7,823 research outputs found
Hybrid Quantum Repeater Protocol With Fast Local Processing
We propose a hybrid quantum repeater protocol combining the advantages of
continuous and discrete variables. The repeater is based on the previous work
of Brask et al. [Phys. Rev. Lett. 105, 160501 (2010)] but we present two ways
of improving this protocol. In the previous protocol entangled single-photon
states are produced and grown into superpositions of coherent states, known as
two-mode cat states. The entanglement is then distributed using homodyne
detection. To improve the protocol, we replace the time-consuming non-local
growth of cat states with local growth of single-mode cat states, eliminating
the need for classical communication during growth. Entanglement is generated
in subsequent connection processes. Furthermore the growth procedure is
optimized. We review the main elements of the original protocol and present the
two modifications. Finally the two protocols are compared and the modified
protocol is shown to perform significantly better than the original protocol.Comment: 14 pages, 7 figure
Ground-State Energy and Spin Gap of Spin-1/2 Kagome Heisenberg Antiferromagnetic Clusters: Large Scale Exact Diagonalization Results
We present a comprehensive list of ground state energies and spin gaps of
finite kagome clusters with up to 42 spins obtained using large-scale exact
diagonalization techniques. This represents the current limit of this exact
approach. For a fixed number of spins N we study several cluster shapes under
periodic boundary conditions in both directions resulting in a toroidal
geometry. The clusters are characterized by their side length and diagonal as
well as the shortest "Manhattan" diameter of the torii. A finite-size scaling
analysis of the ground state energy as well as the spin gap is then performed
in terms of the shortest toroidal diameter as well as the shortest "Manhattan"
diameter. The structure of the spin-spin correlations further supports the
importance of short loops wrapping around the torii.Comment: 4 pages, 4 figures, added one referenc
Dissipative preparation of entanglement in optical cavities
We propose a novel scheme for the preparation of a maximally entangled state
of two atoms in an optical cavity. Starting from an arbitrary initial state, a
singlet state is prepared as the unique fixed point of a dissipative quantum
dynamical process. In our scheme, cavity decay is no longer undesirable, but
plays an integral part in the dynamics. As a result, we get a qualitative
improvement in the scaling of the fidelity with the cavity parameters. Our
analysis indicates that dissipative state preparation is more than just a new
conceptual approach, but can allow for significant improvement as compared to
preparation protocols based on coherent unitary dynamics.Comment: 4 pages, 2 figure
Stability and structure of two coupled boson systems in an external field
The lowest adiabatic potential expressed in hyperspherical coordinates is
estimated for two boson systems in an external harmonic trap. Corresponding
conditions for stability are investigated and the related structures are
extracted for zero-range interactions. Strong repulsion between non-identical
particles leads to two new features, respectively when identical particles
attract or repel each other. For repulsion new stable structures arise with
displaced center of masses. For attraction the mean-field stability region is
restricted due to motion of the center of masses
Bogoliubov theory of entanglement in a Bose-Einstein condensate
We consider a Bose-Einstein condensate which is illuminated by a short
resonant light pulse that coherently couples two internal states of the atoms.
We show that the subsequent time evolution prepares the atoms in an interesting
entangled state called a spin squeezed state. This evolution is analysed in
detail by developing a Bogoliubov theory which describes the entanglement of
the atoms. Our calculation is a consistent expansion in , where
is the number of particles in the condensate, and our theory predict that it is
possible to produce spin squeezing by at least a factor of . Within
the Bogoliubov approximation this result is independent of temperature.Comment: 14 pages, including 5 figures, minor changes in the presentatio
Effective Hamiltonian Theory and Its Applications in Quantum Information
This paper presents a useful compact formula for deriving an effective
Hamiltonian describing the time-averaged dynamics of detuned quantum systems.
The formalism also works for ensemble-averaged dynamics of stochastic systems.
To illustrate the technique we give examples involving Raman processes,
Bloch-Siegert shifts and Quantum Logic Gates.Comment: 5 pages, 3 figures, to be published in Canadian Journal of Physic
Towards low-dimensional hole systems in Be-doped GaAs nanowires
GaAs was central to the development of quantum devices but is rarely used for
nanowire-based quantum devices with InAs, InSb and SiGe instead taking the
leading role. p-type GaAs nanowires offer a path to studying strongly-confined
0D and 1D hole systems with strong spin-orbit effects, motivating our
development of nanowire transistors featuring Be-doped p-type GaAs nanowires,
AuBe alloy contacts and patterned local gate electrodes towards making
nanowire-based quantum hole devices. We report on nanowire transistors with
traditional substrate back-gates and EBL-defined metal/oxide top-gates produced
using GaAs nanowires with three different Be-doping densities and various AuBe
contact processing recipes. We show that contact annealing only brings small
improvements for the moderately-doped devices under conditions of lower anneal
temperature and short anneal time. We only obtain good transistor performance
for moderate doping, with conduction freezing out at low temperature for
lowly-doped nanowires and inability to reach a clear off-state under gating for
the highly-doped nanowires. Our best devices give on-state conductivity 95 nS,
off-state conductivity 2 pS, on-off ratio ~, and sub-threshold slope 50
mV/dec at T = 4 K. Lastly, we made a device featuring a moderately-doped
nanowire with annealed contacts and multiple top-gates. Top-gate sweeps show a
plateau in the sub-threshold region that is reproducible in separate cool-downs
and indicative of possible conductance quantization highlighting the potential
for future quantum device studies in this material system
Spin Squeezing in the Ising Model
We analyze the collective spin noise in interacting spin systems. General
expressions are derived for the short time behaviour of spin systems with
general spin-spin interactions, and we suggest optimum experimental conditions
for the detection of spin squeezing. For Ising models with site dependent
nearest neighbour interactions general expressions are presented for the spin
squeezing parameter for all times. The reduction of collective spin noise can
be used to verify the entangling powers of quantum computer architectures based
on interacting spins.Comment: 7 pages, including 3 figure
Stability of atomic clocks based on entangled atoms
We analyze the effect of realistic noise sources for an atomic clock
consisting of a local oscillator that is actively locked to a spin-squeezed
(entangled) ensemble of atoms. We show that the use of entangled states can
lead to an improvement of the long-term stability of the clock when the
measurement is limited by decoherence associated with instability of the local
oscillator combined with fluctuations in the atomic ensemble's Bloch vector.
Atomic states with a moderate degree of entanglement yield the maximal clock
stability, resulting in an improvement that scales as compared to the
atomic shot noise level.Comment: 4 pages, 2 figures, revtex
Multi-particle entanglement of hot trapped ions
We propose an efficient method to produce multi-particle entangled states of
ions in an ion trap for which a wide range of interesting effects and
applications have been suggested. Our preparation scheme exploits the
collective vibrational motion of the ions, but it works in such a way that this
motion need not be fully controlled in the experiment. The ions may, e.g., be
in thermal motion and exchange mechanical energy with a surrounding heat bath
without detrimental effects on the internal state preparation. Our scheme does
not require access to the individual ions in the trap.Comment: 4 pages, including 3 figures. To appear in Phys. Rev. Lett. This
paper previously appeared under the name "Schrodingers cat in a hot trap".
The paper has been revised according to Phys. Rev. policy on Schrodinger
cats. No cats were harmed during the production of this manuscrip
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