Experimental Realization of a Quantum Refrigerator Driven by Indefinite Causal Orders

TL;DR

This paper experimentally demonstrates a quantum refrigerator driven by indefinite causal order, showing how ICO can be used for heat exchange and improving its efficiency with a novel density matrix exponentiation method.

Contribution

It presents the first experimental realization of an ICO-based quantum refrigerator and introduces a DME framework to enhance its coefficient of performance.

Findings

ICO can be used to control heat exchange in quantum systems.

The DME approach increases the COP by over three times.

The ICO refrigerator operates with bounded COP due to probabilistic measurement outcomes.

Abstract

Indefinite causal order (ICO) is playing a key role in recent quantum technologies. Here, we experimentally study quantum thermodynamics driven by ICO on nuclear spins using the nuclear magnetic resonance system. We realize the ICO of two thermalizing channels to exhibit how the mechanism works, and show that the working substance can be cooled or heated albeit it undergoes thermal contacts with reservoirs of the same temperature. Moreover, we construct a single cycle of the ICO refrigerator based on the Maxwell's demon mechanism, and evaluate its performance by measuring the work consumption and the heat energy extracted from the low-temperature reservoir. Unlike classical refrigerators in which the coefficient of performance (COP) is perversely higher the closer the temperature of the high-temperature and low-temperature reservoirs are to each other, the ICO refrigerator's COP is always bounded to small values due to the non-unit success probability in projecting the ancillary qubit to the preferable subspace. To enhance the COP, we propose and experimentally demonstrate a general framework based on the density matrix exponentiation (DME) approach, as an extension to the ICO refrigeration. The COP is observed to be enhanced by more than three times with the DME approach. Our work demonstrates a new way for non-classical heat exchange, and paves the way towards construction of quantum refrigerators on a quantum system.