922 research outputs found
Open system trajectories specify fluctuating work but not heat
Based on the explicit knowledge of a Hamiltonian of mean force, the classical
statistical mechanics and equilibrium thermodynamics of open systems in contact
with a thermal environment at arbitrary interaction strength can be formulated.
Even though the Hamiltonian of mean force uniquely determines the equilibrium
phase space probability density of a strongly coupled open system the knowledge
of this probability density alone is insufficient to determine the Hamiltonian
of mean force, needed in constructing the underlying statistical mechanics and
thermodynamics. We demonstrate that under the assumption that the Hamiltonian
of mean force is known, an extension of thermodynamic structures from the level
of averaged quantities to fluctuating objects (i.e. a stochastic
thermodynamics) is possible. However, such a construction undesirably involves
also a vast ambiguity. This situation is rooted in the eminent lack of a
physical guiding principle allowing to distinguish a physically meaningful
theory out of a multitude of other equally conceivable ones.Comment: 12 pages, further typos correcte
Decoherence and dissipation during a quantum XOR gate operation
The dynamics of a quantum XOR gate operation in a two-qubit system being
coupled to a bath of quantum harmonic oscillators is investigated. Upon
applying the numerical quasiadiabatic propagator path integral method, we
obtain the numerically precise time-resolved evolution of this interacting
two-qubit system in presence of time-dependent external fields without further
approximations. We simulate the dissipative gate operation for characteristic
experimental realizations of condensed matter qubits; namely, the flux and
charge qubits realized in superconducting Josephson systems and qubits formed
with semiconductor quantum dots. Moreover, we study systematically the quality
of the XOR gate by determining the four characteristic gate quantifiers:
fidelity, purity, the quantum degree, and the entanglement capability of the
gate. Two different types of errors in the qubits have been modelled, i.e.,
bit-flip errors and phase errors. The dependence of the quality of the gate
operation on the environmental temperature, on the friction strength stemming
from the system-bath interaction, and on the strength of the interqubit
coupling is systematically explored: Our main finding is that the four gate
quantifiers depend only weakly on temperature, but are rather sensitive to the
friction strength.Comment: 16 pages including 1 table and 5 figure
Finite Bath Fluctuation Theorem
We demonstrate that a Finite Bath Fluctuation Theorem of the Crooks type
holds for systems that have been thermalized via weakly coupling it to a bath
with energy independent finite specific heat. We show that this theorem reduces
to the known canonical and microcanonical fluctuation theorems in the two
respective limiting cases of infinite and vanishing specific heat of the bath.
The result is elucidated by applying it to a 2D hard disk colliding elastically
with few other hard disks in a rectangular box with perfectly reflecting walls.Comment: 10 pages, 2 figures. Added Sec. V and App.
Decoherence in resonantly driven bistable systems
We study dissipative tunneling in a double well potential that is driven
close to a resonance between the lowest tunnel doublet and a singlet. While the
coherent dynamics can be described well within a three-level approximation,
dissipative transitions to levels outside the singlet and the doublet may play
a crucial role. Moreover, such transitions can enhance the entropy production
significantly.Comment: 12 pages, 7 figures, vch-book.cl
Comment on "Experimental Verification of a Jarzynski-Related Information-Theoretic Equality by a Single Trapped Ion" PRL 120 010601 (2018)
The target paper presents an experimental verification of a
"Jarzynski-related" equality. We show that the latter equality is in fact not
related to the Jarzynski equality.Comment: 1 pag
Quantum Bochkov-Kuzovlev Work Fluctuation Theorems
The quantum version of the Bochkov-Kuzovlev identity is derived on the basis
of the appropriate definition of work as the difference of the measured
internal energies of a quantum system at the beginning and at the end of an
external action on the system given by a prescribed protocol. According to the
spirit of the original Bochkov-Kuzovlev approach, we adopt the "exclusive"
viewpoint, meaning that the coupling to the external work-source is {\it not}
counted as part of the internal energy. The corresponding canonical and
microcanonical quantum fluctuation theorems are derived as well, and are
compared to the respective theorems obtained within the "inclusive" approach.
The relations between the quantum inclusive-work , the exclusive-work
and the dissipated-work , are discussed and clarified. We show by an
explicit example that and are distinct stochastic quantities
obeying different statistics.Comment: 16 page
Fundamental Aspects of Quantum Brownian Motion
With this work we elaborate on the physics of quantum noise in thermal
equilibrium and in stationary non-equilibrium. Starting out from the celebrated
quantum fluctuation-dissipation theorem we discuss some important consequences
that must hold for open, dissipative quantum systems in thermal equilibrium.
The issue of quantum dissipation is exemplified with the fundamental problem of
a damped harmonic quantum oscillator. The role of quantum fluctuations is
discussed in the context of both, the nonlinear generalized quantum Langevin
equation and the path integral approach. We discuss the consequences of the
time-reversal symmetry for an open dissipative quantum dynamics and,
furthermore, point to a series of subtleties and possible pitfalls. The path
integral methodology is applied to the decay of metastable states assisted by
quantum Brownian noise.Comment: 13 pages, 4 figures, RevTeX, submitted to Chaos special issue "100
Years of Brownian Motion
Taxis of Artificial Swimmers in a Spatio-Temporally Modulated Activation Medium
Contrary to microbial taxis, where a tactic response to external stimuli is
controlled by complex chemical pathways acting like sensor-actuator loops,
taxis of artificial microswimmers is a purely stochastic effect associated with
a non-uniform activation of the particles' self-propulsion. We study the tactic
response of such swimmers in a spatio-temporally modulated activating medium by
means of both numerical and analytical techniques. In the opposite limits of
very fast and very slow rotational particle dynamics, we obtain analytic
approximations that closely reproduce the numerical description. A swimmer
drifts on average either parallel or anti-parallel to the propagation direction
of the activating pulses, depending on their speed and width. The drift in line
with the pulses is solely determined by the finite persistence length of the
active Brownian motion performed by the swimmer, whereas the drift in the
opposite direction results from the combination of ballistic and diffusive
properties of the swimmer's dynamics.Comment: 19 pages, 6 figures; Entropy (in press
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