2,602 research outputs found
Tunable refraction in a two dimensional quantum metamaterial
In this paper we consider a two-dimensional metamaterial comprising an array
of qubits (two level quantum objects). Here we show that a two-dimensional
quantum metamaterial may be controlled, e.g. via the application of a magnetic
flux, so as to provide controllable refraction of an input signal. Our results
are consistent with a material that could be quantum birefringent (beam
splitter) or not dependent on the application of this control parameter. We
note that quantum metamaterials as proposed here may be fabricated from a
variety of current candidate technologies from superconducting qubits to
quantum dots. Thus the ideas proposed in this work would be readily testable in
existing state of the art laboratories.Comment: 4 pages, 2 figure
The Quantum Emergence of Chaos
The dynamical status of isolated quantum systems, partly due to the linearity
of the Schrodinger equation is unclear: Conventional measures fail to detect
chaos in such systems. However, when quantum systems are subjected to
observation -- as all experimental systems must be -- their dynamics is no
longer linear and, in the appropriate limit(s), the evolution of expectation
values, conditioned on the observations, closely approaches the behavior of
classical trajectories. Here we show, by analyzing a specific example, that
microscopic continuously observed quantum systems, even far from any classical
limit, can have a positive Lyapunov exponent, and thus be truly chaotic.Comment: 4 pages, 4 figure
Overcoming decoherence in the collapse and revival of spin Schr\"odinger cats
In addition to being a very interesting quantum phenomenon, Schr\"odinger cat
swapping has the potential for application in the preparation of quantum states
that could be used in metrology and other quantum processing. We study in
detail the effects of field decoherence on a cat-swapping system comprising a
set of identical qubits, or spins, all coupled to a field mode. We demonstrate
that increasing the number of spins actually mitigates the effects of field
decoherence on the collapse and revival of a spin Schr\"odinger cat, which
could be of significant utility in quantum metrology and other quantum
processing.Comment: 4 pages, 2 figure
HUMAN SPINAL CORD IMPEDANCE: ITS APPLICATION IN NEUROSURGICAL STEREOTAXIC CORDOTOMY
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/72600/1/j.1749-6632.1970.tb17740.x.pd
Coulomb plasmas in outer envelopes of neutron stars
Outer envelopes of neutron stars consist mostly of fully ionized, strongly
coupled Coulomb plasmas characterized by typical densities about 10^4-10^{11}
g/cc and temperatures about 10^4-10^9 K. Many neutron stars possess magnetic
fields about 10^{11}-10^{14} G. Here we briefly review recent theoretical
advances which allow one to calculate thermodynamic functions and electron
transport coefficients for such plasmas with an accuracy required for
theoretical interpretation of observations.Comment: 4 pages, 2 figures, latex2e using cpp2e.cls (included). Proc. PNP-10
Workshop, Greifswald, Germany, 4-9 Sept. 2000. Accepted for publication in
Contrib. Plasma Phys. 41 (2001) no. 2-
Weak nonlinearities: A new route to optical quantum computation
Quantum information processing (QIP) offers the promise of being able to do
things that we cannot do with conventional technology. Here we present a new
route for distributed optical QIP, based on generalized quantum non-demolition
measurements, providing a unified approach for quantum communication and
computing. Interactions between photons are generated using weak
non-linearities and intense laser fields--the use of such fields provides for
robust distribution of quantum information. Our approach requires only a
practical set of resources, and it uses these very efficiently. Thus it
promises to be extremely useful for the first quantum technologies, based on
scarce resources. Furthermore, in the longer term this approach provides both
options and scalability for efficient many-qubit QIP.Comment: 7 Pages, 4 Figure
Superconducting Analogues of Quantum Optical Phenomena: Macroscopic Quantum Superpositions and Squeezing in a SQUID Ring
In this paper we explore the quantum behaviour of a SQUID ring which has a
significant Josephson coupling energy. We show that that the eigenfunctions of
the Hamiltonian for the ring can be used to create macroscopic quantum
superposition states of the ring. We also show that the ring potential may be
utilised to squeeze coherent states. With the SQUID ring as a strong contender
as a device for manipulating quantum information, such properties may be of
great utility in the future. However, as with all candidate systems for quantum
technologies, decoherence is a fundamental problem. In this paper we apply an
open systems approach to model the effect of coupling a quantum mechanical
SQUID ring to a thermal bath. We use this model to demonstrate the manner in
which decoherence affects the quantum states of the ring.Comment: 9 pages, 10 figures, To be submitted to Phys. Rev. A. (changes for
referee's and editior's comments - replaced to try to get PDF working
Applications of Coherent Population Transfer to Quantum Information Processing
We develop a theoretical framework for the exploration of quantum mechanical
coherent population transfer phenomena, with the ultimate goal of constructing
faithful models of devices for classical and quantum information processing
applications. We begin by outlining a general formalism for weak-field quantum
optics in the Schr\"{o}dinger picture, and we include a general
phenomenological representation of Lindblad decoherence mechanisms. We use this
formalism to describe the interaction of a single stationary multilevel atom
with one or more propagating classical or quantum laser fields, and we describe
in detail several manifestations and applications of electromagnetically
induced transparency. In addition to providing a clear description of the
nonlinear optical characteristics of electromagnetically transparent systems
that lead to ``ultraslow light,'' we verify that -- in principle -- a
multi-particle atomic or molecular system could be used as either a low power
optical switch or a quantum phase shifter. However, we demonstrate that the
presence of significant dephasing effects destroys the induced transparency,
and that increasing the number of particles weakly interacting with the probe
field only reduces the nonlinearity further. Finally, a detailed calculation of
the relative quantum phase induced by a system of atoms on a superposition of
spatially distinct Fock states predicts that a significant quasi-Kerr
nonlinearity and a low entropy cannot be simultaneously achieved in the
presence of arbitrary spontaneous emission rates. Within our model, we identify
the constraints that need to be met for this system to act as a one-qubit and a
two-qubit conditional phase gate.Comment: 25 pages, 14 figure
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