# |QET> talks at the Workshop on Quantum Energetics organized by the Institute for Quantum Studies.

On March 13th, Alexia Auffèves, Léa Bresque and Maria Maffei did four presentations at the Workshop on Quantum Energetics organized in California by the **Institute for Quantum Studies **from the Chapman University.

The first one was the technical talk **Quantum energetics: fundamentals and applications to simple quantum optics experiments **by Alexia Auffèves.

Abstract: Quantum energetics explores the flows of energy, entropy and information in situations of quantum physics where temperature is not necessarily defined. This is the case, e.g. in measurement-powered engines, in driven-dissipative systems. In this talk, I will focus on the canonical case of two coupled systems, otherwise isolated. This captures most situations of quantum optics, where a qubit is coupled to a light field injected in one or several electromagnetic modes. Global energy conservation allows to characterize the nature of energy exchanges, yielding work-like and heat-like quantities. I apply this framework to study the energy cost of work extraction and the role of quantum coherence in quantum batteries, and present recent experimental results obtained with superconducting and semiconducting devices. I finally show how energetic quantities can help probing the quantum nature of a light field.

The second one was the popular talk **Quantum energetics: from measurement powered engines to the energetics of quantum technologies **by Alexia Auffèves.

Abstract: Quantum energetics is an emerging field exploring the relationships between energy, information and entropy at quantum scales – yielding exciting questions e.g. “At the quantum level, the laws of physics are reversible, so is there anything like quantum irreversibility?” “In quantum physics, measuring can put systems in motion – so can I fuel a quantum engine, just by looking at it?”. In this colloquium, I will present the bases of the field and how it relates to thermodynamics. I will finally present how quantum energetics may contribute to keep in check the energy cost of emerging quantum technologies.

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Then, **Thermodynamics with carbon nanotubes and energetics of pre-measurement** by Léa Bresque.

Abstract: When manipulating quantum systems, their quantum nature can affect the heat and work exchanges associated. For instance, erasing the coherence in the energy basis of a quantum system, one can extract useful work. Focusing on the specific example of an oscillating carbon nanotube carrying a spin quantum dot, I will explain how the mechanical oscillations could act as a battery receiving this work. Another intriguing way to affect the energy of quantum systems is to simply measure them. Hence, motivated by quantum measurement engines, I will then discuss the nature of measurement induced energy exchanges. As first pinpointed by Von Neumann, a quantum measurement can be divided into a correlation building step: the pre-measurement followed by a classical measurement of the meter. During the pre-measurement, the situation is that of a bipartite quantum system made of the measured system and the meter system which interact together. Applying a new symmetric and operational formalism to characterize the energy exchanges within closed quantum systems, I will explain under which conditions the energy exchanges during an ideal projective quantum measurement can be solely considered as a work-like quantity.

And also **Energy-efficient entanglement generation and read-out in a spin-photon interface** by Maria Maffei.

Abstract: We consider a quantum interface made of a spinful 2-level atom resonantly coupled with an electromagnetic field propagating in one spatial dimension. We study its potential for energy-efficient generation of spin-light entanglement by comparing incoming light pulses having different photonic statistics. We show that quantum superpositions of vacuum and single photon pulses outperform coherent pulses of light, producing more entanglement with the same energy. We solve analytically the joint spin-light dynamics, derive explicit expressions of the spin-light entangled wavefunctions and hence quantify the amount of entanglement generated as a function of energy and duration of the light pulses, for both photonic statistics considered. In the limit of extremely long (quasi-monochromatic) single-photon pulses, our solution shows that the interaction entangles the spin with the pulses’ phases while preserving their shapes, an effect already employed to implement spin-controlled phase gates on monochromatic photons. Aside from this limit, our solution reveals that the entanglement gets distributed over polarisation, temporal profile and phase of the light pulses becoming harder to detect. We propose a detection scheme capable to achieve this challenging goal and demonstrate its feasibility in state-of-the-art semi-conducting devices. The proposed scheme allows to observe the quantum advantage provided by single-photon states with respect to coherent ones in getting entangled with the spin for any duration of the pulses. We show that the advantage’s detection is robust against system’s imperfections.