-quantum-limited noise temperature of TN ≈ (1.6 ± 0.5)hf at frequency f = 2.8 GHz. It is well theoretically explained through mixed-down quantum noise further affected by the so-called noise compression effect. We discuss further optimization issues through analytic and numerical models describing the device.
18:30 – 20:00 Dinner
20:00 – 21:30 Poster session
chairman Teun Klapwijk
- T.T. Heikkilä (Aalto University): Macroscopic quantum tunneling in nanoelectromechanical systems
- T.T. Heikkilä (Aalto University): Electron-photon coupling in SNS junctions
We study a diffusive superconductor-normal metal-superconductor junction in an environment with intrinsic incoherent fluctuations which couple to the junction through an electromagnetic field. When the temperature of the junction differs from that of the environment, this coupling leads to an energy transfer between the two systems, taking the junction out of equilibrium. We describe this effect in the linear response regime and show that the change in the supercurrent induced by this coupling leads to qualitative changes in the current-phase relation and for a certain range of parameters, an increase in the critical current of the junction. Besides normal metals, similar effects can be expected also in other conducting weak links.
- M.J. Martinez-Perez (Scuola Normale Superiore, Pisa): Prototype for an efficient microkelvin quantum nanocooler
A prototype for an efficient microkelvin nanocooler is presented and discussed. The device consists of a micrometric electronic domain the temperature of which we plan to control down to the microkelvin regime. The realization of such a device will demand us to concentrate on two main issues. The cooling process, on the one hand, will be carried out by means of Quantum Dots (QDs). The use of QDs enables us to tune in-situ the energy of the electrons leaving the central domain therefore maximizing the cooling efficiency for every temperature range. On the other hand, we envisage the use of a non galvanic thermometer to read out the electronic temperature in a non invasive way. A QD directly coupled to the electronic domain will be used to this end. The read out will be performed by means of a Quantum Point Contact (QPC) capacitively coupled to the QD therefore minimizing the amount of energy transmitted to the ultracool zone.
- Matthias Meschke (Aalto University): Towards the ultra-low temperature NIS refrigeratorby M. Meschke, J. V. Koski, J.T. Peltonen, J.T. Muhonen, N.B. Kopnin, and J.P. Pekola
A specially designed solid state NIS cooler offers the possibility to cool the electron system to temperatures below 10 mK when the base temperature is within a reasonable range (about 100 mK). The cooling power of such a device scales with temperature to the power 3/2 and can consequently overpower the electron phonon coupling where the exponent is typically as high as 5. Practically, the realisation is not so straightforward when considering the back side of the cooler, where a noticeable amount of heat has to be deposited into the superconducting electrode. We show in this presentation ways to optimize the cooler following two main directions: (1) effective thermalisation of the superconducting electrode using different approaches of quasi-particle trapping including quasi-particle drain through magnetic vortices, and (2) practical ways of enhancing the efficiency of the cooler itself using a superconductor with an adapted gap.
- Yuri Pashkin (NEC Nano Electronics Research Laboratories, Tsukuba): Charge qubit coupled to a nanomechanical resonator
- Mika Sillanpää (Aalto University): Circuit QED with a hybrid of micromechanical resonator and transmon qubit
Many physical phenomena are governed by the interactions between different kinds of degrees of freedom in hybrid quantum systems. These assemblies can combine the benefits of each subsystem in future quantum technologies, such as the long lifetime of atomic states together with accessible macroscopic electrical circuits in superconducting cavities and qubits. Here we demonstrate the possibility to integrate circuit cavity quantum electrodynamics with a micromechanical resonator, which also allows for a long lifetime and a strong, tunable coupling. We measure an electromechanical analog of the atomic Stark shift, and splitting of the transmon qubit spectral line into mechanical sidebands. In the time domain, we observe a coherent conversion of the qubit excitation into phonons and back in the form of sideband Rabi oscillations. This advance may allow for quantum information applications and studies of strongly coupled quantum systems near the classical limit.
- Matti Silveri (University of Oulu): Motional averaging in a superconducting artificial atom
Motional averaging in a superconducting artificial atom, by J. Li, M.P. Silveri, K.S. Kumar, J.-M. Pirkkalainen, A. Vepsäläinen, W.C. Chien, J. Tuorila, M.A. Sillanpää, P.J. Hakonen, E.V. Thuneberg, and G.S. Paraoanu
Superconducting circuits with Josephson junctions are the most favourable candidates for developing future quantum information technologies. Besides the speeding up of certain calculations, of particular interest is the possibility of simulating effects that typically occur in complex many-body systems. Here, we demonstrate a step in this direction by using a superconducting quantum bit, a transmon, to simulate motional averaging, a phenomenon initially observed in nuclear magnetic resonance (NMR) spectroscopy. To mimic this effect, the flux bias of the transmon is modulated by a controllable pseudo-random telegraph noise, resulting in stochastic jumping of the energy levels between two discrete values. With jumping rates larger than the energy level displacement, the two separated spectral lines merge into a single, motionally averaged line. By driving Rabi oscillations, the modulated system is demonstrated to behave in the averaged regime as a new, hybrid qubit. We also apply regular sinusoidal modulation of the energy levels, resulting in a spectral pattern which can be understood as a Landau-Zener interference on hybridized states.
- Andreas Fleischmann (Universität Heidelberg): Development of a large-area for position and energy resolving detection of molecular fragmentsby Alexandra Kampkötter, Andreas Fleischmann, Loredana Gastaldo, Sebastian Kempf, Andreas Pabinger, and Christian Enss
The recombination of a molecular cation with an electron, followed by fragmentation, is a fundamental reaction in cold, dilute plasmas and plays a key role in interstellar chemistry. To investigate such reactions in laboratory environment with full quantum control, the Max-Planck Institute for Nuclear Physics in Heidelberg is building a cryogenic storage ring to prepare molecular ions in their groundstate. The full kinematics of these recombination processes can be resolved by a position and energy sensitive detection of the reaction products/molecule fragments. We describe the development of a new large-area MMC for position sensitive detection of massive particles with kinetic energies down to a few keV. The detector encompasses sixteen pie-shaped large area absorbers to form a circular whole with a diameter of 36 mm. The temperature sensor is positioned on the outer edge of each absorber. The rise-time of the detector signals varies with the impact location of the particle due to diffusive expansion of heat in the absorbers. We present results of first test measurements where energy was deposited in the absorber at different positions by light pulses from three LEDs. The observed position dependence of the signal shapes agrees well with the one expected from detailed thermal simulations.
- Christian Enss (Universität Heidelberg): maXs: Metallic magnetic calorimeters for high-resolution X-ray spectroscopy in atomic physicsby Christian Pies, Sönke Schäfer, Sebastian Kempf, Jan-Patrick Porst, Simon Uhl, Sebastian Heuser, Thomas Wolf, Loredana Gastaldo, Andreas Fleischmann, and Christian Enss
Highly-charged ions are model systems for the investigation of quantum electrodynamical effects in strong electromagnetic fields. We are developing x-ray detectors based on 1x8 arrays of Metallic Magnetic Calorimeters (MMCs) optimized for x-ray spectroscopy of highly-charged ions at GSI/FAIR and the EBIT facility at the MPI for Nuclear Physics in Heidelberg. One of the detector arrays (maXs-20) is designed to provide an energy resolution below 3 eV (FWHM) and sufficient stopping power for x-rays in the energy range up to 20 keV. The second device (maXs-200) is optimized for the detection of x-rays up to 200 keV and should yield an energy resolution below 30 eV (FWHM). We present detector designs, outline the micro-fabrication process and discuss the results of characterization measurements with 55Fe and 241Am calibration sources including energy resolution, signal shape and cross-talk between adjacent detectors of both arrays.
Tuesday, 20 March, 2012
9:00 - 11:00 Session I: Nanomechanics
chairman Sorin Paraoanu
- 9:00 – 9:30 Florian Marquardt (University of Erlangen): Quantum optomechanics
The study of the interaction between light and nanomechanical motion has given rise to a number of promising results over the past few years.
Very recently, nanomechanical oscillators have been laser-cooled close to the quantum ground state, opening the door to investigations of the quantum regime. In this talk, I will explore theoretically several potentially fruitful future avenues. The light field can be used to couple several mechanical modes and implement quantum gates between those continuous quantum degrees of freedom. In addition, if future setups manage to make the interaction between single photons and single phonons larger than the relevant decay rates, a lot of nonlinear quantum effects will be observable. These include the production of nonclassical mechanical quantum states, where a negative Wigner density could be observed. In addition, there are significant effects on the temporal photon correlations of a light stream transmitted through such a strongly coupled optomechanical system.
- 9:30 – 10:00 Witlef Wieczorek (University of Vienna): Towards the quantum regime of cavity optomechanical systems
Cavity optomechanical systems exploit the coupling of light to mechanical oscillators via radiation pressure. This interaction allows to control the mechanical oscillator as well as to engineer the optical field, as has been demonstrated in experiments on ground-state cooling of the mechanics and on strong coupling of photons and phonons. Ultimately, cavity optomechanical systems will allow to explore macroscopic quantum mechanical experiments provided the optomechanical system is in its quantum regime. In order to reach this regime, ultra-low temperatures are required. We will present experiments towards reaching the quantum regime of mechanical oscillators.
- 10:00 – 10:30 Mika Sillanpää (Aalto University): Towards quantum limited electromechanical microwave amplification
Micromechanical resonators affected by radiation pressure forces allow to address fundamental questions on quantum properties of mechanical objects, or, to explore quantum limits in measurement and amplification. A promising setup for the purpose is an on-chip microwave cavity coupled to a micromechanical resonator. We have implemented a novel setup where a 30 MHz nanowire resonator is capacitively coupled to a high-impedance transmission line microwave cavity via an ultranarrow 10 nanometer vacuum gap. This setup achieves high electromechanical coupling energies of 2 MHz/nm, allowing for studies probing the quantum limit of tangible moving objects, as well as applications related to sensitive measurements. We demonstrate, both theoretically and experimentally, the possibility of using the sideband regime of such a circuit optomechanical system as a microwave amplifier, with noise properties approaching the quantum regime. Under blue sideband irradiation, pump photons will be down-converted, transferring energy into the mechanical resonator. We will here show that addition of a probe signal will induce coherent stimu- lated emission, leading to its amplification up to 25 dB at 7 GHz frequency. A full quantum theory is found to be in an excellent agreement with the experiment. Analyzing its noise properties, we find that our amplifier adds about 20 quanta of noise, comparable to state-of-the-art cryogenic HEMT amplifiers. We predict that under ideal but realistic conditions, the amplifier is expected to reach the Heisenberg limit of lowest added noise of phase-insensitive amplifier, equaling half a quantum at signal frequency. Moreover, by a suitable selection of parameters, we predict phase-sensitive amplification, and, in principle, noise-free amplification of one quadrature of the input signal. In the opposite regime, by pumping the red sideband, we cooled the 30 MHz mechanical mode to thermal occupancy of only 1.8 quanta, limited by heating of the mechanical bath at the highest cooling powers.
- 10:30 – 11:00 Herre van der Zant (Delft University of Technology): Resonating SQUID's
Superconducting Quantum Interference Devices (SQUID’s) are well known as sensitive detectors of magnetic fields. However, SQUID’s are flux-voltage transducers indicating that in a fixed magnetic field they can be used to detect minute changes in their loop area. We have used this principle to demonstrate position detection of a 1 MHz mechanical resonator embedded in the SQUID loop. The SQUID also causes a dynamic Lorentz-force back-action which changes the damping and resonance frequency of the resonator. When the entire SQUID loop is suspended (torsional resonator geometry) the detector-resonator coupling can be so strong that the total damping becomes negative. Once this condition is reached, the measured oscillation amplitude of the resonator grows strongly and settles into a stable limit cycle (self-sustained oscillations). In this regime, the detector resolution is found to be a factor two below the standard quantum limit. High-frequency resonators (> 100 MHz) can be incorporated by employing suspended carbon nanotubes, which act both as moving elements in the SQUID loop and as superconducting junctions. High flux responsivity (0.2 mΦ0/pm) is found making this device attractive for future applications in detecting the quantum properties of carbon nanotube based mechanical resonators.
Work is supported by FOM and the EU (FP7: QNEMS program).
S. Etaki, M. Poot, I. Mahboob, K. Onomitsu, H. Yamaguchi and H.S.J. van der Zant, Nature Physics 4 (2008) 785-788; M. Poot et al., Phys. Rev. Lett. 105 (2010) 207203.
11:00 – 11:30 Coffee
11:30 - 13:00 Session II: Fluctuations and heat transport in nanostructures II
chairman Jukka Pekola
- 11:30 – 12:00 Yuli Nazarov (Delft University of Technology): Towards monitoring big temperature fluctuations in SNS devices
M. A. Laakso, T. T. Heikkilä, Yuli V. Nazarov
We propose a nano-device which exhibits strong and manifestly non-Gaussian fluctuations of energy and temperature when suitably driven out of equilibrium. The setup consists of a normal metal island (N) coupled by tunnel junctions (I) to two superconducting leads (S), forming a SINIS structure, and is biased near the threshold voltage for quasiparticle tunneling. Imortantly, the fluctuations can be measured by monitoring the time-dependent electric current through the system. This makes the setup suitable for the realization of feedback schemes which allow to stabilize the temperature to the desired value.
- 12:00 – 12:30 Klaus Ensslin (ETH Zürich): Irreversibility in single-electron tunneling
The second law of thermodynamics states that a macroscopic system out of thermal equilibrium will irreversibly move toward equilibrium driven by a steady increase of its entropy. This macroscopic irreversibility occurs despite the time-reversal symmetry of the underlying microscopic equations of motion. Also, a microscopic system will undergo an irreversible evolution on a long time scale, but, over a sufficiently short observation time both entropy-producing trajectories as well as their time- reversed entropy-consuming counterparts occur. It is only because of the statistics of these occurrences that a long- term irreversible evolution is established. This phenomenon is described by the fluctuation theorem .
As a step toward the direct test of the fluctuation theorem in the quantum regime, we verify the fluctuation theorem in single- electron tunneling at low temperatures, although our experiment is carried out in the regime of classical charge counting. We employ real-time detection of single- electron charging in quantum dots (QDs). Monitoring the charge state of two QDs that are coupled both in series and to source and drain electrodes allows us to measure the direction-resolved charge flow through this device and consequently the current probability distribution. In order to avoid the spurious backaction, we employ an optimized sample design that combines electron-beam and scanning-probe lithography. It provides the high tunability and electronic stability required for the experiment while maintaining a good QPC-DQD coupling.
- 12:30 – 13:00 Heiner Linke (Lund University): Low-temperature thermoelectric phenomena in nanostructures
Low-dimensional electron systems at low temperatures, such as 0D quantum dots and 1D structures, are interesting as model systems for thermoelectric energy conversion. In addition, thermovoltage measurements on mesoscopic devices can yield information about transport mechanisms that are complementary to traditional conductance measurements.
I will summarize our experimental and theoretical work on thermoelectric energy conversion at subkelvin temperatures in quantum dots, and will discuss a novel, ballistic effect that induces a transverse thermovoltage perpendicular to the applied thermal gradient.
Fahlvik Svensson, S et al., Thermopower Lineshape in Quantum Dots, submitted (2011)
Nakpathomkun et al. Thermoelectric efficiency at maximum power in low-dimensional systems. Phys Rev B (2010) vol. 82 (23) pp. 235428
Hoffmann et al. Measuring Temperature Gradients over Nanometer Length Scales. Nano Lett (2009) vol. 9 (2) pp. 779-783
Humphrey and Linke. Reversible thermoelectric nanomaterials. Phys. Rev. Lett. (2005) vol. 94 (9) pp. 096601
Humphrey et al. Reversible quantum Brownian heat engines for electrons. Phys. Rev. Lett. (2002) vol. 89 (11) pp. 116801
13:00 – 14:30 Lunch
14:30 - 16:00 Session III: Cooling and thermometry
chairman Heiner Linke
- 14:30 – 15:00 Joel Ullom (NIST): Progress on NIS tunnel junction refrigerators
We are developing solid-state refrigerators based on normal metal-insulator-superconductor (NIS) tunnel junctions. These devices are an attractive technology for cooling calorimeter and bolometer sensors to subKelvin operating temperatures. Using large-area, photolithographic junctions, we have previously cooled both bulk objects and thin-film transition-edge sensors (TESs). For example, we used a NIS refrigerator operating at 260 mK to provide an X-ray TES with an effective bath temperature of 160 mK and successfully obtained high-resolution X-ray spectra from the TES. In recent work, we have focused on improving the temperature reduction of our NIS devices. One aspect of this work is a more detailed thermal model that is particularly focused on describing quasiparticle behavior in the superconducting electrode. Using this model, we have designed a new generation of large-area NIS refrigerators that show dramatically increased cooling. We describe these results and also discuss some of the issues related to device thermometry. Finally, we describe ongoing efforts to cool a macroscopic platform for separate, user-supplied electronics.
- 15:00 – 15:30 Dominik Zumbuhl (Universität Basel), Helical Nuclear Order at Ultra-Low Temperatures
The ability to reach low millikelvin or even microkelvin temperatures in nanoscale samples would open up the possibility to discover new physics. For example, mediated by the hyperfine interaction and interacting electrons, helical nuclear spin order in a GaAs 2D electron gas has been predicted around 1 mK at B = 0, constituting a novel type of correlated state. Here, I will present a new method intended to cool nanostructures into the microkelvin regime based on the well established technique of adiabatic nuclear demagnetization. On a parallel network of nuclear refrigerators, temperatures of 185 microK have been reached upon demagnetization, thus completing the first step towards ultracold nanostructures.
Further, I will present evidence of a nuclear spin helix in GaAs cleaved-edge overgrowth quantum wires. In this 1D system, the predicted nuclear ordering temperature is ~75 mK , much higher than in the 2D system. The conductance values of the plateaus are strongly suppressed below the universally expected multiples of 2e2/h . For the lowest mode, the plateau conductance saturates at ~1 e2/h at T < 80 mK, though it is clear that the sample cools well below 80 mK. This suggests that the spin degeneracy is lifted, consistent with recent theory  predicting helical nuclear magnetism in the Luttinger liquid regime.
 "Nuclear magnetism and electron order in interacting one-dimensional conductors", Bernd Braunecker, Pascal Simon, and Daniel Loss, Phys. Rev. B80, 165119 (2009).
 "Nonuniversal Conductance Quantization in Quantum Wires", A. Yacoby, H. L. Stormer, Ned S. Wingreen, L. N. Pfeiffer, K. W. Baldwin, and K. W. West, Phys. Rev. Lett. 77, 4612 (1996).
- 15:30 – 16:00 Hervé Courtois (Université Joseph Fourier & CNRS) Electronic cooling in superconductor-based tunnel junctions: basics and fundamental limitations
A significant electron cooling can be obtained in a micro-cooler made of a pair of Superconductor- Insulator-Normal metal (S-I-N) junctions, when biased at a voltage just below the superconducting gap. Nevertheless, the operation of such electronic coolers appears usually to be less effective and straightforward than expected. After introducing the basic principles at the heart of tunnel junctions-based micro-coolers, I will review several experimental studies devoted to understand how electronic cooling interplays with electron-phonon coupling, Andreev current, hot electron back-tunneling, thermal noise ... I will also describe a new fabrication process for obtaining large-power electronic coolers with excellent characteristics.
16:00 – 16:30 Coffee
16:30 - 18:00 Session IV: Topological insulators and Majorana fermions
chairman Tero Heikkilä
- 16:30 – 17:00 Christoph Bruder (Basel): Interferometric and noise signatures of Majorana fermions in transport experiments
Elementary excitations (often called quasiparticles) of condensed-matter systems can show features that are not displayed by the bare particles that they are composed of. Majorana-like quasiparticles that are their own antiparticles would be a particularly interesting excited state.
Recently, the possibility to realize Majorana-like quasiparticles on the surface of a three-dimensional topological insulator has attracted a lot of attention. It has been theoretically predicted that the domain wall of two superconducting regions support transport channels for Majorana fermions  and the interface of superconducting and magnetic regions give rise to transport channels for chiral Majorana fermions .
We propose to study noise correlations in a Hanbury Brown-Twiss type interferometer and find three signatures of the Majorana nature of the channels . First, the average charge current in the outgoing leads vanishes. Furthermore, we predict an anomalously large shot noise in the output ports for a vanishing average current signal. Adding a quantum point contact to the setup, we find a surprising absence of partition noise which can be traced back to the Majorana nature of the carriers.
Work done in collaboration with G. Struebi (Univ. Basel), W. Belzig (Univ. Konstanz), and M.-S. Choi (Korea University)
 L. Fu and C.L. Kane, Phys. Rev. Lett. 100, 096407 (2008).
 L. Fu and C.L. Kane, Phys. Rev. Lett. 102, 216403 (2009); A.R. Akhmerov, J. Nilsson, and C.W.J. Beenakker, Phys. Rev. Lett. 102, 216404 (2009).
 G. Struebi, W. Belzig, M.-S. Choi, and C. Bruder, Phys. Rev. Lett. 107, 136403 (2011).
- 17:00 – 17:30 Laurens Molenkamp (Würzburg): HgTe as a topological insulator
HgTe is a zincblende-type semiconductor with an inverted band structure. While the bulk material is a semimetal, lowering the crystalline symmetry opens up a gap, turning the compound into a topological insulator. The most straightforward way to do so is by growing a quantum well with (Hg,Cd)Te barriers. Such structures exhibit the quantum spin Hall effect, where a pair of spin polarized helical edge channels develops when the bulk of the material is insulating. Our transport data provide very direct evidence for the existence of this third quantum Hall effect, which now is seen as the prime manifestation of a 2-dimensional topological insulator.
To turn the material into a 3-dimensional topological insulator, we utilize growth induced strain in relatively thick (ca. 100 nm) HgTe epitaxial layers. The high electronic quality of such layers allows a direct observation of the quantum Hall effect of the 2-dimensional topological surface states. Moreover, on contacting these structures with Nb electrodes, a supercurrent is induced in the surface states.
- 17:30 – 18:00 Yuval Oreg (Weizmann institute): What are Majoranas and where to ﬁnd them
Topological quantum computation provides an elegant way around decoherence, as one encodes quantum information in a nonlocal fashion that the environment ﬁnds difﬁcult to corrupt. Zero energy Majorana Fermion states (Majoranas for short) emerges as a key concept for a realization of nonlocal encoding. In this talk we will discuss what are Majoranas? What makes them nonlocal? and how one may create and manipulate them. In particular we will discuss recipes for driving semiconducting wires into a topological phase supporting Majoranas. In this setting Majoranas can be transported, created, and fused by applying locally tunable gates to the wire. More importantly, we will show that networks of such wires allow braiding of Majorana fermions and that they exhibit non-Abelian statistics like vortices in a p+ip superconductor.
- 18:00 – 18:30 Takeshi Mizushima (Okayama University): Surface Majorana fermions and field-induced topological phase transition in superfluid 3He-B
We clarify the interplay of the magnetic field and dipole interaction on surface Majorana fermions of superfluid 3He-B. We first introduce the winding number for the bulk B-phase which may be nontrivial even in the presence of a magnetic field. It is demonstrated that the SO(3) manifold of the B-phase is directly linked to the nontriviality of the winding number, Majorana Ising spins, and the resulting spin susceptibility. Hence, we quantitatively discuss the field-dependences of spin susceptibilities and field-induced topological phase transition, based on the quasiclassical theory taking account of the dipole interaction and magnetic Zeeman energy on equal footing.
18:30 – 20:00 Dinner
20:00 – 21:30 User talks: Nanoelectronics
chairman Erkki Thuneberg
- 20:00 – 20:30 Hung Nguen (Grenoble,Helsinki): Microrefrigerator with enhanced cooling power
We present a novel method for fabricating micron sized electronic coolers by combining photolithography and chemical etching of a superconductor - insulator - normal metal (SIN) multilayer. The process produces SINIS structures with a suspended normal metal part and very high quality junctions. We discuss the enhancement in cooling performances in standard NIS structure with quasiparticle traps and under a small magnetic field, and its guidance in designing an optimized large area cooler. With a direct quasiparticle trap close to the junctions, the cooler approaches its theoretical limit and demonstrates a cooling power beyond 400 pW at 300 mK.
- 20:30 – 21:00 Zuzana Pribulová (Kosice): Local magnetization measurements with a miniature array of Hall probesLocal magnetization measurements with a miniature array of Hall probes, by Z. Pribulová, T. Klein
I shall summarize the work done during my stay in Grenoble in June 2011 which was funded by the Microkelvin Project. Recently we decided to implement a new method of local magnetization measurements in Kosice using miniature Hall probes with the help of an expert from Grenoble. We intend to use probes manufactured in Bratislava which so far have been tested only down to 4.2 K. One of the main aims of my stay was to test them in a new 3He refrigerator, to check their stability and reliability. The tests were performed on a real system – superconducting SrPd2Ge2, isostructural to 122 pnictides. We measured the first penetration field related to the lower critical field of the system. I will discuss the motivation for this research and the results obtained as part of the larger picture.
9:00 - 10:40 Wednesday, 21 March, 2012
Review session I: Summary of progress: milestones and deliverables
Chairman Matti Krusius
- 9:00 – 9:10 Welcome & introduction, Matti Krusius, Microkelvin coordinator
- 9:10 – 9:40 EU Research Infrastructures, Maria Douka, Microkelvin Project Officer
- 9:40 – 10:10 MicroKelvin Collaboration: general matters, Matti Krusius, Microkelvin programme coordinator
- 10:10 – 10:40 Joint Research Activity 1: opening μK regime to nanosciences, Shaun Fisher and George Pickett, activity leaders
10:40 – 11:00 Coffee
11:00 - 13:00 Review session II: Summary of progress: milestones and deliverables
Chairman Matti Krusius
- 11:00 – 11:30 Joint Research Activity 2: nanorefrigeration, Jukka Pekola, activity leader
- 11:30 – 12:00 Joint Research Activity 3: fundamental μK experiments, Henri Godfrin, activity leader
- 12:00 – 12:30 Joint Research Activity 4: methods & devices for μK measurements, Christian Enss, activity leader
- 12:30 – 13:00 Observations and recommendations,
Maria Douka, Project Officer
Advisory Board Members
from the audience: comments & discussion
13:00 – 14:30 Lunch
14:30 – 15:00 Group photograph
15:00 - 16:30 Review session III: Summary of progress: milestones and deliverables
Chairman Matti Krusius
- 15:00 – 15:30 Networking Activities 3: knowledge and technology transfer, Peter Skyba, activity leader
- 15:30 – 16:00 Networking Activities 4: advancement of European low temp research, Henri Godfrin, activity leader
- 16:00 – 16:30 Observations and recommendations:
Maria Douka, Project Officer
Advisory Board Members
from the audience: comments & discussion
16:30 – 17:00 Coffee
17:00 – 18:30 Meetings of Joint Research Activity Packages: plan of actions for the final 12 months
- JRA1: chairmen Shaun Fisher / George Pickett
- JRA2: chairman Jukka Pekola
- JRA3: chairman Henri Godfrin
- JRA4: chairman Christian Enss
18:30 – 20:00 Dinner
20:00 – 21:30 General Assembly Meeting: chairman George Pickett
- review of past 12 months
- plan of actions for the final 12 months
Thursday, 22 March, 2012
9:00 - 11:00 Session I: Nanomechanics and measurement at ultra-low temperatures
chairman Henri Godfrin
- 9:00 – 9:30 Jevak Parpia (Cornell University): Optomechanics of Graphene Resonators
R.A. Barton, I.R. Storch, V. P. Adiga, B. R. Cipriany, R. Sakakibara, B. R. Ilic, S. Wang, P. Ong, T. T. Heikkilä, P. L. McEuen, H. G. Craighead, J. M. Parpia
By virtue of their low mass and stiffness, carbon nanostructures are attractive candidates for use in optomechanics. Here, we demonstrate optical backaction in a graphene mechanical resonator coupled to an optical cavity, with both illuminated by laser light. We demonstrate various gate tuning effects that are modified as the laser power is increased. The continuous wave laser can be used to cool the graphene, to narrow the resonance or to power a graphene-based tunable-frequency optical modulator achieved in self oscillation. Beyond basic physics, the exquisite sensitivity of graphene optomechanical resonators and their ability to operate over a broad range of wavelengths and frequencies makes them attractive for technological applications.
- 9:30 – 10:00 Eddy Collin (Institut Néel, CNRS, Grenoble): Nonlinear dynamics in nanomechanical resonators
Nonlinear dynamics in nanomechanical resonators, by E. Collin, M. Defoort, K. Lulla, J-S. Heron, A. Sultan, T. Moutonet, O. Bourgeois, Yu. Bunkov, H. Godfrin, and F. Pistolesi,
Nanomechanical devices have attracted recently the interest of physicists, for reasons of new applications and fundamental understanding. Indeed, these artificial mechanical devices can be studied and controlled in such an accurate way that they become model systems in which fundamental issues of (here classical)mechanics can be addressed. We present results on a special type of goalpost-shaped nanomechanical devices studied at low temperatures. Particular care is taken in calibrating the setup and understanding the resonance properties of the system . Using a linear excitation technique (magnetomotive) combined with a strongly nonlinear one (capacitive), we can efficiently tune nonlinear effects in the nanoresonator. Parametric amplification with an exceptional gain has been realized and applied to the study of thin films inelasticity . Audio-mechanical mixing has been demonstrated . Finally, the physical issue of dynamic bifurcation, of ubiquitous interest in physics (linked to thermal activation of Josephson junctions, the kinetics of chemical reactions, the magnetization dynamics of macro-molecules…), is presented .
 Article submitted to Rev. Sci. Instrum. (01/2012).
 E. Collin, T. Moutonet, J.-S. Heron, O. Bourgeois, Yu. M. Bunkov, and H. Godfrin, Phys. Rev. B 84, 054108 (2011).
 M. Defoort, K. Lulla, J-S. Heron, O. Bourgeois, E. Collin, and F. Pistolesi, Appl. Phys. Lett. 99, 233107 (2011).
 Article under preparation.
- 10:00 – 10:30 Eva Maria Weig (Ludwig-Maximilians Universität, Munich): Self-sustained oscillation of nanomechanical resonators
Self-oscillation, the generation of a periodic oscillation from a constant input signal in the absence of external modulated driving forces, is a well-known phenomenon in physics. Its ability to convert a direct current (DC) input into a stable oscillation makes it a powerful transduction mechanism for mechanical systems. In particular, the actuation by means of self-oscillation is a viable option for nanomechanical systems where the quest for efficient, non-dissipative driving schemes is ongoing. Notably, this applies to high stress silicon nitride nanoresonators exhibiting quality factors exceeding 100,000 for eigenfrequencies in the 10 MHz range. Self-sustained oscillation of a simple SiN string can be initiated by means of dielectrically coupled cavity-opto- or electromechanics. In a nanomechanical charge shuttle, a device mechanically transporting electrons between two electrodes on a vibrating metal island hosted by a SiN string, DC voltage sustained self-oscillation can be observed. Due to the minimal energy input of this driving scheme, operation at cryogenic temperatures becomes feasible. This may pave the way into the Coulomb blockade regime of discrete mechanical single electron shuttling.
- 10:30 – 11:00 Andrew Armour (University of Nottingham): Dynamics of a driven nanomechanical resonator coupled to a single-electron transistor
A nanomechanical resonator which is coupled capacitively to the island of a single-electron transistor (SET) experiences a fluctuating force due to the flow of electrons. For weak electrostatic coupling the SET damps the mechanical motion and generates Gaussian fluctuations in the position and momentum. The effect of the SET on the resonator is thus like analogous to a thermal bath and the resonator behaves as if it were in thermal equilibrium characterized by an effective temperature. Here we explore the behaviour when the resonator is driven harmonically. We examine the dynamics of the resonator as well as thermodynamic quantities such as the work done on the resonator, paying particular attention to the deviations which arise from what would be expected if the resonator were indeed coupled to a true thermal bath.
11:00 – 11:30 Coffee
11:30 - 13:00 Session II: Exotic fermion systems
chairman Nikolai Kopnin
- 11:30 – 12:00 Mihail Silaev (Physics of Microstructures, RAS, Nizhny Novgorod): Universal mechanism of dissipation in Fermi superfluids at ultralow temperatures
We show that the vortex dynamics in Fermi superfluids at ultralow temperatures is governed by the local heating of the vortex cores creating the heat flux carried by nonequilibrium quasiparticles emitted by moving vortices. This mechanism provides a universal zero temperature limit of dissipation in Fermi superfluids. For the typical experimental conditions realized by the turbulent motion of 3He-B, the temperature of the vortex cores is estimated to be of the order 0.2Tc. The dispersion of Kelvin waves is derived, and the heat flow generated by the Kelvin cascade is shown to have a value close to that experimentally observed.
- 12:00 - 12:30 Erkki Thuneberg (Oulu University): Mechanical forces in a Fermi liquid
The Fermi liquid theory formulated by Landau is a basic paradigm of the behavior of an interacting many-body system. We show that the interactions between quasiparticles lead to a force on a macroscopic object. We show that the oscillation frequency of a pendulum can be increased by immersing it in a Fermi liquid. We apply the Fermi liquid theory to study the mechanical impedance of a vibrating wire immersed in 3He-4He mixtures at low temperatures. We present numerical results based on a direct solution of Landau Boltzmann equation for the 3He quasiparticle distribution in the full scale of the quasiparticle mean-free-paths from the hydrodynamic to the ballistic limit. The results are in fair quantitative agreement with experimental data. In particular, we can reproduce the anomalous increase of the oscillation frequency that has been observed in vibrating wire experiments reaching the ballistic limit. The essential effect of the experimental container and second-sound resonances is demonstrated. Similar approach can be applied to experiments in different geometries. We present preliminary results on a torsional oscillator, and speculate about application to oscillating planar geometry, where puzzling experimental results have been found.
- 12:30 – 13:00 Silke Buehler-Paschen (Institute of Solid State Physics, Vienna University of Technology, Austria): Quantum criticality in heavy-fermion materials
A quantum critical point (QCP) arises in matter when a continuous phase transition is suppressed to absolute zero temperature. Around a QCP strong fluctuations lead to unusual physical properties, and sometimes to the formation of new phases. Heavy-fermion compounds have in recent years emerged as prototypical quantum critical systems. Studies in anisotropic heavy-fermion compounds have shown that different types of QCPs may be induced by the variation of different control parameters (magnetic field, chemical or external pressure), raising the question of the extent to which heavy-fermion quantum criticality is universal. Recently, we have made the unexpected observation of a cubic heavy-fermion material exhibiting a QCP that is accompanied by an abrupt change of Fermi surface. Previously, this kind of QCP was believed to require reduced dimensionality. From these results we have proposed a materials-based global phase diagram that points to the importance of dimensionality - and may serve as guide in the search for a unified theoretical description .
We acknowledge financial support from the European Research Council (ERC Advanced Grant No 227378).  J. Custers, K.-A. Lorenzer, M. Mueller, A. Prokofiev, A. Sidorenko, H. Winkler, A. M. Strydom, Y. Shimura, T. Sakakibara, R. Yu, Q. Si, and S. Paschen, Nature Materials, doi:10.1038/nmat3214.
13:00 – 14:30 Lunch
14:30 - 16:00 Session III: Helium surfaces and surface states
chairman John Saunders
- 14:30 – 15:00 Ryuji Nomura (Tokyo Institute of Technology): Andreev-Majorana bound states on the superfluid 3He-B surface
Surface Andreev bound states of the superfluid 3He B phase are receiving renewed attention as the edge states of a topological superfluid. Topological superfluids are characterized by a non-trivial topological number in the gapped bulk state and gapless edge states on their surfaces. The surface states can be regarded as Majorana fermions as they satisfy the Majorana condition, i.e., a particle and its antiparticle are equivalent, and their linear dispersion is called Majorana cone. We measured the transverse acoustic impedance of the superfluid 3He B phase changing its boundary condition from diffusive scattering up to practically the specular limit via coating the wall with thin layers of superfluid 4He. A growth of low-energy peaks in the transverse acoustic impedance is observed in the superfluid 3He B phase at higher specularities. A self-consistent theoretical calculation reproduces the experiment well and shows that the observed growth of the peak is the reflection of the linear dispersion. Thus, we experimentally confirmed one of the important features of the Majorana fermions supposed to exist on the surface of the superfluid 3He B phase.
- 15:00 – 15:30 James Sauls (Northwestern University, Illinois): Acoustic spectroscopy of fermionic and bosonic excitations in superfluid 3He films
- 15:30 – 16:00 Lev Levitin (Royal Holloway, University of London): Experiments on superfluid 3He in controlled nano-fabricated geometries
by Lev Levitin, Robert Bennett, Andrew Casey, Brian Cowan, Dietmar Drung, Thomas Schurig, Jeevak Parpia, Eugene Surovtsev, and John Saunders.
We present our NMR study of superfluid 3He confined in a well-characterised restricted geometry provided by a nano-fabricated cell with a 0.7 micron thick cavity. NMR is used both to identify the phases and make quantitative measurements of the suppression and distortion of the order parameter. The degree of confinement is continuously tuned with pressure and surface quasiparticle scattering is modified by preplating the walls of the cell with 4He. In a qualitative agreement with theoretical predictions we observe a profound effect of confinement on the phase diagram. This work opens the way for studies of many surface and size phenomena in superfluid 3He and two experiments are currently under preparation.
16:00 – 16:30 Coffee
16:30 - 18:30 Session IV: Low temperature detectors
chairman Christian Enss
- 16:30 – 17:00 Jochem Baselmans (SRON Netherlands Institute for Space Research, Utrecht): Kinetic inductance detectors
Microwave Kinetic Inductance Detectors (MKID) combine device simplicity, intrinsic multiplexing capability and a good sensitivity for radiation detection from the X-ray to the sub-mm part of the electromagnetic spectrum. The latter is achieved by coupling the MKID directly or indirectly to a suitable radiation detecting structure such as an X ray absorber or antenna. Also direct radiation coupling to the MKID itself has been proposed and demonstrated. As a consequence MKIDs are now being developed in a plethora of varieties and for many different applications. Especially at Far Infra-Red-wavelengths band MKIDs offer the possibility to increase the size of detector arrays enormously, which if realized will undoubtedly revolutionize far-infrared astronomy. Arrays for the FIR currently reach a high level of technological maturity, illustrated by the successful demonstration of increasingly large arrays at several ground based observatories such as the CSO, IRAM and APEX. The paper will shortly address the fundamentals of the physics of MKIDs and will elaborate on the various applications of MKID arrays currently under development.
- 17:00 – 17:30 Simon Bandler (NASA, Greenbelt): Performance and understanding of transition-edge sensor microcalorimeters
Transition-Edge Sensor (TES) thermometers are achieving record-setting performance for a wide range of measurements. TES thermometers consist of superconducting thin films electrically biased in the resistive transition. In this presentation I will describe recent results from a variety of different microcalorimeters designed for X-ray spectroscopic measurements in astrophysics and solar physics. These devices combine excellent energy sensitivity and high efficiency, can be fabricated in large numbers using lithographic techniques, and can be read out in large numbers using SQUID amplifiers. Despite the record-setting performance and growing utilization of the technology, a theoretical model of the physics governing TES devices’ superconducting phase transition has until recently proven elusive. Our group at NASA has shown that TESs exhibit weak-link behavior, where, unlike previous models, the average strength of the order parameter varies over the TES. We find our TES measurements have a natural explanation in terms of a spatially varying order parameter. Implications of weak link behavior for microcalorimeter array design, performance and read-out are discussed.
- 17:30 – 18:00 Andreas Fleischmann (Universität Heidelberg): Magnetic calorimeters
Metallic magnetic calorimeters (MMC) are calorimetric particle detectors, typically operated at temperatures below 100 mK, that make use of a paramagnetic temperature sensor to transform the temperature rise upon the absorption of a particle in the detector into a measurable magnetic flux change in a SQUID. During the last years a growing number of groups has started to develop MMC for a wide variety of applications, ranging from alpha-, beta- and gamma-spectrometry over the spatially resolved detection of accelerated molecule fragments to arrays of high resolution x-ray detectors. For soft x-rays an energy resolution of 2.0eV (FWHM) has been demonstrated and we expect that this can be pushed below 1eV in near future. We give an introduction to the physics of MMCs including the typically observed noise contributions and their impact on the energy resolution. We discuss general design considerations, the micro-fabrication of MMCs and the performance of micro-fabricated devices for a couple of applications.
- 18:00 – 18:30 Jörn Beyer (PTB, Berlin): Readout of transition edge sensors and magnetically coupled calorimeters with SQUID current sensors
There are two categories of low-temperature, low-impedance radiation detectors, namely Transition-Edge Sensors (TESs) and Magnetically Coupled Calorimeters (MCCs), that have the potential to significantly improve a variety of photon-sensing applications. For example, TES and MCC detectors and systems are under development to detect single THz photons, to enable the measurement of photon number states at telecom wavelengths with very high quantum efficiency or for high-resolution x ray and gamma ray spectrometers. Owing to their excellent sensitivity and dynamic performance as well as their compatibility with the low operating temperatures, current sensors based upon Superconducting Quantum Interference Devices (SQUIDs) are ubiquitously used to read out TESs and MCCs. The required SQUID performance in terms of input referred current noise, dynamic range, bandwidth, acceptable power dissipation and potential back-action can vary substantially for different TES or MCC detectors. Consequently, suitable SQUID current sensors need to be adapted to the readout configuration at hand. Ground- and satellite-based astronomy instruments that use thousands of TES pixels set particularly stringent requirements on the detector readout and require SQUID-based multiplexers. The contribution will review concepts and performance of state-of-the art SQUID current sensors for single TES and MCC readout as well as SQUID multiplexing techniques.
18:30 – 20:00 Dinner
20:00 – 21:30 Poster session
chairman Carlo Barenghi
- Ryuji Nomura (Tokyo Institute of Technology): Self-organized criticality in quantum crystallization of 4He in aerogel
Two different crystallization processes of 4He in aerogels, observed as creep at high temperatures and avalanche at low temperatures, have been clarified from both the crystallization rate and nucleation probability measurements that the former is via thermal activation and the latter is via macroscopic quantum tunneling. In the quantum tunneling regime, a power law behavior was observed in the avalanche size distribution. This is the first demonstration of self-organized criticality at low temperatures where the quantum nature dominates the dynamical properties of the system. The large-scale cut-off of the power law distribution decreased toward the transition temperature which is probably caused by a dissipation effect on the quantum tunneling.
- Jochem Baselmans (SRON, Utrecht): Generation-recombination noise: the fundamental sensitivity limit of kinetic inductance detectors
- Jochem Baselmans (SRON, Utrecht): Sub-mm photon noise limited detection with lens-antenna coupled microwave kinetic inductance detectors
- Simon Bandler (NASA, Greenbelt): Characterizing weak-link effects in Mo/Au transition-edge sensors
We are developing Mo/Au bilayer transition-edge sensors (TESs) for applications in X-ray astronomy. Critical current measurements on these TESs show they act as weak superconducting links exhibiting oscillatory, Fraunhofer-like, behavior with applied magnetic field. In this contribution we investigate the implications of this behavior for TES detectors, under operational bias conditions. This includes characterizing the logarithmic resistance sensitivity with temperature, α, and current, β, as a function of applied magnetic field and bias point within the resistive transition. Results show that these important device parameters exhibit similar oscillatory behavior with applied magnetic field, which in turn affects the signal responsivity, noise and energy resolution. We compare results for devices that have different geometric contact regions between the absorber and TES, which show different critical current and transition characteristics as a function of applied magnetic field.
- Jörn Beyer (PTB, Berlin): Magnetic Field Fluctuation Thermometers with direct traceability to the PLTS-2000 and GUM-conform uncertainty
Magnetic Field Fluctuation Thermometers with direct traceability to the PLTS-2000 and GUM-conform uncertainty, by J. Beyer, M. Schmidt, J. Engert, G. Wuebbeler, F. Schmaehling, S. AliValiollahi, and H.J. Barthelmess
The Magnetic Field Fluctuation Thermometer (MFFT) is a highly-linear SQUID-based noise thermometer for the temperature range of about 4K to 1mK. In the MFFT a SQUID sensor detects magnetic flux noise above a copper part of about one cubic centimeter volume, which is the actual temperature sensor. For practicality, the spectrum of the magnetic flux noise is analyzed to obtain the temperature information. The MFFT is an easy-to-use thermometer, it principally requires, however, a reference measurement at one known temperature. Aiming at improving the measurement quality of the MFFT and extending its acceptance, we have built a setup for reference measurements with direct traceability to the Provisional Low Temperature Scale (PLTS)-2000. The reference measurements are performed at superconducting transitions temperatures of Cd, Zn, and Al samples that are referenced to the PLTS-2000. The procedure of the MFFT measurements at these reference measurements and at unknown temperatures together with their uncertainty contributions are analyzed. This enables the determination of the total uncertainty of a MFFT in compliance with the Guide to the expression of uncertainty in measurement (GUM).
- Michail Silaev (Physics of Microstructures, RAS, Nizhny Novgorod): Majorana states and longitudinal NMR absorption in a 3He-B film
The topological superfluid 3He-B supports a massless Dirac spectrum of surface bound states which can be described in terms of the self-conjugated Majorana field operators. We discuss here the possible signature of surface bound states in the nuclear magnetic resonance absorption spectrum in a 3He-B film. It is shown that transitions between different branches of the surface states spectrum lead to a nonzero absorption signal in longitudinal NMR when the frequency is larger than the Larmour one.
- Vladimir Dmitriev (Kapitza Institute, Moscow): Measurements of magnetic relaxation in the high-temperature superfluid phase of 3He in nematically ordered aerogel
Measurements of magnetic relaxation in the high-temperature superfluid phase of 3He in nematically ordered aerogel, by V.V. Dmitriev, D.A. Krasnikhin, A.A. Senin, and A.N. Yudin
Magnetic relaxation in the high-temperature superfluid phase of 3He in "nematically ordered" aerogel was measured. It was found that on cooling from the superfluid transition temperature the relaxation rate slowly increases. Then, at a certain temperature, a sharp increase of the rate was observed. Possible origins of this phenomenon will be discussed.
- Piotr Marecki (Universität Duisburg-Essen): Bound states of sound localized on pinned line vortices is superfluids
We present new results related to the dynamics of small disturbances (sound waves) in the presence of a pinned irrotational vortex in a bosonic superfluid. Taking advantage of the acoustic-space description of the problem we show, that the whole problem reduces to solving a form of the Mathieu equation. We present a survey of the most important properties of perturbations localized close to the core, which are similar to what is known as the Kelvin waves in fluid dynamics. However, the gapless mode with the angular number -1 (Kelvin mode), usually discussed in the context of unpinned vortices in superfluid helium or rotating Bose-Einstein condensates, turns out either not to exist or to have completely different dispersion relation if the vortex is pinned. The question of whether or not the acoustic spacetime admits an ergoregion, which is related to the "compactness" of the vortex core, turns out to have a decisive (qualitative) influence on many aspects of sound-propagation phenomena.
- Carlo Barenghi (University of Newcastle): Quantum turbulence: spectra, worms, and decay
by A.W. Baggaley, Y.A. Sergeev, and C.F. Barenghi
Ordinary turbulent flows contain coherent structures, regions of concentrated vorticity called "vortical worms". A turbulent normal fluid is likely to contain similar structures, which may induce the formation of bundles of quantised vortex lines in the superfluid via the mutual friction force (leaving aside problems of stability). In this work we address the simpler but more fundamental problem of the existence of vortex bundles in pure superfluid turbulence at very low temperatures. We shall review the main properties of the quantum turbulence and show numerical evidence of superfluid worms.
- Ladislav Skrbek (Charles University): Steady and decaying quantum turbulence of a bellows-driven 4He superflowby S. Babuin, M. Stammeier, E. Varga, M. Rotter, and L. Skrbek
A low temperature stainless steel bellows assembly has been used to generate turbulent flows of superfluid 4He along channels, in the temperature range 1.3 < T < 2.0 K. Two 11 cm long channels of square cross-section with side 7 and 10 mm have been plugged with superleaks at their ends to cause a net flow of the superfluid component only. The density of quantised vortex lines in the turbulent flow, L, is deduced from the attenuation of second sound propagating perpendicularly to the flow at mid-channel length. For steady state flow velocities v, we found that the square root of the vortex line density is proportional to (v - vc) where vc is the turbulent onset velocity of order 1 mm/s. This functional dependence is in agreement with the Vinen model for vortex lines generation and destruction and with several experiments on thermally generated counterflow and pure superflow. Upon sudden stop of the steady state flow drive, the temporal decay of L has been studied, revealing that the Vinen prediction L ~ 1/t is verified only for relatively low steady state L. Departures to power laws of higher exponent are observed after an initial transient of a few tenths of second. The data is currently under scrutiny to establish whether the assumptions of the Vinen model apply to our apparatus.
- Ladislav Skrbek (Charles University): Visualisation of liquid 4He flowsby M. La Mantia, D. Duda, M. Rotter, and L. Skrbek
An experimental apparatus for the visualization of liquid helium flows has been established at the Charles University in Prague. Several experiments in water have been successfully performed to explore the capabilities of the visualisation system in the analysis of flows similar to those expected at low temperatures. Micron-sized tracer particles, made of hydrogen and deuterium, have been successfully generated by the purpose-made seeding system and injected in liquid helium at various temperatures and mixture ratios. Preliminary results in thermal counterflow have been obtained and show that our flow visualization system seems to be well suited to the task of analysing cryogenic flows. Our main aim is to study in detail the complex interactions between tracer particles, quantized vortices and macroscopic eddies in liquid helium flows. Further thermal counterflow experiments are being planned, as well as experiments on cryogenic flows around oscillating objects, such as cylinders and spheres.
- Andreas Fleischmann (Universität Heidelberg): Investigation of the dephasing of tunneling systems in glasses using two-pulse polarisation echo experimentsby M. Bazrafshan, G. Fickenscher, C. Schötz, M. Schwarze, P. Fassl, A. Fleischmann, and C. Enss
Low temperature properties of glasses are governed by atomic tunneling systems. Many aspects are well described within the phenomenological standard tunneling model. Via their elastic and electric dipole moments tunneling systems interact mutually and with external fields. The dynamics of tunneling systems can be investigated by two-pulse polarisation echo experiments, where the echo amplitude is measured as a function of the delay time between the two excitation pulses. Different dephasing mechanism contribute to the decay of the echo amplitude. In amorphous dielectrics at very low temperatures the dominating dephasing mechanism is spectral diffusion, which is the interaction of resonant tunneling systems with non-resonant thermally fluctuating ones. We have performed such echo decay measurements with an improved setup allowing us to observe echoes at very long delay times where the echo has decayed five orders of magnitude from its original amplitude. The data obtained in this way allows a precision test of the model of spectral diffusion and the distribution of parameters of the tunneling systems given by the standard tunneling model. We will show experimental results from measurement on BK7 and will discuss them in the framework of spectral diffusion and the standard tunneling model.
- Christian Enss (Universität Heidelberg): Magnetic Johnson-noise thermometry at milli-Kelvin temperatures and belowby A. Fleischmann, D. Rothfuß, A. Reiser, and C. Enss
The thermally driven voltage fluctuations of an electrical resistor can be described by the fluctuation-dissipation theorem, providing a fundamental relation between temperature and independently measurable quantities --- one of the prerequisites for primary thermometry. We present a setup for Johnson-noise thermometry at mK and sub-mK temperatures that uses a commercially available dc-SQUID as preamplifier. The noise to be measured is generated by the thermal motion of electrons in a bulk sample of a high purity metal such as gold or copper. These random currents cause fluctuations of magnetic flux in a superconducting pickup coil which is connected to the input coil of a current-sensor dc-SQUID. To characterize the performance of such thermometers we compared prototypes based on different noise source materials to each other, to a PT-NMR thermometer and to a superconducting standard reference device (SRD1000) calibrated against the temperature scale PLTS-2000. The noise thermometer is easy to use, fast and rather insensitive to typical sources of parasitic heating even at lowest temperatures. We present results and discuss general design considerations as well as the dependence of the temperature uncertainty on measurement time.
Friday, 23 March, 2012
9:00 - 11:00 Session I: Coherent NMR modes and Bose-Einstein condensation
chairman Peter Skyba
- 9:00 – 9:30 Yuriy Bunkov (Institut Néel, CNRS, Grenoble): Bose-Einstein condensation of magnons and spin superfluidity in antiferromagnets
The Spin Supercurrent and Bose-Einstein condensation of magnons similar to an atomic BEC was observed in 1984 in superfluid 3He-B. The same phenomena should exist in solid magnetic systems. In this presentation we will report the first observation of magnon BEC in the solid easy plain antiferromagnet CsMnF3. We have observed a magnon BEC in a mode of coupled Nuclear-Electron precession. The dynamical properties of this mode have many similarities with the NMR of superfluid 3He-A, which will be explained. The coupled nuclear electron precession in CsMnF3 shows a very peculiar dynamics. The electron precession frequency drops down to near zero at zero field. The 55Mn NMR frequency in the hyperfine field is very high, about 600 MHz. Due to crossover two mixed modes of precession appear. We will speak about the low frequency electron-nuclear precession mode. Its frequency depends on the orientation of nuclear magnetization. That is similar to the frequency dependence in 3He-A where the BEC of magnons has been established. Furthermore, the involvement of the ordered electron subsystem gives the magnon-magnon interaction, spin waves, and spin supercurrents, while the nuclear subsystem gives the relatively long time of relaxation. That is why the magnon BEC was predicted for CsMnF3 and MnCO3.
- 9:30 – 10:00 Nugzar Suramlishvili (Newcastle University): The relaxation mechanisms and spin-orbital precessing states in 3He-B at ultra-low temperaturesby S.N. Fisher, G.R. Pickett, P. Skyba, and N. Suramlishvili
The B-phase of superfluid 3He can support regions of extremely long-lived coherent spin precession at ultralow temperatures, known as Persistent Precessing Domains (PPD). At the lowest achievable temperatures the orbital viscosity of B-phase becomes vanishingly small, giving rise to the possibility of rapid orbital motions. Leggett's equations are reformulated to allow for orbital dynamics in the absence of dissipation. The resulting non-linear equations of motion couple spin and orbital degrees of freedom resulting in qualitatively new dynamical states.
- 10:00 – 10:30 Vladimir Eltsov (Aalto University): Self trapping and relaxation of magnon condensates in superfluid 3He-B
In superfluid 3He-B traps for magnon excitations can be formed by the order-parameter texture and the applied profile of the static magnetic field. At temperatures around 0.2 Tc and below one can pump magnons into the trap using NMR techniques, to create macroscopic occupation in the ground state or on a selected excited level. Such magnon states form Bose-Einstein condensates as demonstrated by the long-lived coherent spin precession after switching off the pumping. We have found that as the magnon occupation number increases, the orbital texture reorients under the influence of the spin-orbit interaction and the profile of the trap gradually changes from harmonic to a square well, with walls almost impenetrable to magnons. This is the first experimental example of Bose condensation in a box.
With decreasing temperature the life time of the magnon condensates rapidly increases. Our measurements of the relaxation rate in a rotating sample filled with vortex lines clearly demonstrate two contributions to the temperature dependence: One contribution is proportional to the density of the bulk thermal quasiparticles and the other is approximately temperature-independent but rotation-velocity dependent. Whether the latter contribution can be attributed to Majorana fermions, bound to the vortex cores, remains to be established.
- 10:30 – 11:00 Vladimir Dmitriev (Kapitza Institute, Moscow): Phase diagram of superfluid 3He in nematically ordered aerogelby V.V. Dmitriev, D.A. Krasnikhin, A.A. Senin, and A.N. Yudin
Results from experiments with superfluid 3He immersed in a new type of aerogel are described. This aerogel consists of Al2O3 strands which are nearly parallel to each other, we call it "nematically ordered" aerogel. The superfluid phase diagram was measured. Possible origins of the observed superfluid phases will be discussed.
11:00 – 11:30 Coffee
11:30 - 13:00 Session II: Analogue models of quantum fields in the laboratory
chairman George Pickett
- 11:30 – 12:00 Ralf Schützhold (University of Duisburg-Essen): Fundamental quantum field effects in the laboratory?There are several fundamental predictions of quantum field theory, such as Hawking radiation (i.e., black hole evaporation) or the Sauter-Schwinger effect (i.e., electron-positron pair creation out of the quantum vacuum by a strong electric field), which have so far eluded direct experimental verification. However, it should be possible to gain some experimental access to these effects via suitable condensed matter analogues. In this talk, some possibilities for reproducing such fundamental quantum effects in the laboratory are discussed.
- 12:00 – 12:30 Shaun Fisher (Lancaster University): Simulated brane experiments with the AB transition in superfluid 3He
- 12:30 – 13:00 Gil Jannes (Aalto University): Hydrodynamic black hole analogues and the trans-Planckian problem
Surface waves propagating on a background fluid flow experience analogue black/white hole horizons when the velocity of the background flow becomes equal to the propagation speed of the waves themselves. These black hole analogues exhibit a rich phenomenology which depends mainly on their dispersive character, and could have interesting lessons for gravitational black holes. I will discuss some types of analogue black/white hole experiments (mainly with classical fluids) and their possible lessons for the trans-Planckian problem in relativity.
13:00 – 14:30 Lunch
14:30 - 16:00 Session III: Detection of quantized vortices
chairman Shaun Fisher
- 14:30 – 15:00 Yuri Sergeev (Newcastle University): Detection of quantized vortices and vortex structures by Andreev scattering: Theory and numerical analysis
Theoretical and numerical aspects will be discussed of the Andreev scattering technique which is used to detect vortex filaments in superfluid 3He-B. It was found earlier that, in simple configurations ('clusters') of vortices, the Andreev shadow (that is the area of Andreev reflection of thermal quasiparticle excitations) is not necessarily equal to the sum of shadows of individual, isolated vortices. This phenomenon, which we called a 'partial screening', and its implications for interpretation of Andreev reflection measurements will be discussed in some detail. A numerical study will be reported of the Andreev scattering cross-sections of three-dimensional quantized vortex rings in superfluid 3He-B at ultra-low temperatures. Of particular relevance for the analysis of experimental data and observations will be the results which show the dependence of the cross-section on the ring's size and orientation, as well as the calculation of the cross-section averaged over all possible orientations of the vortex ring. Also analyzed will be the role of screening effects for Andreev reflection of quasiparticles by systems of vortex rings. In particular, it is found that the screening factor for a system of unlinked rings depends on the average ring's radius, and that the smaller are the rings the more prominent become the screening effects.
- 15:00 – 15:30 Paul Walmsley (Manchester University): Probing vortex interactions in superfluid 4He using injected electrons
Probing vortex interactions in superfluid 4He using injected electrons, by P.M. Walmsley, P.A. Tompsett and A.I. Golov (School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK)
Electron bubbles in superfluid 4He at low temperature and pressure are attached to quantized vortices either through the nucleation of vortex rings or by sliding along existing vortex lines, allowing forcing and detection of vortex motion by applied electric fields and measurements of current. We present an overview of several recent experiments where we have investigated different types of vortex interactions in superfluid 4He in the low temperature limit. Firstly, we have investigated the collisions between charged vortex rings when their number density is increased. Secondly, we have observed the re-emission of small vortex rings when larger vortex rings collide with vortices in a turbulent tangle. Thirdly, we have used the axial current of electrons sliding along vortices during rotation to probe the highly polarized turbulence that is created when an oscillatory component of rotation is combined with steady rotation.
- 15:30 – 16:00 Paul Williams (Lancaster University): Direct measurement of energy dissipation from decaying quantum turbulence in 3He-B at ultralow temperatures
We describe our experiment on quantum turbulence in superfluid 3He-B in the low temperature limit where the normal fluid fraction becomes vanishingly small. Quantum turbulence, a tangle of quantized vortex lines, is easily generated by mechanical resonators such as vibrating wires and vibrating grids. At low temperatures the kinetic energy contained in the turbulent flow greatly exceeds the thermal energy carried by ballistic quasiparticles. This allows us to directly measure the energy released by the decaying turbulence using black-body radiator techniques. We find that the decay is remarkably similar to that expected for the decay of turbulence in a classical fluid. We discuss recent results.
16:00 – 16:30 Coffee
16:30 - 18:30 Session IV: Dynamics of quantized vortices
chairman Matti Krusius
- 16:30 – 17:00 Jaakko Hosio (Aalto University): Motion of vortex fronts in the zero temperature limit
The equilibrium state of rotating superfluids consists of an array of rectilinear vortex lines oriented parallel to the axis of rotation. With 3He-B in a cylindrical container the transition from an initially vortex-free state to the equilibrium state takes typically place in the form of a propagating vortex front. We describe measurements on superfluid vortex front propagation at temperatures between 0.16-0.4 Tc at 0.5 bar pressure. In the zero-temperature limit, the front velocity tends to a constant value as a function of temperature and to a quadratic dependence on the rotation velocity, with complicated behavior in the transition region between 0.2 and 0.3Tc.
- 17:00 – 17:30 Edouard Sonin (Hebrew University, Jerusalem): Dynamics of helical vortices and twisted vortex bundles
The talk addresses the dynamics of helical vortices forming twisted vortex bundles, which were detected in experimental investigations of turbulence in superfluid 3He-B. Twisted vortex bundles terminating at a lateral wall of a container are analyzed starting from the elementary case when the bundle reduces to a single vortex. The theory considers the laminar regime of the vortex-bundle evolution and investigates the Glaberson-Johnson-Ostermeier instability of the laminar regime, which is a precursor for the transition to the turbulent regime at strong twist of the bundle. It is demonstrated that the vortex front can move with finite velocity even in the absence of mutual friction (the T=0 limit). The dynamics of helical vortex rings is also considered. The analysis based on the canonical Hamilton relation explains anomalous velocities of helical-vortex rings (suppression of the velocity and even inversion of its direction at sufficiently large amplitude of the helical distortion).
- 17:30 – 18:00 Victor Tsepelin (Lancaster University): Power spectrum of vortex line density fluctuations in quantum turbulence in superfluid 3He-B
Quantum turbulence consists of a tangle of quantized vortex lines which interact via their self induced flow. At very low temperatures there is no normal fluid component and no associated viscosity. These are very simple conditions to study turbulence which might eventually lead to a better understanding of turbulence in general. We probe quantum turbulence using vibrating wire detectors in the limit of zero temperature, where the effect of the mutual friction on the evolution of turbulence should be negligible. Our experiments show that this assumption is valid at temperatures below 0.2 Tc. Despite the absence of any normal fluid component and associated viscosity, our measurements show that the frequency power spectrum of vortex line density fluctuations displays a power law of -5/3, reminiscent of the Kolmogorov energy spectrum for classical turbulence. This result is consistent with recent direct measurements of the energy released by freely decaying quantum turbulence. Furthermore at the highest frequencies, the power spectrum exhibits a cross-over to a -3 power law, behaviour that is predicted by various models. Our measurements might provide the first experimental probe of turbulent dynamics at small length scales.
- 18:00 – 18:30 Carlo Barenghi (Newcastle University): Spectra, statistics and worms in quantum turbulence
Tree-algorithms, popular in astrophysics, have made possible better numerical simulations of quantum turbulence. I shall report recent findings concerned with the energy spectrum, the vortex line density spectrum, the statistics of the turbulent velocity field, and the formation of coherent vortex structures. Results will be compared to experimental observations.
18:30 – 20:00 Dinner
20:00 – 21:30 User talks: Ultra-low temperatures
chairman Vladimir Eltsov
- 20:00 – 20:30 Jan Nyeki (Royal Holloway, University of London): Nuclear cooling
In recent years, both the interesting physics and commercial availability of dilution refrigerators have made the microkelvin temperature range an alluring option for solid state physics experiments. Adiabatic nuclear demagnetization is the prime cooling method to cool bulk matter below 1mK. It has served the helium research community well for decades and allowed samples to be cooled below 100 µK. Achieving such low temperatures is, however, still mostly in a “build-it-yourself” phase. Requirements for the design, construction, and operation of a nuclear cooling stage determine the boundary conditions for successful experiments. I will review those and share our experience from research in the microkelvin temperature range [in collaboration with A. Casey, B. Cowan, L. Levitin, C. Lusher, J. Saunders, and A. Shibahara].
- 20:30 – 21:00 David Schmoranzer (Charles University, Prague): Micromechanical resonators in liquid helium
Micromechanical resonators in liquid helium, by D. Schmoranzer and L. Skrbek
The aim of this user report is twofold. First, we summarize recent development in experiments on the properties of quartz tuning forks as probes for quantum fluid dynamics, including their acoustic characteristics as observed in 4He. The second half is centred on processes comprising the design and manufacture of micro- and nano- resonators usable in either of the helium isotopes as thermometers or sensitive quantum turbulence detectors, promising an unprecedented spatial resolution as well as high intrinsic Q-factors at very low temperatures.
- 21:00 – 21:30 Manuel Arrayas (Universidad Rey Juan Carlos, Madrid): Modelling the A-B interface dynamics in superfluid 3He
We investigate the properties of the A-B interface. In some ongoing experiments at Lancaster by the ULT group a shaped magnetic field is used to stabilise and manipulate the phase boundary between the A and B phases. This exploits the influence of a magnetic field on the phase transition between the two, with the B phase being stable up to a critical field of 340 mT, whereupon there is a first-order transition to the A phase. A first-order transition has an energy cost associated with the surface tension between the two phases. We have calculated the equilibrium profile for realistic magnetic fields and boundary conditions, to simulate the interface behaviour when subjected to perturbations, and to see how its properties may be modified by defects that can exist within it. We will discuss some work on progress in order to determine the influence on the dynamics when an inertia mass per unit area of the interface is considered.
Saturday, 24 March, 2012
8:00 – 9:00 Breakfast9:00 – … Departure: transportation to Bratislava/Vienna airports/railway stations