We measure the transverse entanglement of photon pairs on their propagation from the near to the far field of spontaneous parametric down-conversion (SPDC). The Fedorov ratio, depending on the widths of conditional and unconditional intensity measurements, is shown to be only able to characterize entanglement in the near and far field zones of the source. Therefore we also follow a different approach. By evaluating the first-order coherence of a subsystem of the state we can quantify its entanglement. Unlike previous measurements, which determine the Fedorov ratio via intensity correlations, our setup is sensitive to both phase and modulus of the biphoton state and thus always grants experimental access to the full transverse entanglement of the SPDC state. It is shown theoretically that this scheme represents a direct measurement of the Schmidt number.
Nonclassical features of the polarization quasiprobability distribution
Polarization quasiprobability distribution is defined in the space of the Stokes observables. It can be reconstructed with the help of polarization quantum tomography and provides a full description of the so-called polarization sector of quantum states of light. We show here that due to its definition in terms of the discrete-valued Stokes operators, polarization quasiprobability distribution has singularities at integer values of the Stokes observables and takes negative values even for the quantum states typically considered as "classical" ones. In experiments with "bright" multiphoton states, the photon-number resolution is smeared due to the photodetectors' technical limitations. In this case, nonclassical features of the explored quantum states can be revealed by adding a strong coherent beam into the orthogonal polarization.
Macroscopic Hong-Ou-Mandel interference
T. Sh Iskhakov,
K. Yu Spasibko,
M. V. Chekhova,
G. Leuchs
We report on a Hong-Ou-Mandel interference experiment for twin beams with photon numbers per mode as large as 10(6) generated via high-gain parametric down conversion (PDC). The standard technique of coincidence counting leads in this case to a dip with a very low visibility. By measuring, instead of coincidence counting rate, the variance of the photon-number difference, we observe an extremely well-pronounced peak. From the shape of the peak, one can infer information about the spectral properties of the PDC radiation, including the number of frequency/temporal modes.
Multiphoton nonclassical correlations in entangled squeezed vacuum
states
Bhaskar Kanseri,
Timur Iskhakov,
Georgy Rytikov,
Maria Chekhova,
Gerd Leuchs
Photon-number correlation measurements are performed on bright squeezed vacuum states using a standard Bell-test setup, and quantum correlations are observed for conjugate polarization-frequency modes. We further test the entanglement witnesses for these states and demonstrate the violation of the separability criteria, which infers that all of the macroscopic Bell states, containing typically 10(6) photons per pulse, are polarization entangled. The study also reveals the symmetry of macroscopic Bell states with respect to local polarization transformations. DOI: 10.1103/PhysRevA.87.032110
A versatile source of single photons for quantum information processing
Michael Foertsch,
Josef U. Fuerst,
Christoffer Wittmann,
Dmitry Strekalov,
Andrea Aiello,
Maria V. Chekhova,
Christine Silberhorn,
Gerd Leuchs,
Christoph Marquardt
The generation of high-quality single-photon states with controllable narrow spectral bandwidths and central frequencies is key to facilitate efficient coupling of any atomic system to non-classical light fields. Such an interaction is essential in numerous experiments for fundamental science and applications in quantum communication and information processing, as well as in quantum metrology. Here we implement a fully tunable, narrow-band and efficient single-photon source based on a whispering gallery mode resonator. Our disk-shaped, monolithic and intrinsically stable resonator is made of lithium niobate and supports a cavity-assisted spontaneous parametric down-conversion process. The generated photon pairs are emitted into two highly tunable resonator modes. We verify wavelength tuning over 100 nm of both modes with controllable bandwidth between 7.2 and 13 MHz. Heralding of single photons yields anti-bunching with g((2))(0) < 0.2.
The Schmidt modes of biphoton qutrits: Poincare-sphere representation
M. V. Chekhova,
M. V. Fedorov
JOURNAL OF PHYSICS B-ATOMIC MOLECULAR AND OPTICAL PHYSICS
46
(9)
095502
(2013)
| Journal
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 Poincare 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.
Compensation of anisotropy effects in a nonlinear crystal for squeezed
vacuum generation
A. M. Perez,
F. Just,
A. Cavanna,
M. V. Chekhova,
G. Leuchs
Squeezed vacuum 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 spatial spectrum of the generated radiation is affected by the transverse walk-off. In this work we describe a method that reduces the spatial distortions, by using two consecutive crystals instead of one. We show that after anisotropy compensation the two-photon amplitude becomes symmetric, allowing for a simple Schmidt expansion, a procedure that in practice requires states that come from experimental systems free of anisotropy effects. Qualitative experimental observations are made for the case of high-gain PDC.
Polarization assisted fast data encoding and transmission using
coherence based spectral anomalies
Two methods for fast information encoding and free space communication are proposed, which are based on the rapid transitions in coherence-based (spatial and temporal) spectral anomalies called 'spectral switches'. The information (data bits) could be encoded in terms of red and blue shifts in the source spectrum. The encoding process itself could be made fast by polarization assisted switching of spectral anomalies using a polarization selective device such as an electro-optic modulator. The advantages and limitations of this polarization based data processing mechanism are also discussed.
Kontakt
Forschungsgruppe Maria Chekhova
Max-Planck-Institut für die Physik des Lichts Staudtstr. 2 91058 Erlangen
