Two-particle quantum interference in a nonlinear optical medium: a witness of timelike indistinguishability

TL;DR

This paper demonstrates a novel form of quantum interference in a nonlinear medium, revealing timelike indistinguishability of photons, with potential applications in generating complex quantum states for photonic quantum computing.

Contribution

It introduces and experimentally verifies a new regime of nonlinear quantum interference based on timelike indistinguishability of photons.

Findings

Suppression of single-photon pair detection probability at specific gain.

Observation of destructive interference between transmitted and reborn photons.

Extension of quantum interference phenomena into the nonlinear, timelike domain.

Abstract

The Hong-Ou-Mandel effect is a paradigmatic quantum phenomenon demonstrating the interference of two indistinguishable photons that are linearly coupled at a 50:50 beam splitter. Here, we transpose such a two-particle quantum interference effect to the nonlinear regime, when two single photons are impinging on a parametric down-conversion crystal. Formally, this transposition amounts to exchanging space and time variables, giving rise to an unknown form of timelike quantum interference. The two-photon component of the output state is a superposition of the incident photons being either transmitted or reborn, that is, replaced by indistinguishable substitutes due to their interaction with the nonlinear crystal. We experimentally demonstrate the suppression of the probability of detecting precisely one photon pair when the amplification gain is tuned to 2, which arises from the destructive interference between the transmitted and reborn photon pairs. This heretofore unobserved quantum manifestation of indistinguishability in time pushes nonlinear quantum interference towards a new regime with multiple photons. Hence, composing this effect with larger linear optical circuits should provide a tool to generate multimode quantum non-Gaussian states, which are essential resources for photonic quantum computers.