106 research outputs found
Energetic advantages of non-adiabatic drives combined with non-thermal quantum states
Unitary drivings of quantum systems are ubiquitous in experiments and
applications of quantum mechanics and the underlying energetic aspects,
particularly relevant in quantum thermodynamics, are receiving growing
attention. We investigate energetic advantages in unitary driving obtained from
initial non-thermal states. We introduce the non-cyclic ergotropy to quantify
the energetic gains, from which coherent (coherence-based) and incoherent
(population-based) contributions are identified. In particular, initial quantum
coherences appear to be always beneficial whereas non-passive population
distributions not systematically. Additionally, these energetic gains are
accessible only through non-adiabatic dynamics, contrasting with the usual
optimality of adiabatic dynamics for initial thermal states. Finally, following
frameworks established in the context of shortcut-to-adiabaticity, the
energetic cost related to the implementation of the optimal drives are analysed
and, in most situations, are found to be smaller than the energetic cost
associated with shortcut-to-adiabaticity. We treat explicitly the example of a
two-level system and show that energetic advantages increase with larger
initial coherences, illustrating the interplay between initial coherences and
the ability of the dynamics to consume and use coherences.Comment: 6 + 6 pages, 2 + 1 figures. Updated figure
Steady State in Ultrastrong Coupling Regime: Expansion and First Orders
Understanding better the dynamics and steady states of systems strongly coupled to thermal baths is a great theoretical challenge with promising applications in several fields of quantum technologies. Among several strategies to gain access to the steady state, one consists in obtaining approximate expressions of the mean force Gibbs state, the reduced state of the global system-bath thermal state, largely credited to be the steady state. Here, we present analytical expressions of corrective terms to the ultrastrong coupling limit of the mean force Gibbs state, which has been recently derived. We find that the first order term precisely coincides with the first order correction obtained from a dynamical approach—master equation in the strong-decoherence regime. This strengthens the identification of the reduced steady state with the mean force Gibbs state. Additionally, we also compare our expressions with another recent result obtained from a high temperature expansion of the mean force Gibbs state. We observe numerically a good agreement for ultra strong coupling as well as for high temperatures. This confirms the validity of all these results. In particular, we show that, in term of coherences, all three results allow one to sketch the transition from ultrastrong coupling to weak coupling.Quanta 2022; 11: 53–71
Roles of quantum coherences in thermal machines
Some of the oldest and most important applications of thermodynamics are
operations of refrigeration as well as production of useful energy. Part of the
efforts to understand and develop thermodynamics in the quantum regime have
been focusing on harnessing quantum effects to such operations. In this review
we present the recent developments regarding the role of quantum coherences in
the performances of thermal machines --the devices realising the above
thermodynamic operations. While this is known to be an intricate subject, in
part because being largely model-dependent, the review of the recent results
allow us to identify some general tendencies and to suggest some future
directions.Comment: Review paper, 12 pages, 2 figures.Accepted for publication in The
European Physical Journal Special Topic
Heat flow reversals without reversing the arrow of time: the role of internal quantum coherences and correlations
One of the stunning consequences of quantum correlations in thermodynamics is
the reversal of the arrow of time, recently shown experimentally in [K.
Micadei, et al., Nat. Commun. 10:2456 (2019)], and manifesting itself by a
reversal of the heat flow (from the cold system to the hot one). Here, we show
that contrary to what could have been expected, heat flow reversal can happen
without reversal of the arrow of time. Moreover, contrasting with previous
studies, no initial correlations between system and bath is required. Instead,
the heat flow reversal only relies on internal quantum coherences or
correlations, which provides practical advantages over previous schemes: one
does not need to have access to the bath in order to reverse the heat flow. The
underlying mechanism is explained and shown to stem from the collective
system-bath coupling and the impact of non-energetic coherences (coherences
between degenerate energy levels) on apparent temperatures. The phenomenon is
first uncovered in a broad framework valid for diverse quantum systems
containing energy degeneracy. By the end of the paper, aiming at experimental
realisations, more quantitative results are provided for a pair of two-level
systems. Finally, as a curiosity, we mention that our scheme can be adapted as
a correlations-to-energy converter, which have the particularity to be able to
operate at constant entropy, similarly to ideal work sources.Comment: 7 pages of main text, 3 figures, 4 pages of appendices and reference
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