2,421 research outputs found
Preparation of Schr\"odinger cat states with cold ions beyond the Lamb-Dicke limit
A scheme for preparing Schr\"odinger cat (SC) states is proposed beyond the
Lamb-Dicke limit in a Raman--type configuration. It is shown that SC
states can be obtained more efficiently with our scheme than with the former
ones.Comment: RevTex 9 pages, no figures and table
Ensemble Quantum Computation with atoms in periodic potentials
We show how to perform universal quantum computation with atoms confined in
optical lattices which works both in the presence of defects and without
individual addressing. The method is based on using the defects in the lattice,
wherever they are, both to ``mark'' different copies on which ensemble quantum
computation is carried out and to define pointer atoms which perform the
quantum gates. We also show how to overcome the problem of scalability on this
system
Mapping local Hamiltonians of fermions to local Hamiltonians of spins
We show how to map local fermionic problems onto local spin problems on a
lattice in any dimension. The main idea is to introduce auxiliary degrees of
freedom, represented by Majorana fermions, which allow us to extend the
Jordan-Wigner transformation to dimensions higher than one. We also discuss the
implications of our results in the numerical investigation of fermionic
systems.Comment: Added explicit mappin
Bose-Einstein Condensation and strong-correlation behavior of phonons in ion traps
We show that the dynamics of phonons in a set of trapped ions interacting
with lasers is described by a Bose-Hubbard model whose parameters can be
externally adjusted. We investigate the possibility of observing several
quantum many-body phenomena, including (quasi) Bose-Einstein condensation as
well as a superfluid-Mott insulator quantum phase transition.Comment: 5 pages, 3 figure
Quantum emulsion: a glassy phase of bosonic mixtures in optical lattices
We numerically investigate mixtures of two interacting bosonic species with
unequal parameters in one-dimensional optical lattices. In large parameter
regions full phase segregation is seen to minimize the energy of the system,
but the true ground state is masked by an exponentially large number of
metastable states characterized by microscopic phase separation. The ensemble
of these quantum emulsion states, reminiscent of emulsions of immiscible
fluids, has macroscopic properties analogous to those of a Bose glass, namely a
finite compressibility in absence of superfluidity. Their metastability is
probed by extensive quantum Monte Carlo simulations generating a rich
correlated stochastic dynamics. The tuning of the repulsion of one of the two
species via a Feshbach resonance drives the system through a quantum phase
transition to the superfluid state.Comment: 4 pages, 3 figure
Matrix product states represent ground states faithfully
We quantify how well matrix product states approximate exact ground states of
1-D quantum spin systems as a function of the number of spins and the entropy
of blocks of spins. We also investigate the convex set of local reduced density
operators of translational invariant systems. The results give a theoretical
justification for the high accuracy of renormalization group algorithms, and
justifies their use even in the case of critical systems
Equivalence classes of non-local unitary operations
We study when a multipartite non--local unitary operation can
deterministically or probabilistically simulate another one when local
operations of a certain kind -in some cases including also classical
communication- are allowed. In the case of probabilistic simulation and
allowing for arbitrary local operations, we provide necessary and sufficient
conditions for the simulation to be possible. Deterministic and probabilistic
interconversion under certain kinds of local operations are used to define
equivalence relations between gates. In the probabilistic, bipartite case this
induces a finite number of classes. In multiqubit systems, however, two unitary
operations typically cannot simulate each other with non-zero probability of
success. We also show which kind of entanglement can be created by a given
non--local unitary operation and generalize our results to arbitrary operators.Comment: (1) 9 pages, no figures, submitted to QIC; (2) reference added, minor
change
Time-dependent study of disordered models with infinite projected entangled pair states
Infinite projected entangled pair states (iPEPS), the tensor network ansatz
for two-dimensional systems in the thermodynamic limit, already provide
excellent results on ground-state quantities using either imaginary-time
evolution or variational optimisation. Here, we show (i) the feasibility of
real-time evolution in iPEPS to simulate the dynamics of an infinite system
after a global quench and (ii) the application of disorder-averaging to obtain
translationally invariant systems in the presence of disorder. To illustrate
the approach, we study the short-time dynamics of the square lattice Heisenberg
model in the presence of a bi-valued disorder field
Interaction of a two-level atom with squeezed light
We consider a degenerate parametric oscillator whose cavity contains a
two-level atom. Applying the Heisenberg and quantum Langevin equations, we
calculate in the bad-cavity limit the mean photon number, the quadrature
variance, and the power spectrum for the cavity mode in general and for the
signal light and fluorescent light in particular. We also obtain the normalized
second-order correlation function for the fluorescent light. We find that the
presence of the two-level atom leads to a decrease in the degree of squeezing
of the signal light. It so turns out that the fluorescent light is in a
squeezed state and the power spectrum consists of a single peak only.Comment: 9 pages and 9 figures, in press, Opt. Commu
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