1,192 research outputs found
Storage and retrieval of vector beams of light in a multiple-degree-of-freedom quantum memory
The full structuration of light in the transverse plane, including intensity,
phase and polarization, holds the promise of unprecedented capabilities for
applications in classical optics as well as in quantum optics and information
sciences. Harnessing special topologies can lead to enhanced focusing, data
multiplexing or advanced sensing and metrology. Here we experimentally
demonstrate the storage of such spatio-polarization-patterned beams into an
optical memory. A set of vectorial vortex modes is generated via liquid crystal
cell with topological charge in the optic axis distribution, and preservation
of the phase and polarization singularities is demonstrated after retrieval, at
the single-photon level. The realized multiple-degree-of-freedom memory can
find applications in classical data processing but also in quantum network
scenarios where structured states have been shown to provide promising
attributes, such as rotational invariance
Creation and characterization of vector vortex modes for classical and quantum communication
Vector vortex beams are structured states of light that are non-separable in
their polarisation and spatial mode, they are eigenmodes of free-space and many
fibre systems, and have the capacity to be used as a modal basis for both
classical and quantum communication. Here we outline recent progress in our
understanding of these modes, from their creation to their characterization and
detection. We then use these tools to study the propagation behaviour of such
modes in free-space and optical fibre and show that modal cross-talk results in
a decay of vector states into separable scalar modes, with a concomitant loss
of information. We present a comparison between probabilistic and deterministic
detection schemes showing that the former, while ubiquitous, negates the very
benefit of increased dimensionality in quantum communication while reducing
signal in classical communication links. This work provides a useful
introduction to the field as well as presenting new findings and perspectives
to advance it further
Manipulating Transverse Modes of Photons for Quantum Cryptography
Several schemes have been proposed to extend Quantum Key Distribution
protocols aiming at improving their security or at providing new physical
substrates for qubit implementation. We present a toolbox to jointly create,
manipulate and measure qubits stored in polarization and transverse-modes
degrees of freedom of single photons. The toolbox includes local operations on
single qubits, controlled operations between the two qubits and projective
measurements over a wide variety of non-local bases in the four dimensional
space of states. We describe how to implement the toolbox to perform an
extended version of the BB84 protocol for this Hilbert space (ideally
transmitting two key bits per photon). We present the experimental
implementation of the measurement scheme both in the regimes of intense light
beams and with single photons. Thus, we show the feasibility of implementing
the protocol providing an interesting example of a new method for quantum
information processing using the polarization and transverse modes of light as
qubits.Comment: 9 pages, 7 figures, 5 table
Hyperbolic planforms in relation to visual edges and textures perception
We propose to use bifurcation theory and pattern formation as theoretical
probes for various hypotheses about the neural organization of the brain. This
allows us to make predictions about the kinds of patterns that should be
observed in the activity of real brains through, e.g. optical imaging, and
opens the door to the design of experiments to test these hypotheses. We study
the specific problem of visual edges and textures perception and suggest that
these features may be represented at the population level in the visual cortex
as a specific second-order tensor, the structure tensor, perhaps within a
hypercolumn. We then extend the classical ring model to this case and show that
its natural framework is the non-Euclidean hyperbolic geometry. This brings in
the beautiful structure of its group of isometries and certain of its subgroups
which have a direct interpretation in terms of the organization of the neural
populations that are assumed to encode the structure tensor. By studying the
bifurcations of the solutions of the structure tensor equations, the analog of
the classical Wilson and Cowan equations, under the assumption of invariance
with respect to the action of these subgroups, we predict the appearance of
characteristic patterns. These patterns can be described by what we call
hyperbolic or H-planforms that are reminiscent of Euclidean planar waves and of
the planforms that were used in [1, 2] to account for some visual
hallucinations. If these patterns could be observed through brain imaging
techniques they would reveal the built-in or acquired invariance of the neural
organization to the action of the corresponding subgroups.Comment: 34 pages, 11 figures, 2 table
Optical Quantum Computing
In 2001 all-optical quantum computing became feasible with the discovery that
scalable quantum computing is possible using only single photon sources, linear
optical elements, and single photon detectors. Although it was in principle
scalable, the massive resource overhead made the scheme practically daunting.
However, several simplifications were followed by proof-of-principle
demonstrations, and recent approaches based on cluster states or error encoding
have dramatically reduced this worrying resource overhead, making an
all-optical architecture a serious contender for the ultimate goal of a
large-scale quantum computer. Key challenges will be the realization of
high-efficiency sources of indistinguishable single photons, low-loss, scalable
optical circuits, high efficiency single photon detectors, and low-loss
interfacing of these components.Comment: 5 pages, 4 figure
Classical entanglement: Oxymoron or resource?
In this work we review and further develop the controversial concept of
"classical entanglement" in optical beams. We present a unified theory for
different kinds of light beams exhibiting classical entanglement and we
indicate several possible extensions of the concept. Our results shed new light
upon the physics at the debated border between the classical and the quantum
representations of the world.Comment: 9 pages, 6 figures. Version submitted to PR
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