Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Log inSign up
Challenges
Datasets
Workspaces
Papers
Discussions
Leaderboard
Blog
Job Board
Q3AS

© 2026 Aqora Quantum S.A.S.

TermsPrivacyLegal Notice

Physical Review Letters

Generalized Indefinite Causal Orders in an Integrated Quantum Switch

Yaohao Deng, Shuheng Liu, Xiaojiong Chen, Zhaorong Fu, Jueming Bao, Yun Zheng, Qihuang Gong, Qiongyi He, Jianwei Wang

Published Oct 1, 2025• 6 citations
Publisher

Abstract

Indefinite causal order (ICO), a striking feature of quantum mechanics, has attracted broad theoretical and experimental interest. While ICO holds promise for quantum information processing, akin to quantum entanglement, its experimental realization and rigorous verification remain fundamental challenges. Here, we demonstrate and certify generalized multidimensional multipartite ICOs using a programmable integrated-photonic quantum chip implementing a generalized quantum switch. By violating definite causal order bounds with high statistical significance, we experimentally characterize and verify these generalized ICOs. Furthermore, we explore a resource theory of causal nonseparability by implementing a single-copy distillation protocol. Our Letter constitutes a major step toward a generalized framework for quantum causality.

DOI
10.1103/39vh-84n1

Related papers

Toward an Experimental Device-Independent Verification of Indefinite Causal Order

Carla M.D. Richter, Michael Antesberger, Huan Cao +2 more

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.

arXiv:2506.16949•PRX Quantum•Jan 6, 2026•4 citations

Strict Verification of Multiple Indefinite Causal Orders on a Programmable Silicon-Photonic Quantum Chip

Yahohao Deng, Jianwei Wang

We report strict verification of multiple types of indefinite causal orders and demonstrate distillation of indefinite causal order utilizing a programmable silicon-photonic quantum chip. This experimental implementation contributes to the development of a resource theory for indefinite causal order.

•Jan 1, 2024•0 citations

Experimental violation of a Bell-like causal inequality in a photonic quantum switch

Dengke Qu, Quan Lin, Lei Xiao +2 more

Indefinite causal order can be realized by a quantum switch, yet its experimental certification has so far relied on assumptions about the devices. A device-independent verification of indefinite causal order would show, from the observed correlations alone and without relying on a detailed model of the devices, that they are incompatible with any classical description based on a fixed causal order. Here, we implement a photonic quantum switch in a device-independent scenario and test a Bell-like causal inequality using entangled photons. One photon serves as the control qubit of the quantum switch, while the other is measured by a distant party with rapidly and independently chosen settings. We observe a violation of the causal inequality by 24 standard deviations. These results provide experimental evidence for indefinite causal order, albeit in the presence of remaining loopholes, and mark an important step toward a fully loophole-free device-independent verification of causal indefiniteness in quantum processes.

Science Advances•Aug 7, 2026•0 citations

Time-Delocalized Local Measurements in an Indefinite Causal Order

Yann Valibouse, Martí Cladera-Rosselló, Michael Antesberger +3 more

Processes with indefinite causal order (ICO), such as the quantum switch, are an emerging resource for quantum tasks and a fundamental test bed for studies of temporal correlations in quantum mechanics. A limitation of past photonic implementations of the quantum switch, however, is their inability to perform measurements inside the switch without either destroying the superposition of causal orders or delaying readout until the after the quantum switch. Measurements where the results are read out locally are needed for several applications of ICO, but also for a loophole-free verification of ICO. Here, we overcome past limitations by introducing a $\mathit{local}$ measurement scheme and coupling the photon in the switch to a $\mathit{time-delocalized}$ ancilla system. We experimentally realize this protocol using a photonic quantum switch with post-selected linear optical logic gates. Our method ensures that the measurement apparatus interacts with the system at two distinct times and yet yields a single outcome. We use a quantum eraser measurement to preserve the ICO, which we certify by measuring a causal witness and finding a negative value of $\mathcal{C}_W \approx -0.305 (1)$. Furthermore, by explicitly realizing a time-delocalized ancilla system, our protocol not only enables a new class of quantum switch protocols requiring local readout, but also provides a general method for path-coherence-preserving measurements with broad applications beyond ICO.

quant-pharXiv:2604.11878•Apr 15, 2026•0 citations

Semi-device-independent certification of indefinite causal order in a photonic quantum switch

Huan Cao, Jessica Bavaresco, Ningning Wang +7 more

Quantum processes are transformations that act on quantum operations. Their study led to the discovery of the phenomenon of indefinite causal order: some quantum processes, such as the quantum switch, act on independent quantum operations in such a way that the order in which the operations are acted upon not only cannot be determined but is simply undefined. This is the property that we experimentally certify in this work. We report an experimental certification of indefinite causal order that relies only on the characterization of the operations of a single party. We do so in the semi-device-independent scenario with the fewest possible assumptions of characterization of the parties’ local operations in which indefinite causal order can be demonstrated with the quantum switch. To achieve this result, we introduce the concept of semi-device-independent causal inequalities and show that the correlations generated in a photonic quantum switch, in which all parties are able to collect local outcome statistics, achieve a violation of this inequality of 224 standard deviations. This result consists of the experimental demonstration of indefinite causal order with the fewest device-characterization assumptions to date.

arXiv:2202.05346•Optica•Mar 31, 2023•23 citations

Quantum simulation of indefinite causal order induced quantum refrigeration

Huan Cao, Ning-Ning Wang, Zhian Jia +6 more

In the classical world, physical events always happen in a fixed causal order.However, it was recently revealed that quantum mechanics allows events to occur with indefinite causal order (ICO).In this study, we use an optical quantum switch to experimentally investigate the application of ICO in thermodynamic tasks.Specifically, we simulate the working system interacting with two identical thermal reservoirs in an ICO, observing the quantum heat extraction even though they are in thermal equilibrium where heat extraction is inaccessible by traditional thermal contact.Using such a process, we simulate an ICO refrigeration cycle and investigate its properties.We also show that by passing through the ICO channel multiple times, one can extract more heat per cycle and thus obtain a higher refrigeration performance.Our results suggest that the causal nonseparability can be a powerful resource for quantum thermodynamic tasks.

arXiv:2101.07979•Physical Review Research•Aug 16, 2022•32 citations
Indefinite causal order (ICO), a striking feature of quantum mechanics, has attracted broad theoretical and experimental interest. While ICO holds promise for quantum information processing, akin to quantum entanglement, its experimental realization and rigorous verification remain fundamental challenges. Here, we demonstrate and certify generalized multidimensional multipartite ICOs using a programmable integrated-photonic quantum chip implementing a generalized quantum switch. By violating definite causal order bounds with high statistical significance, we experimentally characterize and verify these generalized ICOs. Furthermore, we explore a resource theory of causal nonseparability by implementing a single-copy distillation protocol. Our Letter constitutes a major step toward a generalized framework for quantum causality.
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.
Indefinite causal order can be realized by a quantum switch, yet its experimental certification has so far relied on assumptions about the devices. A device-independent verification of indefinite causal order would show, from the observed correlations alone and without relying on a detailed model of the devices, that they are incompatible with any classical description based on a fixed causal order. Here, we implement a photonic quantum switch in a device-independent scenario and test a Bell-like causal inequality using entangled photons. One photon serves as the control qubit of the quantum switch, while the other is measured by a distant party with rapidly and independently chosen settings. We observe a violation of the causal inequality by 24 standard deviations. These results provide experimental evidence for indefinite causal order, albeit in the presence of remaining loopholes, and mark an important step toward a fully loophole-free device-independent verification of causal indefiniteness in quantum processes.
Processes with indefinite causal order (ICO), such as the quantum switch, are an emerging resource for quantum tasks and a fundamental test bed for studies of temporal correlations in quantum mechanics. A limitation of past photonic implementations of the quantum switch, however, is their inability to perform measurements inside the switch without either destroying the superposition of causal orders or delaying readout until the after the quantum switch. Measurements where the results are read out locally are needed for several applications of ICO, but also for a loophole-free verification of ICO. Here, we overcome past limitations by introducing a local\mathit{local}local measurement scheme and coupling the photon in the switch to a time−delocalized\mathit{time-delocalized}time−delocalized ancilla system. We experimentally realize this protocol using a photonic quantum switch with post-selected linear optical logic gates. Our method ensures that the measurement apparatus interacts with the system at two distinct times and yet yields a single outcome. We use a quantum eraser measurement to preserve the ICO, which we certify by measuring a causal witness and finding a negative value of CW≈−0.305(1)\mathcal{C}_W \approx -0.305 (1)CW​≈−0.305(1). Furthermore, by explicitly realizing a time-delocalized ancilla system, our protocol not only enables a new class of quantum switch protocols requiring local readout, but also provides a general method for path-coherence-preserving measurements with broad applications beyond ICO.
Quantum processes are transformations that act on quantum operations. Their study led to the discovery of the phenomenon of indefinite causal order: some quantum processes, such as the quantum switch, act on independent quantum operations in such a way that the order in which the operations are acted upon not only cannot be determined but is simply undefined. This is the property that we experimentally certify in this work. We report an experimental certification of indefinite causal order that relies only on the characterization of the operations of a single party. We do so in the semi-device-independent scenario with the fewest possible assumptions of characterization of the parties’ local operations in which indefinite causal order can be demonstrated with the quantum switch. To achieve this result, we introduce the concept of semi-device-independent causal inequalities and show that the correlations generated in a photonic quantum switch, in which all parties are able to collect local outcome statistics, achieve a violation of this inequality of 224 standard deviations. This result consists of the experimental demonstration of indefinite causal order with the fewest device-characterization assumptions to date.
In the classical world, physical events always happen in a fixed causal order.However, it was recently revealed that quantum mechanics allows events to occur with indefinite causal order (ICO).In this study, we use an optical quantum switch to experimentally investigate the application of ICO in thermodynamic tasks.Specifically, we simulate the working system interacting with two identical thermal reservoirs in an ICO, observing the quantum heat extraction even though they are in thermal equilibrium where heat extraction is inaccessible by traditional thermal contact.Using such a process, we simulate an ICO refrigeration cycle and investigate its properties.We also show that by passing through the ICO channel multiple times, one can extract more heat per cycle and thus obtain a higher refrigeration performance.Our results suggest that the causal nonseparability can be a powerful resource for quantum thermodynamic tasks.