396 research outputs found
Relativistic Einstein-Podolsky-Rosen correlation and Bell's inequality
We formulate the Einstein-Podolsky-Rosen (EPR) gedankenexperiment within the
framework of relativistic quantum theory to analyze a situation in which
measurements are performed by moving observers. We point out that under certain
conditions the perfect anti-correlation of an EPR pair of spins in the same
direction is deteriorated in the moving observers' frame due to the Wigner
rotation, and show that the degree of the violation of Bell's inequality prima
facie decreases with increasing the velocity of the observers if the directions
of the measurement are fixed. However, this does not imply a breakdown of
non-local correlation since the perfect anti-correlation is maintained in
appropriately chosen different directions. We must take account of this
relativistic effect in utilizing in moving frames the EPR correlation and the
violation of Bell's inequality for quantum communication.Comment: 33 pages, 6 figure
Bose-Einstein droplet in free space
We show that a droplet of a Bose-Einstein condensate can be dynamically
stabilized in free space by rapid oscillations of interatomic interactions
between attractive and repulsive through, e.g., the Feshbach resonance. Energy
dissipation, which is present in realistic situations, is found to play a
crucial role to suppress dynamical instabilities inherent in nonlinear
nonequilibrium systems.Comment: 5 pages, 5 figure
Ferromagnetic fluctuation and possible triplet superconductivity in Na_xCoO_2*yH_2O: Fluctuation-exchange study of multi-orbital Hubbard model
Spin and charge fluctuations and superconductivity in a recently discovered
superconductor Na_xCoO_2*yH_2O are studied based on a multi-orbital Hubbard
model. Tight-binding parameters are determined to reproduce the LDA band
dispersions with the Fermi surface, which consist of a large cylindrical one
around the Gamma-point and six hole pockets near the K-points. By applying the
fluctuation-exchange (FLEX) approximation, we show that the Hund's-rule
coupling between the Co t2g orbitals causes ferromagnetic (FM) spin
fluctuation. Triplet f_{y(y^2-3x^2)}-wave and p-wave pairings are favored by
this FM fluctuation on the hole-pocket band. We propose that, in
Na_xCoO_2*yH_2O, the Co t2g orbitals and inter-orbital Hund's-rule coupling
play important roles on the triplet pairing, and this compound can be a first
example of the triplet superconductor in which the orbital degrees of freedom
play substantial roles.Comment: 5 pages, 3 figure
Multi-copy and stochastic transformation of multipartite pure states
Characterizing the transformation and classification of multipartite
entangled states is a basic problem in quantum information. We study the
problem under two most common environments, local operations and classical
communications (LOCC), stochastic LOCC and two more general environments,
multi-copy LOCC (MCLOCC) and multi-copy SLOCC (MCSLOCC). We show that two
transformable multipartite states under LOCC or SLOCC are also transformable
under MCLOCC and MCSLOCC. What's more, these two environments are equivalent in
the sense that two transformable states under MCLOCC are also transformable
under MCSLOCC, and vice versa. Based on these environments we classify the
multipartite pure states into a few inequivalent sets and orbits, between which
we build the partial order to decide their transformation. In particular, we
investigate the structure of SLOCC-equivalent states in terms of tensor rank,
which is known as the generalized Schmidt rank. Given the tensor rank, we show
that GHZ states can be used to generate all states with a smaller or equivalent
tensor rank under SLOCC, and all reduced separable states with a cardinality
smaller or equivalent than the tensor rank under LOCC. Using these concepts, we
extended the concept of "maximally entangled state" in the multi-partite
system.Comment: 8 pages, 1 figure, revised version according to colleagues' comment
Criteria of off-diagonal long-range order in Bose and Fermi systems based on the Lee-Yang cluster expansion method
The quantum-statistical cluster expansion method of Lee and Yang is extended
to investigate off-diagonal long-range order (ODLRO) in one- and
multi-component mixtures of bosons or fermions. Our formulation is applicable
to both a uniform system and a trapped system without local-density
approximation and allows systematic expansions of one- and multi-particle
reduced density matrices in terms of cluster functions which are defined for
the same system with Boltzmann statistics. Each term in this expansion can be
associated with a Lee-Yang graph. We elucidate a physical meaning of each
Lee-Yang graph; in particular, for a mixture of ultracold atoms and bound
dimers, an infinite sum of the ladder-type Lee-Yang 0-graphs is shown to lead
to Bose-Einstein condensation of dimers below the critical temperature. In the
case of Bose statistics, an infinite series of Lee-Yang 1-graphs is shown to
converge and gives the criteria of ODLRO at the one-particle level.
Applications to a dilute Bose system of hard spheres are also made. In the case
of Fermi statistics, an infinite series of Lee-Yang 2-graphs is shown to
converge and gives the criteria of ODLRO at the two-particle level.
Applications to a two-component Fermi gas in the tightly bound limit are also
made.Comment: 21 pages, 10 figure
Symmetry Breaking and Enhanced Condensate Fraction in a Matter-Wave Bright Soliton
An exact diagonalization study reveals that a matter-wave bright soliton and
the Goldstone mode are simultaneously created in a quasi-one-dimensional
attractive Bose-Einstein condensate by superpositions of quasi-degenerate
low-lying many-body states. Upon formation of the soliton the maximum
eigenvalue of the single-particle density matrix increases dramatically,
indicating that a fragmented condensate converts into a single condensate as a
consequence of the breaking of translation symmetry.Comment: 4 pages, 4 figures, revised versio
Bloch Structures in a Rotating Bose-Einstein Condensate
A rotating Bose-Einstein condensate is shown to exhibit a Bloch band
structure without the need of periodic potential. Vortices enter the condensate
by a mechanism similar to the Bragg reflection, if the frequency of a rotating
drive or the strength of interaction is adiabatically changed. A localized
state analogous to a gap soliton in a periodic system is predicted near the
edge of the Brillouin zone.Comment: 4 pages, 3 figure
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