Parametric Down-conversion Research Articles

Intensified Tpx3Cam, a fast data-driven optical camera with nanosecond timing resolution for single photon detection in quantum applications

We describe a fast data-driven optical camera, Tpx3Cam, with nanosecond scale timing resolution and 80 Mpixel/sec throughput. After the addition of intensifier, the camera is single photon sensitive with quantum efficiency determined primarily by the intensifier photocathode. The single photon performance of the camera was characterized with results on the gain, timing resolution and afterpulsing reported here. The intensified camera was successfully used for measurements in a variety of applications including quantum applications. As an example of such application, which requires simultaneous detection of multiple photons, we describe registration of photon pairs from the spontaneous parametric down-conversion source in a spectrometer. We measured the photon wavelength and timing with respective precisions of 0.15 nm and 3 ns, and also demonstrated that the two photons are anti-correlated in energy.

Mode-dispersion phase matching single photon source based on thin-film lithium niobate

In the domain of integrated quantum photonics, the burgeoning superiority of lithium niobate’s second-order nonlinearity in electro-optic modulation makes thin-film lithium niobate a leading quantum photonic platform after silicon. To date, single-photon sources using thin-film lithium niobate has mainly adopted periodic polarization quasi-phase matching technology, which requires the preparation of complex electrodes for domain inversion in the waveguide to realize quasi-phase matching. This method inevitably introduces complexity, such as complex processing methods, enlarged polarization regions, and compromised integration density. With the development of quantum information technology, the ever-increasing degree of integration constantly creates new demands. Consequently, the development of a streamlined, high-efficiency quantum light source on a lithium niobate platform is a pressing issue. In this study, we propose a novel thin-film lithium niobate parametric down-conversion single-photon source based on mode dispersion phase matching theory. The strategy is different from conventional strategies that utilize periodic polarization to generate single-photon sources in thin-film lithium niobate devices. In contrast to traditional quasi-phase matching techniques that utilize the phase matching between pump fundamental mode light and parametric fundamental mode light, our method employs the phase matching between the pump light’s higher-order mode and the parametric light’s fundamental mode. The pump light’s higher-order mode is obtained by designing an asymmetric directional coupler. The device’s single-photon yield can attain <inline-formula><tex-math id="M2">\begin{document}$3.8\times10^{7}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20230743_M2.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="15-20230743_M2.png"/></alternatives></inline-formula>/(s·mW), satisfying the requirements for optical quantum information processing. This innovative solution is expected to replace the traditional quasi-phase-matching single-photon sources, thus further promoting the study of optical quantum information based on thin-film lithium niobate chips.

Genuine quadripartite steering in three-photon spontaneous parametric down-conversion

The authors show that genuine quadripartite quantum steering can be produced in the case of three-photon spontaneous parametric down-conversion when the quantum property of the pump is considered. They also find that the injected signal can improve the stability of quadripartite quantum steering.

Entangling mechanical vibrations of two massive ferrimagnets by fully exploiting the nonlinearity of magnetostriction

Quantum entanglement in the motion of macroscopic objects is of significance to both fundamental studies and quantum technologies. Here we show how to entangle the mechanical vibration modes of two massive ferrimagnets that are placed in the same microwave cavity. Each ferrimagnet supports a magnon mode and a low-frequency vibration mode coupled by the magnetostrictive force. The two magnon modes are, respectively, coupled to the microwave cavity by the magnetic dipole interaction. We first generate a stationary nonlocal entangled state between the vibration mode of the ferrimagnet-1 and the magnon mode of the ferrimagnet-2. This is realized by continuously driving the ferrimagnet-1 with a strong red-detuned microwave field and the entanglement is achieved by exploiting the magnomechanical parametric down-conversion and the cavity–magnon state-swap interaction. We then switch off the pump on the ferrimagnet-1 and, simultaneously, turn on a red-detuned pulsed drive on the ferrimagnet-2. The latter drive is used to activate the magnomechanical beamsplitter interaction, which swaps the magnonic and mechanical states of the ferrimagnet-2. Consequently, the previously generated phonon–magnon entanglement is transferred to the mechanical modes of two ferrimagnets. The work provides a scheme to prepare entangled states of mechanical motion of two massive objects, which may find applications in various studies exploiting macroscopic entangled states.

Maximizing the validity of the Gaussian approximation for the biphoton state from parametric down-conversion

Spontaneous parametric down-conversion (SPDC) is widely used in quantum applications based on photonic entanglement. The efficiency of photon pair generation is often characterized by means of a $sinc(L\Delta k/2)$-function, where $L$ is the length of the nonlinear medium and $\Delta k$ the phase mismatch between the pump and down-converted fields. In theoretical investigations, the \textit{sinc} behavior of the phase mismatch has often been approximated by a Gaussian function $\exp{(-\alpha x^2)}$ in order to derive analytical expressions for the SPDC process. Different values have been chosen in the literature for the optimization factor $\alpha$, for instance by comparing the widths of \textit{sinc} and Gaussian functions or the momentum of down-converted photons. As a consequence, different values for $\alpha$ provide different theoretical predictions for the same setup. Therefore, an informed and unique choice of this parameter is necessary. In this work, we present a choice of $\alpha$ which maximizes the validity of the Gaussian approximation. Moreover, we also discuss the so-called \textit{super}-Gaussian and \textit{cosine}-Gaussian approximations as practical alternatives with improved predictive power for experiments.

Single-step order shift of Schmidt modes in a linear system

In parametric down-conversion, Schmidt modes are given by special series of wave functions that have widely discussed potential applications in quantum information science. Existing technologies allow the manipulation of such modes one at a time. We report the discovery of a linear process that can simultaneously transform a wide range of Hermite-Gaussian modes, a specific type of Schmidt modes. We examine the scattering process by a single cavity under tunable coupling given by a particular coupling function, and for a wide range of Hermite-Gaussian modes, a single-step shift of mode order is shown as achievable in high fidelity. This result constitutes an effective mean of manipulating Schmidt modes with potential application in quantum information networks.

Spatial and spectral characterization of photon pairs at telecommunication wavelengths from type-0 spontaneous parametric downconversion

The thorough characterization of entangled-photon sources is vital for their optimal use in quantum communication. However, this task is not trivial at telecommunication wavelengths. While cameras and spectrometers are well developed for visible and near-infrared spectra, this does not apply in the mid-infrared range. Here, we present a spatial and spectral characterization of photon pairs emitted in a type-0 phase-matched spontaneous parametric downconverted source. We experimentally show how these photon properties are modified by the crystal temperature. This parameter allows easy modification of photon-pair properties to fit multiplexing schemes based on only one entanglement photon source. Our results pave the way for the optimal design and use of spatial and spectral properties of quantum-correlated photon pairs at telecommunication wavelengths.

Generalized description of the spatio-temporal biphoton state in spontaneous parametric down-conversion

Spontaneous parametric down-conversion (SPDC) is a widely used source for photonic entanglement. Years of focused research have led to a solid understanding of the process, but a cohesive analytical description of the paraxial biphoton state has yet to be achieved. We derive a general expression for the spatio-temporal biphoton state that applies universally across common experimental settings and correctly describes the nonseparability of spatial and spectral modes. We formulate a criterion on how to decrease the coupling of the spatial from the spectral degree of freedom by taking into account the Gouy phase of interacting beams. This work provides new insights into the role of the Gouy phase in SPDC, and also into the preparation of engineered entangled states for multidimensional quantum information processing.

Conversion of transverse momentum correlation of photon pairs into polarization entanglement by using wavefront-splitting interference

The transverse momentum correlation of photon pairs generated by noncollinear spontaneous parametric down-conversion with type-I phase matching is transformed into polarization entanglement. A robust wavefront-splitting interferometer scheme is used, due to which the phase of the entangled state does not require active stabilization. Frequency-degenerate signal and idler photons are separated and coupled into single-mode optical fibers. The concept is demonstrated in a compact and modular setup. A straightforward alignment process for producing any of the Bell states is presented. The entangled source has an output photon flux of $8.33\ifmmode\pm\else\textpm\fi{}1.27$ kcps/mW normalized to pump power (150 mW), and $17\ifmmode\pm\else\textpm\fi{}3%$ heralding ratio. The measured state fidelity with respect to the negative anticorrelated Bell state is $0.951\ifmmode\pm\else\textpm\fi{}0.004$.

Deterministic N-photon state generation using lithium niobate on insulator device

The large-photon-number quantum state is a fundamental but nonresolved request for practical quantum information applications. We propose an N-photon state generation scheme that is feasible and scalable, using lithium niobate on insulator circuits. Such a scheme is based on the integration of a common building block called photon-number doubling unit (PDU) for deterministic single-photon parametric downconversion and upconversion. The PDU relies on a 107-optical-quality-factor resonator and mW-level on-chip power, which is within the current fabrication and experimental limits. N-photon state generation schemes, with cluster and Greenberger–Horne–Zeilinger state as examples, are shown for different quantum tasks.

Benchmarking single-photon sources from an auto-correlation measurement

Here we argue that the probability that a given source produces exactly a single photon is a natural quantity to benchmark single-photon sources as it certifies the absence of multi-photon components and quantifies the efficiency simultaneously. Moreover, this probability can be bounded simply from an auto-correlation measurement — a balanced beam splitter and two photon detectors. Such a bound gives access to various non-classicality witnesses that can be used to certify and quantify Wigner-negativity, in addition to non-Gaussianity and P-negativity of the state produced by the source. We provide tools that can be used in practice to account for an imperfect beam splitter, non-identical and non-unit detection efficiencies, dark counts and other imperfections, take finite statistical effects into account without assuming that identical states are produced in all rounds, and optionally allow one to remove the detector inefficiencies from the analysis. We demonstrate the use of the proposed benchmark, non-classicality witness and measure using a heralded single-photon source based on spontaneous parametric down-conversion. We report on an average probability that a single photon is produced ≥55% and an average measure of the Wigner negativity ≥0.004 with a confidence level of 1−10−10.

Experimental robustness of the fractional topological phase to dephasing noise

We observe experimentally the fractional topological phase (FTP) robustness to the dephasing noise in two-qudit photonic systems. We use a source of photon pairs generated by spontaneous parametric down-conversion, hyperentangled in the polarization and photon path degrees of freedom. The dephasing noise is described theoretically via Kraus maps and experimentally implemented via the spatial light modulator. The FTP is a great candidate to explore in the implementation of quantum processing and communication due to its robustness to dephasing noise.

Anisotropic spatial entanglement

The photon pairs generated through spontaneous parametric down-conversion (SPDC) possess strong correlations in their transverse position and momentum degrees of freedom. For such photon pairs, the transverse position correlation length depends on the crystal thickness and the pump wavelength. In contrast, the transverse momentum correlation length depends on the pump's beam waist and spatial coherence length. By controlling these parameters, it is possible to engineer spatial entanglement. Here, we utilize the circular asymmetry of the pump by using an elliptical-Gaussian beam to change the degree of entanglement in transverse directions, which we call anisotropic entanglement. We show the interrelation between the degree of anisotropic entanglement and the asymmetry in the beam width. In addition, we also show that for a highly asymmetric pump beam, the entanglement vanishes along the direction of the thinner beam width, while it remains intact in the direction of the broader beam width.

Optical parametric oscillator with quantum memory for quantum repeaters

We consider preparing entangled states between single photons and quantum memory by combining two-photon source based on spontaneous parametric down-conversion and multiatomic quantum memory in a common resonator. The scheme allows one to minimize the losses while the photon to be stored is propagating from the source to the quantum memory and avoids the need to synchronize their operating wavelength. In this respect, the scheme is analogous to the cavity-enhanced embedded memory within Duan-Lukin-Cirac-Zoller approach, but it remains possible to generate a second photon at the wavelength of the fiber optic communication channel and use various multiplexing methods inherent in multiatomic quantum memory.

Certification of photon Fock states using second-order nonlinearity

Certifying whether a photon Fock state is present or not while preserving its quantum state is an important task that proves useful in quantum technologies. The existing methods to perform such task apply to single photons only and consume costly quantum resources. In this paper, we propose a resource-efficient scheme for certification of an arbitrary (nonvacuum) photon Fock state by using second-order nonlinearity. In particular, we utilize the correlations in both photon number and polarization of a stimulated parametric downconversion process to herald the presence and the quantum state of an incoming Fock state. The proposed scheme works excellently with a near perfect certification fidelity even when using nonideal photodetectors. We then compare our scheme with the existing ones taking into account relevant figures of merit for each method. We also present applications of our scheme in overcoming transmission loss of photons in quantum channels as well as in preparation of NOON states, $(|N\ensuremath{\rangle}|0\ensuremath{\rangle}+|0\ensuremath{\rangle}|N\ensuremath{\rangle})/\sqrt{2}$. Our paper introduces to the optical toolbox a practical technique that is beneficial for quantum communication and quantum metrology.

Enhanced two-photon excited fluorescence from green fluorescent proteins by ultrafast fluctuations in intense light pulse

The parametric down-conversion process in optical parametric generators causes bunching of light due to ultrafast intensity fluctuations, which enhances the efficiency of nonlinear interactions between light and matter. However, the bunching effect in a sufficiently intense light pulse light required for biological nonlinear imaging has not yet been investigated. We demonstrate enhanced two-photon excited fluorescence by ultrafast fluctuations in intense pulse using a wavelength-tunable optical parametric generator consisting of a periodically poled lithium niobate crystal pumped by nanosecond pulses at a wavelength of 532 nm and emitting pulses with a peak power of about 1 kW. The emission wavelength is tuned to about 927 nm, which is optimal for two-photon excitation of green fluorescent protein. The effect of bunching by ultrafast intensity fluctuations in the pulse is evaluated by an autocorrelator using a green fluorescent protein solution as a two-photon absorber. We found an about 1.9-fold enhancement compared with the coherent state of light. Using this calibrated optical parametric generator, we perform two-photon imaging of green fluorescent protein in brain tissue within a timescale of seconds. These experimental results using intense pulses demonstrate that the bunching effect by ultrafast fluctuations can enhance nonlinear imaging in biology and medicine.

A quantum random number generator on a nanosatellite in low Earth orbit

Quantum random number generators find applications in both quantum and classical communications schemes, particularly in security protocols where they can be used as a source of random seed or key material. In this work, we describe the implementation of a quantum random number generator on-board a nanosatellite deployed in low Earth orbit. Our generator samples shot noise from an entangled photon-pair source based on spontaneous parametric down-conversion, linking the entropy of the output to the quantization of the down-converted beam. We present analyzed data from the orbiting instrument alongside data taken from a ground-based engineering model where the statistical test suites indicate a good match to the output from a uniform distribution. Finally, we use the source to implement a prototype for an off-grid randomness beacon. This work paves the way to future low Earth orbit based public quantum randomness beacons.

Complete Frequency-Bin Bell Basis Synthesizer

We report the experimental generation of all four frequency-bin Bell states in a single versatile setup via successive pumping of spontaneous parametric down-conversion with single and dual spectral lines. Our scheme utilizes intensity modulation to control the pump configuration and offers turn-key generation of any desired Bell state using only off-the-shelf telecommunication equipment. We employ Bayesian inference to reconstruct the density matrices of the generated Bell states, finding fidelities ≥97% for all cases. Additionally, we demonstrate the sensitivity of the frequency-bin Bell states to common-mode and differential-mode temporal delays traversed by the photons comprising the state-presenting the potential for either enhanced resolution or nonlocal sensing enabled by our complete Bell basis synthesizer.

Biphoton engineering using modal spatial overlap on-chip.

Photon pairs generated by spontaneous parametric downconversion are essential for optical quantum information processing, in which the quality of biphoton states is crucial for the performance. To engineer the biphoton wave function (BWF) on-chip, the pump envelope function and the phase matching function are commonly adjusted, while the modal field overlap has been considered as a constant in the frequency range of interest. In this work, by using modal coupling in a system of coupled waveguides, we explore the modal field overlap as a new degree of freedom for biphoton engineering. We provide design examples for on-chip generations of polarization entangled photons and heralded single photons. This strategy can be applied to waveguides of different materials and structures, offering new possibilities for photonic quantum state engineering.

Realistic and general model for quantum key distribution with entangled-photon sources

Entanglement-based quantum key distribution (EB-QKD) has several advantages over prepare-and-measure protocols, such as eliminating the need for decoy states and active modulators. At present, entanglement sources based on the spontaneous parametric down-conversion process have been widely used in EB-QKD experiments. Most of the existing models regard the photon-number distribution of pulse-pumped entanglement sources as thermal. However, doing so ignores any effects of spectral modes, which will lead to the overestimation of the secure key rate. In this paper, we analyze the correlation between the photon-number distribution and the joint spectral distribution and rewrite the quantum state of the photon pairs. Then we apply the results to EB-QKD and present a general model which can be used to calculate the secure key rate not only for any pulse-pumped systems but even for cw-pumped systems.