320 research outputs found
Driven Spin Systems as Quantum Thermodynamic Machines: Fundamental Limits
We show that coupled two level systems like qubits studied in quantum
information can be used as a thermodynamic machine. At least three qubits or
spins are necessary and arranged in a chain. The system is interfaced between
two split baths and the working spin in the middle is externally driven. The
machine performs Carnot-type cycles and is able to work as heat pump or engine
depending on the temperature difference of the baths and the energy
differences in the spin system . It can be shown that the efficiency
is a function of and .Comment: 9 pages, 11 figures, accepted for publication in Phys. Rev.
A Discrete Four Stroke Quantum Heat Engine Exploring the Origin of Friction
The optimal power performance of a first principle quantum heat engine model
shows friction-like phenomena when the internal fluid Hamiltonian does not
commute with the external control field. The model is based on interacting
two-level-systems where the external magnetic field serves as a control
variable.Comment: 4 pages 3 figure
Quantum, cyclic and particle-exchange heat engines
Differences between the thermodynamic behavior of the three-level amplifier
(a quantum heat engine based on a thermally pumped laser) and the classical
Carnot cycle are usually attributed to the essentially quantum or discrete
nature of the former. Here we provide examples of a number of classical and
semiclassical heat engines, such as thermionic, thermoelectric and photovoltaic
devices, which all utilize the same thermodynamic mechanism for achieving
reversibility as the three-level amplifier, namely isentropic (but
non-isothermal) particle transfer between hot and cold reservoirs. This
mechanism is distinct from the isothermal heat transfer required to achieve
reversibility in cyclic engines such as the Carnot, Otto or Brayton cycles. We
point out that some of the qualitative differences previously uncovered between
the three-level amplifier and the Carnot cycle may be attributed to the fact
that they are not the same 'type' of heat engine, rather than to the quantum
nature of the three-level amplifier per se.Comment: 9 pages. Proceedings of 'Frontiers of Quantum and Mesoscopic
Thermodynamics', Prague 200
Performance of discrete heat engines and heat pumps in finite time
The performance in finite time of a discrete heat engine with internal
friction is analyzed. The working fluid of the engine is composed of an
ensemble of noninteracting two level systems. External work is applied by
changing the external field and thus the internal energy levels. The friction
induces a minimal cycle time. The power output of the engine is optimized with
respect to time allocation between the contact time with the hot and cold baths
as well as the adiabats. The engine's performance is also optimized with
respect to the external fields. By reversing the cycle of operation a heat pump
is constructed. The performance of the engine as a heat pump is also optimized.
By varying the time allocation between the adiabats and the contact time with
the reservoir a universal behavior can be identified. The optimal performance
of the engine when the cold bath is approaching absolute zero is studied. It is
found that the optimal cooling rate converges linearly to zero when the
temperature approaches absolute zero.Comment: 45 pages LaTeX, 25 eps figure
Fundamental limitations for quantum and nano thermodynamics
The relationship between thermodynamics and statistical physics is valid in
the thermodynamic limit - when the number of particles becomes very large.
Here, we study thermodynamics in the opposite regime - at both the nano scale,
and when quantum effects become important. Applying results from quantum
information theory we construct a theory of thermodynamics in these limits. We
derive general criteria for thermodynamical state transformations, and as
special cases, find two free energies: one that quantifies the
deterministically extractable work from a small system in contact with a heat
bath, and the other that quantifies the reverse process. We find that there are
fundamental limitations on work extraction from nonequilibrium states, owing to
finite size effects and quantum coherences. This implies that thermodynamical
transitions are generically irreversible at this scale. As one application of
these methods, we analyse the efficiency of small heat engines and find that
they are irreversible during the adiabatic stages of the cycle.Comment: Final, published versio
Thermodynamics of quantum systems under dynamical control
In this review the debated rapport between thermodynamics and quantum
mechanics is addressed in the framework of the theory of
periodically-driven/controlled quantum-thermodynamic machines. The basic model
studied here is that of a two-level system (TLS), whose energy is periodically
modulated while the system is coupled to thermal baths. When the modulation
interval is short compared to the bath memory time, the system-bath
correlations are affected, thereby causing cooling or heating of the TLS,
depending on the interval. In steady state, a periodically-modulated TLS
coupled to two distinct baths constitutes the simplest quantum heat machine
(QHM) that may operate as either an engine or a refrigerator, depending on the
modulation rate. We find their efficiency and power-output bounds and the
conditions for attaining these bounds. An extension of this model to multilevel
systems shows that the QHM power output can be boosted by the multilevel
degeneracy.
These results are used to scrutinize basic thermodynamic principles: (i)
Externally-driven/modulated QHMs may attain the Carnot efficiency bound, but
when the driving is done by a quantum device ("piston"), the efficiency
strongly depends on its initial quantum state. Such dependence has been unknown
thus far. (ii) The refrigeration rate effected by QHMs does not vanish as the
temperature approaches absolute zero for certain quantized baths, e.g.,
magnons, thous challenging Nernst's unattainability principle. (iii)
System-bath correlations allow more work extraction under periodic control than
that expected from the Szilard-Landauer principle, provided the period is in
the non-Markovian domain. Thus, dynamically-controlled QHMs may benefit from
hitherto unexploited thermodynamic resources
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