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.