285 research outputs found
Pattern recognition on a quantum computer
By means of a simple example it is demonstrated that the task of finding and
identifying certain patterns in an otherwise (macroscopically) unstructured
picture (data set) can be accomplished efficiently by a quantum computer.
Employing the powerful tool of the quantum Fourier transform the proposed
quantum algorithm exhibits an exponential speed-up in comparison with its
classical counterpart. The digital representation also results in a
significantly higher accuracy than the method of optical filtering. PACS:
03.67.Lx, 03.67.-a, 42.30.Sy, 89.70.+c.Comment: 6 pages RevTeX, 1 figure, several correction
Efficient and robust entanglement generation in a many-particle system with resonant dipole-dipole interactions
We propose and discuss a scheme for robust and efficient generation of
many-particle entanglement in an ensemble of Rydberg atoms with resonant
dipole-dipole interactions. It is shown that in the limit of complete dipole
blocking, the system is isomorphic to a multimode Jaynes-Cummings model. While
dark-state population transfer is not capable of creating entanglement, other
adiabatic processes are identified that lead to complex, maximally entangled
states, such as the N-particle analog of the GHZ state in a few steps. The
process is robust, works for even and odd particle numbers and the
characteristic time for entanglement generation scales with N^a, with a being
less than unity.Comment: 4 figure
Revisiting consistency conditions for quantum states of systems on closed timelike curves: an epistemic perspective
There has been considerable recent interest in the consequences of closed
timelike curves (CTCs) for the dynamics of quantum mechanical systems. A vast
majority of research into this area makes use of the dynamical equations
developed by Deutsch, which were developed from a consistency condition that
assumes that mixed quantum states uniquely describe the physical state of a
system. We criticise this choice of consistency condition from an epistemic
perspective, i.e., a perspective in which the quantum state represents a state
of knowledge about a system. We demonstrate that directly applying Deutsch's
condition when mixed states are treated as representing an observer's knowledge
of a system can conceal time travel paradoxes from the observer, rather than
resolving them. To shed further light on the appropriate dynamics for quantum
systems traversing CTCs, we make use of a toy epistemic theory with a strictly
classical ontology due to Spekkens and show that, in contrast to the results of
Deutsch, many of the traditional paradoxical effects of time travel are
present.Comment: 10 pages, 6 figures, comments welcome; v2 added references and
clarified some points; v3 published versio
Effect of an inhomogeneous external magnetic field on a quantum dot quantum computer
We calculate the effect of an inhomogeneous magnetic field, which is
invariably present in an experimental environment, on the exchange energy of a
double quantum dot artificial molecule, projected to be used as a 2-qubit
quantum gate in the proposed quantum dot quantum computer. We use two different
theoretical methods to calculate the Hilbert space structure in the presence of
the inhomogeneous field: the Heitler-London method which is carried out
analytically and the molecular orbital method which is done computationally.
Within these approximations we show that the exchange energy J changes slowly
when the coupled dots are subject to a magnetic field with a wide range of
inhomogeneity, suggesting swap operations can be performed in such an
environment as long as quantum error correction is applied to account for the
Zeeman term. We also point out the quantum interference nature of this slow
variation in exchange.Comment: 12 pages, 4 figures embedded in tex
Toward scalable quantum computation with cavity QED systems
We propose a scheme for quantum computing using high-Q cavities in which the
qubits are represented by single cavity modes restricted in the space spanned
by the two lowest Fock states. We show that single qubit operations and
universal multiple qubit gates can be implemented using atoms sequentially
crossing the cavities.Comment: 14 pages, 8 figure
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
A switchable controlled-NOT gate in a spin-chain NMR quantum computer
A method of switching a controlled-NOT gate in a solid-stae NMR quantum
computer is presented. Qubits of I=1/2 nuclear spins are placed periodically
along a quantum spin chain (1-D antiferromagnet) having a singlet ground state
with a finite spin gap to the lowest excited state caused by some quantum
effect. Irradiation of a microwave tuned to the spin gap energy excites a
packet of triplet magnons at a specific part of the chain where control and
target qubits are involved. The packet switches on the Suhl-Nakamura
interaction between the qubits, which serves as a controlled NOT gate. The
qubit initialization is achieved by a qubit initializer consisting of
semiconducting sheets attached to the spin chain, where spin polarizations
created by the optical pumping method in the semiconductors are transferred to
the spin chain. The scheme allows us to separate the initialization process
from the computation, so that one can optimize the computation part without
being restricted by the initialization scheme, which provides us with a wide
selection of materials for a quantum computer.Comment: 8 pages, 5 figure
Proposal of an experimental scheme for realising a translucent eavesdropping on a quantum cryptographic channel
Purpose of this paper is to suggest a scheme, which can be realised with
today's technology and could be used for entangling a probe to a photon qubit
based on polarisation. Using this probe a translucent or a coherent
eavesdropping can be performed.Comment: in pres
Entanglement Sharing in the Two-Atom Tavis-Cummings Model
Individual members of an ensemble of identical systems coupled to a common
probe can become entangled with one another, even when they do not interact
directly. We investigate how this type of multipartite entanglement is
generated in the context of a system consisting of two two-level atoms
resonantly coupled to a single mode of the electromagnetic field. The dynamical
evolution is studied in terms of the entanglements in the different bipartite
partitions of the system, as quantified by the I-tangle. We also propose a
generalization of the so-called residual tangle that quantifies the inherent
three-body correlations in our tripartite system. This enables us to completely
characterize the phenomenon of entanglement sharing in the case of the two-atom
Tavis-Cummings model, a system of both theoretical and experimental interest.Comment: 11 pages, 4 figures, submitted to PRA, v3 contains corrections to
small error
Many particle entanglement in two-component Bose-Einstein Condensates
We investigate schemes to dynamically create many particle entangled states
of a two component Bose-Einstein condensate in a very short time proportional
to 1/N where is the number of condensate particles. For small we
compare exact numerical calculations with analytical semiclassical estimates
and find very good agreement for . We also estimate the effect of
decoherence on our scheme, study possible scenarios for measuring the entangled
states, and investigate experimental imperfections.Comment: 12 pages, 8 figure
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