1,608 research outputs found
Quantum simulation via filtered Hamiltonian engineering: application to perfect quantum transport in spin networks
We propose a method for Hamiltonian engineering in quantum information
processing architectures that requires no local control, but only relies on
collective qubit rotations and field gradients. The technique achieves a
spatial modulation of the coupling strengths via a dynamical construction of a
weighting function combined with a Bragg grating. As an example, we demonstrate
how to generate the ideal Hamiltonian for perfect quantum information transport
between two separated nodes of a large spin network. We engineer a spin chain
with optimal couplings from a large spin network, such as naturally occurring
in crystals, while decoupling all unwanted interactions. For realistic
experimental parameters, our method can be used to drive perfect quantum
information transport at room-temperature. The Hamiltonian engineering method
can be made more robust under coherence and coupling disorder by a novel
apodization scheme. Thus the method is quite general and can be used engineer
the Hamiltonian of many complex spin lattices with different topologies and
interactions.Comment: v2: Extended robustness to decoherenc
Mixed-state quantum transport in correlated spin networks
Quantum spin networks can be used to transport information between separated
registers in a quantum information processor. To find a practical
implementation, the strict requirements of ideal models for perfect state
transfer need to be relaxed, allowing for complex coupling topologies and
general initial states. Here we analyze transport in complex quantum spin
networks in the maximally mixed state and derive explicit conditions that
should be satisfied by propagators for perfect state transport. Using a
description of the transport process as a quantum walk over the network, we
show that it is necessary to phase correlate the transport processes occurring
along all the possible paths in the network. We provide a Hamiltonian that
achieves this correlation, and use it in a constructive method to derive
engineered couplings for perfect transport in complicated network topologies
Classical light vs. nonclassical light: Characterizations and interesting applications
We briefly review the ideas that have shaped modern optics and have led to
various applications of light ranging from spectroscopy to astrophysics, and
street lights to quantum communication. The review is primarily focused on the
modern applications of classical light and nonclassical light. Specific
attention has been given to the applications of squeezed, antibunched, and
entangled states of radiation field. Applications of Fock states (especially
single photon states) in the field of quantum communication are also discussed.Comment: 32 pages, 3 figures, a review on applications of ligh
Stochastic analysis of a full system of two competing populations in a chemostat
This paper formulates two 3D stochastic differential equations (SDEs) of two
microbial populations in a chemostat competing over a single substrate. The two
models have two distinct noise sources. One is general noise whereas the other
is dilution rate induced noise. Nonlinear Monod growth rates are assumed and
the paper is mainly focused on the parameter values where coexistence is
present deterministically. Nondimensionalising the equations around the point
of intersection of the two growth rates leads to a large parameter which is the
nondimensional substrate feed. This in turn is used to perform an asymptotic
analysis leading to a reduced 2D system of equations describing the dynamics of
the populations on and close to a line of steady states retrieved from the
deterministic stability analysis. That reduced system allows the formulation of
a spatially 2D Fokker-Planck equation which when solved numerically admits
results similar to those from simulation of the SDEs. Contrary to previous
suggestions, one particular population becomes dominant at large times.
Finally, we brie y explore the case where death rates are added
Local Quantum Uncertainty in Two-Qubit Separable States: A Case Study
Recent findings suggest, separable states, which are otherwise of no use in
entanglement dependent tasks, can also be used in information processing tasks
that depend upon the discord type general non classical correlations. In this
work, we explore the nature of uncertainty in separable states as measured by
local quantum uncertainty. Particularly in two-qubit system, we find separable
X-state which has maximum local quantum uncertainty. Interestingly, this
separable state coincides with the separable state, having maximum geometric
discord. We also search for the maximum amount of local quantum uncertainty in
separable Bell diagonal states. We indicate an interesting connection to the
tightness of entropic uncertainty with the state of maximum uncertainty.Comment: 11 pages, 2 figures, latex2e, comments welcome, to appear in qi
Fungi in fence posts-- isolation, distribution and interaction of fungal species present and their effect
Imperial Users onl
Optimal pulse spacing for dynamical decoupling in the presence of a purely-dephasing spin-bath
Maintaining quantum coherence is a crucial requirement for quantum
computation; hence protecting quantum systems against their irreversible
corruption due to environmental noise is an important open problem. Dynamical
decoupling (DD) is an effective method for reducing decoherence with a low
control overhead. It also plays an important role in quantum metrology, where
for instance it is employed in multiparameter estimation. While a sequence of
equidistant control pulses (CPMG) has been ubiquitously used for decoupling,
Uhrig recently proposed that a non-equidistant pulse sequence (UDD) may enhance
DD performance, especially for systems where the spectral density of the
environment has a sharp frequency cutoff. On the other hand, equidistant
sequences outperform UDD for soft cutoffs. The relative advantage provided by
UDD for intermediate regimes is not clear. In this paper, we analyze the
relative DD performance in this regime experimentally, using solid-state
nuclear magnetic resonance. Our system-qubits are 13C nuclear spins and the
environment consists of a 1H nuclear spin-bath whose spectral density is close
to a normal (Gaussian) distribution. We find that in the presence of such a
bath, the CPMG sequence outperforms the UDD sequence. An analogy between
dynamical decoupling and interference effects in optics provides an intuitive
explanation as to why the CPMG sequence performs superior to any
non-equidistant DD sequence in the presence of this kind of environmental
noise.Comment: To be published in Phys. Rev. A. 15 pages, 16 figures. Presentation
of the work was improved. One Figure and some Refs. were adde
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