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arXiv:1810.13121 · Physical review. E

Quantum gears from planar rotors

Zheng Liu, Joshua Leong, Stefan Nimmrichter, Valerio Scarani

Published Apr 4, 2019• 5 citations
arXivHTMLPDF

Abstract

We investigate the dynamics of interacting quantum planar rotors as the building blocks of gear trains and nanomachinery operating in the quantum regime. Contrary to a classical hard-gear scenario of rigidly interlocked teeth, we consider the coherent contactless coupling through a finite interlocking potential, and we study the transmission of motion from one externally driven gear to the next as a function of the coupling parameters and gear profile. The transmission is assessed in terms of transferred angular momentum and transferred mechanical work. We highlight the quantum features of the model such as quantum state revivals in the interlocked rotation and interference-enhanced transmission, which could be observed in prospective rotational optomechanics experiments.

DOI
10.1103/physreve.99.042202

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We investigate the dynamics of interacting quantum planar rotors as the building blocks of gear trains and nanomachinery operating in the quantum regime. Contrary to a classical hard-gear scenario of rigidly interlocked teeth, we consider the coherent contactless coupling through a finite interlocking potential, and we study the transmission of motion from one externally driven gear to the next as a function of the coupling parameters and gear profile. The transmission is assessed in terms of transferred angular momentum and transferred mechanical work. We highlight the quantum features of the model such as quantum state revivals in the interlocked rotation and interference-enhanced transmission, which could be observed in prospective rotational optomechanics experiments.
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We study the reciprocal Barnett and Einstein--de Haas effects in an exactly solvable quantum spin--rotor model that conserves the total angular momentum. For a single spin-1/21/21/2, the model describes rotation-dependent spin polarization, coherent transfer of angular momentum between the spin and the rotor, and the accompanying spin--rotor entanglement. At a recoil-compensation point, all angular-momentum sectors become synchronized, allowing complete spin reversal and a one-quantum shift of an arbitrary rotor wave packet. We then extend the model to two spins coupled to the same rotor, where the shared rotational degree of freedom participates dynamically in the generation of spin--spin entanglement. The singlet forms a dark state, while the triplet sector remains coupled to the mechanical motion. At a special parameter point, the rotor returns to its original dynamical branch and disentangles from the spins, leaving behind a maximally entangled two-spin state locally equivalent to a Bell state.
We consider a finite one-dimensional chain of quantum rotors interacting with a set of thermal baths at different temperatures. When the interaction between the rotors is made chiral, such a system behaves as an autonomous thermal motor, converting heat currents into nonvanishing rotational ones. Such a dynamical response is strongly pronounced in the range of the Hamiltonian parameters for which the ground state of the system in the thermodynamic limit exhibits a quantum phase transition. Such working points are associated with large quantum coherence and multipartite quantum correlations within the state of the system. This suggests that the optimal operating regime of such a quantum autonomous motor is one of maximal quantumness.
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