584 research outputs found
Quantum walks and quantum simulations with Bloch oscillating spinor atoms
We propose a scheme for the realization of a quantum walker and a quantum
simulator for the Dirac equation with ultracold spinor atoms in driven optical
lattices. A precise control of the dynamics of the atomic matter wave can be
realized using time-dependent external forces. If the force depends on the spin
state of the atoms, the dynamics will entangle the inner and outer degrees of
freedom which offers unique opportunities for quantum information and quantum
simulation. Here, we introduce a method to realize a quantum walker based on
the state-dependent transport of spinor atoms and a coherent driving of the
internal state. In the limit of weak driving the dynamics is equivalent to that
of a Dirac particle in 1+1 dimensions. Thus it becomes possible to simulate
relativistic effects such as Zitterbewegung and Klein tunneling.Comment: published version, 7 figure
Stochastic resonance driven by quantum shot noise in superradiant Raman scattering
We discuss the effects of noise on the timing and strength of superradiant
Raman scattering from a small dense sample of atoms. We demonstrate a genuine
quantum stochastic resonance effect, where the atomic response is largest for
an appropriate quantum noise level. The peak scattering intensity per atom
assumes its maximum for a specific non-zero value of quantum noise given by the
square root of the number of atoms.Comment: 10 pages, 4 figure
Nonlocal failures in complex supply networks by single link additions
How do local topological changes affect the global operation and stability of
complex supply networks? Studying supply networks on various levels of
abstraction, we demonstrate that and how adding new links may not only promote
but also degrade stable operation of a network. Intriguingly, the resulting
overloads may emerge remotely from where such a link is added, thus resulting
in nonlocal failure. We link this counter-intuitive phenomenon to Braess'
paradox originally discovered in traffic networks. We use elementary network
topologies to explain its underlying mechanism for different types of supply
networks and find that it generically occurs across these systems. As an
important consequence, upgrading supply networks such as communication
networks, biological supply networks or power grids requires particular care
because even adding only single connections may destabilize normal network
operation and induce disturbances remotely from the location of structural
change and even global cascades of failures.Comment: 12 pages, 10 figure
Photon sorters and QND detectors using single photon emitters
We discuss a new method for realizing number-resolving and non-demolition
photo detectors by strong coupling of light to individual single photon
emitters, which act as strong optical non-linearities. As a specific
application we show how these elements can be integrated into an error-proof
Bell state analyzer, whose efficiency exceeds the best possible performance
with linear optics even for a modest atom-field coupling. The methods are
error-proof in the sense that every detection event unambiguously projects the
photon state onto a Fock or Bell state.Comment: revised and enlarged version, 6+ pages, 5 figure
Photon scattering by a three-level emitter in a one-dimensional waveguide
We discuss the scattering of photons from a three-level emitter in a
one-dimensional waveguide, where the transport is governed by the interference
of spontaneously emitted and directly transmitted waves. The scattering problem
is solved in closed form for different level structures. Several possible
applications are discussed: The state of the emitter can be switched
deterministically by Raman scattering, thus enabling applications in quantum
computing such as a single photon transistor. An array of emitters gives rise
to a photonic band gap structure, which can be tuned by a classical driving
laser. A disordered array leads to Anderson localization of photons, where the
localization length can again be controlled by an external driving.Comment: 17 pages, 8 figure
Dissipation induced macroscopic entanglement in an open optical lattice
We introduce a method for the dissipative preparation of strongly correlated
quantum states of ultracold atoms in an optical lattice via localized particle
loss. The interplay of dissipation and interactions enables different types of
dynamics. This ushers a new line of experimental methods to maintain the
coherence of a Bose-Einstein condensate or to deterministically generate
macroscopically entangled quantum states.Comment: 4 figure
Dissipation induced coherence of a two-mode Bose-Einstein condensate
We discuss the dynamics of a Bose-Einstein condensate in a double-well trap
subject to phase noise and particle loss. The phase coherence of a
weakly-interacting condensate as well as the response to an external driving
show a pronounced stochastic resonance effect: Both quantities become maximal
for a finite value of the dissipation rate matching the intrinsic time scales
of the system. Even stronger effects are observed when dissipation acts in
concurrence with strong inter-particle interactions, restoring the purity of
the condensate almost completely and increasing the phase coherence
significantly.Comment: 10 pages, 5 figure
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