Computed tomography of propagating microwave photons

TL;DR

This paper introduces an amplification-free method for Wigner function tomography of propagating microwave photons using a superconducting bolometer, enabling real-time quantum diagnostics in superconducting networks.

Contribution

It presents a novel, amplification-free tomography technique based on computed tomography principles, suitable for propagating microwave photons at the single-photon level.

Findings

Successful implementation of amplification-free Wigner tomography.

Demonstration of tomography for Gaussian states at the single-photon level.

Use of compressed sensing and neural networks to reduce projections without losing quality.

Abstract

Propagating photons serve as essential links for distributing quantum information and entanglement across distant nodes. Knowledge of their Wigner functions not only enables their deployment as active information carriers but also provides error diagnostics when photons passively leak from a quantum processing unit. While well-established for standing waves, characterizing propagating microwave photons requires post-processing of room-temperature signals with excessive amplification noise. Here, we demonstrate amplification-free Wigner function tomography of propagating microwave photons using a superconductor--normal-metal--superconductor bolometer based on the resistive heating effect of absorbed radiation. By introducing two-field interference in power detection, the bolometer acts as a sensitive and broadband quadrature detector that samples the input field at selected angles at millikelvin with no added noise. Adapting the principles of computed tomography (CT) in medical imaging, we implement Wigner function CT by combining quadrature histograms across different projection angles and demonstrate it for Gaussian states at the single-photon level. Compressed sensing and neural networks further reduce the projections to three without compromising the reconstruction quality. These results address the long-standing challenge of characterizing propagating microwave photons in a superconducting quantum network and establish a new avenue for real-time quantum error diagnostics and correction.