Spatiotemporal coherence properties of entangled light beams generated by parametric down-conversion

Spectral Considerations of Entangled Two-Photon Absorption Effects in Hong-Ou-Mandel Interference Experiments

Recently, different experimental methods intended to detect the entangled two-photon absorption (ETPA) phenomenon in a variety of materials have been reported. The present work explores a different approach in which the ETPA process is studied based on the changes induced in the visibility of a Hong-Ou-Mandel (HOM) interferogram. By using an organic solution of Rhodamine B as a model of nonlinear material interacting with entangled photons at ∼800 nm region produced by spontaneous parametric down-conversion (SPDC) Type-II, the conditions that make possible to detect changes in the visibility of a HOM interferogram upon ETPA are investigated. We support the discussion of our results by presenting a model in which the sample is considered as a spectral filtering function which fulfills the energy conservation conditions required by ETPA, allowing us to explain the experimental observations with good agreement. We believe that this work represents a new perspective to studying the ETPA interaction, by using an ultrasensitive quantum interference technique and a detailed mathematical model of the process.

Randomly aperiodically poled LiNbO3 crystal design by Monte Carlo-Metropolis with simulated annealing optimization for ultrabroadband photon pair generation

We report a scheme for generating ultrabroadband two-photon states by spontaneous parametric downconversion (SPDC) using randomly aperiodically poled crystals designed with an optimization algorithm based on the Monte Carlo-Metropolis method with simulated annealing. A particular SPDC source is discussed, showing results of the spectral and temporal properties of the emitted two-photon states, obtaining almost transform-limited SPDC biphoton wave packets. We also analyze the effect of fabrication errors on the SPDC.

Enhancing Entangled Two-Photon Absorption for Picosecond Quantum Spectroscopy

Entangled two-photon absorption (ETPA) is known to create photoinduced transitions with extremely low light intensity, reducing the risk of phototoxicity compared to classical two-photon absorption. Previous works have predicted the ETPA cross-section, σ_e, to vary inversely with the product of entanglement time (T_e) and entanglement area (A_e), i.e., σ_e ∼ 1/A_eT_e. The decreasing σ_e with increasing T_e has limited ETPA to fs-scale T_e, while ETPA applications for ps-scale spectroscopy have been unexplored. However, we show that spectral-spatial coupling, which reduces A_e as the SPDC bandwidth (σ_f) decreases, plays a significant role in determining σ_e when T_e > ∼100 fs. We experimentally measured σ_e for zinc tetraphenylporphyrin at several σ_f values. For type-I ETPA, σ_e increases as σ_f decreases down to 0.1 ps^-1. For type-II SPDC, σ_e is constant for a wide range of σ_f. With a theoretical analysis of the data, the maximum type-I σ_e would occur at σ_f = 0.1 ps^-1 (T_e = 10 ps). At this maximum, σ_e is 1 order of magnitude larger than fs-scale σ_e and 3 orders of magnitude larger than previous predictions of ps-scale σ_e. By utilizing this spectral-spatial coupling, narrowband type-I ETPA provides a new opportunity to increase the efficiency of measuring nonlinear optical signals and to control photochemical reactions requiring ps temporal precision.

Leaving the Limits of Linearity for Light Microscopy

Nonlinear microscopy has enabled additional modalities for chemical contrast, deep penetration into biological tissues, and the ability to collect dynamics on ultrafast timescales across heterogenous samples. The additional light fields introduced to a sample offer seemingly endless possibilities for variation to optimize and customize experimentation and the extraction of physical insight. This perspective highlights three areas of growth in this diverse field: the collection of information across multiple timescales, the selective imaging of interfacial chemistry, and the exploitation of quantum behavior for future imaging directions. Future innovations will leverage the work of the studies reviewed here as well as address the current challenges presented.

Investigations of Thienoacene Molecules for Classical and Entangled Two-Photon Absorption

Investigations of the optical effects in thienoacene chromophores with different central atoms were performed. These chromophores provide a basis for the comparison of the linear, two-photon, and entangled two-photon properties in organic molecules with varying degrees of dipolar or quadrupolar character. Linear absorption and emission as well as time-dependent density functional theory calculations were performed for the chromophores investigated. Measurements of the classical two-photon absorption (TPA), entangled two-photon absorption (ETPA), as well as entangled two-photon fluorescence were experimentally performed for the four chromophores. Electronic structure calculations were utilized to provide estimates of the classical two-photon absorption coefficients. The results of the measured entangled two-photon cross sections were compared with theoretical estimates for the molecules investigated. It is found that the dipole (transition or permanent) pathway can have an effect on the trends in the entangled photon absorption process in dipolar organic chromophores. This study helps predict the properties of the entangled two-photon effect in chromophores with different dipolar and quadrupolar character.

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.

Observation of spontaneous parametric down-conversion excited by high brightness blue LED

We report on what is to our knowledge the first observation of the parametric fluorescence in bulk nonlinear crystals excited by commercial high-brightness incoherent blue LED.

Tailoring the spectral coherence of heralded single photons

Currently, different techniques are known for synthesizing in a tunable fashion the frequency correlations of paired photons generated in spontaneous parametric downconversion (signal-idler correlations). We show that these same techniques can also be used to tune the first-order coherence properties of the signal or idler photons (signal-signal or idler-idler frequency correlations). In consequence, partially coherent radiation can be synthesized through adequate biphoton engineering. Interestingly, when the biphoton is designed to be in a separable state, the signal photons behave as a broadband first-order coherent light source.

Does pump beam intensity affect the efficiency of spontaneous parametric down conversion?

We show that, with proper modification of the pump field description from plane-wave to spherical-like, which serves to incorporate geometric effects that were discarded in the plane-wave model, the efficiency of the spontaneous parametric down conversion process exhibits dependence on pump intensity, contrary to previous theoretical predictions. Experimental investigations of this behavior are carried out under focused pumping conditions, where pump divergence is kept constant, and pairs of photons are collected behind an aperture that is large compared to the overall down-conversion profile. The resulting photon yield is found to exhibit a strong dependence on pump intensity, and an enhancement of the down-conversion efficiency is observed. The modified pump field description is also modeled numerically and calculations are presented, which are found to display good accordance with experimental observations. The enhanced yield of spontaneous parametrically down-converted photons will prove beneficial for many practical applications of entangled photon pairs where a high flux is desired.

Entangled-photon imaging of a pure phase object

We demonstrate experimentally and theoretically that a coherent image of a pure phase object [implemented by a microelectromechanical system (MEMS) micromirror array] may be obtained by use of a spatially incoherent illumination beam. This is accomplished by employing a two-beam source of entangled photons generated by spontaneous parametric down-conversion. One of the beams probes the phase object while the other is scanned. Though each of the beams is, in and of itself, spatially incoherent, the pair of beams exhibits higher-order interbeam coherence.

Maximum-likelihood image estimation using photon-correlated beams

A theory is presented addressing the fundamental limits of image estimation in a setup that uses two photon-correlated beams. These beams have the property that their photon arrivals, as a point process, are ideally synchronized in time and space. The true image represents the spatial distribution of the optical transmittance (or reflectance) of an object. In this setup, one beam is used to probe the image while the other is used as a reference providing additional information on the actual number of photons impinging on the object. This additional information is exploited to reduce the effect of quantum noise associated with the uncertainty in the number of photons per pixel. A stochastic model for the joint statistics of the two observation matrices is developed and used to obtain a local maximum-likelihood estimator of the image. The model captures the nonideal nature of the correlation between the photons of the beams by means of a simple random translation model. The mean-square error of the estimator is evaluated and compared to the corresponding conventional techniques. Conditions for the performance advantage of the proposed estimator are examined in terms of key system parameters. The theoretical predictions are demonstrated by means of simulation.

Role of entanglement in two-photon imaging

The use of entangled photons in an imaging system can exhibit effects that cannot be mimicked by any other two-photon source, whatever the strength of the correlations between the two photons. We consider a two-photon imaging system in which one photon is used to probe a remote (transmissive or scattering) object, while the other serves as a reference. We discuss the role of entanglement versus correlation in such a setting, and demonstrate that entanglement is a prerequisite for achieving distributed quantum imaging.

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.