Dartmouth Events

Abstract:

Mesoscopic quantum systems currently serve as essential building blocks in many quantum information and metrology devices. This thesis investigates the potential of quantum-limited detection in a mesoscopic electrometer named the cavity-embedded Cooper pair transistor (cCPT). As one application, this charge detector can act as the basis for an optomechanical system in the single-photon strong coupling regime. The realization of this scheme would entail near quantum-limited, ultra-sensitive electrometry at the single-photon level, the feasibility of which is studied at length in this thesis.
 

On the one hand, we approach this question using a fundamental, first-principles study, where an operator scattering model is used to analyze the quantum dynamics of this device. While the cCPT is inherently a tunable, strongly nonlinear system affording diverse functionalities, we restrict our analysis to a necessary first investigation of its linear charge sensing capabilities, limiting to low pump powers corresponding to an average cavity photon number ≲ 1. Assuming realizable cCPT parameters, we predict the fundamental, photon shot noise-limited charge sensitivity to be 0.12 μe/√Hz when the pumped cavity has an average of one photon. In practice, this lower bound is difficult to achieve using conventional detection approaches, owing mainly to the low-frequency noise caused by the coupling of two-level systems to the cCPT. Hence we further employ a top-down approach where the gate-dependent tunability of the cCPT is used to implement a feedback scheme derived from the Pound-Drever-Hall locking technique. This scheme effectively reduces the fluctuations due to intrinsic charge noise. In particular, we report a reduction in the resonant frequency fluctuations caused by the internal charge noise over a bandwidth of ∼1.4 kHz when the cavity is driven at an average photon number n = 10, and a bandwidth of 11 Hz for average n = 1. Our technique can be generalized to achieve frequency stabilization in tunable microwave resonators that play a vital role in today’s quantum computing architectures, thereby moderating the limitations in detection caused by the intrinsic 1/f -noise on such circuit devices. As a concluding study, we incorporate these feedback techniques to improve the charge sensitivity of the cCPT, thus demonstrating the potential of near quantum-limited charge detection using this device.