Symmetry Protected Two-Photon Coherence Time

TL;DR

This paper demonstrates that symmetry can protect the coherence time of biphotons generated in atomic media, showing that degenerate biphotons maintain their coherence despite medium losses, unlike nondegenerate biphotons.

Contribution

It provides experimental validation of symmetry protection of two-photon coherence time in biphotons from atomic media, highlighting a new method to control photonic quantum states.

Findings

Degenerate biphotons have coherence times unaffected by medium absorption.

Nondegenerate biphotons exhibit exponential decay of coherence due to absorption.

Symmetry plays a crucial role in preserving quantum coherence in photonic systems.

Abstract

We report the observation of symmetry protected two-photon coherence time of biphotons generated from backward spontaneous four-wave mixing in laser-cooled $^{87}$Rb atoms. When biphotons are nondegenerate, non-symmetric photonic absorption loss results in exponential decay of the temporal waveform of the two-photon joint probability amplitude, leading to shortened coherence time. In contrast, in the case of degenerate biphotons, when both paired photons propagate with the same group velocity and absorption coefficient, the two-photon coherence time, protected by space-time symmetry, remains unaffected by medium absorptive losses. Our experimental results validate these theoretical predictions. This outcome highlights the pivotal role of symmetry in manipulating and controlling photonic quantum states.