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arXiv · 2205.00017

Operational definition of the temperature of a quantum state

Patryk Lipka-Bartosik·Martí Perarnau-Llobet·Nicolas Brunner
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Abstract

Temperature is usually defined for physical systems at thermal equilibrium. Nevertheless one may wonder if it would be possible to attribute a meaningful notion of temperature to an arbitrary quantum state, beyond simply the thermal (Gibbs) state. In this work, we propose such a notion of temperature considering an operational task, inspired by the Zeroth Law of thermodynamics. Specifically, we define two effective temperatures for quantifying the ability of a quantum system to cool down or heat up a thermal environment. In this way we can associate an operationally meaningful notion of temperature to any quantum density matrix. We provide general expressions for these effective temperatures, for both single- and many-copy systems, establishing connections to concepts previously discussed in the literature. Finally, we consider a more sophisticated scenario where the heat exchange between the system and the thermal environment is assisted by a quantum reference frame. This leads to an effect of "coherent quantum catalysis", where the use of a coherent catalyst allows for exploiting quantum energetic coherences in the system, now leading to much colder or hotter effective temperatures.

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Patryk Lipka-Bartosik, Martí Perarnau-Llobet, Nicolas Brunner. 2023-01-31. Operational definition of the temperature of a quantum state. https://doi.org/10.1103/physrevlett.130.040401

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