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Globecom

From the Environment-Assisted Paradigm to the Quantum Switch

Seid Koudia, Angela Sara Cacciapuoti, Marcello Caleffi

Published Dec 1, 2021• 2 citations
Publisher

Abstract

The quantum switch has been witnessing growing attention in the last years due to its advantage in several quantum technologies applications. In particular, it has been proven that the quantum switch can significantly improve the communication rates beyond the limits of conventional quantum Shannon theory. In this paper, we theoretically prove that the quantum switch can be interpreted as a particular instance of the Environment-assisted quantum communication paradigm. The developed analysis is crucial to better understand the limitations of the quantum switch. Furthermore, the analysis is key to shed the light on control strategies within the Environment-assisted communication paradigm.

DOI
10.1109/globecom46510.2021.9685779

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The quantum switch has been witnessing growing attention in the last years due to its advantage in several quantum technologies applications. In particular, it has been proven that the quantum switch can significantly improve the communication rates beyond the limits of conventional quantum Shannon theory. In this paper, we theoretically prove that the quantum switch can be interpreted as a particular instance of the Environment-assisted quantum communication paradigm. The developed analysis is crucial to better understand the limitations of the quantum switch. Furthermore, the analysis is key to shed the light on control strategies within the Environment-assisted communication paradigm.
The quantum switch describes a quantum operation in which two or more quantum channels act on a quantum system with the order of application determined by the state of an order quantum system. And by suitably choosing the state of the order system, one can create a quantum superposition of the different orders of application, which can perform communication tasks impossible within the framework of the standard quantum Shannon theory. In this paper, we consider the scenario of one-shot heralded qubit communication and ask whether there are protocols using a given quantum switch or switches that could outperform the given ones. We answer this question in the affirmative. We define a higher-order quantum switch composed of two quantum switches, with their order of application controlled by another order quantum system. We then show that the quantum switches placed in a quantum superposition of their alternative orders can transmit a qubit, without any error, with a probability higher than that achievable with the quantum switches individually. We demonstrate this communication advantage over quantum switches that are useful as a resource and those that are useless. We also show that there are situations where there is no communication advantage over the individual quantum switches.
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.
A crucial step towards the 6th generation (6G) of networks would be a shift in communication paradigm beyond the limits of Shannon’s theory. In both classical and quantum Shannon’s information theory, communication channels are generally assumed to combine throughclassical trajectories, so that the associated network path traversed by the information carrier is well-defined. Counter-intuitively, quantum mechanics enables a quantum information carrier to propagate through aquantum path, i.e., through a path such that the causal order of the constituting communications channels becomes indefinite. Quantum paths exhibit astonishing features, such as providing non-null capacity even when no information can be sent through any classical path. In this paper, we study the quantum capacity achievable via a quantum path and establish upper and the lower bounds for it. Our findings reveal the substantial advantage achievable with a quantum path over any classical placements of communications channels in terms of ultimate achievable communication rates. Furthermore, we identify the region where a quantum path incontrovertibly outperforms the amount of transmissible information beyond the limits of conventional quantum Shannon’s theory, and we quantify this advantage over classical paths through a conservative estimate.
Abstract Applications of the quantum switch on quantum channels have recently become a topic of intense discussion. In the present work, we show that some useless (for communication) channels may provide useful communication under the action of quantum switch for several information-theoretic tasks: quantum random access codes, quantum steering, etc. We demonstrate that the quantum switch can also be useful in preventing the loss of coherence in a system when only coherence-breaking channels are the available channels for communication. We also show that if a useless quantum channel does not provide useful communication even after using a quantum switch, concatenating the channel with another suitable quantum channel, and subsequently using the switch, one may achieve useful communication. Finally, we discuss how the introduction of noise in the quantum switch can reduce the advantage that the switch provides.
Recent results have shown that quantum theory is compatible with novel causal structures where events happen without a definite causal order. In particular, the quantum switch describes a process in which two quantum channels act in a coherent superposition of their two possible orders. Furthermore, the quantum switch can perform communication tasks that are impossible within the framework of the standard quantum Shannon theory. The present paper considers the scenario of one-shot heralded qubit communication using a higher-order quantum switch constructed from two quantum switches. Specifically, we show that two quantum switches put in a superposition of their alternative causal orders can transmit a qubit, without any error, with a probability strictly higher than that achievable with each quantum switch. We discuss three examples that demonstrate this communication advantage. Notably, a higher-order quantum switch not only can outperform useful quantum switches but also becomes useful as a resource even if the quantum switches making it up are useless.
Protocols that exploit quantum communication technology offer two advantages: they can either extend or render feasible the capabilities of their classical counterparts, or they exhibit functionality entirely unachievable through classical means alone. For an example of the former, quantum key distribution protocols such as E91 [2] and BBM92 [1] can in principle yield information-theoretic security by using entanglement to generate secure key bits. These raw secret key bits can then be distilled into a one-time pad to encode messages sent between two parties. For an example of the latter, distributed quantum sensing frameworks such as [3] and [11] employ entanglement to overcome the standard quantum limit [4].