43,389 research outputs found
Efficient separation of the orbital angular momentum eigenstates of light
Orbital angular momentum (OAM) of light is an attractive degree of freedom
for funda- mentals studies in quantum mechanics. In addition, the discrete
unbounded state-space of OAM has been used to enhance classical and quantum
communications. Unambiguous mea- surement of OAM is a key part of all such
experiments. However, state-of-the-art methods for separating single photons
carrying a large number of different OAM values are limited to a theoretical
separation efficiency of about 77 percent. Here we demonstrate a method which
uses a series of unitary optical transformations to enable the measurement of
lights OAM with an experimental separation efficiency of more than 92 percent.
Further, we demonstrate the separation of modes in the angular position basis,
which is mutually unbiased with respect to the OAM basis. The high degree of
certainty achieved by our method makes it particu- larly attractive for
enhancing the information capacity of multi-level quantum cryptography systems
Polarization-controlled evolution of light transverse modes and associated Pancharatnam geometric phase in orbital angular momentum
We present an easy, efficient and fast method to generate arbitrary linear
combinations of light orbital angular momentum eigenstates
starting from a linearly polarized TEM laser beam. The method exploits
the spin-to-orbital angular momentum conversion capability of a
liquid-crystal-based -plate and a Dove prism inserted in a Sagnac polarizing
interferometer. The nominal generation efficiency is 100\%, being limited only
by reflection and scattering losses in the optical components. When closed
paths are followed on the polarization Poincar\'{e} sphere of the input beam,
the associated Pancharatnam geometric phase is transferred unchanged to the
orbital angular momentum state of the output beam.Comment: 5 pages and 5 figure
Bits from Photons: Oversampled Image Acquisition Using Binary Poisson Statistics
We study a new image sensor that is reminiscent of traditional photographic
film. Each pixel in the sensor has a binary response, giving only a one-bit
quantized measurement of the local light intensity. To analyze its performance,
we formulate the oversampled binary sensing scheme as a parameter estimation
problem based on quantized Poisson statistics. We show that, with a
single-photon quantization threshold and large oversampling factors, the
Cram\'er-Rao lower bound (CRLB) of the estimation variance approaches that of
an ideal unquantized sensor, that is, as if there were no quantization in the
sensor measurements. Furthermore, the CRLB is shown to be asymptotically
achievable by the maximum likelihood estimator (MLE). By showing that the
log-likelihood function of our problem is concave, we guarantee the global
optimality of iterative algorithms in finding the MLE. Numerical results on
both synthetic data and images taken by a prototype sensor verify our
theoretical analysis and demonstrate the effectiveness of our image
reconstruction algorithm. They also suggest the potential application of the
oversampled binary sensing scheme in high dynamic range photography
Secure gated detection scheme for quantum cryptography
Several attacks have been proposed on quantum key distribution systems with
gated single-photon detectors. The attacks involve triggering the detectors
outside the center of the detector gate, and/or using bright illumination to
exploit classical photodiode mode of the detectors. Hence a secure detection
scheme requires two features: The detection events must take place in the
middle of the gate, and the detector must be single-photon sensitive. Here we
present a technique called bit-mapped gating, which is an elegant way to force
the detections in the middle of the detector gate by coupling detection time
and quantum bit error rate. We also discuss how to guarantee single-photon
sensitivity by directly measuring detector parameters. Bit-mapped gating also
provides a simple way to measure the detector blinding parameter in security
proofs for quantum key distribution systems with detector efficiency mismatch,
which up until now has remained a theoretical, unmeasurable quantity. Thus if
single-photon sensitivity can be guaranteed within the gates, a detection
scheme with bit-mapped gating satisfies the assumptions of the current security
proofs.Comment: 7 pages, 3 figure
Strong deflection limit of black hole gravitational lensing with arbitrary source distances
The gravitational field of supermassive black holes is able to strongly bend
light rays emitted by nearby sources. When the deflection angle exceeds ,
gravitational lensing can be analytically approximated by the so-called strong
deflection limit. In this paper we remove the conventional assumption of
sources very far from the black hole, considering the distance of the source as
an additional parameter in the lensing problem to be treated exactly. We find
expressions for critical curves, caustics and all lensing observables valid for
any position of the source up to the horizon. After analyzing the spherically
symmetric case we focus on the Kerr black hole, for which we present an
analytical 3-dimensional description of the higher order caustic tubes.Comment: 20 pages, 8 figures, appendix added. In press on Physical Review
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