490 research outputs found
Quantum control in foundational experiments
We describe a new class of experiments designed to probe the foundations of
quantum mechanics. Using quantum controlling devices, we show how to attain a
freedom in temporal ordering of the control and detection of various phenomena.
We consider wave-particle duality in the context of quantum-controlled and the
entanglement-assisted delayed-choice experiments. Then we discuss a
quantum-controlled CHSH experiment and measurement of photon's transversal
position and momentum in a single set-up.Comment: Contribution to the Proceedings of the workshop Horizons of Quantum
Physics, Taipei, 14-18.10.2012. Published version: two new authors, modified
and streamlined presentation, new section on quantum control in complementary
position/momentum measurement
Quantum entanglement distribution with 810 nm photons through telecom fibers
We demonstrate the distribution of polarization entangled photons of
wavelength 810 nm through standard telecom fibers. This technique allows
quantum communication protocols to be performed over established fiber
infrastructure, and makes use of the smaller and better performing setups
available around 800 nm, as compared to those which use telecom wavelengths
around 1550 nm. We examine the excitation and subsequent quenching of
higher-order spatial modes in telecom fibers up to 6 km in length, and perform
a distribution of high quality entanglement (visibility 95.6%). Finally, we
demonstrate quantum key distribution using entangled 810 nm photons over a 4.4
km long installed telecom fiber link.Comment: 5 pages, 5 figures, 1 tabl
Secure quantum channels with correlated twin laser beams
This work is the development and analysis of the recently proposed quantum
cryptographic protocol, based on the use of the two-mode coherently correlated
states. The protocol is supplied with the cryptographic control procedures. The
quantum noise influence on the channel error properties is examined. State
detection features are proposed
Experimental test of nonlocal realistic theories without the rotational symmetry assumption
We analyze the class of nonlocal realistic theories that was originally
considered by Leggett [Found. Phys. 33, 1469 (2003)] and tested by us in a
recent experiment [Nature (London) 446, 871 (2007)]. We derive an
incompatibility theorem that works for finite numbers of polarizer settings and
that does not require the previously assumed rotational symmetry of the
two-particle correlation functions. The experimentally measured case involves
seven different measurement settings. Using polarization-entangled photon
pairs, we exclude this broader class of nonlocal realistic models by
experimentally violating a new Leggett-type inequality by 80 standard
deviations.Comment: Published versio
Nondegenerate parametric down conversion in coherently prepared two-level atomic gas
We describe parametric down conversion process in a two-level atomic gas,
where the atoms are in a superposition state of relevant energy levels. This
superposition results in splitting of the phase matching condition into three
different conditions. Another, more important, peculiarity of the system under
discussion is the nonsaturability of amplification coefficients with increasing
pump wave intensity, under "sideband" generation conditions
Practical Quantum Key Distribution with Polarization-Entangled Photons
We present an entangled-state quantum cryptography system that operated for
the first time in a real world application scenario. The full key generation
protocol was performed in real time between two distributed embedded hardware
devices, which were connected by 1.45 km of optical fiber, installed for this
experiment in the Vienna sewage system. The generated quantum key was
immediately handed over and used by a secure communication application.Comment: 5 pages, 3 figure
Divergent evolution of protein conformational dynamics in dihydrofolate reductase.
Molecular evolution is driven by mutations, which may affect the fitness of an organism and are then subject to natural selection or genetic drift. Analysis of primary protein sequences and tertiary structures has yielded valuable insights into the evolution of protein function, but little is known about the evolution of functional mechanisms, protein dynamics and conformational plasticity essential for activity. We characterized the atomic-level motions across divergent members of the dihydrofolate reductase (DHFR) family. Despite structural similarity, Escherichia coli and human DHFRs use different dynamic mechanisms to perform the same function, and human DHFR cannot complement DHFR-deficient E. coli cells. Identification of the primary-sequence determinants of flexibility in DHFRs from several species allowed us to propose a likely scenario for the evolution of functionally important DHFR dynamics following a pattern of divergent evolution that is tuned by cellular environment
Entanglement over global distances via quantum repeaters with satellite links
We study entanglement creation over global distances based on a quantum
repeater architecture that uses low-earth orbit satellites equipped with
entangled photon sources, as well as ground stations equipped with quantum
non-demolition detectors and quantum memories. We show that this approach
allows entanglement creation at viable rates over distances that are
inaccessible via direct transmission through optical fibers or even from very
distant satellites.Comment: 5+3 pages, 3+2 figure
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