912 research outputs found
Experimental ancilla-assisted qubit transmission against correlated noise using quantum parity checking
We report the experimental demonstration of a transmission scheme of photonic
qubits over unstabilized optical fibers, which has the plug-and-play feature as
well as the ability to transmit any state of a qubit, regardless of whether it
is known, unknown, or entangled to other systems. A high fidelity to the
noiseless quantum channel was achieved by adding an ancilla photon after the
signal photon within the correlation time of the fiber noise and by performing
quantum parity checking. Simplicity, maintenance-free feature and robustness
against path-length mismatches among the nodes make our scheme suitable for
multi-user quantum communication networks.Comment: 8 pages, 4 figures; published in New J. Phys. and selected in IOP
Selec
High rate, long-distance quantum key distribution over 250km of ultra low loss fibres
We present a fully automated quantum key distribution prototype running at
625 MHz clock rate. Taking advantage of ultra low loss fibres and low-noise
superconducting detectors, we can distribute 6,000 secret bits per second over
100 km and 15 bits per second over 250km
Unusual behaviour of the ferroelectric polarization in PbTiO/SrTiO superlattices
Artificial PbTiO/SrTiO superlattices were constructed using
off-axis RF magnetron sputtering. X-ray diffraction and piezoelectric atomic
force microscopy were used to study the evolution of the ferroelectric
polarization as the ratio of PbTiO to SrTiO was changed. For
PbTiO layer thicknesses larger than the 3-unit cells SrTiO
thickness used in the structure, the polarization is found to be reduced as the
PbTiO thickness is decreased. This observation confirms the primary role
of the depolarization field in the polarization reduction in thin films. For
the samples with ratios of PbTiO to SrTiO of less than one a
surprising recovery of ferroelectricity that cannot be explained by
electrostatic considerations was observed
Robust polarization-based quantum key distribution over collective-noise channel
We present two polarization-based protocols for quantum key distribution. The
protocols encode key bits in noiseless subspaces or subsystems, and so can
function over a quantum channel subjected to an arbitrary degree of collective
noise, as occurs, for instance, due to rotation of polarizations in an optical
fiber. These protocols can be implemented using only entangled photon-pair
sources, single-photon rotations, and single-photon detectors. Thus, our
proposals offer practical and realistic alternatives to existing schemes for
quantum key distribution over optical fibers without resorting to
interferometry or two-way quantum communication, thereby circumventing,
respectively, the need for high precision timing and the threat of Trojan horse
attacks.Comment: Minor changes, added reference
Habitat requirements and ecological niche of two cryptic amphipod species at landscape and local scales
Cryptic species are phylogenetically diverged taxa that are morphologically indistinguishable
and may differ in their ecological and behavioral requirements. This may have important implications for
ecosystem services and conservation of biodiversity. We investigated whether two ecologically important
cryptic species of the freshwater amphipod Gammarus fossarum (types A and B) are associated with different
habitats. We collected data on their occurrence at both the landscape scale (large watersheds) and at the
local scale (river reach) to compare macro- and microscale environmental parameters associated with their
presence. Analysis of the landscape scale data showed that occurrence of types A and B differ with respect
to watershed and river size and, interestingly, human impact on river ecomorphology. Whereas type B was
mainly found in less forested areas with higher human impact, type A showed the opposite occurrence
pattern. Analyses of the local scale data suggested that habitats occupied by type A were characterized by
larger gravel, larger stones and less macrophytes than habitats occupied by type B. The landscape and local
data set showed contradicting patterns with regard to stream size. Overall, the observed differences between
the two types of G. fossarum most likely reflect ecological differences between them, but alternative
explanations (e.g., historical colonization processes) cannot be completely ruled out. Our study underlines
that common cryptic species can differ in their ecology and response to anthropogenic influence. Such
differences in habitat requirements among difficult-to-identify taxa present a challenge for biodiversity
and ecosystem management. Our results emphasize the importance of conservative and precautionary
approaches in maintenance of habitat diversity and environmental heterogeneity
Diodes with Breakdown Voltages Enhanced by the Metal-Insulator Transition of LaAlO-SrTiO Interfaces
Using the metal-insulator transition that takes place as a function of
carrier density at the LaAlO-SrTiO interface, oxide diodes have been
fabricated with room-temperature breakdown voltages of up to 200 V. With
applied voltage, the capacitance of the diodes changes by a factor of 150. The
diodes are robust and operate at temperatures up to 270 C
Efficient generation of tunable photon pairs at 0.8 and 1.6 micrometer
We demonstrate efficient generation of collinearly propagating, highly
nondegenerate photon pairs in a periodically-poled lithium niobate cw
parametric downconverter with an inferred pair generation rate of 1.4*10^7/s/mW
of pump power. Detection of an 800-nm signal photon triggers a
thermoelectrically-cooled 20%-efficient InGaAs avalanche photodiode for the
detection of the 1600-nm conjugate idler photon. Using single-mode fibers as
spatial mode filters, we obtain a signal-conditioned idler-detection
probability of about 3.1%.Comment: 8 pages, 3 figure
Description of a quantum convolutional code
We describe a quantum error correction scheme aimed at protecting a flow of
quantum information over long distance communication. It is largely inspired by
the theory of classical convolutional codes which are used in similar
circumstances in classical communication. The particular example shown here
uses the stabilizer formalism, which provides an explicit encoding circuit. An
associated error estimation algorithm is given explicitly and shown to provide
the most likely error over any memoryless quantum channel, while its complexity
grows only linearly with the number of encoded qubits.Comment: 4 pages, uses revtex4. Minor correction in the encoding and decoding
circuit
Experimental Quantum Cryptography with Qutrits
We produce two identical keys using, for the first time, entangled trinary
quantum systems (qutrits) for quantum key distribution. The advantage of
qutrits over the normally used binary quantum systems is an increased coding
density and a higher security margin. The qutrits are encoded into the orbital
angular momentum of photons, namely Laguerre-Gaussian modes with azimuthal
index l +1, 0 and -1, respectively. The orbital angular momentum is controlled
with phase holograms. In an Ekert-type protocol the violation of a
three-dimensional Bell inequality verifies the security of the generated keys.
A key is obtained with a qutrit error rate of approximately 10 %.Comment: New version includes additional references and a few minor changes to
the manuscrip
Experimental realization of large-alphabet quantum key distribution protocol using orbital angular momentum entanglement
We experimentally demonstrate a quantum key distribution (QKD) protocol using
photon pairs entangled in orbit angular momentum (OAM). In our protocol, Alice
and Bob modulate their OAM states on each entangled pair with spatial light
modulators (SLMs), respectively. Alice uses a fixed phase hologram in her SLM,
while Bob designs different suitable phase holograms and uses them to
represent his -based information in his SLM. With coincidences, Alice can
fully retrieve the key stream sent by Bob without information reconciliation or
privacy amplification. We report the experiment results with N=3 and the sector
states with OAM eigenmodes l=1 and l=-1. Our experiment shows that the
coincidence rates are in relatively distinct value regions for the three
different key elements. Alice could recover fully Bob's keys by the protocol.
Finally, we discuss the security of the protocol both form the light way and
against the general attacks
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