Resonant Cooper Pair Tunneling: Quantum Noise and Measurement Characteristics

TL;DR

This paper investigates the quantum noise and measurement capabilities of a superconducting single-electron transistor at the Cooper pair resonance, highlighting its advantages over non-superconducting devices in quantum measurement efficiency.

Contribution

It provides a detailed density matrix analysis of the measurement back-action and demonstrates improved measurement performance near the Cooper pair resonance.

Findings

Back-action can induce population inversion in the qubit.

Resonant Cooper pair tunneling approaches quantum-limited measurement efficiency.

Superconducting SETs outperform non-superconducting counterparts in measurement precision.

Abstract

We study the quantum charge noise and measurement properties of the double Cooper pair resonance point in a superconducting single-electron transistor (SSET) coupled to a Josephson charge qubit. Using a density matrix approach for the coupled system, we obtain a full description of the measurement back-action; for weak coupling, this is used to extract the quantum charge noise. Unlike the case of a non-superconducting SET, the back-action here can induce population inversion in the qubit. We find that the Cooper pair resonance process allows for a much better measurement than a similar non-superconducting SET, and can approach the quantum limit of efficiency.