type-I Spontaneous Parametric Down-conversion Research Articles

High-dimensional quantum correlation measurements with an adaptively gated hybrid single-photon camera

Efficient measurement of high-dimensional quantum correlations, especially spatial ones, is essential for quantum technologies. We propose and demonstrate an adaptively gated hybrid intensified camera (HIC) that combines the information from a high spatial resolution sensor and a high temporal resolution detector, offering precise control over the number of photons detected within each frame. The HIC facilitates spatially resolved single-photon counting measurements. We study the measurement of momentum correlations of photon pairs generated in type-I spontaneous parametric downconversion with the HIC and demonstrate the possibility of time-tagging the registered photons. With a spatial resolution of multi-megapixels and nanosecond temporal resolution, this system allows for the realization of previously infeasible quantum optics experiments.

First-principles screening of metal–organic frameworks for entangled photon pair generation

The transmission of strong laser light in nonlinear optical materials can generate output photons sources that carry quantum entanglement in multiple degrees of freedom, making this process a fundamentally important tool in optical quantum technology. However, the availability of efficient optical crystals for entangled light generation is severely limited in terms of diversity, thus reducing the prospects for the implementation of next-generation protocols in quantum sensing, communication and computing. To overcome this, we developed and implemented a multi-scale first-principles modeling technique for the computational discovery of novel nonlinear optical devices based on metal–organic framework (MOF) materials that can efficiently generate entangled light via spontaneous parametric down-conversion (SPDC). Using collinear degenerate type-I SPDC as a case study, we computationally screen a database of 114 373 synthesized MOF materials to establish correlations between the structure and chemical composition of MOFs with the brightness and coherence properties of entangled photon pairs. We identify a subset of 49 non-centrosymmetric mono-ligand MOF crystals with high chemical and optical stability that produce entangled photon pairs with intrinsic G(2) correlation times τc∼10−30 fs and pair generation rates in the range 104−1010s−1mW−1mm−1 at 1064 nm. Conditions for optimal type-I phase matching are given for each MOF and relationships between pair brightness, crystal band gap and optical birefringence are discussed. Correlations between the optical properties of crystals and their constituent molecular ligands are also given. Our work paves the way for the computational design of MOF-based devices for optical quantum technology.

Bright narrowband photon pairs generated in an intra-cavity 2N walk-off compensating configuration

We studied theoretically the influence of the walk-off effect on the spectral brightness of the type-I cavity-enhanced spontaneous parametric down-conversion. The results show that the spectral brightness of the generated entangled photon pairs can be improved significantly by using a 2N walk-off compensating configuration, in which N pairs of nonlinear crystals with their walk-off angles of opposite signs are stacked and bonded in sequence along the propagation direction. The spectral brightness of the entangled photon pairs are expected theoretically to be 1.27×105 pairs/(smWMHz) for N = 4 with a submegahertz bandwidth in a triple-resonant cavity configuration. Our results provide a solid theoretical basis for the generation of entangled photon-pair sources of high spectral brightness and narrow bandwidth, which may have potential applications in quantum technology.

Measurement of two-photon position–momentum Einstein–Podolsky–Rosen correlations through single-photon intensity measurements

The measurement of the position–momentum Einstein–Podolsky–Rosen (EPR) correlations of a two-photon state is important for many quantum information applications ranging from quantum key distribution to coincidence imaging. However, all the existing techniques for measuring the position–momentum EPR correlations involve coincidence detection and thus suffer from issues that result in less accurate measurements. In this article, we propose and demonstrate an experimental scheme that does not require coincidence detection for measuring the EPR correlations. Our technique works for two-photon states that are pure, irrespective of whether the state is separable or entangled. We theoretically show that if the pure two-photon state satisfies a certain set of conditions then the position–momentum EPR correlations can be obtained by doing the intensity measurements on only one of the photons. We experimentally demonstrate this technique for pure two-photon states produced by type-I spontaneous parametric down-conversion, and to the best of our knowledge, we report the most accurate experimental measurement of position–momentum EPR correlations.

High-precision quantum transmittometry of DNA and methylene-blue using a frequency-entangled twin-photon beam in type-I SPDC

Using the coincidence-count (CC) measurement of the generated frequency-entangled twin-photon beam (TWB) via the process of type-I spontaneous parametric-down conversion (SPDC) in BBO nonlinear crystal (NLC), we have precisely measured the transmittance of very diluted rabbit- and human-DNA, methylene-blue (MB) as a disinfectant, and a thin-film multilayer at near IR wavelength 810nm with an accuracy in order of %0.01 due to the quantum correlation, while the accuracy of classical-like measurement, single-count (SC), is in order of %0.1 in our setup. Moreover, using quantum measurement of the transmittance, the different types of DNA with the same concentration, and also very diluted (in order of pg/μl) different concentrations of DNA and MB solutions, are distinguished and detected with high-reliability. Interestingly, in the case of human-DNA samples in contrast to our classical-like measurement, we could precisely detect and distinguish two very diluted concentrations 0.01ng/μl and 0.1ng/μl with high reliability while the commercial standard spectrometer device of our DNA-manufacturer could neither detect nor distinguish them. Surprisingly, measurement on the thin-film multilayer illustrates that the introduced method in this work might be performed to cancer/brain tissues or stem cells for cancer therapy and may hopefully open a pave and platform for non-invasive quantum diagnosis in the future.

Gouy phase of type-I SPDC-generated biphotons

We consider a double Gaussian approximation to describe the wavefunction of twin photons (also called a biphoton) created in a nonlinear crystal via a type-I spontaneous parametric downconversion (SPDC) process. We find that the wavefunction develops a Gouy phase while it propagates, being dependent of the two-photon correlation through the Rayleigh length. We evaluate the covariance matrix and show that the logarithmic negativity, useful in quantifying entanglement in Gaussian states, although Rayleigh-dependent, does not depend on the propagation distance. In addition, we show that the two-photon entanglement can be connected to the biphoton Gouy phase as these quantities are Rayleigh-length-related. Then, we focus the double Gaussian biphoton wavefunction using a thin lens and calculate a Gouy phase that is in reasonable agreement with the experimental data of D. Kawase et al. published in Ref. [1].

Measurements of Entangled Two-Photon Absorption in Organic Molecules with CW-Pumped Type-I Spontaneous Parametric Down-Conversion

Entangled photons exhibit strong nonclassical frequency and time correlations simultaneously, which allow them to excite and extract information about molecules in new ways compared to classical sp...

Continuous-variable Bell nonlocality with biphotons produced by spontaneous parametric down-conversion

We revisit a continuous-variable version of the Clauser-Horne-Shimony-Holt inequality with sign binning, first proposed by Bell [J. S. Bell, Speakable and Unspeakable in Quantum Mechanics, 1st ed. (Cambridge University, New York, 1987); Ann. N.Y. Acad. Sci. 480, 263 (1986)]. To explore Bell nonlocality, we use a biphoton effective wave function resulting from a type-I spontaneous parametric down-conversion process. After reviewing the approximations involved in the production and time evolution of such biphotons, we judiciously choose a series expansion for the sinc function part of the biphoton wave function in order to integrate the Wigner function. We conclude that we can qualitatively achieve Bell nonlocality within a regime in which the twin photons are described.

Direct observation of OAM correlations from spatially entangled bi-photon states.

We present spatially-resolved observations of orbital angular momentum (OAM) conservation, via a Laguerre-Gauss (LG) basis decomposition, of spatially-entangled photon pairs produced in type-I collinear spontaneous parametric downconversion (SPDC). These results were obtained with a novel detection system for OAM-entangled photon pairs that combines a projective measurement for the signal photon to a specific value of the azimuthal index ls, with a spatially-resolved measurement for the idler photon using an intensified charge coupled (ICCD) camera. In combination with far-field diffraction of the idler photon through a triangular aperture, we are able to obtain: i) the spatial structure of the heralded idler photon, as governed by the user-selected topological charge of the signal photon; ii) the OAM spectrum; and iii) the topological charge (both magnitude and sign) for the heralded idler photon.

Dispersive Broadening of Two-photon Wave Packets Generated via Type-I and Type-II Spontaneous Parametric Down-conversion

Photons generated via spontaneous parametric down-conversion (SPDC) have broad spectrums and suffer from dispersive broadening of the temporal wave packets when they are transmitted through dispersive media. In this paper we theoretically and experimentally study the detailed amount of the temporal broadening of the two-photon wave packets generated via both type-I and type-II SPDC with β-BaB2O4 of various lengths, by transmitting them through optical fibers. We interpret the results with respect to the spectral properties of the two-photon wave packets. We believe that our results will contribute to implementing protocols involving long-range distributions of photon pairs.

Proposed optical realisation of a two photon, four-qubit entangled χ state

The four-qubit states , exhibiting genuine multi-partite entanglement have been shown to have many interesting properties and have been suggested for novel applications in quantum information processing. In this work we propose a simple quantum circuit and its corresponding optical embodiment with which to prepare photon pairs in the states. Our approach uses hyper-entangled photon pairs, produced by the type-I spontaneous parametric down-conversion process in two contiguous nonlinear crystals, together with a set of simple linear-optical transformations. Our photon pairs are maximally hyper-entangled in both their polarisation and orbital angular momentum. After one of these daughter photons passes through our optical setup, we obtain photon pairs in the hyper-entangled state , and the states can be achieved by further simple transformations.

Research on the time-correlation characterisrtic of correlated photon circles generated by the type-I spontaneous parametric down-conversion

The optical radiometric calibration method based on correlated photons generated by spontaneous parametric down-conversion (SPDC) is a promising method,because it is based on the basic physical phenomena and does not need radiometric traceability.At the present,the SPDC calibration technology is applicable for the photon-counting detector with a highest calibration accuracy of 0.18%.More and more researchers are making effort to improve its calibration accuracy and expand its applicable targets to analog detectors.Correlated photon circles have the high photon rate characteristic.The photon rates of signal and idle light can be improved significantly by using correlated photon circles in calibration.It has very important significance to study the time-correlation characteristic of correlated photon circles. In this paper,the spatial distribution of emission angle of the broadband (450-1000 nm) correlated photons generated by type-I spontaneous parametric down-conversion is presented through the theoretical calculations.The ZEMAX optical software is used to design the optical system which can receive the broadband correlated photon circles and remove the pump laser which is the main source of stray light.The 355 nm wavelength laser is used to pump -barium borate (BBO) crystal to generate the broadband correlated photon circles.A low-light-level CMOS camera is used to adjust and align the opt-mechanical system.Finally,the 430 nm-860 nm correlated photon circles are received by the optical system and detected by the low-light-level CMOS camera.An experimental measurement system is established to measure the time-correlation characteristics of 685 nm and 736.89 nm correlated photon circles.Using 1.0 mW continuous-wave laser to pump BBO crystal,a coincidence peak is observed evidently.The results show that the correlated photon circles generated by type-I SPDC have the characteristics of time-correlation and high photon rates.It can be conductive to improving optical radiometric calibration accuracy based on correlated photons and promoting the development and application in analog detector calibration.

Observation of elliptical rings in type-I spontaneous parametric downconversion

We investigate the transverse spatial profile of down-converted light produced by noncollinear, degenerate, Type-I spontaneous parametric down-conversion in two types of nonlinear crystals. We find that the pattern produced by one crystal, beta barium borate (BBO), produces a circular down-conversion pattern while the other crystal, bismuth triborate (BiBO) produces an elliptical pattern. We show this difference is due to the angle-independent refractive index experienced by the daughter photons in BBO, while they experience an angle-dependent refractive index in BiBO. We image the transverse spatial profile of the generated light to determine the eccentricity produced by each crystal and develop a model to explain our observation. Among other things, this model predicts that there is a wavelength for which the eccentricity from BiBO is nearly zero. Finally, we discuss how the elliptical ring pattern produced in BiBO potentially affects polarization entanglement for experimental setups that collect biphotons around the entire down-conversion ring. We show that the quality of polarization entanglement as measured by the overlap integral of the spectrum of the two rings, can remain high (>99.4%) around the entire ring at the expense of decreased biphoton rate.

Correction to “Enhancing Heralding Efficiency and Biphoton Rate in Type-I Spontaneous Parametric Down-Conversion” [May 15 6400610

Correction to “Enhancing Heralding Efficiency and Biphoton Rate in Type-I Spontaneous Parametric Down-Conversion” [May 15 6400610

Two polarization-entangled sources from the same semiconductor chip

Generating nonclassical states of photons such as polarization entangled states on a monolithic chip is a crucial step towards practical applications of optical quantum information processing such as quantum computing and quantum key distribution. Here we demonstrate two polarization entangled photon sources in a single monolithic semiconductor waveguide. The first source is achieved through the concurrent utilization of two spontaneous parametric down-conversion (SPDC) processes, namely type-0 and type-I SPDC processes. The chip can also generate polarization entangled photons via the type-II SPDC process, enabling the generation of both co-polarized and cross-polarized entangled photons in the same device. In both cases, polarization entanglement is generated directly on the chip without the use of any off-chip compensation, interferometry or bandpass filtering. This enables direct, chip-based generation of both Bell states $(|H,H\rangle+|V,V\rangle)/\sqrt{2}$ and $(|H,V\rangle+|V,H\rangle)/\sqrt{2}$ simultaneously utilizing the same pump source. In addition, based on compound semiconductors, this chip can be directly integrated with it own pump laser. This technique ushers an era of self-contained, integrated, electrically pumped, room-temperature polarization entangled photon sources.

Diffraction of collinear correlated photon pairs by an ultrasonic wave.

The phenomenon of collinear correlated photon pair diffraction by an ultrasonic wave is investigated for Bragg incidence. A beta-barium borate crystal was used for producing collinear correlated photon pairs via type-I spontaneous parametric down-conversion. It is shown experimentally that the Bragg angle for photon pair diffraction is identical to the one corresponding to single-photon diffraction. The numbers of single photon and photon pair counts in discrete diffraction orders were measured as functions of the Raman-Nath parameter. Similarly, the number of coincidence photon counts in separate diffraction orders was also investigated. What is more, simple analytical formulas are derived which perfectly agree with the experimentally obtained data.

Enhancing Heralding Efficiency and Biphoton Rate in Type-I Spontaneous Parametric Down-Conversion

The nonlinear optical process of spontaneous parametric down-conversion (SPDC) is widely studied for applications in quantum information science due to its ability to produce two photons that can be entangled in many degrees of freedom. For applications in quantum communication, two metrics of this process are particularly important: heralding efficiency and total joint rate. Here, we derive expressions for both quantities for a variety of different beam geometries and frequencies. We pay specific attention to the spectrum of both biphotons and individual photons. We reveal the underlying mechanisms responsible for the spectral shape and show they differ for different geometries and frequencies. We then use these spectra to calculate heralding efficiency and joint count rate and examine how each of these metrics changes with different geometries, frequencies, spectral filtering, and beam parameters. Interestingly, we find very high heralding efficiencies are achievable for collinear geometries without spectral filtering, while noncollinear geometries require spectral filtering to achieve the same values. We also find that the spectrum is narrower for nondegenerate SPDC than for degenerate SPDC, leading to lower joint count rates and higher heralding efficiency in the former. In addition to the theory, we verify selected predictions with experimental results.

Spontaneous parametric down-conversion using a pulse train

The process of spontaneous parametric down-conversion (SPDC) is studied for the case in which the pump is a train of pulses. Effects of such a train of pulses (equivalent to a frequency comb) on degenerate collinear type-II SPDC and degenerate collinear type-I SPDC are analyzed using the probability of photon coincidence counts and compared with the case when the medium is pumped by a single pulse.

X-entangled biphotons: Schmidt number for 2D model

We calculate the Schmidt number for a two-dimensional model of the nonfactorable spatiotemporal wave-function of biphotons produced in type-I spontaneous parametric down-conversion with degenerate and collinear phase-matching taking into consideration a major part of the broad spectral and angular bandwidth of the down-converted light. We deduce an analytical expression for the Schmidt number as a function of the filter bandwidth in the limit of spectrally narrow pump and consider a possibility of tailoring a Gaussian model for the description of this kind of entanglement.

Monolithic Source of Photon Pairs

The creation of monolithically integratable sources of single and entangled photons is a top research priority with formidable challenges: The production, manipulation, and measurement of the photons should all occur in the same material platform, thereby fostering stability and scalability. Here we demonstrate efficient photon pair production in a semiconductor platform, gallium arsenide. Our results show type-I spontaneous parametric down-conversion of laser light from a 2.2 mm long Bragg-reflection waveguide, and we estimate its internal pair production efficiency to be 2.0×10(-8) (pairs/pump photon). This is the first time that significant pair production has been demonstrated in a structure that can be electrically self-pumped and which can form the basis for passive optical circuitry, bringing us markedly closer to complete integration of quantum optical technologies.