Trading coherence and entropy by a quantum Maxwell demon

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.Comment: 14 pages, 8 figure

Heat switch and thermoelectric effects based on Cooper-pair splitting and elastic cotunneling

“…The subject of quantum thermodynamics and quan-tum extension of the second law has attracted much attention lately. It has been discovered that under certain circumstances the law in its classical sense can be violated [36][37][38][39]. However, in the present case the transfer of a particle from the warmer left reservoir to the colder right one is still associated with the overall increase in entropy, ∆S = − ε Θ L + ε Θ R > 0, in which sense it does not violate the second law.…”

Verifying detailed fluctuation relations for discrete feedback-controlled quantum dynamics

“…Despite their classical origin, their validity has been extended to driven quantum systems [21][22][23][24][25]. These fundamental relations were also extended for feedback protocols, providing further tools to study information thermodynamics of classical and quantum systems [32][33][34][35][36][37][38][39][40][41][42][43][44][45]. They imply generalized forms of the * p.camati@ufabc.edu.br † serra@ufabc.edu.br second law where thermodynamic and information quantities are treated on an equal footing [46][47][48][49][50][51][52].…”

Quantum systems under frequency modulation

“…This resulted in unexpected connections between quantum physics and thermodynamics -for example in some engine models with a single-mode oscillator as the working medium the dynamical Casimir effect, a purely quantum phenomenon, prevents the machine from reaching absolute zero in a finite time, thus enforcing the third law [308]. Other fundamental connections have been investigated, for example the action of Maxwell's demon and the Landauer erasure principle formulated at the quantum level [309,310]. The thermal distribution and the dynamics of quantum states emerge also in fluctuation relations.…”