Cooper-Pair Pumping

Highlights of recent research: Charge pumping in a Cooper pair sluice [pdf]

Single-electron pumps are known to produce extremely accurate current sources
but their yield in terms of maximum achievable current is very low, far too
small to be applied in metrology. Josephson junction based Cooper pair pumps can
produce larger current, but up to now they have suffered from errors, whose
origins are now under investigation. We work on devices where pumping is
achieved by a combined charge and flux control in a device coined ‘sluice’. The
goals of this work are in two directions. First, there is a fundamental
connection between charge pumped in an adiabatic cycle and geometric phases, in
particular Berry phase. Secondly, the pumping speed and accuracy are to be
optimized to make the device suitable for metrology.

Fig 1:

We have recently measured pumped current in devices where the pump is
embedded in a superconducting loop, with another Josephson junction as a
threshold detector of the pumped current in series. This allows us to observe
the pumped current in a phase-biased configuration, which is needed to observe
the geometric phases. In the preliminary measurements the proof of the concept
has been demonstrated, and more advanced measurements are presently going on.

In another set of experiments we optimized the sluice such that pumped
currents up to 1 nanoampere could be obtained. This is sufficient yield for a
metrological pump. Now we are improving the junction configurations such that
SQUIDs with better switching ratios could be inserted in a sluice in order to
suppress the leakage errors. Eventually the device would serve as the current
source of metrological accuracy in so-called metrological triangle.

Fig 2: Scanning electron micrograph of the small metallic island with four Josephson junctions, which is the
main part in the Cooper-pair sluice.

Fig 3: An example of geometric phase appearing in the physics: while the vector is parallel transported on the surface of a sphere it
accumulates phase which depends on the loop traversed.