Detecting drift of quantum sources: not the de Finetti theorem

TL;DR

This paper introduces a method to detect and analyze drift in nonstationary quantum sources, extending existing techniques to systems of arbitrary size, with practical implementation via the Hong-Ou-Mandel effect for single-photon wavepackets.

Contribution

It generalizes drift detection methods to quantum systems of any dimension and distinguishes between diffusive and systematic drifts.

Findings

The method can identify source drift speed.

It applies to quantum systems of arbitrary Hilbert space.

Hong-Ou-Mandel effect enables implementation for single-photon wavepackets.

Abstract

We propose and analyze a method to detect and characterize the drift of a nonstationary quantum source. It generalizes a standard measurement for detecting phase diffusion of laser fields to quantum systems of arbitrary Hilbert space dimension, qubits in particular. We distinguish diffusive and systematic drifts, and examine how quickly one can determine that a source is drifting. We show that for single-photon wavepackets our measurement is implemented by the Hong-Ou-Mandel effect.