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  • Featured in Physics
  • Open Access

Toward an Experimental Device-Independent Verification of Indefinite Causal Order

Carla M.D. Richter1,*,†, Michael Antesberger1,*, Huan Cao1, Philip Walther1,2, and Lee A. Rozema1,‡

  • 1University of Vienna, Faculty of Physics, Vienna Center for Quantum Science and Technology (VCQ) & Research platform TURIS, Boltzmanngasse 5, 1090 Vienna, Austria
  • 2Christian Doppler Laboratory for Photonic Quantum Computer, Faculty of Physics, University of Vienna, 1090 Vienna, Austria

  • *These two authors contributed equally to this work.
  • †Contact author: carla.richter@univie.ac.at
  • ‡Contact author: lee.rozema@univie.ac.at
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PRX Quantum 7, 010354 – Published 17 March, 2026

DOI: https://doi.org/10.1103/5t2y-ddmt

Abstract

In classical physics, events follow a definite causal order: the past influences the future, but not the reverse. Quantum theory, however, permits superpositions of causal orders—the so-called indefinite causal orders (ICOs)—which can provide operational advantages over classical scenarios. Verifying such phenomena has sparked significant interest, much like earlier efforts devoted to refuting local realism and confirming quantum entanglement. To date, demonstrations of ICO have all been based a process called the quantum switch and have relied on device-dependent or semi-device-independent protocols. Achieving a device-independent verification of ICO would imply that nature allows for correlations that do not respect causality, independent of any experimental assumptions or underlying theoretical description of the experiment. To this end, a recent theoretical development introduced a Bell-like inequality that allows for fully device-independent verification of ICO in a quantum switch. Here we implement this verification by experimentally violating this inequality. In particular, we measure a value of 1.8328±0.0045, which is 18 standard deviations above the definite causal order bound of 1.75. Our work presents the first implementation of a device-independent protocol to verify ICO, albeit in the presence of experimental loopholes. This represents an important step toward the device-independent verification of an ICO and provides a context in which to identify loopholes specifically related to the verification of ICO.

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Research News

Can Before and After Be Superposed?

Published 17 March, 2026

Through a quantum-switch experiment, researchers attempt to reveal “indefinite causal order”—a quantum phenomenon involving events in a before-and-after superposition.

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Popular Summary

Classical intuition dictates a strict order of cause and effect, but quantum theory suggests the possibility of “indefinite causal order” (ICO), where events can occur in a superposition of different temporal sequences. Demonstrating ICO has implications for fundamental physics and could lead to new quantum technologies.

Previous demonstrations of ICO relied on a process called the quantum switch, but required detailed knowledge of the devices used, making them device-dependent. In fact, it was thought that a fully device-independent verification of ICO could not be achieved with the quantum switch. Now, Richter et al. present an experiment that challenges this, taking a significant step toward device-independent verification. The team implemented a quantum switch, where the order of operations is entangled with the polarization of a second photon, and tested a recently developed Bell-like inequality that is violated only if ICO is present.

The experiment successfully violated the inequality, providing strong evidence for ICO using a quantum switch. While the experiment is not loophole-free (it does not eliminate all possible classical explanations), it represents an important advance. The result highlights the need for further research into loophole-free tests of ICO and opens new avenues for exploring the foundations of quantum mechanics and its potential to revolutionize information processing.

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PRX Quantum 7, 010354– Published 17 March, 2026
  • Received 9 September 2025
  • Revised 15 December 2025
  • Accepted 6 January 2026
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