7,827 research outputs found
Quantum enhancement of randomness distribution
The capability of a given channel to transmit information is, a priori, distinct from its capability to distribute random correlations. Despite that, for classical channels, the capacity to distribute information and randomness turns out to be the same, even with the assistance of auxiliary communication. In this work we show that this is no longer true for quantum channels when feedback is allowed. We prove this by constructing a channel that is noisy for the transmission of information but behaves as a virtual noiseless channel for randomness distribution when assisted by feedback communication. Our result can be seen as a way of unlocking quantum randomness internal to the channel
Integer quantum Hall transition in the presence of a long-range-correlated quenched disorder
We theoretically study the effect of long-ranged inhomogeneities on the
critical properties of the integer quantum Hall transition. For this purpose we
employ the real-space renormalization-group (RG) approach to the network model
of the transition. We start by testing the accuracy of the RG approach in the
absence of inhomogeneities, and infer the correlation length exponent nu=2.39
from a broad conductance distribution. We then incorporate macroscopic
inhomogeneities into the RG procedure. Inhomogeneities are modeled by a smooth
random potential with a correlator which falls off with distance as a power
law, r^{-alpha}. Similar to the classical percolation, we observe an
enhancement of nu with decreasing alpha. Although the attainable system sizes
are large, they do not allow one to unambiguously identify a cusp in the
nu(alpha) dependence at alpha_c=2/nu, as might be expected from the extended
Harris criterion. We argue that the fundamental obstacle for the numerical
detection of a cusp in the quantum percolation is the implicit randomness in
the Aharonov-Bohm phases of the wave functions. This randomness emulates the
presence of a short-range disorder alongside the smooth potential.Comment: 10 pages including 6 figures, revised version as accepted for
publication in PR
Collapse of Charge Gap in Random Mott Insulators
Effects of randomness on interacting fermionic systems in one dimension are
investigated by quantum Monte-Carlo techniques. At first, interacting spinless
fermions are studied whose ground state shows charge ordering. Quantum phase
transition due to randomness is observed associated with the collapse of the
charge ordering. We also treat random Hubbard model focusing on the Mott gap.
Although the randomness closes the Mott gap and low-lying states are created,
which is observed in the charge compressibility, no (quasi-) Fermi surface
singularity is formed. It implies localized nature of the low-lying states.Comment: RevTeX with 3 postscript figure
Competition between Kondo and RKKY correlations in the presence of strong randomness
We propose that competition between Kondo and magnetic correlations results
in a novel universality class for heavy fermion quantum criticality in the
presence of strong randomness. Starting from an Anderson lattice model with
disorder, we derive an effective local field theory in the dynamical mean-field
theory (DMFT) approximation, where randomness is introduced into both
hybridization and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. Performing
the saddle-point analysis in the U(1) slave-boson representation, we reveal its
phase diagram which shows a quantum phase transition from a spin liquid state
to a local Fermi liquid phase. In contrast with the clean limit of the Anderson
lattice model, the effective hybridization given by holon condensation turns
out to vanish, resulting from the zero mean value of the hybridization coupling
constant. However, we show that the holon density becomes finite when variance
of hybridization is sufficiently larger than that of the RKKY coupling, giving
rise to the Kondo effect. On the other hand, when the variance of hybridization
becomes smaller than that of the RKKY coupling, the Kondo effect disappears,
resulting in a fully symmetric paramagnetic state, adiabatically connected with
the spin liquid state of the disordered Heisenberg model. .....
Enhancing quantum entropy in vacuum-based quantum random number generator
Information-theoretically provable unique true random numbers, which cannot
be correlated or controlled by an attacker, can be generated based on quantum
measurement of vacuum state and universal-hashing randomness extraction.
Quantum entropy in the measurements decides the quality and security of the
random number generator. At the same time, it directly determine the extraction
ratio of true randomness from the raw data, in other words, it affects quantum
random numbers generating rate obviously. In this work, considering the effects
of classical noise, the best way to enhance quantum entropy in the vacuum-based
quantum random number generator is explored in the optimum dynamical
analog-digital converter (ADC) range scenario. The influence of classical noise
excursion, which may be intrinsic to a system or deliberately induced by an
eavesdropper, on the quantum entropy is derived. We propose enhancing local
oscillator intensity rather than electrical gain for noise-independent
amplification of quadrature fluctuation of vacuum state. Abundant quantum
entropy is extractable from the raw data even when classical noise excursion is
large. Experimentally, an extraction ratio of true randomness of 85.3% is
achieved by finite enhancement of the local oscillator power when classical
noise excursions of the raw data is obvious.Comment: 12 pages,8 figure
Numerical Study of a Two-Dimensional Quantum Antiferromagnet with Random Ferromagnetic Bonds
A Monte Carlo method for finite-temperature studies of the two-dimensional
quantum Heisenberg antiferromagnet with random ferromagnetic bonds is
presented. The scheme is based on an approximation which allows for an analytic
summation over the realizations of the randomness, thereby significantly
alleviating the ``sign problem'' for this frustrated spin system. The
approximation is shown to be very accurate for ferromagnetic bond
concentrations of up to ten percent. The effects of a low concentration of
ferromagnetic bonds on the antiferromagnetism are discussed.Comment: 11 pages + 5 postscript figures (included), Revtex 3.0, UCSBTH-94-2
Highly-symmetric random one-dimensional spin models
The interplay of disorder and interactions is a challenging topic of
condensed matter physics, where correlations are crucial and exotic phases
develop. In one spatial dimension, a particularly successful method to analyze
such problems is the strong-disorder renormalization group (SDRG). This method,
which is asymptotically exact in the limit of large disorder, has been
successfully employed in the study of several phases of random magnetic chains.
Here we develop an SDRG scheme capable to provide in-depth information on a
large class of strongly disordered one-dimensional magnetic chains with a
global invariance under a generic continuous group. Our methodology can be
applied to any Lie-algebra valued spin Hamiltonian, in any representation. As
examples, we focus on the physically relevant cases of SO(N) and Sp(N)
magnetism, showing the existence of different randomness-dominated phases.
These phases display emergent SU(N) symmetry at low energies and fall in two
distinct classes, with meson-like or baryon-like characteristics. Our
methodology is here explained in detail and helps to shed light on a general
mechanism for symmetry emergence in disordered systems.Comment: 26 pages, 12 figure
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