1,899 research outputs found
Upper Bound on the region of Separable States near the Maximally Mixed State
A lower bound on the amount of noise that must be added to a GHZ-like
entangled state to make it separable (also called the random robustness) is
found using the transposition condition. The bound is applicable to arbitrary
numbers of subsystems, and dimensions of Hilbert space, and is shown to be
exact for qubits. The new bound is compared to previous such bounds on this
quantity, and found to be stronger in all cases. It implies that increasing the
number of subsystems, rather than increasing their Hilbert space dimension is a
more effective way of increasing entanglement. An explicit decomposition into
an ensemble of separable states, when the state is not entangled,is given for
the case of qubits.Comment: 2 figures. accepted J. Opt. B: Quantum Semiclass. Opt. (2000
Adiabatic quantum computation along quasienergies
The parametric deformations of quasienergies and eigenvectors of unitary
operators are applied to the design of quantum adiabatic algorithms. The
conventional, standard adiabatic quantum computation proceeds along
eigenenergies of parameter-dependent Hamiltonians. By contrast, discrete
adiabatic computation utilizes adiabatic passage along the quasienergies of
parameter-dependent unitary operators. For example, such computation can be
realized by a concatenation of parameterized quantum circuits, with an
adiabatic though inevitably discrete change of the parameter. A design
principle of adiabatic passage along quasienergy is recently proposed: Cheon's
quasienergy and eigenspace anholonomies on unitary operators is available to
realize anholonomic adiabatic algorithms [Tanaka and Miyamoto, Phys. Rev. Lett.
98, 160407 (2007)], which compose a nontrivial family of discrete adiabatic
algorithms. It is straightforward to port a standard adiabatic algorithm to an
anholonomic adiabatic one, except an introduction of a parameter |v>, which is
available to adjust the gaps of the quasienergies to control the running time
steps. In Grover's database search problem, the costs to prepare |v> for the
qualitatively different, i.e., power or exponential, running time steps are
shown to be qualitatively different. Curiously, in establishing the equivalence
between the standard quantum computation based on the circuit model and the
anholonomic adiabatic quantum computation model, it is shown that the cost for
|v> to enlarge the gaps of the eigenvalue is qualitatively negligible.Comment: 11 pages, 2 figure
SU(N) Coherent States
We generalize Schwinger boson representation of SU(2) algebra to SU(N) and
define coherent states of SU(N) using bosonic harmonic
oscillator creation and annihilation operators. We give an explicit
construction of all (N-1) Casimirs of SU(N) in terms of these creation and
annihilation operators. The SU(N) coherent states belonging to any irreducible
representations of SU(N) are labelled by the eigenvalues of the Casimir
operators and are characterized by (N-1) complex orthonormal vectors describing
the SU(N) manifold. The coherent states provide a resolution of identity,
satisfy the continuity property, and possess a variety of group theoretic
properties.Comment: 25 pages, LaTex, no figure
Entanglement detection from interference fringes in atom-photon systems
A measurement scheme of atomic qubits pinned at given positions is studied by
analyzing the interference pattern obtained when they emit photons
spontaneously. In the case of two qubits, a well-known relation is revisited,
in which the interference visibility is equal to the concurrence of the state
in the infinite spatial separation limit of the qubits. By taking into account
the super-radiant and sub-radiant effects, it is shown that a state tomography
is possible when the qubit spatial separation is comparable to the wavelength
of the atomic transition. In the case of three qubits, the relations between
various entanglement measures and the interference visibility are studied,
where the visibility is defined from the two-qubit case. A qualitative
correspondence among these entanglement relations is discussed. In particular,
it is shown that the interference visibility is directly related to the maximal
bipartite negativity.Comment: 12 pages, 2 figures, published versio
Overcoming decoherence in the collapse and revival of spin Schr\"odinger cats
In addition to being a very interesting quantum phenomenon, Schr\"odinger cat
swapping has the potential for application in the preparation of quantum states
that could be used in metrology and other quantum processing. We study in
detail the effects of field decoherence on a cat-swapping system comprising a
set of identical qubits, or spins, all coupled to a field mode. We demonstrate
that increasing the number of spins actually mitigates the effects of field
decoherence on the collapse and revival of a spin Schr\"odinger cat, which
could be of significant utility in quantum metrology and other quantum
processing.Comment: 4 pages, 2 figure
Quantum Dynamics of Three Coupled Atomic Bose-Einstein Condensates
The simplest model of three coupled Bose-Einstein Condensates (BEC) is
investigated using a group theoretical method. The stationary solutions are
determined using the SU(3) group under the mean field approximation. This
semiclassical analysis using the system symmetries shows a transition in the
dynamics of the system from self trapping to delocalization at a critical value
for the coupling between the condensates. The global dynamics are investigated
by examination of the stable points and our analysis shows the structure of the
stable points depends on the ratio of the condensate coupling to the
particle-particle interaction, undergoes bifurcations as this ratio is varied.
This semiclassical model is compared to a full quantum treatment, which also
displays the dynamical transition. The quantum case has collapse and revival
sequences superposed on the semiclassical dynamics reflecting the underlying
discreteness of the spectrum. Non-zero circular current states are also
demonstrated as one of the higher dimensional effects displayed in this system.Comment: Accepted to PR
Qudit Quantum State Tomography
Recently quantum tomography has been proposed as a fundamental tool for
prototyping a few qubit quantum device. It allows the complete reconstruction
of the state produced from a given input into the device. From this
reconstructed density matrix, relevant quantum information quantities such as
the degree of entanglement and entropy can be calculated. Generally orthogonal
measurements have been discussed for this tomographic reconstruction. In this
paper, we extend the tomographic reconstruction technique to two new regimes.
First we show how non-orthogonal measurement allow the reconstruction of the
state of the system provided the measurements span the Hilbert space. We then
detail how quantum state tomography can be performed for multi qudits with a
specific example illustrating how to achieve this in one and two qutrit
systems.Comment: 6 pages, 4 figures, submitted to PR
Spectral Effects of Strong Chi-2 Non-Linearity for Quantum Processing
Optical non-linearity can be used for parametric amplification
and producing down-converted entangled photon pairs that have broad
applications. It is known that weak non-linear media exhibit dispersion and
produce a frequency response. It is therefore of interest to know how spectral
effects of a strong crystal affect the performance. Here we model
the spectral effects of the dispersion of a strong crystal and
illustrate how this affects its ability to perform Bell measurements and
influence the performance of a quantum gates that employ such a Bell
measurement. We show that a Dyson series expansion of the unitary operator is
necessary in general, leading to unwanted spectral entanglement. We identify a
limiting situation employing periodic poling, in which a Taylor series
expansion is a good approximation and this entanglement can be removed.Comment: Will be submitted to PR
Modified TAP equations for the SK spin glass
The stability of the TAP mean field equations is reanalyzed with the
conclusion that the exclusive reason for the breakdown at the spin glass
instability is an inconsistency for the value of the local susceptibility. A
new alternative approach leads to modified equations which are in complete
agreement with the original ones above the instability. Essentially altered
results below the instability are presented and the consequences for the
dynamical mean field equations are discussed.Comment: 7 pages, 2 figures, final revised version to appear in Europhys. Let
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