Physicists Take Step Toward Device-Independent Test of “Quantum Before-and-After”

19.03.2026

Vienna, Austria – Researchers at the University of Vienna have experimentally demonstrated a new way to test one of the strangest predictions of quantum theory: that events need not occur in a fixed “before–after” order. The work, carried out in the laboratories of Prof. Philip Walther at the Faculty of Physics and the Vienna Center for Quantum Science and Technology (VCQ), is set to appear in PRX Quantum.

In everyday life and classical physics, events always follow a well-defined temporal sequence. Quantum theory, however, allows for indefinite causal order—situations in which events can occur in a superposition of different orders. For more than a decade, experiments using a setup known as the quantum switch have suggested that such indefinite orders can be realized in the lab, and that they may even be useful for quantum technologies.

Until now, however, most demonstrations relied on detailed modeling of the devices involved, using so‑called device‑dependent assumptions. The new experiment goes significantly further: it implements, for the first time, a protocol that can in principle verify indefinite causal order in a device‑independent way. The conclusions of a device independent test are valid for untrusted or uncharacterized apparatus. Device independent tests are thus essential for confirming phenomenon that might have other, more boring, explanations.  For the quantum switch, this means that a device independent test is needed to rule out the possibility that a hidden definite causal order underlies the experiment.

The team realized the device independent test by implementing a recently proposed Bell-like inequality tailored to causal order, introduced by theorists Tein van der Lugt, Jonathan Barrett, and Giulio Chiribella. This “VBC inequality” is designed so that any process with a fixed causal order must obey certain limits. Using a high‑fidelity photonic quantum switch, the team observed correlations that go beyond what is compatible with any definite ordering of events. The result is a clear signature of indefinite causal order, although loopholes in the experimental implementation must still be closed to claim a fully device‑independent experiment.

“Bell’s inequalities gave us a device-independent way to show that nature is incompatible with local realism,” explains Carla Richter, PhD student at the University of Vienna and co-lead author of the study. “What we’re doing here is conceptually similar, but for causal structure instead of locality. Our results show that no theory with a fixed causal description can explain the correlations we observe.”

The Vienna experiment uses pairs of entangled photons at telecom wavelengths. One photon goes to “Bob”, while the other photon enters a time-bin–based quantum switch. In the quantum switch where two intermediate parties (“Alice 1” and “Alice 2”) act on the photon in a superposition of both orders at the same time. A final party, “Charlie,” performs measurements that, together with Bob’s results, are used to test the causal-order inequality. Inside the quantum switch Alice 1 attempts to send a message to Alice 2, while Alice 2 simultaneously tries to message to Alice 1 using the same photon. The experiment essentially checks to see if two Alices can successfully communicate, while Charlie and Bob also violate a Bell Inequality.  VBC showed that this is only possible if the Alices are placed in a true quantum superposition of orders.

“Our experiment is not yet fully device-independent, as there there are still standard Bell-type loopholes and some new ones specific to indefinite causal order that must be closed,” says Senior Scientist Lee Rozema, a member of the Walther lab and senior author of the paper. “But this is the first time anyone has implemented a device independent protocol with a quantum switch. It shows that photonic quantum switches are good enough to reproduce the ideal qualitative behavior, and it provides a concrete platform to discuss loopholes and move towards a loophole free demonstration of indefinite causal order.”

Beyond its foundational impact, confirming indefinite causal order as a genuine physical resource would bolster a wide range of proposed applications, from enhanced communication and metrology to improved noise mitigation and thermodynamic tasks in quantum devices.

“This experiment brings us closer to treating causal order as a resource on the same footing as entanglement,” adds Richter. “If we can fully certify it in a device‑independent way, it will put many of these proposed quantum advantages on a much firmer footing.”

While this study was the first experimental implementation of the VBC protocol, shortly after this work was completed, two complementary photonic implementations of the same theoretical scheme were reported on the arXiv by Qu et al. and Guo et al., underscoring the strong and growing international interest in device‑independent tests of causal structure

Publication:
C. M. D. Richter, M. Antesberger, H. Cao, P. Walther, and L. A. Rozema, “Towards an Experimental Device-Independent Verification of Indefinite Causal Order,” to appear in PRX Quantum.

Related arXiv preprints:

  • D. Qu et al., “Experimental device-independent certification of indefinite causal order,” arXiv:2508.04643 (2025).
  • Y. Guo et al., “Experimental violation of a Bell-like inequality for causal order,” arXiv:2506.20516 (2025).
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To illustrate signalling within a classical causal order, let us imagine a pigeon that can travel along only one of the paths shown above between Alice 1 and Alice 2. Depending on which path it takes, it will visit one of the Alices first, where it can pick up a letter and deliver it to the other. Classically, the pigeon must travel between the Alices in one well-defined order, which means that either Alice 1 can send a message to Alice 2, or vice versa.

In the experiment, however, a quantum switch is used to send a photon (instead of a pigeon) between the Alices in a superposition of both possible orders, creating an indefinite causal order. In the experiment, we then observe that the Alices can send messages in both directions simultaneously. But this is not enough to determine if the order is truly in a quantum superposition.

To determine if the indefinite order is truly quantum, we use a second photon. The polarization of this second photon is entangled with the order in which the first photon travels between the two Alices. A third party, Charlie, then measures the first photon after the quantum switch, while a fourth party, Bob measures the polarization of the second photon. If Charlie and Bob's confirm that entanglement the two photons remain entangled, while the Alices can send messages to each other, this rules out the possibility that the order in the quantum switch can be described by classically.