Spatial properties of spontaneous parametric down-conversion and their effect on induced coherence without induced emission

One-Photon Measurement of Two-Photon Entanglement

Measuring entanglement is an essential step in a wide range of applied and foundational quantum experiments. When a two-particle quantum state is not pure, standard methods to measure the entanglement require detection of both particles. We realize a conceptually new method for verifying and measuring entanglement in a class of two-part (bipartite) mixed states. Contrary to the approaches known to date, in our experiment we verify and measure entanglement in mixed quantum bipartite states by detecting only one subsystem, the other remains undetected. Only one copy of the mixed or pure state is used but that state is in a superposition of having been created in two identical sources. We show that information shared in entangled systems can be accessed through single-particle interference patterns. Our experiment enables entanglement characterization even when one of the subsystems cannot be detected, for example, when suitable detectors are not available.

Hyperspectral infrared microscopy with visible light

Hyperspectral microscopy is an imaging technique that provides spectroscopic information with high spatial resolution. When applied in the relevant wavelength region, such as in the infrared (IR), it can reveal a rich spectral fingerprint across different regions of a sample. Challenges associated with low efficiency and high cost of IR light sources and detector arrays have limited its broad adoption. Here, we introduce a new approach to IR hyperspectral microscopy, where the IR spectral map is obtained with off-the-shelf components built for visible light. The method is based on the nonlinear interference of correlated photons generated via parametric down-conversion. In this proof-of-concept we demonstrate the chemical mapping of a patterned sample, where different areas have distinctive IR spectroscopic fingerprints. The method provides a wide field of view, fast readout, and negligible heat delivered to the sample, which opens prospects for its further development for applications in material and biological studies.

Polarization effects in nonlinear interference of down-converted photons

We study polarization effects in the nonlinear interference of photons generated via frequency nondegenerate spontaneous parametric-down conversion. Signal and idler photons, which are generated in the visible and infrared (IR) range, respectively, are split into different arms of a nonlinear Michelson interferometer, and the interference pattern for signal photons is detected. Due to the effect of induced coherence, the interference pattern for the signal photons depends on the polarization rotation of idler photons, which are introduced by a birefringent sample. Based on this concept, we realize two new methods of measuring sample retardation in the IR range by using well-developed and inexpensive components for visible light. The methods' accuracy reaches specifications that are reported for industrial-grade optical elements. The developed IR polarimetry technique is relevant to material research, optical inspection, and quality control.

Direct transfer of pump amplitude to parametric down-converted photons

In general, the spatial distribution of individual photons (signal or idler) generated by spontaneous parametric down-conversion (SPDC) does not evidently show any particular spatial mode structure because of their randomness in generation and the incoherent nature. Here, we numerically showed that all individual photons generated by the SPDC process carry the transverse amplitude as that of the pump and then confirmed it experimentally. The pump amplitude is revealed in SPDC when individual photons are spatially filtered from the total SPDC distribution. This is observed simply by imaging the photons that are filtered using a minimum-sized aperture. The phase measurements showed that the observed mode distribution does not possess the transverse phase distribution as that of the pump.

Nonlinear infrared spectroscopy free from spectral selection

Infrared (IR) spectroscopy is an indispensable tool for many practical applications including material analysis and sensing. Existing IR spectroscopy techniques face challenges related to the inferior performance and the high cost of IR-grade components. Here, we develop a new method, which allows studying properties of materials in the IR range using only visible light optics and detectors. It is based on the nonlinear interference of entangled photons, generated via Spontaneous Parametric Down Conversion (SPDC). In our interferometer, the phase of the signal photon in the visible range depends on the phase of an entangled IR photon. When the IR photon is traveling through the media, its properties can be found from observations of the visible photon. We directly acquire the SPDC signal with a visible range CCD camera and use a numerical algorithm to infer the absorption coefficient and the refraction index of the sample in the IR range. Our method does not require the use of a spectrometer and a slit, thus it allows achieving higher signal-to-noise ratio than the earlier developed method.

Quantifying the momentum correlation between two light beams by detecting one

We report a measurement of the transverse momentum correlation between two photons by detecting only one of them. Our method uses two identical sources in an arrangement in which the phenomenon of induced coherence without induced emission is observed. In this way, we produce an interference pattern in the superposition of one beam from each source. We quantify the transverse momentum correlation by analyzing the visibility of this pattern. Our approach might be useful for the characterization of correlated photon pair sources and may lead to an experimental measure of continuous variable entanglement, which relies on the detection of only one of two entangled particles.

Spatial properties of twin-beam correlations at low- to high-intensity transition

It is shown that spatial correlation functions measured for correlated photon pairs at the single-photon level correspond to speckle patterns visible at high intensities. This correspondence is observed for the first time in one experimental setup by using different acquisition modes of an intensified CCD camera in low and high intensity regimes. The behavior of intensity auto- and cross-correlation functions in dependence on pump-beam parameters including power and transverse profile is investigated.

Quantum imaging of nonlocal spatial correlations induced by orbital angular momentum

Through scanned coincidence counting, we probe the quantum image produced by parametric down-conversion with a pump-beam carrying orbital angular momentum. Nonlocal spatial correlations are manifested through splitting of the coincidence spot into two.

Orbital and intrinsic angular momentum of single photons and entangled pairs of photons generated by parametric down-conversion

States of light that are simultaneously eigenstates of orbital, intrinsic, and total angular momentum are found and eigenstates describing pairs of photons from spontaneous parametric down-conversion reveal classes of photon conversion with angular momentum conservation and cases where the initial angular momentum from the pump laser beam is shared between the converted photons and the generating medium. These angular momentum conservation laws open up a wide range of applications to be explored.