53 research outputs found
Universal unitary gate for single-photon spinorbit four-dimensional states
The recently demonstrated possibility of entangling opposite values of the
orbital angular momentum (OAM) of a photon with its spin enables the
realization of nontrivial one-photon spinorbit four-dimensional states for
quantum information purposes. Hitherto, however, an optical device able to
perform arbitrary unitary transformations on such spinorbit photon states has
not been proposed yet. In this work we show how to realize such a ``universal
unitary gate'' device, based only on existing optical technology, and describe
its operation. Besides the quantum information field, the proposed device may
find applications wherever an efficient and convenient manipulation of the
combined OAM and spin of light is required.Comment: 7 pages, 2 figure
Photon spin-to-orbital angular momentum conversion via an electrically tunable -plate
Exploiting electro-optic effects in liquid crystals, we achieved real-time
control of the retardation of liquid- crystal-based -plates through an
externally applied voltage. The newly conceived electro-optic -plates can be
operated as electrically driven converters of photon spin into orbital angular
momentum, enabling a variation of the orbital angular momentum probabilities of
the output photons over a time scale of milliseconds.Comment: 4 pages, 5 figures, submitte
Efficient generation and control of different order orbital angular momentum states for communication links
We present a novel optical device to encode and decode two bits of
information into different Orbital Angular Momentum (OAM) states of a paraxial
optical beam. Our device generates the four angular momentum states of order
and by Spin-To-Orbital angular momentum Conversion (STOC) in a
triangular optical loop arrangement. The switching among the four OAM states is
obtained by changing the polarization state of the circulating beam by two
quarter wave plates and the two-bit information is transferred to the beam OAM
exploiting a single -plate. The polarization of the exit beam is left free
for additional one bit of information. The transmission bandwidth of the device
may be as large as several megahertz if electro-optical switches are used to
change the beam polarization. This may be particularly useful in communication
system based on light OAM.Comment: 5 pages, 5 figures, 1 table. Submitte
Reconstructing the Poynting vector skew angle and wave-front of optical vortex beams via two-channel moir\'e deflectometery
A novel approach based on the two-channel moir\'e deflectometry has been used
to measure both wave-front and transverse component of the Poynting vector of
an optical vortex beam. Generated vortex beam by the q-plate, an inhomogeneous
liquid crystal cell, has been analyzed with such technique. The measured
topological charge of generated beams are in an excellent agreement with
theoretical prediction.Comment: 3 pages, 2 figure
Polarization-controlled evolution of light transverse modes and associated Pancharatnam geometric phase in orbital angular momentum
We present an easy, efficient and fast method to generate arbitrary linear
combinations of light orbital angular momentum eigenstates
starting from a linearly polarized TEM laser beam. The method exploits
the spin-to-orbital angular momentum conversion capability of a
liquid-crystal-based -plate and a Dove prism inserted in a Sagnac polarizing
interferometer. The nominal generation efficiency is 100\%, being limited only
by reflection and scattering losses in the optical components. When closed
paths are followed on the polarization Poincar\'{e} sphere of the input beam,
the associated Pancharatnam geometric phase is transferred unchanged to the
orbital angular momentum state of the output beam.Comment: 5 pages and 5 figure
Experimental optimal cloning of four-dimensional quantum states of photons
Optimal quantum cloning is the process of making one or more copies of an
arbitrary unknown input quantum state with the highest possible fidelity. All
reported demonstrations of quantum cloning have so far been limited to copying
two-dimensional quantum states, or qubits. We report the experimental
realization of the optimal quantum cloning of four-dimensional quantum states,
or ququarts, encoded in the polarization and orbital angular momentum degrees
of freedom of photons. Our procedure, based on the symmetrization method, is
also shown to be generally applicable to quantum states of arbitrarily high
dimension -- or qudits -- and to be scalable to an arbitrary number of copies,
in all cases remaining optimal. Furthermore, we report the bosonic coalescence
of two single-particle entangled states.Comment: 5 pages, 3 figure
Experimental optical phase measurement approaching the exact Heisenberg limit
The use of quantum resources can provide measurement precision beyond the
shot-noise limit (SNL). The task of ab initio optical phase measurement---the
estimation of a completely unknown phase---has been experimentally demonstrated
with precision beyond the SNL, and even scaling like the ultimate bound, the
Heisenberg limit (HL), but with an overhead factor. However, existing
approaches have not been able---even in principle---to achieve the best
possible precision, saturating the HL exactly. Here we demonstrate a scheme to
achieve true HL phase measurement, using a combination of three techniques:
entanglement, multiple samplings of the phase shift, and adaptive measurement.
Our experimental demonstration of the scheme uses two photonic qubits, one
double passed, so that, for a successful coincidence detection, the number of
photon-passes is . We achieve a precision that is within of the HL,
surpassing the best precision theoretically achievable with simpler techniques
with . This work represents a fundamental achievement of the ultimate
limits of metrology, and the scheme can be extended to higher and other
physical systems.Comment: (12 pages, 6 figures), typos correcte
Free-space quantum key distribution by rotation-invariant twisted photons
Twisted photons are photons carrying a well-defined nonzero value of orbital
angular momentum (OAM). The associated optical wave exhibits a helical shape of
the wavefront (hence the name) and an optical vortex at the beam axis. The OAM
of light is attracting a growing interest for its potential in photonic
applications ranging from particle manipulation, microscopy and
nanotechnologies, to fundamental tests of quantum mechanics, classical data
multiplexing and quantum communication. Hitherto, however, all results obtained
with optical OAM were limited to laboratory scale. Here we report the
experimental demonstration of a link for free-space quantum communication with
OAM operating over a distance of 210 meters. Our method exploits OAM in
combination with optical polarization to encode the information in
rotation-invariant photonic states, so as to guarantee full independence of the
communication from the local reference frames of the transmitting and receiving
units. In particular, we implement quantum key distribution (QKD), a protocol
exploiting the features of quantum mechanics to guarantee unconditional
security in cryptographic communication, demonstrating error-rate performances
that are fully compatible with real-world application requirements. Our results
extend previous achievements of OAM-based quantum communication by over two
orders of magnitudes in the link scale, providing an important step forward in
achieving the vision of a worldwide quantum network
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