153 research outputs found
Effect of an External Field on Decoherence
"Decoherence of quantum superpositions through coupling to engineered
reservoirs" is the topic of a recent article by Myatt et al. [Nature
{\underline{403}}, 269 (2000)] which has attracted much interest because of its
relevance to current research in fundamental quantum theory, quantum
computation, teleportation, entanglement and the quantum-classical interface.
However, the preponderance of theoretical work on decoherence does not consider
the effect of an {\underline{external field}}. Here, we present an analysis of
such an effect in the case of the random delta-correlated force discussed by
Myatt et al
Polariton Analysis of a Four-Level Atom Strongly Coupled to a Cavity Mode
We present a complete analytical solution for a single four-level atom
strongly coupled to a cavity field mode and driven by external coherent laser
fields. The four-level atomic system consists of a three-level subsystem in an
EIT configuration, plus an additional atomic level; this system has been
predicted to exhibit a photon blockade effect. The solution is presented in
terms of polaritons. An effective Hamiltonian obtained by this procedure is
analyzed from the viewpoint of an effective two-level system, and the dynamic
Stark splitting of dressed states is discussed. The fluorescence spectrum of
light exiting the cavity mode is analyzed and relevant transitions identified.Comment: 12 pages, 9 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.
An Analysis of Mutual Communication between Qubits by Capacitive Coupling
A behavior of a two qubit system coupled by the electric capacitance has been
studied quantum mechanically. We found that the interaction is essentially the
same as the one for the dipole-dipole interaction; i.e., qubit-qubit coupling
of the NMR quantum gate. Therefore a quantum gate could be constructed by the
same operation sequence for the NMR device if the coupling is small enough. The
result gives an information to the effort of development of the devices
assuming capacitive coupling between qubits.Comment: 8 pages, 2 figures Revised and Replaced on Apr. 8 200
Low-temperature electrical transport in bilayer manganite LaSrMnO
The temperature and magnetic field dependence of anisotropic in-plane
and out-of-plane resistivities have been investigated in
single crystals of the bilayer manganite LaSrMnO.
Below the Curie transition temperature 125 K, and
display almost the same temperature dependence with an up-turn around 50 K. In
the metallic regime (50 K 110 K), both and
follow a dependence, consistent with the two-magnon
scattering. We found that the value of the proportionality coefficient
and the ratio of the exchange interaction obtained
by fitting the data are in excellent agreement with the calculated
based on the two-magnon model and deduced from neutron scattering,
respectively. This provides further support for this scattering mechanism. At
even lower , in the non-metallic regime ( 50 K), {\it both} the in-plane
and out-of-plane conductivities obey a
dependence, consistent with weak localization effects. Hence, this demonstrates
the three-dimensional metallic nature of the bilayer manganite
LaSrMnO at .Comment: 7 pages and 5 figures, accepted for publication in Phys. Rev.
Scheme for the preparation of the multi-particle entanglement in cavity QED
Here we present a quantum electrodynamics (QED) model involving a
large-detuned single-mode cavity field and identical two-level atoms. One
of its applications for the preparation of the multi-particle states is
analyzed. In addition to the Greenberger-Horne-Zeilinger (GHZ) state, the W
class states can also be generated in this scheme. The further analysis for the
experiment of the model of case is also presented by considering the
possible three-atom collision.Comment: 5 Pages, 1 Figure. Minor change
Reducing the communication complexity with quantum entanglement
We propose a probabilistic two-party communication complexity scenario with a
prior nonmaximally entangled state, which results in less communication than
that is required with only classical random correlations. A simple all-optical
implementation of this protocol is presented and demonstrates our conclusion.Comment: 4 Pages, 2 Figure
Grover search with pairs of trapped ions
The desired interference required for quantum computing may be modified by
the wave function oscillations for the implementation of quantum
algorithms[Phys.Rev.Lett.84(2000)1615]. To diminish such detrimental effect, we
propose a scheme with trapped ion-pairs being qubits and apply the scheme to
the Grover search. It can be found that our scheme can not only carry out a
full Grover search, but also meet the requirement for the scalable hot-ion
quantum computing. Moreover, the ion-pair qubits in our scheme are more robust
against the decoherence and the dissipation caused by the environment than
single-particle qubits proposed before.Comment: RevTe
A Self Assembled Nanoelectronic Quantum Computer Based on the Rashba Effect in Quantum Dots
Quantum computers promise vastly enhanced computational power and an uncanny
ability to solve classically intractable problems. However, few proposals exist
for robust, solid state implementation of such computers where the quantum
gates are sufficiently miniaturized to have nanometer-scale dimensions. Here I
present a new approach whereby a complete computer with nanoscale gates might
be self-assembled using chemical synthesis. Specifically, I demonstrate how to
self-assemble the fundamental unit of this quantum computer - a 2-qubit
universal quantum controlled-NOT gate - based on two exchange coupled
multilayered quantum dots. Then I show how these gates can be wired using
thiolated conjugated molecules as electrical connectors. A qubit is encoded in
the ground state of a quantum dot spin-split by the Rashba interaction.
Arbitrary qubit rotations are effected by bringing the spin splitting energy in
a target quantum dot in resonance with a global ac magnetic field by applying a
potential pulse of appropriate amplitude and duration to the dot. The
controlled dynamics of the 2-qubit controlled-NOT operation (XOR) can be
realized by exploiting the exchange coupling with the nearest neighboring dot.
A complete prescription for initialization of the computer and data
input/output operations is presented.Comment: 22 pages, 4 figure
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