1,255 research outputs found
Testing nonlocality over 12.4 km of underground fiber with universal time-bin qubit analyzers
We experimentally demonstrate that the nonlocal nature of time-bin entangled
photonic qubits persists when one or two qubits of the pair are converted to
polarization qubits. This is possible by implementing a novel Universal
Time-Bin Qubit Analyzer (UTBA), which, for the first time, allows analyzing
time-bin qubits in any basis. We reveal the nonlocal nature of the emitted
light by violating the Clauser-Horne-Shimony-Holt inequality with measurement
bases exploring all the dimensions of the Bloch sphere. Moreover, we conducted
experiments where one qubit is transmitted over a 12.4 km underground fiber
link and demonstrate the suitability of our scheme for use in a real-world
setting. The resulting entanglement can also be interpreted as hybrid
entanglement between different types of degrees of freedom of two physical
systems, which could prove useful in large scale, heterogeneous quantum
networks. This work opens new possibilities for testing nonlocality and for
implementing new quantum communication protocols with time-bin entanglement.Comment: 6 pages, 5 figure
Quantum memory for non-stationary light fields based on controlled reversible inhomogeneous broadening
We propose a new method for efficient storage and recall of non-stationary
light fields, e.g. single photon time-bin qubits, in optically dense atomic
ensembles. Our approach to quantum memory is based on controlled, reversible,
inhomogeneous broadening. We briefly discuss experimental realizations of our
proposal.Comment: 4 page
Non-local two-photon correlations using interferometers physically separated by 35 meters
An experimental demonstration of quantum correlations is presented. Energy
and time entangled photons at wavelengths of 704 and 1310 nm are produced by
parametric downconversion in KNbO3 and are sent through optical fibers into a
bulk-optical (704 nm) and an all-fiber Michelson-interferometer (1310 nm),
respectively. The two interferometers are located 35 meters aside from one
another. Using Faraday-mirrors in the fiber-interferometer, all birefringence
effects in the fibers are automatically compensated. We obtained two-photon
fringe visibilities of up to 95 % from which one can project a violation of
Bell's inequality by 8 standard deviations. The good performance and the
auto-aligning feature of Faraday-mirror interferometers show their potential
for a future test of Bell's inequalities in order to examine
quantum-correlations over long distances.Comment: 9 pages including 3 postscript figures, to be published in Europhys.
Let
Quantum Cryptography using entangled photons in energy-time Bell states
We present a setup for quantum cryptography based on photon pairs in
energy-time Bell states and show its feasability in a laboratory experiment.
Our scheme combines the advantages of using photon pairs instead of faint laser
pulses and the possibility to preserve energy-time entanglement over long
distances. Moreover, using 4-dimensional energy-time states, no fast random
change of bases is required in our setup : Nature itself decides whether to
measure in the energy or in the time base.Comment: 4 pages including 2 figure
Security of Quantum Key Distribution with entangled quNits
We consider a generalisation of Ekert's entanglement-based quantum
cryptographic protocol where qubits are replaced by quits (i.e.,
N-dimensional systems). In order to study its robustness against optimal
incoherent attacks, we derive the information gained by a potential
eavesdropper during a cloning-based individual attack. In doing so, we
generalize Cerf's formalism for cloning machines and establish the form of the
most general cloning machine that respects all the symmetries of the problem.
We obtain an upper bound on the error rate that guarantees the confidentiality
of quNit generalisations of the Ekert's protocol for qubits.Comment: 15 pages, equation 15 and conclusions corrected the 14th of April
2003, new results adde
An experimental test of all theories with predictive power beyond quantum theory
According to quantum theory, the outcomes of future measurements cannot (in
general) be predicted with certainty. In some cases, even with a complete
physical description of the system to be measured and the measurement
apparatus, the outcomes of certain measurements are completely random. This
raises the question, originating in the paper by Einstein, Podolsky and Rosen,
of whether quantum mechanics is the optimal way to predict measurement
outcomes. Established arguments and experimental tests exclude a few specific
alternative models. Here, we provide a complete answer to the above question,
refuting any alternative theory with significantly more predictive power than
quantum theory. More precisely, we perform various measurements on distant
entangled photons, and, under the assumption that these measurements are chosen
freely, we give an upper bound on how well any alternative theory could predict
their outcomes. In particular, in the case where quantum mechanics predicts two
equally likely outcomes, our results are incompatible with any theory in which
the probability of a prediction is increased by more than ~0.19. Hence, we can
immediately refute any already considered or yet-to-be-proposed alternative
model with more predictive power than this.Comment: 13 pages, 4 figure
Counter-Intuitive Vacuum-Stimulated Raman Scattering
Vacuum-stimulated Raman scattering in strongly coupled atom-cavity systems
allows one to generate free-running single photon pulses on demand. Most
properties of the emitted photons are well defined, provided spontaneous
emission processes do not contribute. Therefore, electronic excitation of the
atom must not occur, which is assured for a system adiabatically following a
dark state during the photon-generation process. We experimentally investigate
the conditions that must be met for adiabatic following in a time-of-flight
driven system, with atoms passing through a cavity and a pump beam oriented
transverse to the cavity axis. From our results, we infer the optimal intensity
and relative pump-beam position with respect to the cavity axis.Comment: 4 pages, 4 figure
Lifting Bell inequalities
A Bell inequality defined for a specific experimental configuration can always be extended to a situation involving more observers, measurement settings, or measurement outcomes. In this article, such "liftings" of Bell inequalities are studied. It is shown that if the original inequality defines a facet of the polytope of local joint outcome probabilities then the lifted one also defines a facet of the more complex polytope
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