Photon State Tomography for Two-Mode Correlated Itinerant Microwave Fields

TL;DR

This paper demonstrates the experimental reconstruction of two-mode correlated microwave fields, revealing strong quadrature correlations and paving the way for observing entanglement in microwave photons.

Contribution

It introduces a method for full quantum state tomography of two-mode microwave fields using heterodyne detection and covariance matrix analysis.

Findings

Successful generation of two-mode correlated states with a Josephson parametric amplifier

Full reconstruction of the four-dimensional Wigner function of the microwave fields

Identification of strong quadrature amplitude correlations between the two modes

Abstract

Continuous variable entanglement between two modes of a radiation field is usually studied at optical frequencies. As an important step towards the observation of entanglement between propagating microwave photons we demonstrate the experimental state reconstruction of two field modes in the microwave domain. In particular, we generate two-mode correlated states with a Josephson parametric amplifier and detect all four quadrature components simultaneously in a two-channel heterodyne setup using amplitude detectors. Analyzing two-dimensional phase space histograms for all possible pairs of quadratures allows us to determine the full covariance matrix and reconstruct the four-dimensional Wigner function. We demonstrate strong correlations between the quadrature amplitude noise in the two modes. Under ideal conditions two-mode squeezing below the standard quantum limit should be observable in future experiments.