465 research outputs found
Scalable quantum computation with fast gates in two-dimensional microtrap arrays of trapped ions
We theoretically investigate the use of fast pulsed two-qubit gates for
trapped ion quantum computing in a two-dimensional microtrap architecture. In
one dimension, such fast gates are optimal when employed between nearest
neighbours, and we examine the generalisation to a two-dimensional geometry. We
demonstrate that fast pulsed gates are capable of implementing high-fidelity
entangling operations between ions in neighbouring traps faster than the
trapping period, with experimentally demonstrated laser repetition rates.
Notably, we find that without increasing the gate duration, high-fidelity gates
are achievable even in large arrays with hundreds of ions. To demonstrate the
usefulness of this proposal, we investigate the application of these gates to
the digital simulation of a 40-mode Fermi-Hubbard model. This also demonstrates
why shorter chains of gates required to connect arbitrary pairs of ions makes
this geometry well suited for large-scale computation
XMDS2: Fast, scalable simulation of coupled stochastic partial differential equations
XMDS2 is a cross-platform, GPL-licensed, open source package for numerically
integrating initial value problems that range from a single ordinary
differential equation up to systems of coupled stochastic partial differential
equations. The equations are described in a high-level XML-based script, and
the package generates low-level optionally parallelised C++ code for the
efficient solution of those equations. It combines the advantages of high-level
simulations, namely fast and low-error development, with the speed, portability
and scalability of hand-written code. XMDS2 is a complete redesign of the XMDS
package, and features support for a much wider problem space while also
producing faster code.Comment: 9 pages, 5 figure
Stabilizing an atom laser using spatially selective pumping and feedback
We perform a comprehensive study of stability of a pumped atom laser in the
presence of pumping, damping and outcoupling. We also introduce a realistic
feedback scheme to improve stability by extracting energy from the condensate
and determine its effectiveness. We find that while the feedback scheme is
highly efficient in reducing condensate fluctuations, it usually does not alter
the stability class of a particular set of pumping, damping and outcoupling
parameters.Comment: 7 figure
Pulse retrieval and soliton formation in a non-standard scheme for dynamic electromagnetically induced transparency
We examine in detail an alternative method of retrieving the information
written into an atomic ensemble of three-level atoms using electromagnetically
induced transparency. We find that the behavior of the retrieved pulse is
strongly influenced by the relative collective atom-light coupling strengths of
the two relevant transitions. When the collective atom-light coupling strength
for the retrieval beam is the stronger of the two transitions, regeneration of
the stored pulse is possible. Otherwise, we show the retrieval process can lead
to creation of soliton-like pulses.Comment: 11 figure
Controlling chaos in the quantum regime using adaptive measurements
The continuous monitoring of a quantum system strongly influences the
emergence of chaotic dynamics near the transition from the quantum regime to
the classical regime. Here we present a feedback control scheme that uses
adaptive measurement techniques to control the degree of chaos in the
driven-damped quantum Duffing oscillator. This control relies purely on the
measurement backaction on the system, making it a uniquely quantum control, and
is only possible due to the sensitivity of chaos to measurement. We quantify
the effectiveness of our control by numerically computing the quantum Lyapunov
exponent over a wide range of parameters. We demonstrate that adaptive
measurement techniques can control the onset of chaos in the system, pushing
the quantum-classical boundary further into the quantum regime
Multimode quantum limits to the linewidth of an atom laser
The linewidth of an atom laser can be limited by excitation of higher energy
modes in the source Bose-Einstein condensate, energy shifts in that condensate
due to the atomic interactions, or phase diffusion of the lasing mode due to
those interactions. The first two are effects that can be described with a
semiclassical model, and have been studied in detail for both pumped and
unpumped atom lasers. The third is a purely quantum statistical effect, and has
been studied only in zero dimensional models. We examine an unpumped atom laser
in one dimension using a quantum field theory using stochastic methods based on
the truncated Wigner approach. This allows spatial and statistical effects to
be examined simultaneously, and the linewidth limit for unpumped atom lasers is
quantified in various limits.Comment: 8 Figure
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