2,064 research outputs found
Sub shot noise phase quadrature measurement of intense light beams
We present a setup to perform sub shot noise measurements of the phase
quadrature for intense pulsed light without the use of a separate local
oscillator. A Mach--Zehnder interferometer with an unbalanced arm length is
used to detect the fluctuations of the phase quadrature at a single side band
frequency. Using this setup, the non--separability of a pair of quadrature
entangled beams is demonstrated experimentally.Comment: 9 pages, 2 figures, accepted for publication in Optics Letter
Quantum key distribution using gaussian-modulated coherent states
Quantum continuous variables are being explored as an alternative means to
implement quantum key distribution, which is usually based on single photon
counting. The former approach is potentially advantageous because it should
enable higher key distribution rates. Here we propose and experimentally
demonstrate a quantum key distribution protocol based on the transmission of
gaussian-modulated coherent states (consisting of laser pulses containing a few
hundred photons) and shot-noise-limited homodyne detection; squeezed or
entangled beams are not required. Complete secret key extraction is achieved
using a reverse reconciliation technique followed by privacy amplification. The
reverse reconciliation technique is in principle secure for any value of the
line transmission, against gaussian individual attacks based on entanglement
and quantum memories. Our table-top experiment yields a net key transmission
rate of about 1.7 megabits per second for a loss-free line, and 75 kilobits per
second for a line with losses of 3.1 dB. We anticipate that the scheme should
remain effective for lines with higher losses, particularly because the present
limitations are essentially technical, so that significant margin for
improvement is available on both the hardware and software.Comment: 8 pages, 4 figure
Long-distance quantum communication with atomic ensembles and linear optics
Quantum communication holds a promise for absolutely secure transmission of
secret messages and faithful transfer of unknown quantum states. Photonic
channels appear to be very attractive for physical implementation of quantum
communication. However, due to losses and decoherence in the channel, the
communication fidelity decreases exponentially with the channel length. We
describe a scheme that allows to implement robust quantum communication over
long lossy channels. The scheme involves laser manipulation of atomic
ensembles, beam splitters, and single-photon detectors with moderate
efficiencies, and therefore well fits the status of the current experimental
technology. We show that the communication efficiency scale polynomially with
the channel length thereby facilitating scalability to very long distances.Comment: 2 tex files (Main text + Supplement), 4 figure
Entanglement Percolation in Quantum Networks
Quantum networks are composed of nodes which can send and receive quantum
states by exchanging photons. Their goal is to facilitate quantum communication
between any nodes, something which can be used to send secret messages in a
secure way, and to communicate more efficiently than in classical networks.
These goals can be achieved, for instance, via teleportation. Here we show that
the design of efficient quantum communication protocols in quantum networks
involves intriguing quantum phenomena, depending both on the way the nodes are
displayed, and the entanglement between them. These phenomena can be employed
to design protocols which overcome the exponential decrease of signals with the
number of nodes. We relate the problem of establishing maximally entangled
states between nodes to classical percolation in statistical mechanics, and
demonstrate that quantum phase transitions can be used to optimize the
operation of quantum networks.Comment: Accepted for publication in Nature Physics. This is the original
submitted versio
Experimental entanglement distillation of mesoscopic quantum states
The distribution of entangled states between distant parties in an optical
network is crucial for the successful implementation of various quantum
communication protocols such as quantum cryptography, teleportation and dense
coding [1-3]. However, owing to the unavoidable loss in any real optical
channel, the distribution of loss-intolerant entangled states is inevitably
inflicted by decoherence, which causes a degradation of the transmitted
entanglement. To combat the decoherence, entanglement distillation, which is
the process of extracting a small set of highly entangled states from a large
set of less entangled states, can be used [4-14]. Here we report on the
mesoscopic distillation of deterministically prepared entangled light pulses
that have undergone non-Gaussian noise. The entangled light pulses [15-17] are
sent through a lossy channel, where the transmission is varying in time
similarly to light propagation in the atmosphere. By employing linear optical
components and global classical communication, the entanglement is
probabilistically increased.Comment: 13 pages, 4 figures. It's the first submitted version to the Nature
Physics. The final version is already published on Nature Physics vol.4,
No.12, 919 - 923 (2008
Noiseless Linear Amplification and Distillation of Entanglement
The idea of signal amplification is ubiquitous in the control of physical
systems, and the ultimate performance limit of amplifiers is set by quantum
physics. Increasing the amplitude of an unknown quantum optical field, or more
generally any harmonic oscillator state, must introduce noise. This linear
amplification noise prevents the perfect copying of the quantum state, enforces
quantum limits on communications and metrology, and is the physical mechanism
that prevents the increase of entanglement via local operations. It is known
that non-deterministic versions of ideal cloning and local entanglement
increase (distillation) are allowed, suggesting the possibility of
non-deterministic noiseless linear amplification. Here we introduce, and
experimentally demonstrate, such a noiseless linear amplifier for
continuous-variables states of the optical field, and use it to demonstrate
entanglement distillation of field-mode entanglement. This simple but powerful
circuit can form the basis of practical devices for enhancing quantum
technologies. The idea of noiseless amplification unifies approaches to cloning
and distillation, and will find applications in quantum metrology and
communications.Comment: Submitted 10 June 200
Entangled light in transition through the generation threshold
We investigate continuous variable entangling resources on the base of
two-mode squeezing for all operational regimes of a nondegenerate optical
parametric oscillator with allowance for quantum noise of arbitrary level. The
results for the quadrature variances of a pair of generated modes are obtained
by using the exact steady-state solution of Fokker-Planck equation for the
complex P-quasiprobability distribution function. We find a simple expression
for the squeezed variances in the near-threshold range and conclude that the
maximal two-mode squeezing reaches 50% relative to the level of vacuum
fluctuations and is achieved at the pump field intensity close to the
generation threshold. The distinction between the degree of two-mode squeezing
for monostable and bistable operational regimes is cleared up.Comment: 7 pages, 4 figures; Content changed: more details added to the
discussion. To be published in Phys. Rev.
Entanglement concentration of continuous variable quantum states
We propose two probabilistic entanglement concentration schemes for a single
copy of two-mode squeezed vacuum state. The first scheme is based on the
off-resonant interaction of a Rydberg atom with the cavity field while the
second setup involves the cross Kerr interaction, auxiliary mode prepared in a
strong coherent state and a homodyne detection. We show that the
continuous-variable entanglement concentration allows us to improve the
fidelity of teleportation of coherent states.Comment: 7 pages, 7 figure
Preparation of distilled and purified continuous variable entangled states
The distribution of entangled states of light over long distances is a major
challenge in the field of quantum information. Optical losses, phase diffusion
and mixing with thermal states lead to decoherence and destroy the
non-classical states after some finite transmission-line length. Quantum
repeater protocols, which combine quantum memory, entanglement distillation and
entanglement swapping, were proposed to overcome this problem. Here we report
on the experimental demonstration of entanglement distillation in the
continuous-variable regime. Entangled states were first disturbed by random
phase fluctuations and then distilled and purified using interference on beam
splitters and homodyne detection. Measurements of covariance matrices clearly
indicate a regained strength of entanglement and purity of the distilled
states. In contrast to previous demonstrations of entanglement distillation in
the complementary discrete-variable regime, our scheme achieved the actual
preparation of the distilled states, which might therefore be used to improve
the quality of downstream applications such as quantum teleportation
Von Bezold assimilation effect reverses in stereoscopic conditions
Lightness contrast and lightness assimilation are opposite phenomena: in contrast,
grey targets appear darker when bordering bright surfaces (inducers) rather than dark ones; in
assimilation, the opposite occurs. The question is: which visual process favours the occurrence
of one phenomenon over the other? Researchers provided three answers to this question. The
first asserts that both phenomena are caused by peripheral processes; the second attributes their
occurrence to central processes; and the third claims that contrast involves central processes,
whilst assimilation involves peripheral ones. To test these hypotheses, an experiment on an IT
system equipped with goggles for stereo vision was run. Observers were asked to evaluate the
lightness of a grey target, and two variables were systematically manipulated: (i) the apparent
distance of the inducers; and (ii) brightness of the inducers. The retinal stimulation was kept
constant throughout, so that the peripheral processes remained the same. The results show that
the lightness of the target depends on both variables. As the retinal stimulation was kept constant, we
conclude that central mechanisms are involved in both lightness contrast and lightness assimilation
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