253 research outputs found
Classical entanglement: Oxymoron or resource?
In this work we review and further develop the controversial concept of
"classical entanglement" in optical beams. We present a unified theory for
different kinds of light beams exhibiting classical entanglement and we
indicate several possible extensions of the concept. Our results shed new light
upon the physics at the debated border between the classical and the quantum
representations of the world.Comment: 9 pages, 6 figures. Version submitted to PR
Quantum Cloning of Binary Coherent States - Optimal Transformations and Practical Limits
The notions of qubits and coherent states correspond to different physical
systems and are described by specific formalisms. Qubits are associated with a
two-dimensional Hilbert space and can be illustrated on the Bloch sphere. In
contrast, the underlying Hilbert space of coherent states is
infinite-dimensional and the states are typically represented in phase space.
For the particular case of binary coherent state alphabets these otherwise
distinct formalisms can equally be applied. We capitalize this formal
connection to analyse the properties of optimally cloned binary coherent
states. Several practical and near-optimal cloning schemes are discussed and
the associated fidelities are compared to the performance of the optimal
cloner.Comment: 12 pages, 12 figure
Transverse angular momentum of photons
We develop the quantum theory of transverse angular momentum of light beams.
The theory applies to paraxial and quasi-paraxial photon beams in vacuum, and
reproduces the known results for classical beams when applied to coherent
states of the field. Both the Poynting vector, alias the linear momentum, and
the angular momentum quantum operators of a light beam are calculated including
contributions from first-order transverse derivatives. This permits a correct
description of the energy flow in the beam and the natural emergence of both
the spin and the angular momentum of the photons. We show that for collimated
beams of light, orbital angular momentum operators do not satisfy the standard
commutation rules. Finally, we discuss the application of our theory to some
concrete cases.Comment: 10 pages, 2 figure
Trojan-horse attacks threaten the security of practical quantum cryptography
A quantum key distribution system may be probed by an eavesdropper Eve by
sending in bright light from the quantum channel and analyzing the
back-reflections. We propose and experimentally demonstrate a setup for
mounting such a Trojan-horse attack. We show it in operation against the
quantum cryptosystem Clavis2 from ID~Quantique, as a proof-of-principle. With
just a few back-reflected photons, Eve discerns Bob's secret basis choice, and
thus the raw key bit in the Scarani-Ac\'in-Ribordy-Gisin 2004 protocol, with
higher than 90% probability. This would clearly breach the security of the
cryptosystem. Unfortunately in Clavis2 Eve's bright pulses have a side effect
of causing high level of afterpulsing in Bob's single-photon detectors,
resulting in a high quantum bit error rate that effectively protects this
system from our attack. However, in a Clavis2-like system equipped with
detectors with less-noisy but realistic characteristics, an attack strategy
with positive leakage of the key would exist. We confirm this by a numerical
simulation. Both the eavesdropping setup and strategy can be generalized to
attack most of the current QKD systems, especially if they lack proper
safeguards. We also propose countermeasures to prevent such attacks.Comment: 22 pages including appendix and references, 6+2 figure
Optimal working points for continuous-variable quantum channels
The most important ability of a quantum channel is to preserve the quantum
properties of transmitted quantum states. We experimentally demonstrate a
continuous-variable system for efficient benchmarking of quantum channels. We
probe the tested quantum channels for a wide range of experimental parameters
such as amplitude, phase noise and channel lengths up to 40 km. The data is
analyzed using the framework of effective entanglement. We subsequently are
able to deduce an optimal point of operation for each quantum channel with
respect to the rate of distributed entanglement. This procedure is a promising
candidate for benchmarking quantum nodes and individual links in large quantum
networks of different physical implementations.Comment: 5 pages, 4 (colour) figures; v2 changes: Added PACS numbers,
corrections to citations/page numbers, minor rephrasin
Quantum Uncertainty in the Beam Width of Spatial Optical Modes
We theoretically investigate the quantum uncertainty in the beam width of
transverse optical modes and, for this purpose, define a corresponding quantum
operator. Single mode states are studied as well as multimode states with small
quantum noise. General relations are derived, and specific examples of
different modes and quantum states are examined. For the multimode case, we
show that the quantum uncertainty in the beam width can be completely
attributed to the amplitude quadrature uncertainty of one specific mode, which
is uniquely determined by the field under investigation. This discovery
provides us with a strategy for the reduction of the beam width noise by an
appropriate choice of the quantum state
Nonlinear and Quantum Optics with Whispering Gallery Resonators
Optical Whispering Gallery Modes (WGMs) derive their name from a famous
acoustic phenomenon of guiding a wave by a curved boundary observed nearly a
century ago. This phenomenon has a rather general nature, equally applicable to
sound and all other waves. It enables resonators of unique properties
attractive both in science and engineering. Very high quality factors of
optical WGM resonators persisting in a wide wavelength range spanning from
radio frequencies to ultraviolet light, their small mode volume, and tunable
in- and out- coupling make them exceptionally efficient for nonlinear optical
applications. Nonlinear optics facilitates interaction of photons with each
other and with other physical systems, and is of prime importance in quantum
optics. In this paper we review numerous applications of WGM resonators in
nonlinear and quantum optics. We outline the current areas of interest,
summarize progress, highlight difficulties, and discuss possible future
development trends in these areas.Comment: This is a review paper with 615 references, submitted to J. Op
Atmospheric continuous-variable quantum communication
We present a quantum communication experiment conducted over a point-to-point
free-space link of 1.6 km in urban conditions. We study atmospheric influences
on the capability of the link to act as a continuous-variable (CV) quantum
channel. Continuous polarization states (that contain the signal encoding as
well as a local oscillator in the same spatial mode) are prepared and sent over
the link in a polarization multiplexed setting. Both signal and local
oscillator undergo the same atmospheric fluctuations. These are intrinsically
auto-compensated which removes detrimental influences on the interferometric
visibility. At the receiver, we measure the Q-function and interpret the data
using the framework of effective entanglement. We compare different state
amplitudes and alphabets (two-state and four-state) and determine their optimal
working points with respect to the distributed effective entanglement. Based on
the high entanglement transmission rates achieved, our system indicates the
high potential of atmospheric links in the field of CV QKD.Comment: 13 pages, 7 figure
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