In-line detection and reconstruction of multi-photon quantum states

TL;DR

This paper introduces a method for in-line detection and reconstruction of multi-photon quantum states in integrated photonic circuits, enabling real-time quantum state monitoring without disturbing the transmitted photons.

Contribution

The paper proposes a novel in-line measurement technique using photon correlations on coupled waveguides, with proof-of-principle experiments demonstrating its feasibility with classical light.

Findings

Successful theoretical reconstruction of quantum states from photon correlations

Proof-of-principle experiments with classical light emulate single-photon detection

Method enables practical, fast in-line quantum measurements

Abstract

Integrated single-photon detectors open new possibilities for monitoring inside quantum photonic circuits. We present a concept for the in-line measurement of spatially-encoded multi-photon quantum states, while keeping the transmitted ones undisturbed. We theoretically establish that by recording photon correlations from optimally positioned detectors on top of coupled waveguides with detuned propagation constants, one can perform robust reconstruction of the density matrix describing the amplitude, phase, coherence and quantum entanglement. We report proof-of-principle experiments using classical light, which emulates single-photon regime. Our method opens a pathway towards practical and fast in-line quantum measurements for diverse applications in quantum photonics.