Multiplying and detecting propagating microwave photons using inelastic Cooper-pair tunneling

TL;DR

This paper proposes a theoretical method to convert and detect propagating microwave photons by leveraging the nonlinearity of inelastic Cooper-pair tunneling in Josephson junctions, enabling photon multiplication and single-photon detection.

Contribution

It introduces a novel scheme for microwave photon multiplication and detection using inelastic Cooper-pair tunneling, achieving reflectionless conversion and high photon number states.

Findings

Multiplication ratios n > 2 are achievable with realistic parameters.

The process creates frequency-entangled, highly bunched multi-photon states.

The scheme enables single-photon detection without dead time.

Abstract

The interaction between propagating microwave fields and Cooper-pair tunneling across a DC voltage-biased Josephson junction can be highly nonlinear. We show theoretically that this nonlinearity can be used to convert an incoming single microwave photon into an outgoing $n$-photon Fock state in a different mode. In this process, the electrostatic energy released in a Cooper-pair tunneling event is transferred to the outgoing Fock state, providing energy gain. The created multi-photon Fock state is frequency entangled and highly bunched. The conversion can be made reflectionless (impedance-matched) so that all incoming photons are converted to $n$-photon states. With realistic parameters multiplication ratios $n > 2$ can be reached. By two consecutive multiplications, the outgoing Fock-state number can get sufficiently large to accurately discriminate it from vacuum with linear post-amplification and power measurement. Therefore, this amplification scheme can be used as single-photon detector without dead time.