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1. Quantum and Classical Ergotropy from Relative Entropies

   https://doi.org/10.3390/e23091107  

   Sone, Akira; Deffner, Sebastian (September 2021, Entropy)

   null (Ed.)

   The quantum ergotropy quantifies the maximal amount of work that can be extracted from a quantum state without changing its entropy. Given that the ergotropy can be expressed as the difference of quantum and classical relative entropies of the quantum state with respect to the thermal state, we define the classical ergotropy, which quantifies how much work can be extracted from distributions that are inhomogeneous on the energy surfaces. A unified approach to treat both quantum as well as classical scenarios is provided by geometric quantum mechanics, for which we define the geometric relative entropy. The analysis is concluded with an application of the conceptual insight to conditional thermal states, and the correspondingly tightened maximum work theorem. 

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2. Non-adiabatic quantum control of quantum dot arrays with fixed exchange using Cartan decomposition

   https://doi.org/10.1098/rsta.2021.0275  

   Kanaar, David W.; Güngördü, Utkan; Kestner, J. P. (December 2022, Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences)

   In semiconductor spin qubits which typically interact through short-range exchange coupling, shuttling of spin is a practical way to generate quantum operations between distant qubits. Although the exchange is often tunable through voltages applied to gate electrodes, its minimal value can be significantly large, which hinders the applicability of existing shuttling protocols to such devices, requiring a different approach. In this work, we extend our previous results for double- and triple-dot systems, and describe a method for implementing spin state transfer in long chains of singly occupied quantum dots in a non-adiabatic manner. We make use of Cartan decomposition to break down the interacting problem into simpler problems in a systematic way, and use dynamical invariants to design smooth non-adiabatic pulses that can be implemented in devices with modest control bandwidth. Finally, we discuss the extensibility of our results to directed shuttling of spin states on two-dimensional lattices of quantum dots with fixed coupling. This article is part of the theme issue ‘Shortcuts to adiabaticity: theoretical, experimental and interdisciplinary perspectives’. 

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3. Thermally driven quantum refrigerator autonomously resets a superconducting qubit

   https://doi.org/10.1038/s41567-024-02708-5  

   Aamir, Mohammed Ali; Jamet_Suria, Paul; Marín_Guzmán, José Antonio; Castillo-Moreno, Claudia; Epstein, Jeffrey M; Yunger_Halpern, Nicole; Gasparinetti, Simone (February 2025, Nature Physics)

   Abstract Although classical thermal machines power industries and modern living, quantum thermal engines have yet to prove their utility. Here, we demonstrate a useful quantum absorption refrigerator formed from superconducting circuits. We use it to cool a transmon qubit to a temperature lower than that achievable with any one available bath, thereby resetting the qubit to an initial state suitable for quantum computing. The process is driven by a thermal gradient and is autonomous, requiring no external feedback. The refrigerator exploits an engineered three-body interaction between the target qubit and two auxiliary qudits. Each auxiliary qudit is coupled to a physical heat bath, realized with a microwave waveguide populated with synthesized quasithermal radiation. If the target qubit is initially fully excited, its effective temperature reaches a steady-state level of approximately 22 mK, lower than what can be achieved by existing state-of-the-art reset protocols. Our results demonstrate that superconducting circuits with propagating thermal fields can be used to experimentally explore quantum thermodynamics and apply it to quantum information-processing tasks. 

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4. Approaching the motional ground state of a 10-kg object

   https://doi.org/10.1126/science.abh2634  

   Whittle, Chris; Hall, Evan D.; Dwyer, Sheila; Mavalvala, Nergis; Sudhir, Vivishek; Abbott, R.; Ananyeva, A.; Austin, C.; Barsotti, L.; Betzwieser, J.; et al (June 2021, Science)

   The motion of a mechanical object, even a human-sized object, should be governed by the rules of quantum mechanics. Coaxing them into a quantum state is, however, difficult because the thermal environment masks any quantum signature of the object’s motion. The thermal environment also masks the effects of proposed modifications of quantum mechanics at large mass scales. We prepared the center-of-mass motion of a 10-kilogram mechanical oscillator in a state with an average phonon occupation of 10.8. The reduction in temperature, from room temperature to 77 nanokelvin, is commensurate with an 11 orders-of-magnitude suppression of quantum back-action by feedback and a 13 orders-of-magnitude increase in the mass of an object prepared close to its motional ground state. Our approach will enable the possibility of probing gravity on massive quantum systems. 

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5. Long-lived quantum coherent dynamics of a Λ-system driven by a thermal environment

   https://doi.org/10.1063/5.0102808  

   Koyu, Suyesh; Tscherbul, Timur V. (September 2022, The Journal of Chemical Physics)

   We present a theoretical study of quantum coherent dynamics of a three-level Λ-system driven by a thermal environment (such as blackbody radiation), which serves as an essential building block of photosynthetic light-harvesting models and quantum heat engines. By solving nonsecular Bloch–Redfield master equations, we obtain analytical results for the ground-state population and coherence dynamics and classify the dynamical regimes of the incoherently driven Λ-system as underdamped and overdamped depending on whether the ratio Δ/[ rf( p)] is greater or less than one, where Δ is the ground-state energy splitting, r is the incoherent pumping rate, and f( p) is a function of the transition dipole alignment parameter p. In the underdamped regime, we observe long-lived coherent dynamics that lasts for τ c ≃ 1/ r, even though the initial state of the Λ-system contains no coherences in the energy basis. In the overdamped regime for p = 1, we observe the emergence of coherent quasi-steady states with the lifetime τ c = 1.34( r/Δ 2 ), which have a low von Neumann entropy compared to conventional thermal states. We propose an experimental scenario for observing noise-induced coherent dynamics in metastable He* atoms driven by x-polarized incoherent light. Our results suggest that thermal excitations can generate experimentally observable long-lived quantum coherent dynamics in the ground-state subspace of atomic and molecular Λ-systems in the absence of coherent driving. 

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