Experimental demonstration of input-output indefiniteness in a single quantum device

TL;DR

This paper demonstrates experimentally that a single quantum device can exhibit indefinite input-output directionality, revealing a new resource for quantum information processing and enabling simulations of quantum indefiniteness in time.

Contribution

It introduces a theoretical method to witness input-output indefiniteness and experimentally verifies it using a photonic setup with high statistical significance.

Findings

Successful experimental demonstration with >69 standard deviations significance

Provides a new way to characterize input-output indefiniteness as a quantum resource

Enables tabletop simulations of quantum scenarios with indefinite temporal order

Abstract

Quantum theory allows information to flow through a single device in a coherent superposition of two opposite directions, resulting into situations where the input-output direction is indefinite. Here we introduce a theoretical method to witness input-output indefiniteness in a single quantum device, and we experimentally demonstrate it by constructing a photonic setup that exhibits input-output indefiniteness with a statistical significance exceeding 69 standard deviations. Our results provide a way to characterize input-output indefiniteness as a resource for quantum information and photonic quantum technologies and enable table-top simulations of hypothetical scenarios exhibiting quantum indefiniteness in the direction of time.