305 research outputs found
On the properties of Circular-Beams
Circular-Beams were introduced as a very general solution of the paraxial
wave equation carrying Orbital Angular Momentum. Here we study their
properties, by looking at their normalization and their expansion in terms of
Laguerre-Gauss modes. We also study their far-field divergence and, for
particular cases of the beam parameters, their possible experimental
generation.Comment: 5 page
The role of beam waist in Laguerre-Gauss expansion of vortex beam
Laguerre-Gauss (LG) modes represent an orthonormal basis set of solutions of
the paraxial wave equation. LG are characterized by two integer parameters
and that are related to the radial and azimuthal profile of the beam.
The physical dimension of the mode is instead determined by the beam waist
parameter : only LG modes with the same satisfy the orthogonality
relation. Here, we derive the scalar product between two LG modes with
different beam waists and show how this result can be exploited to derive
different expansions of a generic beam in terms of LG modes. In particular, we
apply our results to the recently introduced Circular Beams, by deriving a
previously unknown expansion. We finally show how the waist parameter must be
chosen in order to optimize such expansion.Comment: 5 page
EPR-steering: closing the detection loophole with non-maximally entangled states and arbitrary low efficiency
Quantum steering inequalities allow to demonstrate the presence of
entanglement between two parties when one of the two measurement device is not
trusted. In this paper we show that quantum steering can be demonstrated for
arbitrary low detection efficiency by using two-qubit non-maximally entangled
states. Our result can have important applications in one-sided
device-independent quantum key distribution.Comment: Revtex, 5 pages, 3 figure
Strong measurements give a better direct measurement of the quantum wave function
Weak measurements have thus far been considered instrumental in the so-called
direct measurement of the quantum wavefunction [Nature (London) 474, 188
(2011)]. Here we show that direct measurement of the wavefunction can be
obtained by using measurements of arbitrary strength. In particular, in the
case of strong measurements, i.e. those in which the coupling between the
system and the measuring apparatus is maximum, we compared the precision and
the accuracy of the two methods, by showing that strong measurements outperform
weak measurements in both for arbitrary quantum states in most cases. We also
give the exact expression of the difference between the reconstructed and
original wavefunctions obtained by the weak measurement approach: this will
allow to define the range of applicability of such method.Comment: Updated version, 5 pages + Supplementary Informatio
Random bits, true and unbiased, from atmospheric turbulence
Random numbers represent a fundamental ingredient for numerical simulation,
games, informa- tion science and secure communication. Algorithmic and
deterministic generators are affected by insufficient information entropy. On
the other hand, suitable physical processes manifest intrinsic unpredictability
that may be exploited for generating genuine random numbers with an entropy
reaching the ideal limit. In this work, we present a method to extract genuine
random bits by using the atmospheric turbulence: by sending a laser beam along
a 143Km free-space link, we took advantage of the chaotic behavior of air
refractive index in the optical propagation. Random numbers are then obtained
by converting in digital units the aberrations and distortions of the received
laser wave-front. The generated numbers, obtained without any post-processing,
pass the most selective randomness tests. The core of our extracting algorithm
can be easily generalized for other physical processes
Source-Device-Independent Ultrafast Quantum Random Number Generation
Secure random numbers are a fundamental element of many applications in science, statistics, cryptography and more in general in security protocols. We present a method that enables the generation of high-speed unpredictable random numbers from the quadratures of an electromagnetic field without any assumption on the input state. The method allows us to eliminate the numbers that can be predicted due to the presence of classical and quantum side information. In particular, we introduce a procedure to estimate a bound on the conditional min-entropy based on the entropic uncertainty principle for position and momentum observables of infinite dimensional quantum systems. By the above method, we experimentally demonstrated the generation of secure true random bits at a rate greater than 1.7 Gbit/s
Loss tolerant device-independent quantum key distribution: a proof of principle
We here present the rate analysis and a proof of principle realization of a
device-independent quantum key distribution (QKD) protocol requiring the lowest
detection efficiency necessary to achieve a secure key compared to
device-independent protocols known so far. The protocol is based on
non-maximally entangled state and its experimental realization has been
performed by two-photon bipartite entangled states. The improvement with
respect to protocols involving maximally entangled states has been estimated.Comment: 8 pages, 4 figure + appendi
Extremal Quantum Correlations: Experimental Study with Two-qubit States
We explore experimentally the space of two-qubit quantum correlated mixed
states, including frontier ones as defined by the use of quantum discord and
von Neumann entropy. Our experimental setup is flexible enough to allow for the
high-quality generation of a vast variety of states. We address quantitatively
the relation between quantum discord and a recently suggested alternative
measure of quantum correlations.Comment: 5 pages, 2 figure
Source-device-independent heterodyne-based quantum random number generator at 17 Gbps
For many applications, quantum random number generation should be fast and independent from assumptions on the apparatus. Here, the authors devise and implement an approach which assumes a trusted detector but not a trusted source, and allows random bit generations at ~17 Gbps using off-the-shelf components
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