118 research outputs found
Fiber-optic realization of anisotropic depolarizing quantum channels
We employed an electrically-driven polarization controller to implement
anisotropic depolarizing quantum channels for the polarization state of single
photons. The channels were characterized by means of ancilla-assisted quantum
process tomography using polarization-entangled photons generated in the
process of spontaneous parametric down-conversion. The demonstrated
depolarization method offers good repeatability, low cost, and compatibility
with fiber-optic setups. It does not perturb the modal structure of single
photons, and therefore can be used to verify experimentally protocols for
managing decoherence effects based on multiphoton interference.Comment: 7 pages, 6 figure
Experimental characterization of three-wave mixing in a multimode nonlinear KTiOPO_4 waveguide
We report experimental determination of the phase-matching function for
type-II three-wave mixing in a periodically poled KTiOPO_4 waveguide in the
792-815 nm spectral region. The measurement was performed by sum-frequency
generation of spectrally tuned fundamental components. Strong dependence of the
observed signal on the excited spatial modes in the waveguide has been observed
and fully interpreted. These results indicate a route to employ the waveguide
for spontaneous parametric down-conversion producing photon pairs in
well-defined spatial modes.Comment: 4 pages, 4 figure
Generation of spatially pure photon pairs in a multimode nonlinear waveguide using intermodal dispersion
We present experimental realization of type-II spontaneous parametric
down-conversion in a periodically poled potassium titanyl phosphate (KTiOPO4)
nonlinear waveguide. We demonstrate that by careful exploitation of intermodal
dispersion in the waveguide it is feasible to produce photon pairs in well
defined transverse modes without any additional spatial filtering at the
output. Spatial characteristics is verified by measurements of the M2 beam
quality factors. We also prepared a postselected polarization-entangled
two-photon state shown to violate Bell's inequality. Similar techniques based
on intermodal dispersion can be used to generate spatial entanglement and
hyperentanglement.Comment: 11 pages, 5 figures, submitted to Proceedings of Quantum
Communications and Quantum Imaging X Conference at SPIE 2012 Optics +
Photonics, San Diego, 12-16 August 201
Quantum mechanical which-way experiment with an internal degree of freedom
For a particle travelling through an interferometer, the trade-off between
the available which-way information and the interference visibility provides a
lucid manifestation of the quantum mechanical wave-particle duality. Here we
analyze this relation for a particle possessing an internal degree of freedom
such as spin. We quantify the trade-off with a general inequality that paints
an unexpectedly intricate picture of wave-particle duality when internal states
are involved. Strikingly, in some instances which-way information becomes
erased by introducing classical uncertainty in the internal degree of freedom.
Furthermore, even imperfect interference visibility measured for a suitable set
of spin preparations can be sufficient to infer absence of which-way
information. General results are illustrated with a proof-of-principle single
photon experiment.Comment: 8 pages, 3 figure
Linear optics schemes for entanglement distribution with realistic single-photon sources
We study the operation of linear optics schemes for entanglement distribution
based on nonlocal photon subtraction when input states, produced by imperfect
single-photon sources, exhibit both vacuum and multiphoton contributions. Two
models for realistic photon statistics with radically different properties of
the multiphoton "tail" are considered. The first model assumes occasional
emission of double photons and linear attenuation, while the second one is
motivated by heralded sources utilizing spontaneous parametric down-conversion.
We find conditions for the photon statistics that guarantee generation of
entanglement in the relevant qubit subspaces and compare it with classicality
criteria. We also quantify the amount of entanglement that can be produced with
imperfect single-photon sources, optimized over setup parameters, using as a
measure entanglement of formation. Finally, we discuss verification of the
generated entanglement by testing Bell's inequalities. The analysis is carried
out for two schemes. The first one is the well-established one-photon scheme,
which produces a photon in a delocalized superposition state between two nodes,
each of them fed with one single photon at the input. As the second scheme, we
introduce and analyze a linear-optics analog of the robust scheme based on
interfering two Stokes photons emitted by atomic ensembles, which does not
require phase stability between the nodes.Comment: 12 pages, 7 figures, title change, minor corrections in the tex
High-visibility nonclassical interference of photon pairs generated in a multimode nonlinear waveguide
We report measurements of two-photon interference using a cw-pumped type-II
spontaneous parametric down-conversion source based on a multimode perodically
poled potassium titanyl phosphate waveguide. We have used the recently
demonstrated technique of controlling the spatial characteristics of the
down-conversion process via intermodal dispersion to generate photon pairs in
fundamental transverse modes, thus ensuring their spatial indistinguishability.
Good spatial overlap of photon modes within pairs has been verified using the
Hong-Ou-Mandel interferometer and the preparation of polarization entanglement
in the Shih-Alley configuration, yielding visibilities consistently above 90%.Comment: 9 pages, 6 figure
Determination of the Wigner function from photon statistics
We present an experimental realisation of the direct scheme for measuring the
Wigner function of a single quantized light mode. In this method, the Wigner
function is determined as the expectation value of the photon number parity
operator for the phase space displaced quantum state.Comment: 4 pages LaTeX, contribution to proceedings of 6th central-european
workshop on quantum optics; see also
http://www.fuw.edu.pl/~kbanasz/QOLab/ExpWigner
Experimental demonstration of entanglement-enhanced classical communication over a quantum channel with correlated noise
We present an experiment demonstrating entanglement-enhanced classical
communication capacity of a quantum channel with correlated noise. The channel
is modelled by a fiber optic link exhibiting random birefringence that
fluctuates on a time scale much longer than the temporal separation between
consecutive uses of the channel. In this setting, introducing entanglement
between two photons travelling down the fiber allows one to encode reliably up
to one bit of information into their joint polarization degree of freedom. When
no quantum correlations between two separate uses of the channel are allowed,
this capacity is reduced by a factor of more than three. We demonstrated this
effect using a fiber-coupled source of entagled photon pairs based on
spontaneous parametric down-conversion, and a linear-optics Bell state
measurement.Comment: 4 pages, 2 figures, REVTe
Dispersion-based control of modal characteristics for parametric down-conversion in a multimode waveguide
We report generation of near-infrared photon pairs in fundamental spatial
modes via type-II spontaneous parametric down-conversion in a periodically
poled potassium titanyl phosphate (KTiOPO4) nonlinear waveguide supporting
multiple transverse modes. This demonstrates experimentally a versatile scheme
for controlling the spatial characteristics of the produced nonclassical light
based on exploitation of intermodal dispersion. The down-converted photons are
characterized by the measurement of the beam quality factors in the heralded
regime.Comment: 4 pages, 3 figures. To appear in Optics Letter
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