Measuring the quadrature coherence scale on a cloud quantum computer

TL;DR

This paper reports the first measurement of the quadrature coherence scale (QCS) on a cloud quantum computer, demonstrating its sensitivity to decoherence and its potential for certifying quantum features in photonic states.

Contribution

It introduces a method to measure the QCS using a cloud-based quantum optical setup, bridging theory and experiment for a new quantum coherence quantifier.

Findings

QCS can be effectively measured on a cloud quantum computer.

Measured data aligns well with theoretical predictions.

Demonstrates the utility of interferometers and photon-counting in quantum state certification.

Abstract

Coherence underlies quantum phenomena, yet it is manifest in classical theories; delineating coherence's role is a fickle business. The quadrature coherence scale (QCS) was invented to remove such ambiguity, quantifying quantum features of any single-mode bosonic system without choosing a preferred orientation of phase space. The QCS is defined for any state, reducing to well-known quantities in appropriate limits, including Gaussian and pure states, and perhaps most importantly for a coherence measure, it is highly sensitive to decoherence. Until recently, it was unknown how to measure the QCS; we here report on an initial measurement of the QCS for squeezed light and thermal states of light. This is performed using Xanadu's machine Borealis, accessed through the cloud, which offers the configurable beam splitters and photon-number-resolving detectors essential for measuring the QCS. The data and theory match well, certifying the usefulness of interferometers and photon-counting devices in certifying quantumness.