54 research outputs found

### Electric response of DNA hairpins to magnetic fields

We study the electric properties of single-stranded DNA molecules with
hairpin-like shapes in the presence of a magnetic flux. It is shown that the
current amplitude can be modulated by the applied field. The details of the
electric response strongly depend on the twist angles. The current exhibits
periodicity for geometries where the flux through the plaquettes of the ladder
can be cancelled pairwise (commensurate twist). Further twisting the geometry
and changing its length causes complex aperiodic oscillations. We also study
persistent currents: They reduce to simple harmonic oscillations if the system
is commensurate, otherwise deviations occur due to the existence of closed
paths leading to a washboard shape.Comment: 11 pages, 4 figure

### Thermodynamic arrow of time of quantum projective measurements

We investigate a thermodynamic arrow associated with quantum projective
measurements in terms of the Jensen-Shannon divergence between the probability
distribution of energy change caused by the measurements and its time reversal
counterpart. Two physical quantities appear to govern the asymptotic values of
the time asymmetry. For an initial equilibrium ensemble prepared at a high
temperature, the energy fluctuations determine the convergence of the time
asymmetry approaching zero. At low temperatures, finite survival probability of
the ground state limits the time asymmetry to be less than $\ln 2$. We
illustrate our results for a concrete system and discuss the fixed point of the
time asymmetry in the limit of infinitely repeated projections.Comment: 6 pages in two columns, 1 figure, to appear in EP

### Single-Temperature Quantum Engine Without Feedback Control

A cyclically working quantum mechanical engine that operates at a single
temperature is proposed. Its energy input is delivered by a quantum
measurement. The functioning of the engine does not require any feedback
control. We analyze work, heat, and the efficiency of the engine for the case
of a working substance that is governed by the laws of quantum mechanics and
that can be adiabatically compressed and dilated. The obtained general
expressions are exemplified for a spin in an adiabatically changing magnetic
field and a particle moving in a potential with slowly changing shape

### Statistics of work and fluctuation theorems for microcanonical initial states

The work performed on a system in a microcanonical state by changes in a
control parameter is characterized in terms of its statistics. The transition
probabilities between eigenstates of the system Hamiltonians at the beginning
and the end of the parameter change obey a detailed balance-like relation from
which various forms of the microcanonical fluctuation theorem are obtained. As
an example, sudden deformations of a two dimensional harmonic oscillator
potential are considered and the validity of the microcanonical Jarzynski
equality connecting the degrees of degeneracy of energy eigenvalues before and
after the control parameter change is confirmed.Comment: 16 pages, 5 figure

### Comparison of free energy estimators and their dependence on dissipated work

The estimate of free energy changes based on Bennett's acceptance ratio
method is examined in several limiting cases and compared with other estimates
based on the Jarzynski equality and on the Crooks relation. While the absolute
amount of dissipated work, defined as the surplus of average work over the free
energy difference, limits the practical applicability of Jarzynski's and
Crooks' methods, the reliability of Bennett's approach is restricted by the
difference of the dissipated works in the forward and the backward process. We
illustrate these points by considering a Gaussian chain and a hairpin chain
which both are extended during the forward and accordingly compressed during
the backward protocol. The reliability of the Crooks relation predominantly
depends on the sample size; for the Jarzynski estimator the slowness of the
work protocol is crucial, and the Bennett method is shown to give precise
estimates irrespective of the pulling speed and sample size as long as the
dissipated works are the same for the forward and the backward process as it is
the case for Gaussian work distributions. With an increasing dissipated work
difference the Bennett estimator also acquires a bias which increases roughly
in proportion to this difference. A substantial simplification of the Bennett
estimator is provided by the 1/2-formula which expresses the free energy
difference by the algebraic average of the Jarzynski estimates for the forward
and the backward processes. It agrees with the Bennett estimate in all cases
when the Jarzynski and the Crooks estimates fail to give reliable results

### Work fluctuations for Bose particles in grand canonical initial states

We consider bosons in a harmonic trap and investigate the fluctuations of the
work performed by an adiabatic change of the trap curvature. Depending on the
reservoir conditions such as temperature and chemical potential that provide
the initial equilibrium state, the exponentiated work average (EWA) defined in
the context of the Crooks relation and the Jarzynski equality may diverge if
the trap becomes wider. We investigate how the probability distribution
function (PDF) of the work signals this divergence. It is shown that at low
temperatures the PDF is highly asymmetric with a steep fall off at one side and
an exponential tail at the other side. For high temperatures it is closer to a
symmetric distribution approaching a Gaussian form. These properties of the
work PDF are discussed in relation to the convergence of the EWA and to the
existence of the hypothetical equilibrium state to which those thermodynamic
potential changes refer that enter both the Crooks relation and the Jarzynski
equality.Comment: 9 pages, 4 figure

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