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For identical quantum channels, order matters

In (a) and (b), a quantum particle travels through two channels, N1 and N2, in a fixed order. In (c), a quantum switch creates an overlap of the two configurations in (a) and (b). Credit: Ebler et al. © 2018 American Physical Society

Physicists have shown that the use of two quantum channels in different orders can improve the ability of a communication network to transmit information, even, intuitively, when the channels are identical. This result contrasts with the way things work with identical classical channels (or almost anything else that is identical), where using them in a different order should not make any difference.

Physicists Daniel Ebler, Sina Salek and Giulio Chiribella have published an article on this unusual feature of quantum channels and their potential advantages for quantum communication in a recent issue of Physical Review Letters .

"This is a new paradigm of quantum communication," said Salek Phys.org . "Not only are the carriers of information quantum, but also communication channels can be combined quantumly, in this new paradigm, it is possible to communicate in situations where it would not normally be possible to communicate."

Theory of information promoted by the seminal work of Claude Shannon, was originally formulated as a classical theory, but in recent years has given rise to the quantum theory of Shannon. Although quantum communication networks use quantum particles and quantum processes to encode and decode information, real channels are still connected in a classical manner, that is, in a fixed order. This means that the quantum particles that travel through the network will always pass through the channels in the same order each time.

In the new study, physicists investigated the possibility of connecting two identical channels in a quantum superposition of different orders. To do this, they used an operation called "quantum switch" that takes two identical channels as inputs and creates a new channel in which the order of the two input channels is entangled with a control system. They then showed that the resulting quantum superposition of the channel commands can be used to communicate classical information in this network, which is impossible to do when the order is fixed.

As the physicists explain, the results may seem paradoxical because the exchange of the order of two identical channels does not seem to have any effect on an ordinary quantum circuit. However, quantum channels are intrinsically noisy, so each channel can be broken down into a random mix of different processes. Some of these processes are not commuted, that is, the use of processes in different orders produces different results, so these differences are transferred to the channels themselves.

This underlying randomness leads to the ability to create a channel that transmits information-information that is not contained either in the state of the system alone or in the state of control alone, but rather in the correlations between them.

Physicists calculated the maximum amount of information that can be transmitted by changing two identical channels, and they hope it will be possible to communicate more information using additional copies of these channels. In collaboration with Professor Philip Walther's group in Vienna, they are now planning to implement their communication protocol with photons.

"The goal is to develop a complete theory of communication, extending Shannon's theory to situations where different transmission lines can be combined in a quantum fashion," said Salek.

Explore more:
Possible bidirectional signaling with a single quantum particle

More information:
Daniel Ebler, Sina Salek and Giulio Chiribella. "Improved communication with assistance of undefined causal order". Physical Review Letters . DOI: 10.1103 / PhysRevLett.120.120502

Also in: arXiv: 1711.10165 [quant-ph]

Newspaper reference:
Physical revision letters

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