7,416 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
The Generic, Incommensurate Transition in the two-dimensional Boson Hubbard Model
The generic transition in the boson Hubbard model, occurring at an
incommensurate chemical potential, is studied in the link-current
representation using the recently developed directed geometrical worm
algorithm. We find clear evidence for a multi-peak structure in the energy
distribution for finite lattices, usually indicative of a first order phase
transition. However, this multi-peak structure is shown to disappear in the
thermodynamic limit revealing that the true phase transition is second order.
These findings cast doubts over the conclusion drawn in a number of previous
works considering the relevance of disorder at this transition.Comment: 13 pages, 10 figure
The primordial deuterium abundance at z = 2.504 from a high signal-to-noise spectrum of Q1009+2956
The spectrum of the quasar Q1009+2956 has been observed
extensively on the Keck telescope. The Lyman limit absorption system was previously used to measure D/H by Burles & Tytler using a
spectrum with signal to noise approximately 60 per pixel in the continuum near
Ly {\alpha} at . The larger dataset now available combines
to form an exceptionally high signal to noise spectrum, around 147 per pixel.
Several heavy element absorption lines are detected in this LLS, providing
strong constraints on the kinematic structure. We explore a suite of absorption
system models and find that the deuterium feature is likely to be contaminated
by weak interloping Ly {\alpha} absorption from a low column density H I cloud,
reducing the expected D/H precision. We find D/H =
for this system. Combining this new
measurement with others from the literature and applying the method of Least
Trimmed Squares to a statistical sample of 15 D/H measurements results in a
"reliable" sample of 13 values. This sample yields a primordial deuterium
abundance of (D/H). The
corresponding mean baryonic density of the Universe is . The quasar absorption data is of the same precision as, and
marginally inconsistent with, the 2015 CMB Planck (TT+lowP+lensing)
measurement, . Further quasar and more
precise nuclear data are required to establish whether this is a random
fluctuation.Comment: accepted by MNRAS, 18 pages, 12 figures, 6 table
Extending the memory times of trapped-ion qubits with error correction and global entangling operations
The technical demands to perform quantum error correction are considerable.
The task requires the preparation of a many-body entangled state, together with
the ability to make parity measurements over subsets of the physical qubits of
the system to detect errors. Here we propose two trapped-ion experiments to
realise error-correcting codes of variable size to protect a single encoded
qubit from dephasing errors. Novel to our schemes is the use of a global
entangling phase gate, which could be implemented in both Penning traps and
Paul traps. We make use of this entangling operation to significantly reduce
the experimental complexity of state preparation and syndrome measurements. We
also show, in our second scheme, that storage times can be increased further by
repeatedly teleporting the logical information between two codes supported by
the same ion Coulomb crystal to learn information about the locations of
errors. We estimate that a logical qubit encoded in such a crystal will
maintain high coherence for times more than an order of magnitude longer than
each physical qubit would.Comment: 18 pages, 8 figures. The authors list has changed since the first
version of this draf
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
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
Generation and detection of a sub-Poissonian atom number distribution in a one-dimensional optical lattice
We demonstrate preparation and detection of an atom number distribution in a
one-dimensional atomic lattice with the variance dB below the Poissonian
noise level. A mesoscopic ensemble containing a few thousand atoms is trapped
in the evanescent field of a nanofiber. The atom number is measured through
dual-color homodyne interferometry with a pW-power shot noise limited probe.
Strong coupling of the evanescent probe guided by the nanofiber allows for a
real-time measurement with a precision of atoms on an ensemble of some
atoms in a one-dimensional trap. The method is very well suited for
generating collective atomic entangled or spin-squeezed states via a quantum
non-demolition measurement as well as for tomography of exotic atomic states in
a one-dimensional lattice
- …