Seeded and unseeded high order parametric down conversion

Spontaneous parametric down conversion (SPDC) has been one of the foremost tools in quantum optics for over five decades. Over that time it has been used to demonstrate some of the curious features that arise from quantum mechanics. Despite the success of SPDC, its higher-order analogues have never been observed, even though it has been suggested that they generate far more unique and exotic states than SPDC. An example of this is the emergence of non-Gaussian states without the need for post-selection. Here we calculate the expected rate of emission for nth-order SPDC with and without external stimulation (seeding). Focusing primarily on third-order parametric down-conversion (TOPDC), we estimate the photon detection rates in a rutile crystal, for both the unseeded and seeded regimes.Comment: 11 pages, 6 figure

Two-Color Bright Squeezed Vacuum

In a strongly pumped non-degenerate traveling-wave OPA, we produce two-color squeezed vacuum with up to millions of photons per pulse. Our approach to registering this macroscopic quantum state is direct detection of a large number of transverse and longitudinal modes, which is achieved by making the detection time and area much larger than the coherence time and area, respectively. Using this approach, we obtain a record value of twin-beam squeezing for direct detection of bright squeezed vacuum. This makes direct detection of macroscopic squeezed vacuum a practical tool for quantum information applications.Comment: 4 pages, 4 figure

Time-resolved purification of photon pairs from ultrasmall sources

Generation of entangled photons through spontaneous parametric down-conversion (SPDC) from ultrasmall sources like thin films, metasurfaces, or nanoantennas, offers unprecedented freedom in quantum state engineering. However, as the source of SPDC gets smaller, the role of photoluminescence increases, which leads to the contamination of two-photon states with thermal background. Here we propose and implement a solution to this problem: by using pulsed SPDC and time distillation, we increase the purity and the heralding efficiency of the photon pairs. In the experiment, we increase the purity of two-photon states generated in a 7 $\mu$m film of lithium niobate from 0.002 to 0.99. With the higher purity, we were able to observe and characterize different polarization states of photon pairs generated simultaneously due to relaxed phase matching. In particular, we showed the presence of orthogonally polarized photons, potentially usable for the generation of polarization entanglement

Autonomous absolute calibration of an ICCD camera in single-photon detection regime

Intensified charge coupled device (ICCD) cameras are widely used in various applications such as microscopy, astronomy, spectroscopy. Often they are used as single-photon detectors, with thresholding being an essential part of the readout. In this paper, we measure the quantum efficiency of an ICCD camera in the single-photon detection mode using the Klyshko absolute calibration technique. The quantum efficiency is obtained as a function of the threshold value and of the wavelength of the detected light. In addition, we study the homogeneity of the photon sensitivity over the camera chip area. The experiment is performed in the autonomous regime, without using any additional detectors. We therefore demonstrate the self-calibration of an ICCD camera.Comment: 8 pages, 3 figure

Transverse Entanglement of Biphotons

We measure the transverse entanglement of photon pairs on their propagation from the near to the far field of spontaneous parametric downconversion (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.Comment: 11 pages, 6 figure

Wigner Function Tomography via Optical Parametric Amplification

We demonstrate a method of Wigner function tomography that is tolerant to detection loss and noise. The method employs optical parametric amplification to reconstruct quadrature distributions from photon-number distributions measured via direct detection. By applying the method to a single mode filtered from multimode squeezed vacuum, we obtained a Wigner function showing a squeezing of $-7.5$ $\pm$ 0.4 dB and a purity of $0.91^{+0.09}_{-0.08}$ despite more than $90\%$ detection losses. The method can be applied to faint as well as bright non-Gaussian states, which are very susceptible to loss. In addition, our scheme is suitable for the tomography of spatially and temporally multimode quantum states, and being detection loss-tolerant, the method allows for the simultaneous tomography of multiple modes. This makes it a powerful tool for optical quantum information

Multi-photon nonclassical correlations in entangled squeezed vacuum states

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 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.Comment: 5 pages, 4 figure