3,677 research outputs found
Kondo Resonance of a Microwave Photon
We emulate renormalization group models, such as the Spin-Boson Hamiltonian
or the anisotropic Kondo model, from a quantum optics perspective by
considering a superconducting device. The infra-red confinement involves photon
excitations of two tunable transmission lines entangled to an artificial
spin-1/2 particle or double-island charge qubit. Focusing on the propagation of
microwave light, in the underdamped regime of the Spin-Boson model, we identify
a many-body resonance where a photon is absorbed at the renormalized qubit
frequency and reemitted forward in an elastic manner. We also show that
asymptotic freedom of microwave light is reached by increasing the input signal
amplitude at low temperatures which allows the disappearance of the
transmission peak.Comment: Final Version: Main text and Supplementary Materia
Noninvasive Probe of Charge Fractionalization in Quantum Spin-Hall Insulators
When an electron with well-defined momentum tunnels into a nonchiral
Luttinger liquid, it breaks up into two separate wave packets that carry
fractional charges and move in opposite directions. A direct observation of
this phenomenon has proven elusive, mainly due to single-particle and plasmon
backscattering caused by measurement probes. This paper theoretically
introduces two topological insulator devices that are naturally suited for
detecting fractional charges and their velocities directly and in a noninvasive
fashion.Comment: Revised and extended version. To appear in PR
Probabilistic analysis of a differential equation for linear programming
In this paper we address the complexity of solving linear programming
problems with a set of differential equations that converge to a fixed point
that represents the optimal solution. Assuming a probabilistic model, where the
inputs are i.i.d. Gaussian variables, we compute the distribution of the
convergence rate to the attracting fixed point. Using the framework of Random
Matrix Theory, we derive a simple expression for this distribution in the
asymptotic limit of large problem size. In this limit, we find that the
distribution of the convergence rate is a scaling function, namely it is a
function of one variable that is a combination of three parameters: the number
of variables, the number of constraints and the convergence rate, rather than a
function of these parameters separately. We also estimate numerically the
distribution of computation times, namely the time required to reach a vicinity
of the attracting fixed point, and find that it is also a scaling function.
Using the problem size dependence of the distribution functions, we derive high
probability bounds on the convergence rates and on the computation times.Comment: 1+37 pages, latex, 5 eps figures. Version accepted for publication in
the Journal of Complexity. Changes made: Presentation reorganized for
clarity, expanded discussion of measure of complexity in the non-asymptotic
regime (added a new section
Chaotic quantum ratchets and filters with cold atoms in optical lattices: properties of Floquet states
Recently, cesium atoms in optical lattices subjected to cycles of
unequally-spaced pulses have been found to show interesting behavior: they
represent the first experimental demonstration of a Hamiltonian ratchet
mechanism, and they show strong variability of the Dynamical Localization
lengths as a function of initial momentum. The behavior differs qualitatively
from corresponding atomic systems pulsed with equal periods, which are a
textbook implementation of a well-studied quantum chaos paradigm, the quantum
delta-kicked particle (delta-QKP). We investigate here the properties of the
corresponding eigenstates (Floquet states) in the parameter regime of the new
experiments and compare them with those of the eigenstates of the delta-QKP at
similar kicking strengths. We show that, with the properties of the Floquet
states, we can shed light on the form of the observed ratchet current as well
as variations in the Dynamical Localization length.Comment: 9 pages, 9 figure
-Kicked Quantum Rotors: Localization and `Critical' Statistics
The quantum dynamics of atoms subjected to pairs of closely-spaced
-kicks from optical potentials are shown to be quite different from the
well-known paradigm of quantum chaos, the singly--kicked system. We
find the unitary matrix has a new oscillating band structure corresponding to a
cellular structure of phase-space and observe a spectral signature of a
localization-delocalization transition from one cell to several. We find that
the eigenstates have localization lengths which scale with a fractional power
and obtain a regime of near-linear spectral variances
which approximate the `critical statistics' relation , where is related to the fractal
classical phase-space structure. The origin of the exponent
is analyzed.Comment: 4 pages, 3 fig
Detecting Quantum Critical Points using Bipartite Fluctuations
We show that the concept of bipartite fluctuations F provides a very
efficient tool to detect quantum phase transitions in strongly correlated
systems. Using state of the art numerical techniques complemented with
analytical arguments, we investigate paradigmatic examples for both quantum
spins and bosons. As compared to the von Neumann entanglement entropy, we
observe that F allows to find quantum critical points with a much better
accuracy in one dimension. We further demonstrate that F can be successfully
applied to the detection of quantum criticality in higher dimensions with no
prior knowledge of the universality class of the transition. Promising
approaches to experimentally access fluctuations are discussed for quantum
antiferromagnets and cold gases.Comment: 5 pages, 6 figures + suppl. material; final version, Phys. Rev. Lett.
(in press
General Relation between Entanglement and Fluctuations in One Dimension
In one dimension very general results from conformal field theory and exact
calculations for certain quantum spin systems have established universal
scaling properties of the entanglement entropy between two parts of a critical
system. Using both analytical and numerical methods, we show that if particle
number or spin is conserved, fluctuations in a subsystem obey identical scaling
as a function of subsystem size, suggesting that fluctuations are a useful
quantity for determining the scaling of entanglement, especially in higher
dimensions. We investigate the effects of boundaries and subleading corrections
for critical spin and bosonic chains.Comment: 4 pages, 2 figures. Minor changes, references added
Andreev scattering in the asymmetric ladder with preformed bosonic pairs
We discuss the phase coherence which emanates from the ladder-like proximity
effect between a ``weak superconductor'' with preformed bosonic pairs (here, a
single-chain Luther-Emery liquid with superconducting correlations that decay
approximately as ) and a Fermi gas with unpaired fermions. Carefully
studying tunneling mechanism(s), we show that the boson-mediated Cooper pairing
between remaining unpaired electrons results in a quasi long-range
superconductivity: Superconducting correlations decay very slowly as
with . This process is reminiscent of the coupling
of fermions to preformed bosonic pairs introduced in the context of high-Tc
cuprates.Comment: 5 pages, final version (To appear in PRB Rapid Communication
Seeding sustainability through social innovation in fashion design, Proceedings of the Crafting the Future
Double-gap superconducting proximity effect in nanotubes
We theoretically explore the possibility of a superconducting proximity
effect in single-walled metallic carbon nanotubes due to the presence of a
superconducting substrate. An unconventional double-gap situation can arise in
the two bands for nanotubes of large radius wherein the tunneling is (almost)
symmetric in the two sublattices. In such a case, a proximity effect can take
place in the symmetric band below a critical experimentally-accessible Coulomb
interaction strength in the nanotube. Furthermore, due to interactions in the
nanotube, the appearance of a BCS gap in this band stabilizes superconductivity
in the other band at lower temperatures. We also discuss the scenario of highly
asymmetric tunneling and show that this case too supports double-gap
superconductivity.Comment: 4 pages, 2 figure
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