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Journal of Physics B Atomic Molecular and Optical Physics

A nuclear quadrupolar spin quantum heat engine

Selçuk Çakmak, Ferdi Altintas

Published May 17, 2022• 5 citations
Publisher

Abstract

Abstract We give an implementable scheme which uses intrinsic quadrupolar nuclear spin interactions to harvest efficient energy from a quantum Otto cycle. We employ realistic parameter regimes for the 23Na nucleus in sodium nitrate. The processes of the cycle are accomplished by orienting the sample with respect to the static magnetic field. The effects of stroke duration on the work output and efficiency are revealed in detail. Finite-time adiabatic transformations leading to quantum friction are found to substantially reduce cycle outputs which are stimulated from the non-secular parts of the quadrupolar interaction. An estimation for the power output at maximum efficiency is also given. We show that with the precise control and manipulation of the intrinsic nuclear spin interactions, for example in an advanced nuclear magnetic resonance setup, makes our scheme implement as a powerful quantum Otto cycle.

DOI
10.1088/1361-6455/ac7097

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Abstract We give an implementable scheme which uses intrinsic quadrupolar nuclear spin interactions to harvest efficient energy from a quantum Otto cycle. We employ realistic parameter regimes for the 23Na nucleus in sodium nitrate. The processes of the cycle are accomplished by orienting the sample with respect to the static magnetic field. The effects of stroke duration on the work output and efficiency are revealed in detail. Finite-time adiabatic transformations leading to quantum friction are found to substantially reduce cycle outputs which are stimulated from the non-secular parts of the quadrupolar interaction. An estimation for the power output at maximum efficiency is also given. We show that with the precise control and manipulation of the intrinsic nuclear spin interactions, for example in an advanced nuclear magnetic resonance setup, makes our scheme implement as a powerful quantum Otto cycle.
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Abstract The efficiency of a quantum heat engine is maximum when the unitary strokes of the quantum Otto cycle are adiabatic. On the other hand, this may not be always possible due to small energy gaps in the system, especially at the critical point (CP) where the gap between the ground state and the first excited state vanishes and the system gets excited. With the aim to regain this lost adiabaticity, we modify one of the unitary strokes of the Otto cycle by allowing the system to first evolve with a time dependent Hamiltonian as in the case of a usual Otto cycle, followed by an additional evolution with a different time independent Hamiltonian so that the system reaches a less excited state. This will help in increasing the magnitude of the heat absorbed from the hot bath so that the work output and efficiency of the engine can be increased. We demonstrate this method using an integrable model and a non-integrable model as the working medium and discuss the generality and limitations of this method. In the case of a two spin system, the optimal value for the time till which the system needs to be freely evolved is calculated analytically in the adiabatic limit. The results show that implementing this modified unitary stroke significantly improves the work output and efficiency of the engine, especially when it crosses the CP.
The concept of inner friction, by which a quantum heat engine is unable to follow adiabatically its strokes and thus dissipates useful energy, is illustrated in an exact physical model where the working substance consists of an ensemble of misaligned spins interacting with a magnetic field and performing the Otto cycle. The effect of this static disorder under a finite-time cycle gives a new perspective of the concept of inner friction under realistic settings. We investigate the efficiency and power of this engine and relate its performance to the amount of friction from misalignment and to the temperature difference between heat baths. Finally we propose an alternative experimental implementation of the cycle where the spin is encoded in the degree of polarization of photons.
The practical untenability of the quasi-static assumption makes any realistic engine intrinsically irreversible and its operating time finite, thus implying friction effects at short cycle times. An important technological goal is thus the design of maximally efficient engines working at the maximum possible power. We show that, by utilising shortcuts to adiabaticity in a quantum engine cycle, one can engineer a thermodynamic cycle working at finite power and zero friction. Our findings are illustrated using a harmonic oscillator undergoing a quantum Otto cycle.
Developments in the thermodynamics of small quantum systems envisage nonclassical thermal machines. In this scenario, energy fluctuations play a relevant role in the description of irreversibility. We experimentally implement a quantum heat engine based on a spin-1/2 system and nuclear magnetic resonance techniques. Irreversibility at a microscope scale is fully characterized by the assessment of energy fluctuations associated with the work and heat flows. We also investigate the efficiency lag related to the entropy production at finite time. The implemented heat engine operates in a regime where both thermal and quantum fluctuations (associated with transitions among the instantaneous energy eigenstates) are relevant to its description. Performing a quantum Otto cycle at maximum power, the proof-of-concept quantum heat engine is able to reach an efficiency for work extraction (η≈42%) very close to its thermodynamic limit (η=44%).