63 research outputs found
Physical implementations of quantum absorption refrigerators
Absorption refrigerators are autonomous thermal machines that harness the
spontaneous flow of heat from a hot bath into the environment in order to
perform cooling. Here we discuss quantum realizations of absorption
refrigerators in two different settings: namely, cavity and circuit quantum
electrodynamics. We first provide a unified description of these machines in
terms of the concept of virtual temperature. Next, we describe the two
different physical setups in detail and compare their properties and
performance. We conclude with an outlook on future work and open questions in
this field of research.Comment: Patrick P. Potts was formerly known as Patrick P. Hofe
Probing the dynamic structure factor of a neutral Fermi superfluid along the BCS-BEC crossover using atomic impurity qubits
We study an impurity atom trapped by an anharmonic potential, immersed within
a cold atomic Fermi gas with attractive interactions that realizes the
crossover from a Bardeen-Cooper-Schrieffer (BCS) superfluid to a Bose-Einstein
condensate (BEC). Considering the qubit comprising the lowest two vibrational
energy eigenstates of the impurity, we demonstrate that its dynamics probes the
equilibrium density fluctuations encoded in the dynamic structure factor of the
superfluid. Observing the impurity's evolution is thus shown to facilitate
nondestructive measurements of the superfluid order parameter and the contact
between collective and single-particle excitation spectra. Our setup
constitutes a novel model of an open quantum system interacting with a thermal
reservoir, the latter supporting both bosonic and fermionic excitations that
are also coupled to each other.Comment: Updated to final author version. 9+7 pages, 18 figure
Thermometry by correlated dephasing of impurities in a 1D Fermi gas
We theoretically investigate the pure dephasing dynamics of two static
impurity qubits embedded within a common environment of ultracold fermionic
atoms, which are confined to one spatial dimension. Our goal is to understand
how bath-mediated interactions between impurities affect their performance as
nonequilibrium quantum thermometers. By solving the dynamics exactly using a
functional determinant approach, we show that the impurities become correlated
via retarded interactions of the Ruderman-Kittel-Kasuya-Yosida type. Moreover,
we demonstrate that these correlations can provide a metrological advantage,
enhancing the sensitivity of the two-qubit thermometer beyond that of two
independent impurities. This enhancement is most prominent in the limit of low
temperature and weak collisional coupling between the impurities and the gas.
We show that this precision advantage can be exploited using standard Ramsey
interferometry, with no need to prepare correlated initial states nor to
individually manipulate or measure the impurities. We also quantitatively
assess the impact of ignoring these correlations when constructing a
temperature estimate, finding that acceptable precision can still be achieved
from a simplified model of independent impurities. Our results demonstrate the
rich nonequilibrium physics of impurities dephasing in a common Fermi gas, and
may help to provide better temperature estimates at ultralow temperatures.Comment: v1: 16 pages, 6 figures. Comments and feedback welcome as always v2:
included temperature dependent decoherence and added reference
Charging a quantum battery with linear feedback control
Energy storage is a basic physical process with many applications. When
considering this task at the quantum scale, it becomes important to optimise
the non-equilibrium dynamics of energy transfer to the storage device or
battery. Here, we tackle this problem using the methods of quantum feedback
control. Specifically, we study the deposition of energy into a quantum battery
via an auxiliary charger. The latter is a driven-dissipative two-level system
subjected to a homodyne measurement whose output signal is fed back linearly
into the driving field amplitude. We explore two different control strategies,
aiming to stabilise either populations or quantum coherences in the state of
the charger. In both cases, linear feedback is shown to counteract the
randomising influence of environmental noise and allow for stable and effective
battery charging. We analyse the effect of realistic control imprecisions,
demonstrating that this good performance survives inefficient measurements and
small feedback delays. Our results highlight the potential of continuous
feedback for the control of energetic quantities in the quantum regime.Comment: v1: 10 pages, 8 figures. Comments welcome! v2: Final versio
Robust nonequilibrium edge currents with and without band topology
We study two-dimensional bosonic and fermionic lattice systems under
nonequilibrium conditions corresponding to a sharp gradient of temperature
imposed by two thermal baths. In particular, we consider a lattice model with
broken time-reversal symmetry that exhibits both topologically trivial and
nontrivial phases. Using a nonperturbative approach, we characterize the
nonequilibrium current distribution in different parameter regimes. For both
bosonic and fermionic systems weakly coupled to the reservoirs, we find chiral
edge currents that are robust against defects on the boundary or in the bulk.
This robustness not only originates from topological effects at zero
temperature but, remarkably, also persists as a result of dissipative
symmetries in regimes where band topology plays no role. Chirality of the edge
currents implies that energy locally flows against the temperature gradient
without any external work input. In the fermionic case, there is also a regime
with topologically protected boundary currents, which nonetheless do not
circulate around all system edges.Comment: 5+4 pages, 4+2 figures. Comments welcom
Thermodynamics of precision in quantum nanomachines
Fluctuations strongly affect the dynamics and functionality of nanoscale thermal machines. Recent developments in stochastic thermodynamics have shown that fluctuations in many far-from-equilibrium systems are constrained by the rate of entropy production via so-called thermodynamic uncertainty relations. These relations imply that increasing the reliability or precision of an engine's power output comes at a greater thermodynamic cost. Here we study the thermodynamics of precision for small thermal machines in the quantum regime. In particular, we derive exact relations between the power, power fluctuations, and entropy production rate for several models of few-qubit engines (both autonomous and cyclic) that perform work on a quantized load. Depending on the context, we find that quantum coherence can either help or hinder where power fluctuations are concerned. We discuss design principles for reducing such fluctuations in quantum nanomachines and propose an autonomous three-qubit engine whose power output for a given entropy production is more reliable than would be allowed by any classical Markovian model
Speed-ups to isothermality: Enhanced quantum thermal machines through control of the system-bath coupling
Isothermal transformations are minimally dissipative but slow processes, as
the system needs to remain close to thermal equilibrium along the protocol.
Here, we show that smoothly modifying the system-bath interaction can
significantly speed up such transformations. In particular, we construct
protocols where the overall dissipation decays with the total
time of the protocol as , where each value can be obtained by a suitable
modification of the interaction, whereas corresponds to a standard
isothermal process where the system-bath interaction remains constant.
Considering heat engines based on such speed-ups, we show that the
corresponding efficiency at maximum power interpolates between the
Curzon-Ahlborn efficiency for and the Carnot efficiency for . Analogous enhancements are obtained for the coefficient of
performance of refrigerators. We confirm our analytical results with two
numerical examples where , namely the time-dependent
Caldeira-Leggett and resonant-level models, with strong system-environment
correlations taken fully into account. We highlight the possibility of
implementing our proposed speed-ups with ultracold atomic impurities and
mesoscopic electronic devices.Comment: 21 pages, 14 figures. Final author versio
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