490 research outputs found
Excising das All: Evolving Maxwell waves beyond scri
We study the numerical propagation of waves through future null infinity in a
conformally compactified spacetime. We introduce an artificial cosmological
constant, which allows us some control over the causal structure near null
infinity. We exploit this freedom to ensure that all light cones are tilted
outward in a region near null infinity, which allows us to impose
excision-style boundary conditions in our finite difference code. In this
preliminary study we consider electromagnetic waves propagating in a static,
conformally compactified spacetime.Comment: 13 pages; incorporated material from gr-qc/051216
System Design for a Long-Line Quantum Repeater
We present a new control algorithm and system design for a network of quantum
repeaters, and outline the end-to-end protocol architecture. Such a network
will create long-distance quantum states, supporting quantum key distribution
as well as distributed quantum computation. Quantum repeaters improve the
reduction of quantum-communication throughput with distance from exponential to
polynomial. Because a quantum state cannot be copied, a quantum repeater is not
a signal amplifier, but rather executes algorithms for quantum teleportation in
conjunction with a specialized type of quantum error correction called
purification to raise the fidelity of the quantum states. We introduce our
banded purification scheme, which is especially effective when the fidelity of
coupled qubits is low, improving the prospects for experimental realization of
such systems. The resulting throughput is calculated via detailed simulations
of a long line composed of shorter hops. Our algorithmic improvements increase
throughput by a factor of up to fifty compared to earlier approaches, for a
broad range of physical characteristics.Comment: 12 pages, 13 figures. v2 includes one new graph, modest corrections
to some others, and significantly improved presentation. to appear in
IEEE/ACM Transactions on Networkin
On the Effect of Quantum Interaction Distance on Quantum Addition Circuits
We investigate the theoretical limits of the effect of the quantum
interaction distance on the speed of exact quantum addition circuits. For this
study, we exploit graph embedding for quantum circuit analysis. We study a
logical mapping of qubits and gates of any -depth quantum adder
circuit for two -qubit registers onto a practical architecture, which limits
interaction distance to the nearest neighbors only and supports only one- and
two-qubit logical gates. Unfortunately, on the chosen -dimensional practical
architecture, we prove that the depth lower bound of any exact quantum addition
circuits is no longer , but . This
result, the first application of graph embedding to quantum circuits and
devices, provides a new tool for compiler development, emphasizes the impact of
quantum computer architecture on performance, and acts as a cautionary note
when evaluating the time performance of quantum algorithms.Comment: accepted for ACM Journal on Emerging Technologies in Computing
System
A high bandwidth quantum repeater
We present a physical- and link-level design for the creation of entangled
pairs to be used in quantum repeater applications where one can control the
noise level of the initially distributed pairs. The system can tune
dynamically, trading initial fidelity for success probability, from high
fidelity pairs (F=0.98 or above) to moderate fidelity pairs. The same physical
resources that create the long-distance entanglement are used to implement the
local gates required for entanglement purification and swapping, creating a
homogeneous repeater architecture. Optimizing the noise properties of the
initially distributed pairs significantly improves the rate of generating
long-distance Bell pairs. Finally, we discuss the performance trade-off between
spatial and temporal resources.Comment: 5 page
Arithmetic on a Distributed-Memory Quantum Multicomputer
We evaluate the performance of quantum arithmetic algorithms run on a
distributed quantum computer (a quantum multicomputer). We vary the node
capacity and I/O capabilities, and the network topology. The tradeoff of
choosing between gates executed remotely, through ``teleported gates'' on
entangled pairs of qubits (telegate), versus exchanging the relevant qubits via
quantum teleportation, then executing the algorithm using local gates
(teledata), is examined. We show that the teledata approach performs better,
and that carry-ripple adders perform well when the teleportation block is
decomposed so that the key quantum operations can be parallelized. A node size
of only a few logical qubits performs adequately provided that the nodes have
two transceiver qubits. A linear network topology performs acceptably for a
broad range of system sizes and performance parameters. We therefore recommend
pursuing small, high-I/O bandwidth nodes and a simple network. Such a machine
will run Shor's algorithm for factoring large numbers efficiently.Comment: 24 pages, 10 figures, ACM transactions format. Extended version of
Int. Symp. on Comp. Architecture (ISCA) paper; v2, correct one circuit error,
numerous small changes for clarity, add reference
Binary black hole merger dynamics and waveforms
We study dynamics and radiation generation in the last few orbits and merger
of a binary black hole system, applying recently developed techniques for
simulations of moving black holes. Our analysis of the gravitational radiation
waveforms and dynamical black hole trajectories produces a consistent picture
for a set of simulations with black holes beginning on circular-orbit
trajectories at a variety of initial separations. We find profound agreement at
the level of one percent among the simulations for the last orbit, merger and
ringdown. We are confident that this part of our waveform result accurately
represents the predictions from Einstein's General Relativity for the final
burst of gravitational radiation resulting from the merger of an astrophysical
system of equal-mass non-spinning black holes. The simulations result in a
final black hole with spin parameter a/m=0.69. We also find good agreement at a
level of roughly 10 percent for the radiation generated in the preceding few
orbits.Comment: 11 pages, 11 figures, submitted to PRD, update citations, minor
change
Quantum Repeater with Encoding
We propose a new approach to implement quantum repeaters for long distance
quantum communication. Our protocol generates a backbone of encoded Bell pairs
and uses the procedure of classical error correction during simultaneous
entanglement connection. We illustrate that the repeater protocol with simple
Calderbank-Shor-Steane (CSS) encoding can significantly extend the
communication distance, while still maintaining a fast key generation rate.Comment: 11 pages, 5 figures (add new section III with an explicit example and
new appendix A
- …