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1. Thermodynamically-informed Air-based Soft Heat Engine Design

   Charles Xiao, Luke F. ( January 2021 , ArXivorg)

   null (Ed.)

   Soft heat engines are poised to play a vital role in future soft robots due to their easy integration into soft structures and low-voltage power requirements. Recent works have demonstrated soft heat engines relying on liquid-to-gas phase change materials. However, despite the fact that many soft robots have air as a primary component, soft air cycles are not a focus of the field. In this paper, we develop theory for air-based soft heat engines design and efficiency, and demonstrate experimentally that efficiency can be improved through careful cycle design. We compare a simple constant-load cycle to a designed decreasing-load cycle, inspired by the Otto cycle. While both efficiencies are relatively low, the Otto-like cycle improves efficiency by a factor of 11.3, demonstrating the promise of this approach. Our results lay the foundation for the development of air-based soft heat engines as a new option for powering soft robots. 

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2. Boosting engine performance with Bose–Einstein condensation

   https://doi.org/10.1088/1367-2630/ac47cc  

   Myers, Nathan M. ; Peña, Francisco J. ; Negrete, Oscar ; Vargas, Patricio ; De Chiara, Gabriele ; Deffner, Sebastian ( February 2022 , New Journal of Physics)

   At low-temperatures a gas of bosons will undergo a phase transition into a quantum state of matter known as a Bose–Einstein condensate (BEC), in which a large fraction of the particles will occupy the ground state simultaneously. Here we explore the performance of an endoreversible Otto cycle operating with a harmonically confined Bose gas as the working medium. We analyze the engine operation in three regimes, with the working medium in the BEC phase, in the gas phase, and driven across the BEC transition during each cycle. We find that the unique properties of the BEC phase allow for enhanced engine performance, including increased power output and higher efficiency at maximum power.

    

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3. Coherently Coupled Mechanical Oscillators in the Quantum Regime

   https://doi.org/10.21203/rs.3.rs-1774123/v1  

   Hou, P-Y. ; Wu, J.J. ; Erickson, S.D. ; Cole, D.C. ; Zarantonello, G. ; Brandt, A.D. ; Wilson, A.C. ; Slichter, D.H. ; Leibfried, D. ( July 2022 , ArXivorg)

   Coupled harmonic oscillators are ubiquitous in physics and play a prominent role in quantum science. They are a cornerstone of quantum mechanics and quantum field theory, where second quantization relies on harmonic oscillator operators to create and annihilate particles. Descriptions of quantum tunneling, beamsplitters, coupled potential wells, "hopping terms", decoherence and many other phenomena rely on coupled harmonic oscillators. Despite their prominence, only a few experimental systems have demonstrated direct coupling between separate harmonic oscillators; these demonstrations lacked the capability for high-fidelity quantum control. Here, we realize coherent exchange of single motional quanta between harmonic oscillators -- in this case, spectrally separated harmonic modes of motion of a trapped ion crystal where the timing, strength, and phase of the coupling are controlled through the application of an oscillating electric field with suitable spatial variation. We demonstrate high-fidelity quantum state transfer, entanglement of motional modes, and Hong-Ou-Mandel-type interference. We also project a harmonic oscillator into its ground state by measurement and preserve that state during repetitions of the projective measurement, an important prerequisite for non-destructive syndrome measurement in continuous-variable quantum error correction. Controllable coupling between harmonic oscillators has potential applications in quantum information processing with continuous variables, quantum simulation, and precision measurements. It can also enable cooling and quantum logic spectroscopy involving motional modes of trapped ions that are not directly accessible. 

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4. Quantum simulation of bosons with the contracted quantum eigensolver

   https://doi.org/10.1088/1367-2630/acf9c3  

   Wang, Yuchen ; Sager-Smith, LeeAnn M. ; Mazziotti, David A. ( October 2023 , New Journal of Physics)

   Quantum computers are promising tools for simulating many-body quantum systems due to their potential scaling advantage over classical computers. While significant effort has been expended on many-fermion systems, here we simulate a model entangled many-boson system with the contracted quantum eigensolver (CQE). We generalize the CQE to many-boson systems by encoding the bosonic wavefunction on qubits. The CQE provides a compact ansatz for the bosonic wave function whose gradient is proportional to the residual of a contracted Schrödinger equation. We apply the CQE to a bosonic system, whereNquantum harmonic oscillators are coupled through a pairwise quadratic repulsion. The model is relevant to the study of coupled vibrations in molecular systems on quantum devices. Results demonstrate the potential efficiency of the CQE in simulating bosonic processes such as molecular vibrations with good accuracy and convergence even in the presence of noise.

    

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5. 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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