252 research outputs found
The Schmidt modes of biphoton qutrits: Poincare-sphere representation
For a general-form polarization biphoton qutrit, physically corresponding to
a pair of arbitrarily polarized photons in a single frequency and wavevector
mode, we explicitly find polarization Schmidt modes. A simple method is
suggested for factorizing the state vector and the explicit expressions for the
factorizing photon creation operators are found. The degrees of entanglement
and polarization of a qutrit are shown to depend directly on the commutation
features of the factorizing operators. Clear graphic representations for the
Stokes vectors of the qutrit state as a whole, its Schmidt modes, and
factorizing single-photon creation operators are given, based on the Poincar\'e
sphere. An experimental scheme is proposed for measuring the parameters of the
Schmidt decomposition as well as for demonstrating the operational meaning of
qutrit entanglement.Comment: 20 pages, 3 figure
Accessing higher order correlations by time-multiplexing
We experimentally measured higher order normalized correlation functions
(nCF) of pulsed light with a time-multiplexing-detector. We demonstrate
excellent performance of our device by verifying unity valued nCF up to the
eighth order for coherent light, and factorial dependence of the nCF for
pseudothermal light. We applied our measurement technique to a type-II
parametric downconversion source to investigate mutual two-mode correlation
properties and ascertain nonclassicality.Comment: 5 pages, 3 figure
Effects of anisotropy in a nonlinear crystal for squeezed vacuum generation
Squeezed vacuum (SV) can be obtained by an optical parametric amplifier (OPA)
with the quantum vacuum state at the input. We are interested in a degenerate
type-I OPA based on parametric down-conversion (PDC) where due to phase
matching requirements, an extraordinary polarized pump must impinge onto a
birefringent crystal with a large \chi(2) nonlinearity. As a consequence of the
optical anisotropy of the medium, the direction of propagation of the pump
wavevector does not coincide with the direction of propagation of its energy,
an effect known as transverse walk-off. For certain pump sizes and crystal
lengths, the transverse walk-off has a strong influence on the spatial spectrum
of the generated radiation, which in turn affects the outcome of any experiment
in which this radiation is employed. In this work we propose a method that
reduces the distortions of the two-photon amplitude (TPA) of the states
considered, by using at least two consecutive crystals instead of one. We show
that after anisotropy compensation the TPA becomes symmetric, allowing for a
simple Schmidt expansion, a procedure that in practice requires states that
come from experimental systems free of anisotropy effects
Testing Ultrafast Two-Photon Spectral Amplitudes via Optical Fibres
We test two-dimensional TPSA of biphoton light emitted via ultrafast
spontaneous parametric down-conversion (SPDC) using the effect of
group-velocity dispersion in optical fibres. Further, we apply this technique
to demonstrate the engineering of biphoton spectral properties by acting on the
pump pulse shape
Compensation of anisotropy effects in the generation of two-photon light
We analyse a method to compensate for anisotropy effects in the spatial
distribution of parametric down-conversion (PDC) radiation in bulk crystals. In
this method, a single nonlinear crystal is replaced by two consecutive crystals
with opposite transverse walk-off directions. We implement a simple numerical
model to calculate the spatial distribution of intensity and correlations, as
well as the Schmidt mode structure, with an account for the anisotropy.
Experimental results are presented which prove the validity of both the model
and the method.Comment: 9 pages, 8 figure
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