54 research outputs found
Anisotropically high entanglement of biphotons generated in spontaneous parametric down conversion
We show that the wave packet of a biphoton generated via spontaneous
parametric down conversion is strongly anisotropic. Its anisotropic features
manifest themselves very clearly in comparison of measurements performed in two
different schemes: when the detector scanning plane is perpendicular or
parallel to the plane containing the crystal optical axis and the laser axis.
The first of these two schemes is traditional whereas the second one gives rise
to such unexpected new results as anomalously strong narrowing of the biphoton
wave packet measured in the coincidence scheme and very high degree of
entanglement. The results are predicted theoretically and confirmed
experimentally
Projective filtering of a single spatial radiation eigenmode
Lossless filtering of a single coherent (Schmidt) mode from spatially
multimode radiation is a problem crucial for optics in general and for quantum
optics in particular. It becomes especially important in the case of
nonclassical light that is fragile to optical losses. An example is bright
squeezed vacuum generated via high-gain parametric down conversion or four-wave
mixing. Its highly multiphoton and multimode structure offers a huge increase
in the information capacity provided that each mode can be addressed
separately. However, the nonclassical signature of bright squeezed vacuum,
photon-number correlations, are highly susceptible to losses. Here we
demonstrate lossless filtering of a single spatial Schmidt mode by projecting
the spatial spectrum of bright squeezed vacuum on the eigenmode of a
single-mode fiber. Moreover, we show that the first Schmidt mode can be
captured by simply maximizing the fiber-coupled intensity. Importantly, the
projection operation does not affect the targeted mode and leaves it usable for
further applications.Comment: 10 pages, 9 figure
Angular Schmidt Modes in Spontaneous Parametric Down-Conversion
We report a proof-of-principle experiment demonstrating that appropriately
chosen set of Hermite-Gaussian modes constitutes a Schmidt decomposition for
transverse momentum states of biphotons generated in the process of spontaneous
parametric down conversion. We experimentally realize projective measurements
in Schmidt basis and observe correlations between appropriate pairs of modes.
We perform tomographical state reconstruction in the Schmidt basis, by direct
measurement of single-photon density matrix eigenvalues.Comment: 5 pages, 4 figure
Learning the tensor network model of a quantum state using a few single-qubit measurements
The constantly increasing dimensionality of artificial quantum systems
demands for highly efficient methods for their characterization and
benchmarking. Conventional quantum tomography fails for larger systems due to
the exponential growth of the required number of measurements. The conceptual
solution for this dimensionality curse relies on a simple idea - a complete
description of a quantum state is excessive and can be discarded in favor of
experimentally accessible information about the system. The probably
approximately correct (PAC) learning theory has been recently successfully
applied to a problem of building accurate predictors for the measurement
outcomes using a dataset which scales only linearly with the number of qubits.
Here we present a constructive and numerically efficient protocol which learns
a tensor network model of an unknown quantum system. We discuss the limitations
and the scalability of the proposed method.Comment: 10 pages, 11 figure
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