Photon-number resolution with microwave Josephson photomultipliers

TL;DR

This paper investigates how a Josephson photomultiplier can be used to count photons in a microwave resonator, optimizing measurement regimes and analyzing resolution limits mainly due to relaxation effects.

Contribution

It introduces optimal operational regimes and measurement techniques for photon counting with JPM, including positive operator-valued measures and resolution measures.

Findings

Photon-number resolution decreases mainly due to JPM relaxation.

Two counting techniques analyzed: total clicks and first no-click event.

Results applicable for testing nonclassical microwave radiation properties.

Abstract

We study counting photons confined in a mode of a microwave resonator via repeated measurements by a Josephson photomultiplier (JPM). The considered JPM is essentially a flux-biased phase qubit operating as a single-photon detector. We identify optimal operational regimes that maximize photon-number resolution within a predetermined range. Two counting techniques are studied. The first is to count the total number of clicks in the measurement sequence. The second involves counting the number of clicks until the occurrence of either the first no-click event or the end of the measurement sequence. Our theoretical methods employ the derived positive operator-valued measures for the considered photocounting techniques and the introduced measure of the photon-number resolution. The results reveal that the resolution decrease in both cases is mainly caused by the JPM relaxation. As an example, we show how the obtained results can be used for practical testing nonclassical properties of electromagnetic radiation in a microwave resonator.