274 research outputs found
Single-Quadrature Continuous-Variable Quantum Key Distribution
Most continuous-variable quantum key distribution schemes are based on the
Gaussian modulation of coherent states followed by continuous quadrature
detection using homodyne detectors. In all previous schemes, the Gaussian
modulation has been carried out in conjugate quadratures thus requiring two
independent modulators for their implementations. Here, we propose and
experimentally test a largely simplified scheme in which the Gaussian
modulation is performed in a single quadrature. The scheme is shown to be
asymptotically secure against collective attacks, and considers asymmetric
preparation and excess noise. A single-quadrature modulation approach renders
the need for a costly amplitude modulator unnecessary, and thus facilitates
commercialization of continuous-variable quantum key distribution.Comment: 13 pages, 7 figure
Continuous Variable Quantum Key Distribution with a Noisy Laser
Existing experimental implementations of continuous-variable quantum key
distribution require shot-noise limited operation, achieved with shot-noise
limited lasers. However, loosening this requirement on the laser source would
allow for cheaper, potentially integrated systems. Here, we implement a
theoretically proposed prepare-and-measure continuous-variable protocol and
experimentally demonstrate the robustness of it against preparation noise
stemming for instance from technical laser noise. Provided that direct
reconciliation techniques are used in the post-processing we show that for
small distances large amounts of preparation noise can be tolerated in contrast
to reverse reconciliation where the key rate quickly drops to zero. Our
experiment thereby demonstrates that quantum key distribution with
non-shot-noise limited laser diodes might be feasible.Comment: 10 pages, 6 figures. Corrected plots for reverse reconciliatio
The Cauchy problem for Lorentzian Dirac operators under non-local boundary conditions
Non-local boundary conditions, such as the Atiyah-Patodi-Singer (APS)
conditions, for Dirac operators on Riemannian manifolds are well understood
while not much is known for such operators on spacetimes with timelike
boundary. We define a class of Lorentzian boundary conditions that are local in
time and non-local in the spatial directions and show that they lead to a
well-posed Cauchy problem for the Dirac operator. This applies in particular to
the APS conditions imposed on each level set of a given Cauchy temporal
function.Comment: 37 pages, 4 figure
MDI-QKD: Continuous- versus discrete-variables at metropolitan distances
In a comment, Xu, Curty, Qi, Qian, and Lo claimed that discrete-variable (DV)
measurement device independent (MDI) quantum key distribution (QKD) would
compete with its continuous-variable (CV) counterpart at metropolitan
distances. Actually, Xu et al.'s analysis supports exactly the opposite by
showing that the experimental rate of our CV protocol (achieved with practical
room-temperature devices) remains one order of magnitude higher than their
purely-numerical and over-optimistic extrapolation for qubits, based on
nearly-ideal parameters and cryogenic detectors (unsuitable solutions for a
realistic metropolitan network, which is expected to run on cheap
room-temperature devices, potentially even mobile). The experimental rate of
our protocol (expressed as bits per relay use) is confirmed to be two-three
orders of magnitude higher than the rate of any realistic simulation of
practical DV-MDI-QKD over short-medium distances. Of course this does not mean
that DV-MDI-QKD networks should not be investigated or built, but increasing
their rate is a non-trivial practical problem clearly beyond the analysis of Xu
et al. Finally, in order to clarify the facts, we also refute a series of
incorrect arguments against CV-MDI-QKD and, more generally, CV-QKD, which were
made by Xu et al. with the goal of supporting their thesis.Comment: Updated reply to Xu, Curty, Qi, Qian and Lo (arXiv:1506.04819),
including a point-to-point rebuttal of their new "Appendix E: Addendum
Quantum cryptography with an ideal local relay
We consider two remote parties connected to a relay by two quantum channels.
To generate a secret key, they transmit coherent states to the relay, where the
states are subject to a continuous-variable (CV) Bell detection. We study the
ideal case where Alice's channel is lossless, i.e., the relay is locally
situated in her lab and the Bell detection is performed with unit efficiency.
This configuration allows us to explore the optimal performances achievable by
CV measurement-device-independent (MDI) quantum key distribution (QKD). This
corresponds to the limit of a trusted local relay, where the detection loss can
be re-scaled. Our theoretical analysis is confirmed by an experimental
simulation where 10^-4 secret bits per use can potentially be distributed at
170km assuming ideal reconciliation.Comment: in Proceedings of the SPIE Security + Defence 2015 conference on
Quantum Information Science and Technology, Toulouse, France (21-24 September
2015) - Paper 9648-4
Continuous-variable quantum computing on encrypted data
The ability to perform computations on encrypted data is a powerful tool for
protecting a client's privacy, especially in today's era of cloud and
distributed computing. In terms of privacy, the best solutions that classical
techniques can achieve are unfortunately not unconditionally secure in the
sense that they are dependent on a hacker's computational power. Here we
theoretically investigate, and experimentally demonstrate with Gaussian
displacement and squeezing operations, a quantum solution that achieves the
unconditional security of a user's privacy using the practical technology of
continuous variables. We demonstrate losses of up to 10 km both ways between
the client and the server and show that security can still be achieved. Our
approach offers a number of practical benefits, which can ultimately allow for
the potential widespread adoption of this quantum technology in future
cloud-based computing networks.Comment: Main text (6 pages) plus Appendices (14 pages), 13 figure
An Evaluation of Moreau’s Time-Stepping Scheme for the Simulation of a Legged Robot
International audienceA state-of-the-art simulation technique that solves the equations of motion together with the set-valued contact and impulse laws by the time-stepping scheme of Moreau is introduced to the legged robotics community. An analysis is given that shows which of the many variations of the method fits best to legged robots. Two different methods to solve the discretized normal cone inclusions are compared: the projected over-relaxed Jacobi and Gauss-Seidel iteration. The methods are evaluated for an electrically-driven quadrupedal robot in terms of robustness, accuracy, speed and ease of use. Furthermore, the dependence of the simulation speed on the choice of the generalized coordinates is examined. The proposed technique is implemented in C++ and compared to a fast and simple approach based on compliant contact models. In conclusion, the introduced method with hard contacts is very beneficial for the simulation of legged robots
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