Observation of OAM non-conservation in entangled photon generation

TL;DR

This paper experimentally demonstrates non-conservation of orbital angular momentum in Type-I SPDC, challenging the current understanding, using a high-sensitivity detector and a novel theoretical framework.

Contribution

It introduces a high-sensitivity OAM detector and provides experimental evidence of OAM non-conservation in Type-I SPDC, contradicting previous assumptions.

Findings

Demonstrated non-conservation of OAM in Type-I SPDC.

Developed a high-sensitivity two-photon OAM detector.

Proved non-conservation using a new theoretical framework.

Abstract

Orbital angular momentum (OAM)-entangled states produced by spontaneous parametric down-conversion (SPDC) are considered ideal for realizing high-dimensional entangled states, which have several advantages for quantum technologies. However, the limited sensitivity of current two-photon OAM detectors is a major roadblock not only for realizing such technologies but also for resolving foundational questions, such as OAM conservation in SPDC. The current theoretical understanding is that OAM is not conserved in Type-II SPDC but is conserved in Type-I. Experimentally, although non-conservation in TypeII has not been demonstrated, conservation in Type-I has been reported frequently and has become an underlying assumption for techniques generating high-dimensional OAM entangled states. In this work, we experimentally demonstrate a high-sensitivity two-photon OAM detector, using which, contrary to the current understanding, we report non-conservation of OAM in Type-I SPDC. We attribute this to a spatial walk-off effect and prove it using a framework free of standard phase-matching approximations.