169 research outputs found
Breaking a quantum key distribution system through a timing side channel
The security of quantum key distribution relies on the validity of quantum
mechanics as a description of nature and on the non-existence of leaky degrees
of freedom in the practical implementations. We experimentally demonstrate how,
in some implementations, timing information revealed during public discussion
between the communicating parties can be used by an eavesdropper to
undetectably access a significant portion of the ``secret'' key.Comment: 6 pages, 4 figures. Added additional references and extended
analysis. Identical to published versio
Clock synchronization by remote detection of correlated photon pairs
We present an algorithm to detect the time and frequency difference of
independent clocks based on observation of time-correlated photon pairs. This
enables remote coincidence identification in entanglement-based quantum key
distribution schemes without dedicated coincidence hardware, pulsed sources
with a timing structure or very stable reference clocks. We discuss the method
for typical operating conditions, and show that the requirement in reference
clock accuracy can be relaxed by about 5 orders of magnitude in comparison with
previous schemes.Comment: 14 pages, 6 figure
Experimental Quantum Cloning of Single Photons
Although perfect copying of unknown quantum systems is forbidden by the laws
of quantum mechanics, approximate cloning is possible. A natural way of
realizing quantum cloning of photons is by stimulated emission. In this context
the fundamental quantum limit to the quality of the clones is imposed by the
unavoidable presence of spontaneous emission. In our experiment a single input
photon stimulates the emission of additional photons from a source based on
parametric down-conversion. This leads to the production of quantum clones with
near optimal fidelity. We also demonstrate universality of the copying
procedure by showing that the same fidelity is achieved for arbitrary input
states.Comment: 4 pages, 2 figure
Daylight operation of a free space, entanglement-based quantum key distribution system
Many quantum key distribution (QKD) implementations using a free space
transmission path are restricted to operation at night time in order to
distinguish the signal photons used for a secure key establishment from
background light. Here, we present a lean entanglement-based QKD system
overcoming that imitation. By implementing spectral, spatial and temporal
filtering techniques, we were able to establish a secure key continuously over
several days under varying light and weather conditions.Comment: 13 pages, 6 figure
Symmetrical clock synchronization with time-correlated photon pairs
We demonstrate a point-to-point clock synchronization protocol based on
bidirectionally exchanging photons produced in spontaneous parametric down
conversion (SPDC). The technique exploits tight timing correlations between
photon pairs to achieve a precision of 51ps in 100s with count rates of order
200s. The protocol is distance independent, secure against symmetric
delay attacks and provides a natural complement to techniques based on Global
Navigation Satellite Systems (GNSS). The protocol works with mobile parties and
can be augmented to provide authentication of the timing signal via a Bell
inequality check.Comment: 5 pages, 5 figure
A universal setup for active control of a single-photon detector
The influence of bright light on a single-photon detector has been described
in a number of recent publications. The impact on quantum key distribution
(QKD) is important, and several hacking experiments have been tailored to fully
control single-photon detectors. Special attention has been given to avoid
introducing further errors into a QKD system. We describe the design and
technical details of an apparatus which allows to attack a
quantum-cryptographic connection. This device is capable of controlling
free-space and fiber-based systems and of minimizing unwanted clicks in the
system. With different control diagrams, we are able to achieve a different
level of control. The control was initially targeted to the systems using BB84
protocol, with polarization encoding and basis switching using beamsplitters,
but could be extended to other types of systems. We further outline how to
characterize the quality of active control of single-photon detectors.Comment: 10 pages, 10 figure
Experimental violation of a spin-1 Bell inequality using maximally-entangled four-photon states
We demonstrate the first experimental violation of a spin-1 Bell inequality.
The spin-1 inequality is a calculation based on the Clauser, Horne, Shimony and
Holt formalism. For entangled spin-1 particles the maximum quantum mechanical
prediction is 2.552 as opposed to a maximum of 2, predicted using local hidden
variables. We obtained an experimental value of 2.27 using the
four-photon state generated by pulsed, type-II, stimulated parametric
down-conversion. This is a violation of the spin-1 Bell inequality by more than
13 standard deviations.Comment: 5 pages, 3 figures, Revtex4. Problem with figures resolve
Robustness of noise-present Bell's inequality violation by entangled state
The robustness of Bell's inequality (in CHSH form) violation by entangled
state in the simultaneous presence of colored and white noise in the system is
considered. A twophoton polarization state is modeled by twoparameter density
matrix. Setting parameter values one can vary the relative fraction of pure
entangled Bell's state as well as white and colored noise fractions. Bell's
operator-parameter dependence analysis is made. Computational results are
compared with experimental data [quant-ph/0511265] and with results computed
using a oneparameter density matrix [doi: 10.1103/PhysRevA.72.052112], which
one can get as a special case of the model considered in this work.Comment: 9 pages, 6 figure
- …