Theory of Josephson Photomultipliers: Optimal Working Conditions and Back Action

TL;DR

This paper analyzes the back action of a Josephson photomultiplier in microwave-photon detection, revealing how measurement duration influences the operator's nature and identifying optimal conditions for quantum state tomography.

Contribution

It provides a detailed theoretical description of the back action operator of JPM and identifies optimal conditions for measurement, enabling multiple measurements and improved quantum state tomography.

Findings

Back action depends on measurement duration, resembling photon annihilation at short times.

Optimal conditions differ from quantum information processing, with short T2 suppressing dephasing.

Understanding back action enables multiple measurements for efficient state tomography.

Abstract

We describe the back action of microwave-photon detection via a Josephson photomultiplier (JPM), a superconducting qubit coupled strongly to a high-quality microwave cavity. The back action operator depends qualitatively on the duration of the measurement interval, resembling the regular photon annihilation operator at short interaction times and approaching a variant of the photon subtraction operator at long times. The optimal operating conditions of the JPM differ from those considered optimal for processing and storing of quantum information, in that a short $T_2$ of the JPM suppresses the cavity dephasing incurred during measurement. Understanding this back action opens the possibility to perform multiple JPM measurements on the same state, hence performing efficient state tomography.