221 research outputs found
Layer by layer generation of cluster states
Cluster states can be used to perform measurement-based quantum computation.
The cluster state is a useful resource, because once it has been generated only
local operations and measurements are needed to perform universal quantum
computation. In this paper, we explore techniques for quickly and
deterministically building a cluster state. In particular we consider
generating cluster states on a qubus quantum computer, a computational
architecture which uses a continuous variable ancilla to generate interactions
between qubits. We explore several techniques for building the cluster, with
the number of operations required depending on whether we allow the ability to
destroy previously created controlled-phase links between qubits. In the case
where we can not destroy these links, we show how to create an n x m cluster
using just 3nm -2n -3m/2 + 3 operations. This gives more than a factor of 2
saving over a naive method. Further savings can be obtained if we include the
ability to destroy links, in which case we only need (8nm-4n-4m-8)/3
operations. Unfortunately the latter scheme is more complicated so choosing the
correct order to interact the qubits is considerably more difficult. A half way
scheme, that keeps a modular generation but saves additional operations over
never destroying links requires only 3nm-2n-2m+4 operations. The first scheme
and the last scheme are the most practical for building a cluster state because
they split up the generation into the repetition of simple sections.Comment: 16 pages, 11 figure
Entanglement in spin chains and lattices with long-range Ising-type interactions
We consider N initially disentangled spins, embedded in a ring or
d-dimensional lattice of arbitrary geometry, which interact via some
long--range Ising--type interaction. We investigate relations between
entanglement properties of the resulting states and the distance dependence of
the interaction in the limit N to infinity. We provide a sufficient condition
when bipartite entanglement between blocks of L neighboring spins and the
remaining system saturates, and determine S_L analytically for special
configurations. We find an unbounded increase of S_L as well as diverging
correlation and entanglement length under certain circumstances. For
arbitrarily large N, we can efficiently calculate all quantities associated
with reduced density operators of up to ten particles.Comment: 4 pages, 2 figures; V2: presentation improved, references adde
Secondary Structures in Long Compact Polymers
Compact polymers are self-avoiding random walks which visit every site on a
lattice. This polymer model is used widely for studying statistical problems
inspired by protein folding. One difficulty with using compact polymers to
perform numerical calculations is generating a sufficiently large number of
randomly sampled configurations. We present a Monte-Carlo algorithm which
uniformly samples compact polymer configurations in an efficient manner
allowing investigations of chains much longer than previously studied. Chain
configurations generated by the algorithm are used to compute statistics of
secondary structures in compact polymers. We determine the fraction of monomers
participating in secondary structures, and show that it is self averaging in
the long chain limit and strictly less than one. Comparison with results for
lattice models of open polymer chains shows that compact chains are
significantly more likely to form secondary structure.Comment: 14 pages, 14 figure
Coherence in Microchip Traps
We report the coherent manipulation of internal states of neutral atoms in a
magnetic microchip trap. Coherence lifetimes exceeding 1 s are observed with
atoms at distances of m from the microchip surface. The coherence
lifetime in the chip trap is independent of atom-surface distance within our
measurement accuracy, and agrees well with the results of similar measurements
in macroscopic magnetic traps. Due to the absence of surface-induced
decoherence, a miniaturized atomic clock with a relative stability in the
range can be realized. For applications in quantum information
processing, we propose to use microwave near-fields in the proximity of chip
wires to create potentials that depend on the internal state of the atoms.Comment: Revised version, accepted for publication in Phys. Rev. Lett., 4
pages, 4 figure
Limitations of Quantum Simulation Examined by Simulating a Pairing Hamiltonian using Nuclear Magnetic Resonance
Quantum simulation uses a well-known quantum system to predict the behavior
of another quantum system. Certain limitations in this technique arise,
however, when applied to specific problems, as we demonstrate with a
theoretical and experimental study of an algorithm to find the low-lying
spectrum of a Hamiltonian. While the number of elementary quantum gates does
scale polynomially with the size of the system, it increases inversely to the
desired error bound . Making such simulations robust to decoherence
using fault-tolerance constructs requires an additional factor of
gates. These constraints are illustrated by using a three qubit nuclear
magnetic resonance system to simulate a pairing Hamiltonian, following the
algorithm proposed by Wu, Byrd, and Lidar.Comment: 6 pages, 2 eps figure
Optimal estimation of quantum dynamics
We construct the optimal strategy for the estimation of an unknown unitary
transformation . This includes, in addition to a convenient
measurement on a probe system, finding which is the best initial state on which
is to act. When , such an optimal strategy can be applied to
estimate simultaneously both the direction and the strength of a magnetic
field, and shows how to use a spin 1/2 particle to transmit information about a
whole coordinate system instead of only a direction in space.Comment: 4 pages, REVTE
KIC InnoEnergy Project Neptune: development of a floating LiDAR buoy for wind, wave and current measurements
The KIC-InnoEnergy project âNEPTUNEâ develops a floating Lidar buoy and a hindcast- and forecast model for wind- wave- and current measurements of offshore wind farms. In this paper just the lidar buoy is presented and discussed: Main challenges, the design ideas and the steps to develop, test and prototype this product, which â according to the KIC-InnoEnergy project idea â should be commercialized after the project end, foreseen for the end of 2014. KIC-InnoEnergy is funded from the European Institute of Technology, EIT.Peer ReviewedPostprint (published version
Correlated quantum percolation in the lowest Landau level
Our understanding of localization in the integer quantum Hall effect is
informed by a combination of semi-classical models and percolation theory.
Motivated by the effect of correlations on classical percolation we study
numerically electron localization in the lowest Landau level in the presence of
a power-law correlated disorder potential. Careful comparisons between
classical and quantum dynamics suggest that the extended Harris criterion is
applicable in the quantum case. This leads to a prediction of new localization
quantum critical points in integer quantum Hall systems with power-law
correlated disorder potentials. We demonstrate the stability of these critical
points to addition of competing short-range disorder potentials, and discuss
possible experimental realizations.Comment: 15 pages, 12 figure
Effective Spin Quantum Phases in Systems of Trapped Ions
A system of trapped ions under the action of off--resonant standing--waves
can be used to simulate a variety of quantum spin models. In this work, we
describe theoretically quantum phases that can be observed in the simplest
realization of this idea: quantum Ising and XY models. Our numerical
calculations with the Density Matrix Renormalization Group method show that
experiments with ion traps should allow one to access general properties of
quantum critical systems. On the other hand, ion trap quantum spin models show
a few novel features due to the peculiarities of induced effective spin--spin
interactions which lead to interesting effects like long--range quantum
correlations and the coexistence of different spin phases.Comment: 11 pages, 13 figure
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