Thermodynamically free quantum measurements

TL;DR

This paper extends the concept of thermal channels to thermal instruments in quantum thermodynamics, showing they only measure Hamiltonian-commuting observables and thermalize systems, revealing a thermodynamic information-disturbance trade-off.

Contribution

It introduces thermal instruments as thermodynamically free quantum measurements and analyzes their properties, including measurement limitations and thermodynamic costs.

Findings

Thermal instruments only measure observables commuting with the Hamiltonian.

Performing a complete measurement thermalizes the measured system.

Thermodynamically free measurements do not violate the second law.

Abstract

Thermal channels -- the free processes allowed in the resource theory of quantum thermodynamics -- are generalised to thermal instruments, which we interpret as implementing thermodynamically free quantum measurements; a Maxwellian demon using such measurements never violates the second law of thermodynamics. Further properties of thermal instruments are investigated and, in particular, it is shown that they only measure observables commuting with the Hamiltonian, and they thermalise the measured system when performing a complete measurement, the latter of which indicates a thermodynamically induced information-disturbance trade-off. The demarcation of measurements that are not thermodynamically free paves the way for a resource-theoretic quantification of their thermodynamic cost.