Ultrasensitive Charge Detection Using a Cavity Embedded Single Cooper Pair Transistor

Abstract

Abstract : We have investigated an alternate approach to standard charge sensing of quantum dots Dsbased on the radio-frequency single electron transistor (RF-SET) or quantum point contact (RF-QPC). Our approach should be more sensitive and less invasive than standard approaches, and allow higher device density. By embedding a superconducting SET known as a Cooper pair transistor (CPT) in a superconducting microwave cavity, it is possible to use the charge sensitivity of the CPT's quantum inductance to dispersively tune the cavity resonant frequency. A carrier wave at the resonant frequency then experiences a charge-dependent phase shift. Charge detection is in turn achieved via interferometric measurement of the phase of a transmitted carrier wave. In this detection scheme the CPT is operated on its supercurrent branch and contributes no shot noise, allowing the usual shot-noise limit on charge sensitivity to be beaten. Furthermore, multiple CPTs could be embedded in the same cavity allowing at least four times the areal device density possible for the RF-SET. Finally, the detection process should be quantum limited, having the minimal effect on a measured QD allowed by quantum mechanics.