Surpassing Carnot efficiency with relativistic motion

Relativistic thermal devices offer a unique platform for understanding the interplay between motion, quantum fields, and thermodynamics, revealing phenomena inaccessible to stationary systems. We consider a two-qubit SWAP heat engine whose working medium consists of inertially moving Unruh-DeWitt qubit detectors, each coupled to a scalar quantum field in thermal equilibrium at a distinct temperature. Relativistic motion causes the qubits to perceive frequency-dependent effective temperatures that are either hotter or colder than their respective reservoir temperature. We show that the relativistic temperature shift, perhaps the qubit velocity, can be harnessed as a thermodynamic resource to enhance the work output and the efficiency at maximum power of the heat engine. We derive a generalized second law for a heat engine with a moving working medium and demonstrate that it can exceed the standard Carnot bound defined by rest-frame temperatures.