Trading coherence and entropy by a quantum Maxwell demon

TL;DR

This paper proposes a quantum Maxwell demon setup that enables entropy reduction in a quantum system through a non-unital, energy-conserving channel, leading to a novel quantum-thermodynamic engine capable of work extraction without waste heat.

Contribution

It introduces a new quantum engine based on a non-unital, energy-conserving channel that achieves entropy decrease and work extraction, a departure from traditional thermodynamic principles.

Findings

Entropy can decrease in a quantum system via a non-unital, energy-conserving channel.

A quantum Maxwell demon can be realized with a mesoscopic four-lead scatterer and a spin micro-environment.

The proposed engine operates with two cycles, enabling work extraction without local waste heat.

Abstract

The Second Law of Thermodynamics states that the entropy of a closed system is non-decreasing. Discussing the Second Law in the quantum world poses new challenges and provides new opportunities, involving fundamental quantum-information-theoretic questions and novel quantum-engineered devices. In quantum mechanics, systems with an evolution described by a so-called unital quantum channel evolve with a non-decreasing entropy. Here, we seek the opposite, a system described by a non-unital and, furthermore, energy-conserving channel that describes a system whose entropy decreases with time. We propose a setup involving a mesoscopic four-lead scatterer augmented by a micro-environment in the form of a spin that realizes this goal. Within this non-unital and energy-conserving quantum channel, the micro-environment acts with two non-commuting operations on the system in an autonomous way. We find, that the process corresponds to a partial exchange or swap between the system and environment quantum states, with the system's entropy decreasing if the environment's state is more pure. This entropy-decreasing process is naturally expressed through the action of a quantum Maxwell demon and we propose a quantum-thermodynamic engine with four qubits that extracts work from a single heat reservoir when provided with a reservoir of pure qubits. The special feature of this engine, which derives from the energy-conservation in the non-unital quantum channel, is its separation into two cycles, a working cycle and an entropy cycle, allowing to run this engine with no local waste heat.