Quantum Time: experimental multi-time correlations

TL;DR

This paper experimentally demonstrates a quantum time mechanism using single photons, illustrating two-time correlations and testing Leggett-Garg inequality violations from different observational perspectives.

Contribution

It provides an experimental realization of the Page-Wootters quantum time mechanism with a novel photonic implementation.

Findings

Violation of Leggett-Garg inequality observed from internal perspective

External observer perceives a time-independent global state

Successful encoding of clock in photon position

Abstract

In this paper we provide an experimental illustration of Page and Wootters' quantum time mechanism that is able to describe two-time quantum correlation functions. This allows us to test a Leggett-Garg inequality, showing a violation from the "internal" observer point of view. The "external" observer sees a time-independent global state. Indeed, the scheme is implemented using a narrow-band single photon where the clock degree of freedom is encoded in the photon's position. Hence, the internal observer that measures the position can track the flow of time, while the external observer sees a delocalized photon that has no time evolution in the experiment time-scale.