{
  "id": "2001.07696",
  "version": "v1",
  "published": "2020-01-21T18:54:04.000Z",
  "updated": "2020-01-21T18:54:04.000Z",
  "title": "Charging by thermalization",
  "authors": [
    "Karen V. Hovhannisyan",
    "Felipe Barra",
    "Alberto Imparato"
  ],
  "comment": "13+17 pages, 3 figures",
  "categories": [
    "quant-ph",
    "cond-mat.stat-mech"
  ],
  "abstract": "A strongly coupled subsystem of a thermal system will generally be in an athermal state, and in some cases, it will be possible to unitarily extract work from such a state. At the same time, being part of a thermal equilibrium, the subsystem can maintain its state indefinitely and for free. We put these observations to use by devising a battery charging and storing unit, made up of just a thermal bath and put in action by system-bath coupling. The charging cycle, consisting of connecting the system to the bath, letting them thermalize, disconnecting the system, and extracting work from it, requires very little external control and fine-tuning. As a consequence of the second law of thermodynamics, we show that the ratio between the work that can be stored and extracted and the total work spent on connecting and disconnecting the system---the efficiency---is always $\\leq 1$, which is a single-bath analogue of the Carnot bound. Moreover, coupling, being a resource for the machine, is also a source of dissipation: the entropy production per charging cycle is always significant, which strongly limits the efficiency in all coupling strength regimes. We show that our results also hold for generic microcanonical baths. Finally, we illustrate our theory on the Caldeira-Leggett model with a harmonic oscillator coupled to a harmonic bath. We derive general asymptotic formulas in both weak and ultrastrong coupling regimes, for arbitrary Ohmic spectral densities. Also, we analyze the model in the high-temperature limit, showing, along the way, that energy equipartition holds both for potential and kinetic energies. Lastly, we show that the efficiency can be increased by connecting several copies of the system to the bath.",
  "revisions": [
    {
      "version": "v1",
      "updated": "2020-01-21T18:54:04.000Z"
    }
  ],
  "analyses": {
    "keywords": [
      "thermalization",
      "arbitrary ohmic spectral densities",
      "charging cycle",
      "little external control",
      "energy equipartition holds"
    ],
    "note": {
      "typesetting": "TeX",
      "pages": 17,
      "language": "en",
      "license": "arXiv",
      "status": "editable"
    }
  }
}