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Complex Light and Optical Forces

Measuring the impossible by ignorance of events

Melvee George, William Gunn, Jaden McKinlay, Aaron Z. Goldberg, Khabat Heshami, Markus Rambach, Luis Lorenzo Sánchez-Soto, Andrew G. White

Published Mar 5, 2026• 0 citations
Publisher

Abstract

In this talk I’ll introduce indefinite causal order— quantum superposition of ordering of events—and show you how it can be done with photons, the fundamental particle of light. We’ll then explore using this trick to communicate via perfectly noisy channels—which is classically impossible—and to measure rotations with completely noisy probes via a channel that erases all information—again, classically impossible. Questions before, during, and after the talk are highly encouraged.

DOI
10.1117/12.3088727

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In this talk I’ll introduce indefinite causal order— quantum superposition of ordering of events—and show you how it can be done with photons, the fundamental particle of light. We’ll then explore using this trick to communicate via perfectly noisy channels—which is classically impossible—and to measure rotations with completely noisy probes via a channel that erases all information—again, classically impossible. Questions before, during, and after the talk are highly encouraged.
Investigating the role of causal order in quantum mechanics has recently revealed that the causal relations of events may not be a priori well defined in quantum theory. Although this has triggered a growing interest on the theoretical side, creating processes without a causal order is an experimental task. We report the first decisive demonstration of a process with an indefinite causal order. To do this, we quantify how incompatible our setup is with a definite causal order by measuring a "causal witness." This mathematical object incorporates a series of measurements that 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 nonordered process." Using this information, we experimentally determine a causal witness, demonstrating by almost 7 SDs that the experimentally implemented process does not have a definite causal order.