Observation of quantum capacitance in the Cooper-pair transistor

TL;DR

This paper reports the experimental observation of quantum capacitance in a Cooper-pair transistor, demonstrating its potential for charge qubit readout by measuring the reflected rf signal.

Contribution

The study provides the first measurement of quantum capacitance in a CPT, highlighting its dependence on gate voltage and its significance in quantum device applications.

Findings

Quantum capacitance detected via rf reflection measurements.

Quantum capacitance can be significantly larger than geometric capacitance.

Potential application in charge qubit readout systems.

Abstract

We have fabricated a Cooper-pair transistor (CPT) with parameters such that for appropriate voltage biases, the sub-gap charge transport takes place via slow tunneling of quasiparticles that link two Josephson-coupled charge manifolds. In between the quasiparticle tunneling events, the CPT behaves essentially like a single Cooper-pair box (SCB). The effective capacitance of a SCB can be defined as the derivative of the induced charge with respect to gate voltage. This capacitance has two parts, the geometric capacitance, C_geom, and the quantum capacitance C_Q. The latter is due to the level anti-crossing caused by the Josephson coupling. It depends parametrically on the gate voltage and is dual to the Josephson inductance. Furthermore, it's magnitude may be substantially larger than C_geom. We have been able to detect C_Q in our CPT, by measuring the in-phase and quadrature rf-signal reflected from a resonant circuit in which the CPT is embedded. C_Q can be used as the basis of a charge qubit readout by placing a Cooper-pair box in such a resonant circuit.