Demonstration of superior communication through thermodynamically free channels in an optical quantum switch

TL;DR

This paper demonstrates that an optical quantum switch can enhance communication efficiency through thermodynamically free channels, especially when considering the control qubit, and explores non-Markovian effects in this process.

Contribution

It experimentally shows thermodynamically free communication advantages in a quantum switch considering the control qubit, extending prior theoretical proposals.

Findings

Quantum switch acts as a resource when including the control qubit.

Thermally free channels can be exploited for enhanced communication.

Non-Markovian effects are characterized by information backflows.

Abstract

The release of causal structure of physical events from a well-defined order to an indefinite one stimulates remarkable enhancements in various quantum information tasks. Some of these advantages, however, are questioned for the ambiguous role of the control system in the quantum switch that is an experimentally realized process with indefinite causal structure. In communications, for example, not only the superposition of alternative causal orders, but also the superposition of alternative trajectories can accelerate information transmissions. Here, we follow the proposal of Liu et al. [Phys. Rev. Lett. 129, 230604 (2022)], and examine the information enhancement effect of indefinite causal orders with the toolkit of thermodynamics in a photonic platform. Specifically, we simulate the thermal interaction between a system qubit and two heat baths embedded in a quantum switch by implementing the corresponding switched thermal channels. Although its action on the system qubit only is thermally free, our results suggest that the quantum switch should be seen as a resource when the control qubit is also considered. Moreover, we characterize the non-Markovian property in this scenario by measuring the information backflows from the heat baths to the system qubit.