Source-linked AI summary
Enhanced communication with the assistance of indefinite causal order
Daniel Ebler, Sina Salek, Giulio Chiribella
TL;DR
Standard quantum Shannon theory assumes classical interlinking of quantum channels, limiting communication through constant channels. This paper adds the quantum SWITCH, showing that a superposition of channel orders activates two identical completely depolarizing channels for classical communication.
Problem
Standard quantum Shannon theory treats channel interconnections classically, and constant channels cannot transmit information within its allowed operations.
Method
The paper extends quantum Shannon theory by adding the quantum SWITCH, which combines channels in an order controlled by a receiver-accessible quantum control state.
Results
Combining two completely depolarizing channels with the quantum SWITCH enables non-zero-rate classical communication, unlike fixed-order or classically controlled combinations.
Takeaways & Limitations
Self-switching shows that identical noisy channels can provide a communication advantage because their underlying non-commuting processes respond nontrivially to quantum order control.
Takeaways & Limitations
The optimal rate is established for product encodings with two channels, while more copies and entangled inputs may yield higher rates; additivity remains unresolved.
Abstract
from arXiv · showhide
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, non-commuting processes, whose order (or lack thereof) can affect the ability to transmit information.