161 research outputs found
Devil's crevasse and macroscopic entanglement in two-component Bose-Einstein condensates
Spin coherent states are the matter equivalent of optical coherent states,
where a large number of two component particles form a macroscopic state
displaying quantum coherence. Here we give a detailed study of entanglement
generated between two spin-1/2 BECs due to an Sz1 Sz2 interaction. The states
that are generated show a remarkably rich structure showing fractal
characteristics. In the limit of large particle number N, the entanglement
shows a strong dependence upon whether the entangling gate times are a rational
or irrational multiple of pi/4. We discuss the robustness of various states
under decoherence and show that despite the large number of particles in a
typical BEC, entanglement on a macroscopic scale should be observable as long
as the gate times are less than hbar/J sqrt[N], where J is the effective
BEC-BEC coupling energy. Such states are anticipated to be useful for various
quantum information applications such as quantum teleportation and quantum
algorithms
Time dynamics of Bethe ansatz solvable models
We develop a method for finding the time evolution of exactly solvable models
by Bethe ansatz. The dynamical Bethe wavefunction takes the same form as the
stationary Bethe wavefunction except for time varying Bethe parameters and a
complex phase prefactor. From this, we derive a set of first order nonlinear
coupled differential equations for the Bethe parameters, called the dynamical
Bethe equations. We find that this gives the exact solution to particular types
of exactly solvable models, including the Bose-Hubbard dimer and Tavis-Cummings
model. These models go beyond the Gaudin class, and offers an interesting
possibility for performing time evolution in exactly solvable models.Comment: 9 pages, 1 figur
Skyrmion quantum spin Hall effect
The quantum spin Hall effect is conventionally thought to require a strong
spin-orbit coupling, producing an effective spin-dependent magnetic field.
However, spin currents can also be present without transport of spins, for
example, in spin-waves or skyrmions. In this paper, we show that topological
skyrmionic spin textures can be used to realize a quantum spin Hall effect.
From basic arguments relating to the single-valuedness of the wave function, we
deduce that loop integrals of the derivative of the Hamiltonian must have a
spectrum that is integer multiples of . By relating this to the spin
current, we form a new quantity called the quantized spin current which obeys a
precise quantization rule. This allows us to derive a quantum spin Hall effect,
which we illustrate with an example of a spin-1 Bose-Einstein condensate.Comment: 7 pages, 2 figures (published in PRB
Light mediated non-Gaussian atomic ensemble entanglement
We analyze a similar scheme for producing light-mediated entanglement between
atomic ensembles, as first realized by Julsgaard, Kozhekin and Polzik [Nature
{\bf 413}, 400 (2001)]. In the standard approach to modeling the scheme, a
Holstein-Primakoff approximation is made, where the atomic ensembles are
treated as bosonic modes, and is only valid for short interaction times. In
this paper, we solve the time evolution without this approximation, which
extends the region of validity of the interaction time. For short entangling
times, we find this produces a state with similar characteristics as a two-mode
squeezed state, in agreement with standard predictions. For long entangling
times, the state evolves into a non-Gaussian form, and the two-mode squeezed
state characteristics start to diminish. This is attributed to more exotic
types of entangled states being generated. We characterize the states by
examining the Fock state probability distributions, Husimi distributions,
and non-local entanglement between the ensembles. We compare and connect
several quantities obtained using the Holstein-Primakoff approach and our exact
time evolution methods
Entanglement generation in quantum networks of Bose-Einstein condensates
Two component (spinor) Bose-Einstein condensates (BECs) are considered as the
nodes of an interconnected quantum network. Unlike standard single-system
qubits, in a BEC the quantum information is duplicated in a large number of
identical bosonic particles, thus can be considered to be a "macroscopic"
qubit. One of the difficulties with such a system is how to effectively
interact such qubits together in order to transfer quantum information and
create entanglement. Here we propose a scheme of cavities containing spinor
BECs coupled by optical fiber in order to achieve this task. We discuss
entanglement generation and quantum state transfer between nodes using such
macroscopic BEC qubits.Comment: 17 pages, 4 figure
Suppression of ac Stark shift scattering rate due to non-Markovian behavior
The ac Stark shift in the presence of spontaneous decay is typically
considered to induce an effective dephasing with a scattering rate equal to , where is the spontaneous decay
rate, is the laser transition coupling, and is the
detuning. We show that under realistic circumstances this dephasing rate may be
strongly modifed due to non-Markovian behavior. The non-Markovian behavior
arises due to an effective modification of the light-atom coupling in the
presence of the ac Stark shift laser. An analytical formula for the
non-Markovian ac Stark shift induced dephasing is derived. We obtain that for
narrow laser linewidths the effective dephasing rate is suppressed by a factor
of , where is the quality factor of the laser.Comment: Accepted in PRA Rapid Communication
Landau-Zener transition stabilized by the enhanced quantum Zeno effect in the bosonic system
We study the Landau-Zener transition with the quantum Zeno effect in an open
dissipative system populated by a large number of bosons. Given the quantum
Zeno effect is strong enough, both discrete and continuous quantum Zeno
measurements are found to stabilize the Landau-Zener transition. Both the
-type longitudinal relaxation and -type transverse
relaxation in the bosonic system are analyzed as a model of continuous quantum
Zeno measurements. While both of them improve the signal-to-noise ratio in
terms of the ground state population, the -type relaxation can
further boost measurement sensitivity and thus lead to a polynomial speedup
with the number of bosons in the system. For a system that contains a large
number of bosons such as in a Bose-Einstein condensate with more than
bosons, this equates to several orders of magnitude speedup.Comment: 9 pages, 3 figure
Optimization using Bose-Einstein condensation and measurement-feedback circuits
We investigate a computational device that harnesses the effects of
Bose-Einstein condensation (BEC) to accelerate the speed of finding the
solution of a given optimization problem. Many computationally difficult
problems, including NP-complete problems, can be formulated as a ground state
search problem. In a BEC, below the critical temperature, bosonic particles
have a natural tendency to accumulate in the ground state. Furthermore, the
speed of attaining this configuration is enhanced as a result of final state
stimulation. We propose a physical device that incorporates these basic
properties of bosons into the optimization problem, such that an optimized
solution is found by a simple cooling of the physical temperature of the
device. We find that the speed of convergence to the ground state can be sped
up by a factor of at a given error, where N is the boson number per site.Comment: 10 pages, 3 figure
Covariance matrix entanglement criterion for an arbitrary set of operators
We generalize entanglement detection with covariance matrices for an
arbitrary set of observables. A generalized uncertainty relation is constructed
using the covariance and commutation matrices, then a criterion is established
by performing a partial transposition on the operators. The method is highly
efficient and versatile in the sense that the set of measurement operators can
be freely chosen, do not need to be complete, and there is no constraint on the
commutation relations. The method is particularly suited for systems with
higher dimensionality since the computations do not scale with the dimension of
the Hilbert space rather they scale with the number of chosen observables which
can always be kept small. We illustrate the approach by examining the
entanglement between two spin ensembles, and show that it detects entanglement
in a basis independent way
Quantum coherence of planar spin models with Dzyaloshinsky-Moriya interaction
The quantum coherence of one dimensional planar spin models with the
Dzyaloshinsky-Moriya interaction is investigated. The anisotropic XY model, the
isotropic XX model and the transverse field model are studied in the large
N-limit using the two qubit reduced density matrices and the two point
correlation functions. From our investigations we find that the coherence as
measured using the Jensen-Shannon divergence can be used to detect the quantum
phase transitions and the quantum critical points. The derivative of coherence
shows non-analytic behavior at the critical points leading to the conclusions
that these transitions are of second order. Further we show that the presence
of the Dzyaloshinsky-Moriya coupling suppresses the phase transition due to the
residual ferromagnetism which is caused by spin canting.Comment: accepted for publication in Physical Review
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