14,822 research outputs found
Variational quantum Monte Carlo simulations with tensor-network states
We show that the formalism of tensor-network states, such as the matrix
product states (MPS), can be used as a basis for variational quantum Monte
Carlo simulations. Using a stochastic optimization method, we demonstrate the
potential of this approach by explicit MPS calculations for the transverse
Ising chain with up to N=256 spins at criticality, using periodic boundary
conditions and D*D matrices with D up to 48. The computational cost of our
scheme formally scales as ND^3, whereas standard MPS approaches and the related
density matrix renromalization group method scale as ND^5 and ND^6,
respectively, for periodic systems.Comment: 4+ pages, 2 figures. v2: improved data, comparisons with exact
results, to appear in Phys Rev Let
Three qubits can be entangled in two inequivalent ways
Invertible local transformations of a multipartite system are used to define
equivalence classes in the set of entangled states. This classification
concerns the entanglement properties of a single copy of the state.
Accordingly, we say that two states have the same kind of entanglement if both
of them can be obtained from the other by means of local operations and
classical communcication (LOCC) with nonzero probability. When applied to pure
states of a three-qubit system, this approach reveals the existence of two
inequivalent kinds of genuine tripartite entanglement, for which the GHZ state
and a W state appear as remarkable representatives. In particular, we show that
the W state retains maximally bipartite entanglement when any one of the three
qubits is traced out. We generalize our results both to the case of higher
dimensional subsystems and also to more than three subsystems, for all of which
we show that, typically, two randomly chosen pure states cannot be converted
into each other by means of LOCC, not even with a small probability of success.Comment: 12 pages, 1 figure; replaced with revised version; terminology
adapted to earlier work; reference added; results unchange
Effects of critical temperature inhomogeneities on the voltage-current characteristics of a planar superconductor near the Berezinskii-Kosterlitz-Thouless transition
We analyze numerically how the voltage-current (V-I) characteristics near the
so-called Berezinskii-Kosterlitz-Thouless (BKT) transition of 2D
superconductors are affected by a random spatial Gaussian distribution of
critical temperature inhomogeneities with long characteristic lengths (much
larger than the in-plane superconducting coherence length amplitude). Our
simulations allow to quantify the broadening around the average BKT transition
temperature of both the exponent alpha in V I^alpha and of the resistance V/I.
These calculations reveal that strong spatial redistributions of the local
current will occur around the transition as either I or the temperature T are
varied. Our results also support that the condition alpha=3 provides a good
estimate for the location of the average BKT transition temperature, and that
extrapolating to alpha->1 the alpha(T) behaviour well below the transition
provides a good estimate for the average mean-field critical temperature.Comment: 18 pages; pdfLaTeX; 1 TeX file + 8 PDF files for figures
(figs.1,2,3a,3b,4,5a,5b,6
Electromagnetic dipole moments of charged baryons with bent crystals at the LHC
We propose a unique program of measurements of electric and magnetic dipole
moments of charm, beauty and strange charged baryons at the LHC, based on the
phenomenon of spin precession of channeled particles in bent crystals. Studies
of crystal channeling and spin precession of positively- and negatively-charged
particles are presented, along with feasibility studies and expected
sensitivities for the proposed experiment using a layout based on the LHCb
detector.Comment: 19 pages, 13 figure
Boundary quantum critical phenomena with entanglement renormalization
We extend the formalism of entanglement renormalization to the study of
boundary critical phenomena. The multi-scale entanglement renormalization
ansatz (MERA), in its scale invariant version, offers a very compact
approximation to quantum critical ground states. Here we show that, by adding a
boundary to the scale invariant MERA, an accurate approximation to the critical
ground state of an infinite chain with a boundary is obtained, from which one
can extract boundary scaling operators and their scaling dimensions. Our
construction, valid for arbitrary critical systems, produces an effective chain
with explicit separation of energy scales that relates to Wilson's RG
formulation of the Kondo problem. We test the approach by studying the quantum
critical Ising model with free and fixed boundary conditions.Comment: 8 pages, 12 figures, for a related work see arXiv:0912.289
Magnetoelectric metglas/bidomain y + 140°-cut lithium niobate composite for sensing fT magnetic fields
We investigated the magnetoelectric properties of a new laminate composite material based on y+140°-cut congruent lithium niobate piezoelectric plates with an antiparallel polarized “head-to-head” bidomain structure and metglas used as a magnetostrictive layer. A series of bidomain lithium niobate crystals were prepared by annealing under conditions of Li2O outdiffusion from LiNbO3 with a resultant growth of an inversion domain. The measured quasi-static magnetoelectric coupling coefficient achieved |αE31| = 1.9 V·(cm·Oe)–1. At a bending resonance frequency of 6862 Hz, we found a giant |αE31| value up to 1704 V·(cm·Oe)–1. Furthermore, the equivalent magnetic noise spectral density of the investigated composite material was only 92 fT/Hz1/2, a record value for such a low operation frequency. The magnetic-field detection limit of the laminated composite was found to be as low as 200 fT in direct measurements without any additional shielding from external noises.publishe
Quantum dynamics in high codimension tilings: from quasiperiodicity to disorder
We analyze the spreading of wavepackets in two-dimensional quasiperiodic and
random tilings as a function of their codimension, i.e. of their topological
complexity. In the quasiperiodic case, we show that the diffusion exponent that
characterizes the propagation decreases when the codimension increases and goes
to 1/2 in the high codimension limit. By constrast, the exponent for the random
tilings is independent of their codimension and also equals 1/2. This shows
that, in high codimension, the quasiperiodicity is irrelevant and that the
topological disorder leads in every case, to a diffusive regime, at least in
the time scale investigated here.Comment: 4 pages, 5 EPS figure
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