Quantum state tomography of an itinerant squeezed microwave field

TL;DR

This paper demonstrates quantum state tomography of an itinerant squeezed microwave field using Josephson parametric amplifiers, achieving improved measurement efficiency and revealing significant squeezing below vacuum noise.

Contribution

It introduces a method to perform state tomography of itinerant microwave squeezed states with enhanced quantum efficiency using a second JPA for pre-amplification.

Findings

Quantum efficiency improved from 2% to 36% using a second JPA.

Minimum quadrature variance observed at 69% of vacuum variance.

Reconstructed state shows a minimum variance less than 40% of vacuum.

Abstract

We perform state tomography of an itinerant squeezed state of the microwave field prepared by a Josephson parametric amplifier (JPA). We use a second JPA as a pre-amplifier to improve the quantum efficiency of the field quadrature measurement (QM) from 2% to 36 +/- 4%. Without correcting for the detection inefficiency we observe a minimum quadrature variance which is 69 +/- 8% of the variance of the vacuum. We reconstruct the state's density matrix by a maximum likelihood method and infer that the squeezed state has a minimum variance less than 40% of the vacuum, with uncertainty mostly caused by calibration systematics.