This article sets up a new formalism to investigate stochastic thermodynamics in the quantum regime, where stochasticity and irreversibility primarily come from quantum measurement. In the absence of any bath, we define a purely quantum component to heat exchange, that corresponds to energy fluctuations caused by measurement back-action. Energetic and entropic signatures of measurement induced irreversibility are then investigated for canonical experiments of quantum optics, and the energetic cost of counter-acting decoherence is characterized on a simple state-stabilizing protocol. By placing quantum measurement in a central position, our formalism contributes to bridge a gap between experimental quantum optics and quantum thermodynamics

The essence of both classical and quantum engines is to extract useful energy (work) from stochastic energy sources, e.g. thermal baths. In Maxwell's demon engines, work extraction is assisted by a feedback control based on measurements performed by a demon, whose memory is erased at some nonzero energy cost. Here we propose a new type of quantum Maxwell's demon engine where work is directly extracted from the measurement channel, such that no heat bath is required. We show that in the Zeno regime of frequent measurements, memory erasure costs eventually vanish. Our findings provide a new paradigm to analyze quantum heat engines and work extraction in the quantum world.Introduction. Thermodynamics was originally developed to optimize machines that would extract work from reservoirs at various temperatures, by exploiting the transformations of some working agent. These machines may be assisted by a so-called Maxwell's demon, that exploits information acquired on the agent to enhance work extraction, at the energy expense of resetting the demon's memory. Maxwell's demons and Szilard's engines have been investigated in several theoretical proposals [1][2][3][4][5][6][7][8][9][10], including the thermodynamics of feedback control [11][12][13], and experimentally realized in various systems, e.g. Brownian particles [14,15], single electron transistors [16,17] and visible light [18]. Latest experiments have started addressing the regime where the working agent exhibits quantum coherences [2,19]. The potential extraction of work from quantum coherence leads to interesting open questions related to the energetic aspects of quantum information technologies [21][22][23][24][25][26]. Furthermore, novel designs for quantum engines, based on various kinds of quantum non-equilibrium reservoirs have been suggested [27][28][29][30][31][32] and experimentally investigated [33,34].Most quantum engines considered so far involve a hot reservoir, which is the primary source of energy. In this framework, measurements performed by the demon are practical steps where information is extracted, without changing the energy of the working agent. Ultimately, measurement (just like decoherence) can appear as a detrimental step of the thermodynamic cycle as it destroys quantum coherences, further preventing to extract work from them [35]. Here we adopt a different approach and show that measurement itself can be exploited as a fuel in a new kind of quantum engine. Originally here, the demon can perform measurements that are sensitive to states in an arbitrary basis of the system Hilbert space. It was recently shown [36-39] * Electronic address: alexia.auffeves@neel.cnrs.fr FIG. 1:Maxwell's demon assisted engines. a) Thermally driven engine. The working agent is a qubit of transition frequency ω0. A demon measuring in the qubit energy basis {|0 ; |1 } allows to convert the heat Q hot extracted from a bath into work Wext. The demon's memory is erased by some extra-work source at the minimal work cost Wer while the heat Q cold is evacuated in a ...

We propose quantum engines powered entirely by a position-resolving measurement performed on a quantum particle. These engines produce work by moving the quantum particle against a force. Unlike classical information-driven engines (e.g., Maxwell's demon), the energy is not extracted from a thermal hot source but directly from the observation process via a partial wave-function collapse of the particle. We present results for the work done and the efficiency for different values of the engine parameters. Feedback is required for optimal performance. We find that unit efficiency can be approached when one measurement outcome prepares the initial state of the next engine cycle, while the other outcomes leave the original state nearly unchanged.