Generation Of Photon Pairs Research Articles

Design and demonstration of an efficient pump rejection filter for silicon photonic applications.

Photon pair generation via spontaneous four-wave mixing in silicon waveguides/microring resonators integrated with a high extinction pump rejection filter is very much in demand for futuristic large-scale integrated quantum photonics circuits. Ideally, a distributed Bragg reflector (DBR) can be designed to offer desired pump rejection. However, fabricated DBRs suffer degradation in pump extinction due to roughness-induced unwanted scattering waves in the forward direction around the Bragg wavelength. It is therefore inferred that the roughness-induced forward scattering can be reduced significantly by integrating a DBR structure in one of the sidewalls (instead of two sidewalls) of a multimode rib waveguide (instead of a single mode strip waveguide). Therefore, we studied a single-stage DBR filter with this design which exhibits a significantly higher stop band extinction (∼63 dB), in comparison with that of earlier reported results (<50 dB). To validate the pump rejection efficiency of such fabricated devices in quantum photonic applications, we have carried out on-chip stimulated four-wave mixing experiments and shown that the pump laser within the rejection band could be attenuated to the level of idler power.

Enhanced generation of angle correlated photon-pairs in nonlinear metasurfaces

We reveal that strongly enhanced generation of photon pairs with narrow frequency spectra and sharp angular correlations can be realised through spontaneous parametric down-conversion in metasurfaces. This is facilitated by creating meta-gratings through nano-structuring of nonlinear films of sub-wavelength thickness to support the extended bound state in the continuum resonances, associated with ultra-high Q-factors, at the biphoton wavelengths across a wide range of emission angles. Such spectral features of photons can be beneficial for various applications, including quantum imaging. Our modelling demonstrates a pronounced enhancement, compared to unpatterned films, of the total photon-pair generation rate normalized to the pump power reaching 1.75 kHz mW−1, which is robust with respect to the angular bandwidth of the pump, supporting the feasibility of future experimental realisations.

Generation and dynamic manipulation of frequency degenerate polarization entangled Bell states by a silicon quantum photonic circuit

A silicon quantum photonic circuit was proposed and realized for the generation and the dynamic manipulation of telecom-band frequency-degenerate polarization entangled Bell states. Frequency degenerate biphoton states were generated in four silicon waveguides by spontaneous four wave mixing. They were transformed to polarization entangled Bell states through on-chip quantum interference and quantum superposition, and then coupled to optical fibers. The property of polarization entanglement in generated photon pairs was demonstrated by two-photon interference under two non-orthogonal polarization bases. The output state could be dynamically switched between two Bell states, which was demonstrated by the simplified Bell state measurement. The experiment results indicated that the manipulation speed supported a modulation rate of several tens kHz, showing its potential on applications of quantum communication and quantum information processing requiring Bell state encoding and dynamic control.

Spectrally pure photons generated in a quasi-phase matched xenon-filled hollow-core photonic crystal fiber.

Spectrally pure photons heralded from unentangled photon pair sources are crucial for any quantum optical system reliant on the multiplexing of heralded photons from independent sources. Generation of unentangled photon pairs in gas-filled hollow-core photonic crystal fibers specifically remains an attractive architecture for integration into quantum-optical fiber networks. The dispersion design offered by selection of fiber microstructures and gas pressure allows considerable control over the group-velocity profile which dictates the wavelengths of photon pairs that can be generated without spectral entanglement. Here, we expand on this design flexibility, which has previously been implemented for four-wave mixing, by modeling the use of a static, periodically poled electric field to achieve an effective quasi-phase-matched three-wave mixing nonlinearity that creates spontaneous parametric downconversion. Electric-field-induced quasi-phase-matched spontaneous parametric downconversion enables control of phase matching conditions that is independent of the group velocity, allowing phase matching at arbitrary wavelengths without affecting the entanglement of photons at those wavelengths. This decoupling of entanglement engineering and phase matching facilitates spectrally pure photon pair generation with efficiency and wavelength-tunability that is otherwise unprecedented.

Spontaneous Parametric Down-Conversion from GaAs Nanowires at Telecom Wavelength

We report on the generation of photon pairs at 1550 nm from free-standing epitaxially grown self-assisted micrometre long GaAs nanowires. The efficiency of the spontaneous parametric down-conversion process has a rate of 320 GHz/Wm normalized to the transmission of the setup, the pump intensity, and the volume of the nanostructure. GaAs is a high index dielectric that can support electromagnetic Mie modes, therefore we model how shorter nanowires could improve the second-harmonic signal and we found that sub-micro long nanowires (600 nm length and 250 nm diameter) can support quality factors up to 15 at the pump wavelength (780 nm). We anticipate that the near field enhancement compared to micrometre long nanowires will boost the second-harmonic generation and, correspondingly, the biphoton rate efficiency.

Cooperative Spontaneous Four-wave Mixing in Single-channel and Dualchannel Sequences of Side-coupled Ring Resonators

Cooperative photon pair generation by Spontaneous Four-Wave Mixing (SFWM) process in singlechannel and dual-channel side-coupled ring resonator sequences is investigated. Our analysis shows that superlinear growth of generation rate with respect to the number of rings is possible even in presence of loss. Experimental evidence of super-SFWM is provided by comparing individual and collective generation rates obtained from a dual-channel ring resonator sequence. The results are in good agreement with theory and suggest that high photon pair generation rates can be achieved from integrated silicon ring resonator sequences without initiating nonlinear absorption processes.

Mode-Entanglement in Silicon Waveguides Through Intermodal Four-Wave Mixing

Silicon photonics is rapidly emerging as a promising, scalable platform for quantum information applications. As the technology matures, sources of high-quality quantum-optical states are increasingly important. We discuss the use of intermodal four-wave mixing in silicon waveguides for creating entangled photon pairs. We describe a framework to numerically analyze the state of produced photon pairs and discuss a scheme for generating photon pairs that are purely entangled in their transverse waveguide mode. Using this scheme, we find a theoretical fidelity of 99.6 % to a true Bell state in the transverse mode with no spectral correlations.

Mid-infrared photon pair generation in AgGaS2

We demonstrate nondegenerate photon pair generation by spontaneous parametric downconversion in a silver gallium sulfide AgGaS2 crystal. By tuning the pump wavelength, we achieve phase matching over a large spectral range. This allows to generate idler photons in the mid-infrared spectral range above a 6 μm wavelength with corresponding signal photons in the visible. Also, we show photon pair generation with broad spectral bandwidth. These results are a valuable step toward the development of quantum imaging and sensing techniques in the mid-infrared.

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.

Submegahertz spectral width photon pair source based on fused silica microspheres

High-efficiency submegahertz bandwidth photon pair generators will enable the field of quantum technology to transition from laboratory demonstrations to transformational applications involving information transfer from photons to atoms. While spontaneous parametric processes are able to achieve high-efficiency photon pair generation, the spectral bandwidth tends to be relatively large, as defined by phase-matching constraints. To solve this fundamental limitation, we use an ultrahigh quality factor ( Q ) fused silica microsphere resonant cavity to form a photon pair generator. We present the full theory for the spontaneous four-wave mixing (SFWM) process in these devices, fully taking into account all relevant source characteristics in our experiments. The exceptionally narrow (down to kilohertz-scale) linewidths of these devices result in a reduction in the bandwidth of the photon pair generation, allowing submegahertz spectral bandwidth to be achieved. Specifically, using a pump source centered around 1550 nm, photon pairs with the signal and idler modes at wavelengths close to 1540 and 1560 nm, respectively, are demonstrated. We herald a single idler-mode photon by detecting the corresponding signal photon, filtered via transmission through a wavelength division multiplexing channel of choice. We demonstrate the extraction of the spectral profile of a single peak in the single-photon frequency comb from a measurement of the signal–idler time of emission distribution. These improvements in device design and experimental methods enabled the narrowest spectral width ( Δ ν = 366 kHz ) to date in a heralded single-photon source based on SFWM.

Scalable multiphoton quantum metrology with neither pre- nor post-selected measurements

The quantum statistical fluctuations of electromagnetic fields establish a limit, known as the shot-noise limit, on the sensitivity of optical measurements performed with classical technologies. However, quantum technologies are not constrained by this shot-noise limit. In this regard, the possibility of using every photon produced by quantum sources of light to estimate small physical parameters, beyond the shot-noise limit, constitutes one of the main goals of quantum optics. Here, we experimentally demonstrate a scalable protocol for quantum-enhanced optical phase estimation across a broad range of phases, with neither pre- nor post-selected measurements. This is achieved through the efficient design of a source of spontaneous parametric downconversion in combination with photon-number-resolving detection. The robustness of two-mode squeezed vacuum states against loss allows us to outperform schemes based on N00N states, in which the loss of a single photon is enough to remove all phase information from a quantum state. In contrast to other schemes that rely on N00N states or conditional measurements, the sensitivity of our technique could be improved through the generation and detection of high-order photon pairs. This unique feature of our protocol makes it scalable. Our work is important for quantum technologies that rely on multiphoton interference such as quantum imaging, boson sampling, and quantum networks.

Photonic analog of Mollow triplet with on-chip photon-pair generation in dressed modes.

Making analogy with atomic physics is a powerful tool for photonic technology, witnessed by the recent development in topological photonics and non-Hermitian photonics based on parity-time symmetry. The Mollow triplet is a prominent atomic effect with both fundamental and technological importance. Here we demonstrate the analog of the Mollow triplet with quantum photonic systems. Photonic entanglement is generated with spontaneous nonlinear processes in dressed photonic modes, which are introduced through coherent multimode coupling. We further demonstrate the possibility of the photonic system to realize different configurations of dressed states, leading to modification of the Mollow triplet. Our work would enable the investigation of complex atomic processes and the realization of unique quantum functionalities based on photonic systems.

Enhanced generation of nondegenerate photon pairs in nonlinear metasurfaces

We reveal a novel regime of photon-pair generation driven by the interplay of multiple bound states in the continuum resonances in nonlinear metasurfaces. This non-degenerate photon-pair generation is derived from the hyperbolic topology of the transverse phase-matching and can enable orders-of-magnitude enhancement of the photon rate and spectral brightness, as compared to the degenerate regime. We show that the entanglement of the photon-pairs can be tuned by varying the pump polarization, which can underpin future advances and applications of ultra-compact quantum light sources.

Ultrafast electric control of cavity mediated single-photon and photon-pair generation with semiconductor quantum dots

Employing the ultrafast control of electronic states of a semiconductor quantum dot in a cavity, we introduce a novel approach to achieve on-demand emission of single photons with almost perfect indistinguishability and photon pairs with near ideal entanglement. Our scheme is based on optical excitation off-resonant to a cavity mode followed by ultrafast control of the electronic states using the time-dependent quantum-confined Stark effect, which then allows for cavity-resonant emission. Our theoretical analysis takes into account cavity-loss mechanisms, the Stark effect, and phonon-induced dephasing allowing realistic predictions for finite temperatures.

Comparison of photon-pair generation rate between the sideband and fundamental modes of spontaneous four-wave mixing

Spontaneous four-wave mixing (SpFWM) in fiber optics has been investigated due to its high photon-pair generation rate and negligible connection loss to fiber optic systems. Photon pairs generated in the fundamental or sideband modes satisfy SpFWM’s phase matching condition, and the pair-generation efficiency of both modes has been considered nearly identical. Here, we directly compare the pair-generation efficiency of the fundamental and sideband modes. Dispersion in optical fiber varies unevenly, and SpFWM in the sideband mode is more sensitive to the medium’s dispersion properties than in the fundamental mode, inducing lower pair-generation efficiency. These strong non-uniformity effects in the sideband mode will affect the photon-pair generation and four-wave mixing based quantum and nonlinear applications.

High-performance quantum entanglement generation via cascaded second-order nonlinear processes

In this paper, we demonstrate the generation of high-performance entangled photon-pairs in different degrees of freedom from a single piece of fiber pigtailed periodically poled LiNbO3 (PPLN) waveguide. We utilize cascaded second-order nonlinear optical processes, i.e., second-harmonic generation (SHG) and spontaneous parametric downconversion (SPDC), to generate photon-pairs. Previously, the performance of the photon-pairs is contaminated by Raman noise photons. Here by fiber-integrating the PPLN waveguide with noise-rejecting filters, we obtain a coincidence-to-accidental ratio (CAR) higher than 52,600 with photon-pair generation and detection rate of 52.36 kHz and 3.51 kHz, respectively. Energy-time, frequency-bin, and time-bin entanglement is prepared by coherently superposing correlated two-photon states in these degrees of freedom, respectively. The energy-time entangled two-photon states achieve the maximum value of CHSH-Bell inequality of S = 2.71 ± 0.02 with two-photon interference visibility of 95.74 ± 0.86%. The frequency-bin entangled two-photon states achieve fidelity of 97.56 ± 1.79% with a spatial quantum beating visibility of 96.85 ± 2.46%. The time-bin entangled two-photon states achieve the maximum value of CHSH-Bell inequality of S = 2.60 ± 0.04 and quantum tomographic fidelity of 89.07 ± 4.35%. Our results provide a potential candidate for the quantum light source in quantum photonics.

Integrated Si3N4 microresonator-based quantum light sources with high brightness using a subtractive wafer-scale platform.

The silicon nitride (Si3N4) platform, demonstrating a moderate third-order optical nonlinearity and a low optical loss compared with those of silicon, is suitable for integrated quantum photonic circuits. However, it is challenging to develop a crack-free, wafer-scale, thick Si3N4 platform in a single deposition run using a subtractive complementary metal-oxide-semiconductor (CMOS)-compatible fabrication process suitable for dispersion-engineered quantum light sources. In this paper, we demonstrate our unique subtractive fabrication process by introducing a stress-release pattern prior to the single Si3N4 film deposition. Our Si3N4 platform enables 950 nm-thick and 8 μm-wide microring resonators supporting whispering-gallery modes for quantum light sources at 1550 nm wavelengths. We report a high photon-pair generation rate of ∼1.03 MHz/mW2, with a high spectral brightness of ∼5×106 pairs/s/mW2/GHz. We demonstrate the first heralded single-photon measurement on the Si3N4 platform, which exhibits a high quality of conditional self-correlation gH(2)(0) of 0.008 ± 0.003.

On-Demand Generation of Entangled Photon Pairs in the Telecom C-Band with InAs Quantum Dots

Entangled photons are an integral part in quantum optics experiments and a key resource in quantum imaging, quantum communication, and photonic quantum information processing. Making this resource available on-demand has been an ongoing scientific challenge with enormous progress in recent years. Of particular interest is the potential to transmit quantum information over long distances, making photons the only reliable flying qubit. Entangled photons at the telecom C-band could be directly launched into single-mode optical fibers, enabling worldwide quantum communication via existing telecommunication infrastructure. However, the on-demand generation of entangled photons at this desired wavelength window has been elusive. Here, we show a photon pair generation efficiency of 69.9 ± 3.6% in the telecom C-band by an InAs/GaAs semiconductor quantum dot on a metamorphic buffer layer. Using a robust phonon-assisted two-photon excitation scheme we measure a maximum concurrence of 91.4 ± 3.8% and a peak fidelity to the Φ+ state of 95.2 ± 1.1%, verifying on-demand generation of strongly entangled photon pairs and marking an important milestone for interfacing quantum light sources with our classical fiber networks.

Suppression of Parasitic Nonlinear Processes in Spontaneous Four-Wave Mixing with Linearly Uncoupled Resonators.

We report on a signal-to-noise ratio characterizing the generation of identical photon pairs of more than 4 orders of magnitude in a ring resonator system. Parasitic noise, associated with single-pump spontaneous four-wave mixing, is essentially eliminated by employing a novel system design involving two resonators that are linearly uncoupled but nonlinearly coupled. This opens the way to a new class of integrated devices exploiting the unique properties of identical photon pairs in the same optical mode.

Observation of frequency-uncorrelated photon pairs generated by counter-propagating spontaneous parametric down-conversion

We report the generation of frequency-uncorrelated photon pairs from counter-propagating spontaneous parametric down-conversion in a periodically-poled KTP waveguide. The joint spectral intensity of photon pairs is characterized by measuring the corresponding stimulated process, namely, the difference frequency generation process. The experimental result shows a clear uncorrelated joint spectrum, where the backward-propagating photon has a narrow bandwidth of 7.46 GHz and the forward-propagating one has a bandwidth of 0.23 THz like the pump light. The heralded single-photon purity estimated through Schmidt decomposition is as high as 0.996, showing a perspective for ultra-purity and narrow-band single-photon generation. Such unique feature results from the backward-wave quasi-phase-matching condition and does not has a strict limitation on the material and working wavelength, thus fascinating its application in photonic quantum technologies.