Monolithic Entangled Photon Sources Using Second Order Optical Nonlinearities

We explore the potential of versatile and efficient entangled photon pair generation by spontaneous parametric downconversion in Bragg reflection waveguides. By employing a quantum treatment of modes in channel waveguides, and by accounting for group velocity dispersion in the modes, the quantum state of the generated biphotons is realistically calculated. The pair production rate is predicted to reach 4×108 pairs/s/nm/mW of pump light in a 2 mm-long structure, on par with or exceeding the performance of previously reported designs. This is attributable to an enhanced nonlinear interaction through tight mode confinement in the waveguide. Strategies for device performance optimization and phase matching wavelength tunability are outlined and numerically demonstrated. The proposed design platform is versatile and allows photon pair generation with controllable flux, bandwidth, Schmidt number, and degree of polarization entanglement. The possibility of monolithic integration with a diode laser pump offers a way to design an electrically pumped entangled photon source.

Entangled photon pairs are a crucial resource in quantum information processing. However, common sources of entangled photon pairs, such as generation by spontaneous parametric down-conversion, impose a Poisson distribution on the pairs, limiting their applicability to quantum information processing. Therefore, generating entangled photon pairs exhibiting a sub-Poisson distribution (i.e., entangled photon bundle) would be crucial/an important advancement in the current research context. Recently, it has been proposed that antibunched photon pairs can be generated using pulsed excitation of quantum dots based on photon blockade. In this work we propose a scheme to generate entangled photon pairs with strong antibunching properties via continuous driving. Under two-photon resonant continuous pumping, a quantum dot is first excited to a biexciton state and then emits a pair of entangled photons into a two-mode cavity. Owing to the strong coupling between the quantum dot and optical modes of the cavity, at most one pair of left-/right-polarized photons exists at a time in the cavity. Specifically, the antibunching effect, average photon number, and entanglement of the left- or right-polarized photons can be resonantly enhanced when the coupling strengths between the exciton (biexciton) transition and cavity modes are equal. Moreover, our results show that the binding energy of the biexciton substantially affects the antibunching effect, average photon number, and entanglement of the left-/right-polarized photons. With suitable parameters, we can realize photon pairs with strong antibunching, a large average photon number, and deep entanglement. Hence, the studied quantum dot-cavity system is a promising source of antibunched entangled photon pairs.

Energy-time entangled photon pairs (EPPs), which are at the heart of numerous quantum light applications, are commonly generated in nonlinear crystals. Some highly sensitive quantum applications require the use of ultra-broadband entangled photons that cannot be generated in nonlinear crystals due to phase-matching requirements. Here, we investigate the possibility of using metallic nanoparticles (MNPs) as a means for generating entangled photons through spontaneous parametric down-conversion (SPDC). MNPs are known for their strong light-matter coupling at their localized surface plasmon resonance, and since the propagation length through them is negligible relative to optical wavelengths, we consider them as excellent candidates to serve as non-phase matched sources of ultra-broadband entangled photons. To that end, we report experimental results of classical-light second-harmonic generation in silver nanotriangles and nanocubes embedded in polyvinyl alcohol. Based on the results of our experiments, performed using the reference-free hyper-Rayleigh scattering method [A. Ashkenazy et al., J. Phys. B 145401, 2019], we present an estimation of the characteristics of SPDC in MNPs. We show that, despite of the metals' centrosymmetric structure, MNPs exhibit second order nonlinearity that is mainly of an electric dipole nature, and so they are suitable for SPDC. Moreover, we show that localized surface-plasmon resonance can play a significant role in enhancing the generated EPPs flux. Finally, we compare the SPDC capabilities of MNPs to that of commonly used nonlinear crystals and show that the expected EPPs flux from MNPs is weaker but the EPPs have very large bandwidth, which could be helpful for advanced quantum sensing, spectroscopy and communication applications.

Deterministic sources of high-fidelity entangled qubit pairs encoded in the dual-rail photonic basis, i.e., presence of a single photon in one of two orthogonal modes, are a key enabling technology of many applications of quantum information processing, including high-rate, high-fidelity quantum communications over long distances. The most popular and mature sources of such photonic entanglement, e.g., those that leverage spontaneous parametric down-conversion (SPDC) or spontaneous four-wave mixing, generate an entangled (so-called continuous-variable) quantum state that contains contributions from high-order photon terms that lie outside the span of the dual-rail basis, which is detrimental to most applications. One often uses low pump power to mitigate the effects of those high-order terms. However, that reduces the pair generation rate, and the source becomes inherently probabilistic. We investigate a cascaded source that performs a linear-optical entanglement swap between two SPDC sources, to generate a heralded photonic entangled state that has a higher fidelity (to the ideal Bell state) compared to a free-running SPDC source. Furthermore, with the Bell swap providing a heralding trigger, we show how to build a multiplexed source, which despite reasonable switching losses and detector loss and noise, yields a fidelity versus success probability trade-off of a high-efficiency source of high-fidelity dual-rail photonic entanglement. We find, however, that there is a threshold of $1.5$ dB of loss per switch, beyond which multiplexing hurts the fidelity versus success probability trade-off.

In quantum technology, entanglement is the key resource to perform experiments in quantum computing, quantum communication, quantum sensing and metrology. Generation, characterization and distribution of entanglement deterministically is still an active research area. In contemporary, probabilistic entanglement sources based on parametric down-conversion process remain the workhorse for experimental demonstrations and practical applications, despite their inherent limitations of on-demand entangled photon generation. Over the years, three main technological architectures have evolved for generating probabilistic entangled-photon source; bulk-optics, waveguides and integrated photonic chips, each comprising several distinct architectural approaches tailored to different applications. This review article talks about the state of the art in probabilistic entangled photon source, examining the advancement from free space bulk non-linear optics to fiber based waveguides and scalable photonic chips with their current limitations, presenting a road map for researchers to choose the most appropriate platform for their quantum photonic applications.

Quantum state engineering, the cornerstone of quantum photonic technologies, mainly relies on spontaneous parametric downconversion and four-wave mixing, where one or two pump photons spontaneously decay into a photon pair. Both of these nonlinear effects require momentum conservation for the participating photons, which strongly limits the versatility of the resulting quantum states. Nonlinear metasurfaces have subwavelength thickness and allow the relaxation of this constraint; when combined with resonances, they greatly expand the possibilities of quantum state engineering. Here, we generated entangled photons via spontaneous parametric downconversion in semiconductor metasurfaces with high-quality factor, quasi-bound state in the continuum resonances. By enhancing the quantum vacuum field, our metasurfaces boost the emission of nondegenerate entangled photons within multiple narrow resonance bands and over a wide spectral range. A single resonance or several resonances in the same sample, pumped at multiple wavelengths, can generate multifrequency quantum states, including cluster states. These features reveal metasurfaces as versatile sources of complex states for quantum information.

The successful employment of high-dimensional quantum correlations and its integration in telecommunication infrastructures is vital in cutting-edge quantum technologies for increasing robustness and key generation rate. Position-momentum Einstein-Podolsky-Rosen (EPR) entanglement of photon pairs are a promising resource of such high-dimensional quantum correlations. Here, we experimentally certify EPR correlations of photon pairs generated by spontaneous parametric down-conversion (SPDC) in a nonlinear crystal with type-0 phase-matching at telecommunication wavelength for the first time. To experimentally observe EPR entanglement, we perform scanning measurements in the near- and far-field planes of the signal and idler modes. We certify EPR correlations with high statistical significance of up to 45 standard deviations. Furthermore, we determine the entanglement of formation of our source to be greater than one, indicating a dimensionality of greater than 2. Operating at telecommunication wavelengths around 1550 nm, our source is compatible with today’s deployed telecommunication infrastructure, thus paving the way for integrating sources of high-dimensional entanglement into quantum-communication infrastructures.

Non-phase-matched spontaneous parametric down-conversion (SPDC) in atomically thin materials provides new degrees of freedom and enhanced quantum information capacity compared to conventional phase-matched sources. These systems emerged as promising platforms for quantum computing, communication, and imaging, with the potential to support higher-order nonlinear processes. However, direct observation of photon-pair emission from a monolayer has remained experimentally challenging. In this work, we theoretically modeled SPDC emission across the full angular space from a monolayer GaSe film and experimentally validated the model through measurements of both co- and counter-propagating photon pairs. We demonstrated two-photon quantum correlations in the telecom C-band from a monolayer SPDC source. The spatially symmetric, broadband emission predicted by theory was confirmed experimentally. Furthermore, we observed high-fidelity Bell states in the counter-propagating configuration. Our results revealed the emission characteristics of SPDC in the deeply subwavelength, non-phase-matched regime, and introduced atomically thin, counter-propagating SPDC as a scalable and integrable platform for programmable quantum state generation, extendable via moiré superlattice engineering.

Entangled photons are a crucial resource for linear optical quantum communication and quantum computation. Besides the remarkable progress of photon state engineering using atomic memories (Kimble (2008); Yuan et al. (2008)) the majority of current experiments is based on the production of photon pairs in the process of spontaneous parametric down-conversion (SPDC), where the entangled photon pair is concluded from post-selection of randomly occurring coincidences. Here we present new insights into the heralded generation of photon states (Barz et al. (2010); Wagenknecht et al. (2010)) that are maximally entangled in polarization (Schrodinger (1935)) with linear optics and standard photon detection from SPDC (Kwiat et al. (1995)). We utilize the down-conversion state corresponding to the generation of three pairs of photons, where the coincident detection of four auxiliary photons unambiguously heralds the successful preparation of the entangled state (Śliwa & Banaszek (2003)). This controlled generation of entangled photon states is a significant step towards the applicability of a linear optics quantum network (Nielsen & Chuang (2000)), in particular for entanglement distribution (Bennett et al. (1996)), entanglement swapping (Kaltenbaek et al. (2009); Pan et al. (1998)), quantum teleportation (Bouwmeester et al. (1997)), quantum cryptography (Bennett & Brassard (1984); Ekert (1991); Jennewein et al. (2000)) and scalable approaches towards photonics-based quantum computing schemes (Browne & Rudolph (2004); Gottesman & Chuang (1999); Knill et al. (2001)).

Sources of entangled photons are cornerstones of photonic quantum technologies, providing the basic resources for a wide range of quantum protocols for computing, simulation and sensing. A typical technique to generate entangled photon pairs exploits spontaneous parametric down conversion (SPDC) in non-linear crystals. In this context, an integrated-optics approach based on non-linear waveguides [1] can boost source brightness due to the enhanced mode confinement, and can take great advantage from the unique compactness and stability of on-chip devices. However, integrated devices are often specialized to produce a given quantum state.

We report on the generation of a bright photon source at telecom wavelengths using spontaneous parametric down-conversion in a type II periodically poled potassium titanyl phosphate crystal. Simultaneous resonances of the pump and the two down-converted fields are achieved with a properly designed optical cavity. This triple-resonance optical parametric oscillator operates far below threshold, generating photon pairs at 1560 nm wavelength with a bandwidth of 8 MHz. Time correlation measurements produce an estimate of 27.7 ns for the coherence time, and production rate of 134±25  s−1 MHz−1 mW−1 for photon pairs. As photon pairs in the telecom regime are suitable for long-distance transmission, this source may find application in quantum communications.

We describe the simplest single–photon encoding schemes and introduce the concept of a quantum optical gate. These gates can be used to build up arbitrary entangled states of many initially separate single photons. Experimental realization of such a gate involves the development of nonlinear phase shift elements sensitice at the single quantum level. We report our experimental efforts using linear optical elements and non–classical sources where we are able to demonstrate interference and entanglement of initially separate single–photon pulses.

Fine structure splitting (FSS) is a bottleneck in quantum dot (QD) based solid-state entangled photon pair sources for application in quantum key distribution (QKD). In QDs, entangle photon pairs are generated through a cascaded emission process: biexciton to exciton to the ground state. The FSS of the excitonic states destroys the entanglement of the photon pairs; hence, it needs to be eliminated. For numerical investigation of FSS and design optimization, a multiscale-multiphysics many-body calculation is required. In this article, we report the coupling of a full configuration interaction (FCI) method with a 10-band (sp <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> s*-spin) tight-binding (TB) model to calculate the excitonic energetics of realistically sized InGaN/GaN dot-innanowire structures. Model benchmarking has been done against a recently reported InGaN/GaN multiple quantum well (MQW) structure in the a -plane orientation. Computational methodology of implementing a hexagonal-base truncated pyramid shaped QD has been presented. The effects of QD shape/thickness, material composition, and crystal growth direction (polar c-plane and non-polar m -plane and α-plane) on the FSS of InGaN/GaN based photon emitters have been investigated. Polarization profiles of the emitted photons from the excitonic transitions have been derived quantum mechanically from transition dipole moments. With the smallest FSS, the non-polar m-plane device has been found to be most promising for the QKD application.

We present a theoretical and experimental investigation of the emission characteristics and the flux of photon pairs generated by spontaneous parametric downconversion in quasi-phase matched bulk crystals for the use in quantum communication sources. We show that, by careful design, one can attain well defined modes close to the fundamental mode of optical fibers and obtain high coupling efficiencies also for bulk crystals, these being more easily aligned than crystal waveguides. We distinguish between singles coupling, ${\ensuremath{\gamma}}_{\mathrm{s}}$ and ${\ensuremath{\gamma}}_{\mathrm{i}}$, conditional coincidence, ${\ensuremath{\mu}}_{\mathrm{i}\ensuremath{\mid}\mathrm{s}}$, and pair coupling, ${\ensuremath{\gamma}}_{\mathrm{c}}$, and show how each of these parameters can be maximized by varying the focusing of the pump mode and the fiber-matched modes using standard optical elements. Specifically we analyze a periodically poled KTP-crystal pumped by a $532\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ laser creating photon pairs at $810\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and $1550\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. Numerical calculations lead to coupling efficiencies above 93% at optimal focusing, which is found by the geometrical relation $L∕{z}_{R}$ to be $\ensuremath{\approx}1$ to 2 for the pump mode and $\ensuremath{\approx}2$ to 3 for the fiber-modes, where $L$ is the crystal length and ${z}_{R}$ is the Rayleigh-range of the mode-profile. These results are independent on $L$. By showing that the single-mode bandwidth decreases $\ensuremath{\propto}1∕L$, we can therefore design the source to produce and couple narrow bandwidth photon pairs well into the fibers. Smaller bandwidth means both less chromatic dispersion for long propagation distances in fibers, and that telecom Bragg gratings can be utilized to compensate for broadened photon packets---a vital problem for time-multiplexed qubits. Longer crystals also yield an increase in fiber photon flux $\ensuremath{\propto}\sqrt{L}$, and so, assuming correct focusing, we can only see advantages using long crystals.

Quantum-photonic chips, which integrate quantum light sources alongside active and passive optical elements, as well as single-photon detectors, show great potential for photonic quantum information processing and quantum technology. Mature semiconductor nanofabrication processes allow for scaling such photonic integrated circuits to on-chip networks of increasing complexity. Second-order nonlinear materials are the method of choice for generating photonic quantum states in the overwhelming majority of linear optic experiments using bulk components, but integration with waveguide circuitry on a nanophotonic chip proved to be challenging. Here, we demonstrate such an on-chip parametric down-conversion source of photon pairs based on second-order nonlinearity in an aluminum-nitride microring resonator. We show the potential of our source for quantum information processing by measuring the high visibility anti-bunching of heralded single photons with nearly ideal state purity. Our down-conversion source yields measured coincidence rates of 80 Hz, which implies MHz generation rates of correlated photon pairs. Low noise performance is demonstrated by measuring high coincidence-to-accidental ratios. The generated photon pairs are spectrally far separated from the pump field, providing great potential for realizing sufficient on-chip filtering and monolithic integration of quantum light sources, waveguide circuits and single-photon detectors.