125 research outputs found
Orbital entanglement and violation of Bell inequalities in mesoscopic conductors
We propose a spin-independent scheme to generate and detect two-particle
entanglement in a mesoscopic normal-superconductor system. A superconductor,
weakly coupled to the normal conductor, generates an orbitally entangled state
by injecting pairs of electrons into different leads of the normal conductor.
The entanglement is detected via violation of a Bell inequality, formulated in
terms of zero-frequency current cross-correlators. It is shown that the Bell
inequality can be violated for arbitrary strong dephasing in the normal
conductor.Comment: 4 pages, 2 figure
Cyclical Quantum Memory for Photonic Qubits
We have performed a proof-of-principle experiment in which qubits encoded in
the polarization states of single-photons from a parametric down-conversion
source were coherently stored and read-out from a quantum memory device. The
memory device utilized a simple free-space storage loop, providing a cyclical
read-out that could be synchronized with the cycle time of a quantum computer.
The coherence of the photonic qubits was maintained during switching operations
by using a high-speed polarizing Sagnac interferometer switch.Comment: 4 pages, 5 figure
Symmetric qubits from cavity states
Two-mode cavities can be prepared in quantum states which represent symmetric
multi-qubit states. However, the qubits are impossible to address individually
and as such cannot be independently measured or otherwise manipulated. We
propose two related schemes to coherently transfer the qubits which the cavity
state represents onto individual atoms, so that the qubits can then be
processed individually. In particular, our scheme can be combined with the
quantum cloning scheme of Simon and coworkers [C. Simon et al, PRL 84, 2993
(2000)] to allow the optimal clones which their scheme produces to be spatially
separated and individually utilized.Comment: 8 pages, 4 figures, minor typographical errors correcte
Electron Entanglement via a Quantum Dot
This Letter presents a method of electron entanglement generation. The system
under consideration is a single-level quantum dot with one input and two output
leads. The leads are arranged such that the dot is empty, single electron
tunneling is suppressed by energy conservation, and two-electron virtual
co-tunneling is allowed. This yields a pure, non-local spin-singlet state at
the output leads. Coulomb interaction is the nonlinearity essential for
entanglement generation, and, in its absence, the singlet state vanishes. This
type of electron entanglement is a four-wave mixing process analogous to the
photon entanglement generated by a Chi-3 parametric amplifier.Comment: 4 page
Electrical current noise of a beam splitter as a test of spin-entanglement
We investigate the spin entanglement in the superconductor-quantum dot system
proposed by Recher, Sukhorukov and Loss, coupling it to an electronic
beam-splitter. The superconductor-quantum dot entangler and the beam-splitter
are treated within a unified framework and the entanglement is detected via
current correlations. The state emitted by the entangler is found to be a
linear superposition of non-local spin-singlets at different energies, a
spin-entangled two-particle wavepacket. Colliding the two electrons in the
beam-splitter, the singlet spin-state gives rise to a bunching behavior,
detectable via the current correlators. The amount of bunching depends on the
relative positions of the single particle levels in the quantum dots and the
scattering amplitudes of the beam-splitter. The singlet spin entanglement,
insensitive to orbital dephasing but suppressed by spin dephasing, is
conveniently quantified via the Fano factors. It is found that the
entanglement-dependent contribution to the Fano factor is of the same magnitude
as the non-entangled, making an experimental detection feasible. A detailed
comparison between the current correlations of the non-local spin-singlet state
and other states, possibly emitted by the entangler, is performed. This
provides conditions for an unambiguous identification of the non-local singlet
spin entanglement.Comment: 13 pages, 8 figures, section on quantification of entanglement adde
High nuclear polarization of helium-3 at low and high pressure by metastability exchange optical pumping at 1.5 Tesla
We perform metastability exchange optical pumping of helium-3 in a strong
magnetic field of 1.5 T. The achieved nuclear polarization, from 80% at 1.33
mbar to 25% at 67 mbar, shows a substantial improvement at high pressures with
respect to standard low-field optical pumping. The specific mechanisms of
metastability exchange optical pumping at high field are investigated,
advantages and intrinsic limitations are discussed. From a practical point of
view, our results open the way to alternative technological solutions for
polarized helium-3 applications and in particular for magnetic resonance
imaging of human lungs.Comment: accepted for publication in Europhysics Letter
Engineering cavity-field states by projection synthesis
We propose a reliable scheme for engineering a general cavity-field state.
This is different from recently presented strategies,where the cavity is
supposed to be initially empty and the field is built up photon by photon
through resonant atom-field interactions. Here, a coherent state is previously
injected into the cavity. So, the Wigner distribution function of the desired
state is constructed from that of the initially coherent state. Such an
engineering process is achieved through an adaptation of the recently proposed
technique of projection synthesis to cavity QED phenomena.Comment: 5 ps pages plus 3 included figure
Generating entangled atom-photon pairs from Bose-Einstein condensates
We propose using spontaneous Raman scattering from an optically driven
Bose-Einstein condensate as a source of atom-photon pairs whose internal states
are maximally entangled. Generating entanglement between a particle which is
easily transmitted (the photon) and one which is easily trapped and coherently
manipulated (an ultracold atom) will prove useful for a variety of
quantum-information related applications. We analyze the type of entangled
states generated by spontaneous Raman scattering and construct a geometry which
results in maximum entanglement
Universal Quantum Cloning in Cavity QED
We propose an implementation of an universal quantum cloning machine [UQCM,
Hillery and Buzek, Phys. Rev. A {\bf 56}, 3446 (1997)] in a Cavity Quantum
Electrodynamics (CQED) experiment. This UQCM acts on the electronic states of
atoms that interact with the electromagnetic field of a high cavity. We
discuss here the specific case of the cloning process using either a
one- or a two-cavity configuration
Quantum computation with mesoscopic superposition states
We present a strategy to engineer a simple cavity-QED two-bit universal
quantum gate using mesoscopic distinct quantum superposition states. The
dissipative effect on decoherence and amplitude damping of the quantum bits are
analyzed and the critical parameters are presented.Comment: 9 pages, 5 Postscript and 1 Encapsulated Postscript figures. To be
published in Phys. Rev.
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