Information gain and loss for a quantum Maxwell's demon

TL;DR

This paper experimentally investigates the information dynamics of a quantum Maxwell's demon using superconducting qubits, demonstrating information-to-work conversion, verifying quantum fluctuation theorems, and exploring measurement backaction effects.

Contribution

It provides the first experimental study of quantum Maxwell's demon with continuous weak measurements and quantum feedback, revealing information loss mechanisms and the role of initial state purity.

Findings

Quantum feedback enables work extraction from a single quantum system.

Quantum backaction can cause information loss during imperfect measurements.

The transition between information gain and loss depends on the initial qubit purity.

Abstract

We use continuous weak measurements of a driven superconducting qubit to experimentally study the information dynamics of a quantum Maxwell's demon. We show how information gained by a demon who can track single quantum trajectories of the qubit can be converted into work using quantum coherent feedback. We verify the validity of a quantum fluctuation theorem with feedback by utilizing information obtained along single trajectories. We demonstrate, in particular, that quantum backaction can lead to a loss of information in imperfect measurements. We furthermore probe the transition between information gain and loss by varying the initial purity of the qubit.