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

arXiv:1711.10165 · Physical Review Letters

Enhanced Communication with the Assistance of Indefinite Causal Order

Daniel Ebler, Sina Salek, Giulio Chiribella

Published Mar 22, 2018• 254 citations
arXivHTMLPDF

Abstract

In quantum Shannon theory, the way information is encoded and decoded takes advantage of the laws of quantum mechanics, while the way communication channels are interlinked is assumed to be classical. In this Letter, we relax the assumption that quantum channels are combined classically, showing that a quantum communication network where quantum channels are combined in a superposition of different orders can achieve tasks that are impossible in conventional quantum Shannon theory. In particular, we show that two identical copies of a completely depolarizing channel become able to transmit information when they are combined in a quantum superposition of two alternative orders. This finding runs counter to the intuition that if two communication channels are identical, using them in different orders should not make any difference. The failure of such intuition stems from the fact that a single noisy channel can be a random mixture of elementary, noncommuting processes, whose order (or lack thereof) can affect the ability to transmit information.

DOI
10.1103/physrevlett.120.120502

Related papers

Increasing communication capacity via superposition of order

Kaumudibikash Goswami, Y. Cao, Gerardo A. Paz-Silva +2 more

Classically, no information can be transmitted through a depolarising, which is a completely noisy, channel.We show that by combining a depolarising channel with another channel in an indefinite causal order-that is, when there is a superposition of the order that these two channels were applied-it becomes possible to transmit significant information.We consider two limiting cases.When both channels are fully depolarising, the ideal limit is communication of 0.049 bits; experimentally we achieve (3.4 ± 0.2) × 10 -2 bits.When one channel is fully depolarising, and the other is a known unitary, the ideal limit is communication of 1 bit.We experimentally achieve 0.64 ± 0.02 bits.Our results offer intriguing possibilities for future communication strategies beyond conventional quantum Shannon theory.

arXiv:1807.07383•Physical Review Research•Aug 24, 2020•90 citations

Quantum and Classical Data Transmission through Completely Depolarizing Channels in a Superposition of Cyclic Orders

Giulio Chiribella, Matthew Wilson, H. F. Chau

Completely depolarizing channels are often regarded as the prototype of physical processes that are useless for communication: any message that passes through them along a well-defined trajectory is completely erased. When two such channels are used in a quantum superposition of two alternative orders, they become able to transmit some amount of classical information, but still no quantum information can pass through them. Here, we show that the ability to place N completely depolarizing channels in a superposition of N alternative causal orders enables a high-fidelity heralded transmission of quantum information with error vanishing as 1/N. This phenomenon highlights a fundamental difference with the N=2 case, where completely depolarizing channels are unable to transmit quantum data, even when placed in a superposition of causal orders. The ability to place quantum channels in a superposition of orders also leads to an increase of the classical communication capacity with N, which we rigorously prove by deriving an exact single-letter expression. Our results highlight the more complex patterns of correlations arising from multiple causal orders, which are similar to the more complex patterns of entanglement arising in multipartite quantum systems.

arXiv:2005.00618•Physical Review Letters•Nov 5, 2021•49 citations

Experimental investigating communication in a superposition of causal orders

Yu Chen Guo, Xiao‐Min Hu, Zhibo Hou +6 more

Quantum mechanics allows for the superposition of quantum states, thereby bringing about the great success of quantum communications. Recently, in the so-called quantum switch, the relative orders between quantum operations can also be coherently controlled with the help of an auxiliary system. In conventional quantum Shannon theory, however, communication channels are assumed to be combined in a classical way. Relaxing this assumption, it was shown theoretically that one can transmit information through a quantum switch. Here, we experimentally demonstrate this enhancement on the rates of communication for both classical and quantum information with the help of an optical quantum switch. Our results present evidence of the advantages of the superposition of causal orders in the communications domain.

•Nov 19, 2018•6 citations

Quantum communication in a superposition of causal orders

Sina Salek, Daniel Ebler, Giulio Chiribella

Quantum mechanics allows for situations where the relative order between two processes is entangled with a quantum degree of freedom. Here we show that such entanglement can enhance the ability to transmit quantum information over noisy communication channels. We consider two completely dephasing channels, which in normal conditions are unable to transmit any quantum information. We show that, when the two channels are traversed in an indefinite order, a quantum bit sent through them has a 25% probability to reach the receiver without any error. For partially dephasing channels, a similar advantage takes place deterministically: the amount of quantum information that can travel through two channels in a superposition of orders can be larger than the amount of quantum information that can travel through each channel individually.

arXiv:1809.06655•Sep 18, 2018•47 citations

Communication using an indefinite causal structure

Lorenzo M. Procopio, Francisco Delgado, Marco Enríquez +2 more

It has recently been shown that if two completely depolarizing channels are in a superposition of different orders, classical information can be transmitted that would be otherwise lost [1]. Here, we present a generalization of the results of [1] to study communication with arbitrary depolarizing channels and putting to good use indefinite causal order, a new paradigm for quantum information/computation.

Quantum Information and Measurement (QIM)•Jan 1, 2019•0 citations

Indefinite causal order enables perfect quantum communication with zero capacity channels

Giulio Chiribella, Manik Banik, Some Sankar Bhattacharya +6 more

Abstract Quantum mechanics is compatible with scenarios where the relative order between two events can be indefinite. Here we show that two independent instances of a noisy process can behave as a perfect quantum communication channel when used in a coherent superposition of two alternative orders. This phenomenon occurs even if the original process has zero capacity to transmit quantum information. In contrast, perfect quantum communication does not occur when the message is sent directly from the sender to the receiver through a superposition of alternative paths, with an independent noise process acting on each path. The possibility of perfect quantum communication through independent noisy channels highlights a fundamental difference between the superposition of orders in time and the superposition of paths in space.

arXiv:1810.10457•New Journal of Physics•Feb 18, 2021•142 citations
In quantum Shannon theory, the way information is encoded and decoded takes advantage of the laws of quantum mechanics, while the way communication channels are interlinked is assumed to be classical. In this Letter, we relax the assumption that quantum channels are combined classically, showing that a quantum communication network where quantum channels are combined in a superposition of different orders can achieve tasks that are impossible in conventional quantum Shannon theory. In particular, we show that two identical copies of a completely depolarizing channel become able to transmit information when they are combined in a quantum superposition of two alternative orders. This finding runs counter to the intuition that if two communication channels are identical, using them in different orders should not make any difference. The failure of such intuition stems from the fact that a single noisy channel can be a random mixture of elementary, noncommuting processes, whose order (or lack thereof) can affect the ability to transmit information.
Classically, no information can be transmitted through a depolarising, which is a completely noisy, channel.We show that by combining a depolarising channel with another channel in an indefinite causal order-that is, when there is a superposition of the order that these two channels were applied-it becomes possible to transmit significant information.We consider two limiting cases.When both channels are fully depolarising, the ideal limit is communication of 0.049 bits; experimentally we achieve (3.4 ± 0.2) × 10 -2 bits.When one channel is fully depolarising, and the other is a known unitary, the ideal limit is communication of 1 bit.We experimentally achieve 0.64 ± 0.02 bits.Our results offer intriguing possibilities for future communication strategies beyond conventional quantum Shannon theory.
Completely depolarizing channels are often regarded as the prototype of physical processes that are useless for communication: any message that passes through them along a well-defined trajectory is completely erased. When two such channels are used in a quantum superposition of two alternative orders, they become able to transmit some amount of classical information, but still no quantum information can pass through them. Here, we show that the ability to place N completely depolarizing channels in a superposition of N alternative causal orders enables a high-fidelity heralded transmission of quantum information with error vanishing as 1/N. This phenomenon highlights a fundamental difference with the N=2 case, where completely depolarizing channels are unable to transmit quantum data, even when placed in a superposition of causal orders. The ability to place quantum channels in a superposition of orders also leads to an increase of the classical communication capacity with N, which we rigorously prove by deriving an exact single-letter expression. Our results highlight the more complex patterns of correlations arising from multiple causal orders, which are similar to the more complex patterns of entanglement arising in multipartite quantum systems.
Quantum mechanics allows for the superposition of quantum states, thereby bringing about the great success of quantum communications. Recently, in the so-called quantum switch, the relative orders between quantum operations can also be coherently controlled with the help of an auxiliary system. In conventional quantum Shannon theory, however, communication channels are assumed to be combined in a classical way. Relaxing this assumption, it was shown theoretically that one can transmit information through a quantum switch. Here, we experimentally demonstrate this enhancement on the rates of communication for both classical and quantum information with the help of an optical quantum switch. Our results present evidence of the advantages of the superposition of causal orders in the communications domain.
Quantum mechanics allows for situations where the relative order between two processes is entangled with a quantum degree of freedom. Here we show that such entanglement can enhance the ability to transmit quantum information over noisy communication channels. We consider two completely dephasing channels, which in normal conditions are unable to transmit any quantum information. We show that, when the two channels are traversed in an indefinite order, a quantum bit sent through them has a 25% probability to reach the receiver without any error. For partially dephasing channels, a similar advantage takes place deterministically: the amount of quantum information that can travel through two channels in a superposition of orders can be larger than the amount of quantum information that can travel through each channel individually.
Abstract Quantum mechanics is compatible with scenarios where the relative order between two events can be indefinite. Here we show that two independent instances of a noisy process can behave as a perfect quantum communication channel when used in a coherent superposition of two alternative orders. This phenomenon occurs even if the original process has zero capacity to transmit quantum information. In contrast, perfect quantum communication does not occur when the message is sent directly from the sender to the receiver through a superposition of alternative paths, with an independent noise process acting on each path. The possibility of perfect quantum communication through independent noisy channels highlights a fundamental difference between the superposition of orders in time and the superposition of paths in space.