451 research outputs found
Effect of couplings weakening and reversing in ferromagnetic Ising systems - Rigorous inequalities
We consider Ising systems where all the many-spin couplings are
positive. We show that the absolute value of all the many-spin correlations
does not increase when the value of any of the couplings is reduced, taking any
value in the interval . Results of this type are motivated by work
in systems such as random field Ising models.Comment: ps, 5 pages, no figure
The theory of heating of the quantum ground state of trapped ions
Using a displacement operator formalism, I analyse the depopulation of the
vibrational ground state of trapped ions. Two heating times, one characterizing
short time behaviour, the other long time behaviour are found. The short time
behaviour is analyzed both for single and multiple ions, and a formula for the
relative heating rates of different modes is derived. The possibility of
correction of heating via the quantum Zeno effect, and the exploitation of the
suppression of heating of higher modes to reduce errors in quantum computation
is considered.Comment: 9 pages, 2 figure
Cooling the Collective Motion of Trapped Ions to Initialize a Quantum Register
We report preparation in the ground state of collective modes of motion of
two trapped 9Be+ ions. This is a crucial step towards realizing quantum logic
gates which can entangle the ions' internal electronic states. We find that
heating of the modes of relative ion motion is substantially suppressed
relative to that of the center-of-mass modes, suggesting the importance of
these modes in future experiments.Comment: 5 pages, including 3 figures. RevTeX. PDF and PostScript available at
http://www.bldrdoc.gov/timefreq/ion/qucomp/papers.htm . final (published)
version. Eq. 1 and Table 1 slightly different from original submissio
A device for feasible fidelity, purity, Hilbert-Schmidt distance and entanglement witness measurements
A generic model of measurement device which is able to directly measure
commonly used quantum-state characteristics such as fidelity, overlap, purity
and Hilbert-Schmidt distance for two general uncorrelated mixed states is
proposed. In addition, for two correlated mixed states, the measurement
realizes an entanglement witness for Werner's separability criterion. To
determine these observables, the estimation only one parameter - the visibility
of interference, is needed. The implementations in cavity QED, trapped ion and
electromagnetically induced transparency experiments are discussed.Comment: 6 pages, 3 figure
Speed of ion trap quantum information processors
We investigate theoretically the speed limit of quantum gate operations for
ion trap quantum information processors. The proposed methods use laser pulses
for quantum gates which entangle the electronic and vibrational degrees of
freedom of the trapped ions. Two of these methods are studied in detail and for
both of them the speed is limited by a combination of the recoil frequency of
the relevant electronic transition, and the vibrational frequency in the trap.
We have experimentally studied the gate operations below and above this speed
limit. In the latter case, the fidelity is reduced, in agreement with our
theoretical findings. //
Changes: a) error in equ. 24 and table III repaired b) reference Jonathan et
al, quant-ph/ 0002092, added (proposes fast quantum gates using the AC-Stark
effect)Comment: 10 pages, 4 figure
Quantum computation with two-level trapped cold ions beyond Lamb-Dicke limit
We propose a simple scheme for implementing quantum logic gates with a string
of two-level trapped cold ions outside the Lamb-Dicke limit. Two internal
states of each ion are used as one computational qubit (CQ) and the collective
vibration of ions acts as the information bus, i.e., bus qubit (BQ). Using the
quantum dynamics for the laser-ion interaction as described by a generalized
Jaynes-Cummings model, we show that quantum entanglement between any one CQ and
the BQ can be coherently manipulated by applying classical laser beams. As a
result, universal quantum gates, i.e. the one-qubit rotation and two-qubit
controlled gates, can be implemented exactly. The required experimental
parameters for the implementation, including the Lamb-Dicke (LD) parameter and
the durations of the applied laser pulses, are derived. Neither the LD
approximation for the laser-ion interaction nor the auxiliary atomic level is
needed in the present scheme.Comment: 12 pages, no figures, to appear in Phys. Rev.
Decoherence Bounds on Quantum Computation with Trapped Ions
Using simple physical arguments we investigate the capabilities of a quantum
computer based on cold trapped ions. From the limitations imposed on such a
device by spontaneous decay, laser phase coherence, ion heating and other
sources of error, we derive a bound between the number of laser interactions
and the number of ions that may be used. The largest number which may be
factored using a variety of species of ion is determined.Comment: 5 pages in RevTex, 2 figures, the paper is also avalaible at
http://qso.lanl.gov/qc
Dynamical Stability and Quantum Chaos of Ions in a Linear Trap
The realization of a paradigm chaotic system, namely the harmonically driven
oscillator, in the quantum domain using cold trapped ions driven by lasers is
theoretically investigated. The simplest characteristics of regular and chaotic
dynamics are calculated. The possibilities of experimental realization are
discussed.Comment: 24 pages, 17 figures, submitted to Phys. Rev
Engineering arbitrary motional ionic state through realistic intensity-fluctuating laser pulses
We present a reliable scheme for engineering arbitrary motional ionic states
through an adaptation of the projection synthesis technique for trapped-ion
phenomena. Starting from a prepared coherent motional state, the Wigner
function of the desired state is thus sculpted from a Gaussian distribution.
The engineering process has also been developed to take into account the errors
arising from intensity fluctuations in the exciting-laser pulses required for
manipulating the electronic and vibrational states of the trapped ion. To this
end, a recently developed phenomenological-operator approach that allows for
the influence of noise will be applied. This approach furnishes a
straightforward technique to estimate the fidelity of the prepared state in the
presence of errors, precluding the usual extensive ab initio calculations. The
results obtained here by the phenomenological approach, to account for the
effects of noise in our engineering scheme, can be directly applied to any
other process involving trapped-ion phenomena.Comment: more information at http://www.df.ufscar.br/~quantum
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