5,308 research outputs found
Maximally entangled mixed states of two atoms trapped inside an optical cavity
In some off-resonant cases, the reduced density matrix of two atoms
symmetrically coupled with an optical cavity can very approximately approach to
maximally entangled mixed states or maximal Bell violation mixed states in
their evolution. The influence of phase decoherence on the generation of
maximally entangled mixed state is also discussed.
PACS numbers: 03.67.-a, 03.65.UdComment: 7 pages, 4 figures, Latex, have a major revision of content
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
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
Microwave probes Dipole Blockade and van der Waals Forces in a Cold Rydberg Gas
We show that microwave spectroscopy of a dense Rydberg gas trapped on a
superconducting atom chip in the dipole blockade regime reveals directly the
dipole-dipole many-body interaction energy spectrum. We use this method to
investigate the expansion of the Rydberg cloud under the effect of repulsive
van der Waals forces and the breakdown of the frozen gas approximation. This
study opens a promising route for quantum simulation of many-body systems and
quantum information transport in chains of strongly interacting Rydberg atoms.Comment: PACS: 03.67.-a, 32.80.Ee, 32.30.-
Non-Markovian decoherence in the adiabatic quantum search algorithm
We consider an adiabatic quantum algorithm (Grover's search routine) weakly
coupled to a rather general environment, i.e., without using the Markov
approximation. Markovian errors generally require high-energy excitations (of
the reservoir) and tend to destroy the scalability of the adiabatic quantum
algorithm. We find that, under appropriate conditions (such as low
temperatures), the low-energy (i.e., non-Markovian) modes of the bath are most
important. Hence the scalability of the adiabatic quantum algorithm depends on
the infra-red behavior of the environment: a reasonably small coupling to the
three-dimensional electromagnetic field does not destroy the scaling behavior,
whereas phonons or localized degrees of freedom can be problematic. PACS:
03.67.Pp, 03.67.Lx, 03.67.-a, 03.65.Yz
Contextual Realization of the Universal Quantum Cloning Machine and of the Universal-NOT gate by Quantum Injected Optical Parametric Amplification
A simultaneous, contextual experimental demonstration of the two processes of
cloning an input qubit and of flipping it into the orthogonal qubit is
reported. The adopted experimental apparatus, a Quantum-Injected Optical
Parametric Amplifier (QIOPA) is transformed simultaneously into a Universal
Optimal Quantum Cloning Machine (UOQCM) and into a Universal NOT
quantum-information gate. The two processes, indeed forbidden in their exact
form for fundamental quantum limitations, will be found to be universal and
optimal, i.e. the measured fidelity of both processes F<1 will be found close
to the limit values evaluated by quantum theory. A contextual theoretical and
experimental investigation of these processes, which may represent the basic
difference between the classical and the quantum worlds, can reveal in a
unifying manner the detailed structure of quantum information. It may also
enlighten the yet little explored interconnections of fundamental axiomatic
properties within the deep structure of quantum mechanics. PACS numbers:
03.67.-a, 03.65.Ta, 03.65.UdComment: 27 pages, 7 figure
Flat bands with higher Chern number in pyrochlore slabs
A large number of recent works point to the emergence of intriguing analogs
of fractional quantum Hall states in lattice models due to effective
interactions in nearly flat bands with Chern number C=1. Here, we provide an
intuitive and efficient construction of almost dispersionless bands with higher
Chern numbers. Inspired by the physics of quantum Hall multilayers and
pyrochlore-based transition-metal oxides, we study a tight-binding model
describing spin-orbit coupled electrons in N parallel kagome layers connected
by apical sites forming N-1 intermediate triangular layers (as in the
pyrochlore lattice). For each N, we find finite regions in parameter space
giving a virtually flat band with C=N. We analytically express the states
within these topological bands in terms of single-layer states and thereby
explicitly demonstrate that the C=N wave functions have an appealing structure
in which layer index and translations in reciprocal space are intricately
coupled. This provides a promising arena for new collective states of matter.Comment: 5+3 pages. Title extended, as publishe
Quantum Discord for Generalized Bloch Sphere States
In this study for particular states of bipartite quantum system in 2n?2m
dimensional Hilbert space state, similar to m or n-qubit density matrices
represented in Bloch sphere we call them generalized Bloch sphere states(GBSS),
we give an efficient optimization procedure so that analytic evaluation of
quantum discord can be performed. Using this optimization procedure, we find an
exact analytical formula for the optimum positive operator valued measure
(POVM) that maximize the measure of the classical correlation for these states.
The presented optimization procedure also is used to show that for any concave
entropy function the same POVMs are sufficient for quantum discord of mentioned
states. Furthermore, We show that such optimization procedure can be used to
calculate the geometric measure of quantum discord (GMQD) and then an explicit
formula for GMQD is given. Finally, a complete geometric view is presented for
quantum discord of GBSS. Keywords: Quantum Discord, Generalized Bloch Sphere
States, Dirac matrices, Bipartite Quantum System. PACs Index: 03.67.-a,
03.65.Ta, 03.65.UdComment: 26 pages. arXiv admin note: text overlap with arXiv:1107.5174 by
other author
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