2,287 research outputs found
A measure of majorisation emerging from single-shot statistical mechanics
The use of the von Neumann entropy in formulating the laws of thermodynamics
has recently been challenged. It is associated with the average work whereas
the work guaranteed to be extracted in any single run of an experiment is the
more interesting quantity in general. We show that an expression that
quantifies majorisation determines the optimal guaranteed work. We argue it
should therefore be the central quantity of statistical mechanics, rather than
the von Neumann entropy. In the limit of many identical and independent
subsystems (asymptotic i.i.d) the von Neumann entropy expressions are recovered
but in the non-equilbrium regime the optimal guaranteed work can be radically
different to the optimal average. Moreover our measure of majorisation governs
which evolutions can be realized via thermal interactions, whereas the
nondecrease of the von Neumann entropy is not sufficiently restrictive. Our
results are inspired by single-shot information theory.Comment: 54 pages (15+39), 9 figures. Changed title / changed presentation,
same main results / added minor result on pure bipartite state entanglement
(appendix G) / near to published versio
Scanning Tunneling Spectroscopy of Bi2Sr2CuO6+d: New Evidence for the Common Origin of the Pseudogap and Superconductivity
Using scanning tunneling spectroscopy, we investigated the temperature dependence of the quasiparticle density of states of overdoped Bi2Sr2CuO6+δ between 275 mK and 82 K. Below Tc = 10 K, the spectra show a gap with well-defined coherence peaks at ±Δp≃12 meV, which disappear at Tc. Above Tc, the spectra display a clear pseudogap of the same magnitude, gradually filling up and vanishing at T*≃68 K. The comparison with Bi2Sr2CaCu2O8+δ demonstrates that the pseudogap and the superconducting gap scale with each other, providing strong evidence that they have a common origin
The work value of information
We present quantitative relations between work and information that are valid
both for finite sized and internally correlated systems as well in the
thermodynamical limit. We suggest work extraction should be viewed as a game
where the amount of work an agent can extract depends on how well it can guess
the micro-state of the system. In general it depends both on the agent's
knowledge and risk-tolerance, because the agent can bet on facts that are not
certain and thereby risk failure of the work extraction. We derive strikingly
simple expressions for the extractable work in the extreme cases of effectively
zero- and arbitrary risk tolerance respectively, thereby enveloping all cases.
Our derivation makes a connection between heat engines and the smooth entropy
approach. The latter has recently extended Shannon theory to encompass finite
sized and internally correlated bit strings, and our analysis points the way to
an analogous extension of statistical mechanics.Comment: 5 pages, 4 figure
Linear and field-independent relation between vortex core state energy and gap in Bi2Sr2CaCu2O8+d
We present a scanning tunneling spectroscopy study on quasiparticle states in vortex cores in Bi2Sr2CaCu2O8+δ. The energy of the observed vortex core states shows an approximately linear scaling with the superconducting gap in the region just outside the core. This clearly distinguishes them from conventional localized core states and is a signature of the mechanism responsible for their discrete appearance in high-temperature superconductors. The energy scaling of the vortex core states also suggests a common nature of vortex cores in Bi2Sr2CaCu2O8+δ and YBa2Cu3O7-δ. Finally, these states do not show any dependence on the applied magnetic field between 1 and 6 T
The Uncertainty Principle in the Presence of Quantum Memory
The uncertainty principle, originally formulated by Heisenberg, dramatically
illustrates the difference between classical and quantum mechanics. The
principle bounds the uncertainties about the outcomes of two incompatible
measurements, such as position and momentum, on a particle. It implies that one
cannot predict the outcomes for both possible choices of measurement to
arbitrary precision, even if information about the preparation of the particle
is available in a classical memory. However, if the particle is prepared
entangled with a quantum memory, a device which is likely to soon be available,
it is possible to predict the outcomes for both measurement choices precisely.
In this work we strengthen the uncertainty principle to incorporate this case,
providing a lower bound on the uncertainties which depends on the amount of
entanglement between the particle and the quantum memory. We detail the
application of our result to witnessing entanglement and to quantum key
distribution.Comment: 5 pages plus 12 of supplementary information. Updated to match the
journal versio
Asymptotic Entanglement Dynamics and Geometry of Quantum States
A given dynamics for a composite quantum system can exhibit several distinct
properties for the asymptotic entanglement behavior, like entanglement sudden
death, asymptotic death of entanglement, sudden birth of entanglement, etc. A
classification of the possible situations was given in [M. O. Terra Cunha,
{\emph{New J. Phys}} {\bf{9}}, 237 (2007)] but for some classes there were no
known examples. In this work we give a better classification for the possibile
relaxing dynamics at the light of the geometry of their set of asymptotic
states and give explicit examples for all the classes. Although the
classification is completely general, in the search of examples it is
sufficient to use two qubits with dynamics given by differential equations in
Lindblad form (some of them non-autonomous). We also investigate, in each case,
the probabilities to find each possible behavior for random initial states.Comment: 9 pages, 2 figures; revised version accepted for publication in J.
Phys. A: Math. Theo
Decoupling with unitary approximate two-designs
Consider a bipartite system, of which one subsystem, A, undergoes a physical
evolution separated from the other subsystem, R. One may ask under which
conditions this evolution destroys all initial correlations between the
subsystems A and R, i.e. decouples the subsystems. A quantitative answer to
this question is provided by decoupling theorems, which have been developed
recently in the area of quantum information theory. This paper builds on
preceding work, which shows that decoupling is achieved if the evolution on A
consists of a typical unitary, chosen with respect to the Haar measure,
followed by a process that adds sufficient decoherence. Here, we prove a
generalized decoupling theorem for the case where the unitary is chosen from an
approximate two-design. A main implication of this result is that decoupling is
physical, in the sense that it occurs already for short sequences of random
two-body interactions, which can be modeled as efficient circuits. Our
decoupling result is independent of the dimension of the R system, which shows
that approximate 2-designs are appropriate for decoupling even if the dimension
of this system is large.Comment: Published versio
Itinerant in-plane magnetic fluctuations and many-body correlations in NaCoO
Based on the {\it ab-initio} band structure for NaCoO we derive the
single-electron energies and the effective tight-binding description for the
bands using projection procedure. Due to the presence of the
next-nearest-neighbor hoppings a local minimum in the electronic dispersion
close to the point of the first Brillouin zone forms. Correspondingly,
in addition to a large Fermi surface an electron pocket close to the
point emerges at high doping concentrations. The latter yields the new
scattering channel resulting in a peak structure of the itinerant magnetic
susceptibility at small momenta. This indicates dominant itinerant in-plane
ferromagnetic fluctuations above certain critical concentration , in
agreement with neutron scattering data. Below the magnetic susceptibility
shows a tendency towards the antiferromagnetic fluctuations. We further analyze
the many-body effects on the electronic and magnetic excitations using various
approximations applicable for different ratio.Comment: 10 page
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