2,716 research outputs found
Micrometre-scale refrigerators
A superconductor with a gap in the density of states or a quantum dot with
discrete energy levels is a central building block in realizing an electronic
on-chip cooler. They can work as energy filters, allowing only hot
quasiparticles to tunnel out from the electrode to be cooled. This principle
has been employed experimentally since the early 1990s in investigations and
demonstrations of micrometre-scale coolers at sub-kelvin temperatures. In this
paper, we review the basic experimental conditions in realizing the coolers and
the main practical issues that are known to limit their performance. We give an
update of experiments performed on cryogenic micrometre-scale coolers in the
past five years
Finite-size effects in dynamics of zero-range processes
The finite-size effects prominent in zero-range processes exhibiting a
condensation transition are studied by using continuous-time Monte Carlo
simulations. We observe that, well above the thermodynamic critical point, both
static and dynamic properties display fluid-like behavior up to a density
{\rho}c (L), which is the finite-size counterpart of the critical density
{\rho}c = {\rho}c (L \rightarrow \infty). We determine this density from the
cross-over behavior of the average size of the largest cluster. We then show
that several dynamical characteristics undergo a qualitative change at this
density. In particular, the size distribution of the largest cluster at the
moment of relocation, the persistence properties of the largest cluster and
correlations in its motion are studied.Comment: http://pre.aps.org/abstract/PRE/v82/i3/e03111
Experimental determination of the Berry phase in a superconducting charge pump
We present the first measurements of the Berry phase in a superconducting
Cooper pair pump. A fixed amount of Berry phase is accumulated to the
quantum-mechanical ground state in each adiabatic pumping cycle, which is
determined by measuring the charge passing through the device. The dynamic and
geometric phases are identified and measured quantitatively from their
different response when pumping in opposite directions. Our observations, in
particular, the dependencies of the dynamic and geometric effects on the
superconducting phase bias across the pump, agree with the basic theoretical
model of coherent Cooper pair pumping.Comment: 4 pages, 3 figure
On the structure of covariant phase observables
We study the mathematical structure of covariant phase observables. Such an
observable can alternatively be expressed as a phase matrix, as a sequence of
unit vectors, as a sequence of phase states, or as an equivalent class of
covariant trace-preserving operations. Covariant generalized operator measures
are defined by structure matrices which form a W*-algebra with phase matrices
as its subset. The properties of the Radon-Nikodym derivatives of phase
probability measures are studied.Comment: 11 page
Completely positive maps on modules, instruments, extremality problems, and applications to physics
Convex sets of completely positive maps and positive semidefinite kernels are
considered in the most general context of modules over -algebras and a
complete charaterization of their extreme points is obtained. As a byproduct,
we determine extreme quantum instruments, preparations, channels, and extreme
autocorrelation functions. Various applications to quantum information and
measurement theories are given. The structure of quantum instruments is
analyzed thoroughly.Comment: 32 page
Extreme commutative quantum observables are sharp
It is well known that, in the description of quantum observables, positive
operator valued measures (POVMs) generalize projection valued measures (PVMs)
and they also turn out be more optimal in many tasks. We show that a
commutative POVM is an extreme point in the convex set of all POVMs if and only
if it is a PVM. This results implies that non-commutativity is a necessary
ingredient to overcome the limitations of PVMs.Comment: 5 pages, minor corrections in v
Characteristics of the polymer transport in ratchet systems
Molecules with complex internal structure in time-dependent periodic
potentials are studied by using short Rubinstein-Duke model polymers as an
example. We extend our earlier work on transport in stochastically varying
potentials to cover also deterministic potential switching mechanisms,
energetic efficiency and non-uniform charge distributions. We also use currents
in the non-equilibrium steady state to identify the dominating mechanisms that
lead to polymer transportation and analyze the evolution of the macroscopic
state (e.g., total and head-to-head lengths) of the polymers. Several numerical
methods are used to solve the master equations and nonlinear optimization
problems. The dominating transport mechanisms are found via graph optimization
methods. The results show that small changes in the molecule structure and the
environment variables can lead to large increases of the drift. The drift and
the coherence can be amplified by using deterministic flashing potentials and
customized polymer charge distributions. Identifying the dominating transport
mechanism by graph analysis tools is found to give insight in how the molecule
is transported by the ratchet effect.Comment: 35 pages, 17 figures, to appear in Phys. Rev.
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