Experimental Verification of an Indefinite Causal Order

TL;DR

This paper experimentally demonstrates a process with an indefinite causal order in quantum mechanics by measuring a causal witness, providing the first decisive verification of such a phenomenon.

Contribution

It presents the first experimental verification of an indefinite causal order using a superposition of causal orders and a causal witness measurement.

Findings

Successfully measured a causal witness indicating indefinite causal order

Demonstrated coherence preservation in a superposition of causal orders

Established statistical significance of almost seven standard deviations

Abstract

Investigating the role of causal order in quantum mechanics has recently revealed that the causal distribution of events may not be a-priori well-defined in quantum theory. While this has triggered a growing interest on the theoretical side, creating processes without a causal order is an experimental task. Here we report the first decisive demonstration of a process with an indefinite causal order. To do this, we quantify how incompatible our set-up is with a definite causal order by measuring a 'causal witness'. This mathematical object incorporates a series of measurements which are designed to yield a certain outcome only if the process under examination is not consistent with any well-defined causal order. In our experiment we perform a measurement in a superposition of causal orders - without destroying the coherence - to acquire information both inside and outside of a 'causally non-ordered process'. Using this information, we experimentally determine a causal witness, demonstrating by almost seven standard deviations that the experimentally implemented process does not have a definite causal order.