Generating correlated photons is an essential technology for photonic quantum information experiments. Increasing the generation efficiency of photon pairs via the enhancement of material nonlinearity is important for various applications including a near-on-demand single photon source based on multiplexed photon pair sources [1]. In this regard, photonic crystal nanocavities are attractive nonlinear media thanks to a high Q value and a wavelength-sized mode volume, which lead to the significant nonlinearity enhancement. However, such cavities generally support single resonant mode, whereas multiple resonance frequencies are required for photon pair generation processes. Coupled photonic crystal nanocavities have been studied to obtain multiple resonance frequencies [2-4]. However, there has been no report on the resonant photon pair generation from the coupled photonic crystal cavities.
For large-scale, multi-partite photonic quantum information system, stable on-demand single photon sources are particularly important building blocks. A scheme to implement the source is to multiplex many heralded single photon sources of identical photon pair generators. To achieve this, we must increase the integration density of the photon pair generators. Slow light enhanced nonlinearity in Si photonic waveguides has proven useful for reducing the device size. So far, we demonstrated that coupled-resonator optical waveguides (CROWs) based on photonic crystal (PhC) nanocavities exhibited the largest nonlinear constant γ (/W/m) in Si-core waveguides thanks to the slow light enhancement and a strong spatial field confinement achieved by PhC nanocavities. Using the PhC-cavity-based CROW, we demonstrate efficient correlated photon pair generation.
Photon pair sources with the capability to tune the wavelength and correlation properties of the generated photons can be useful for quantum applications such as communication, sensing, and computing. Silicon devices based on the SOI (silicon-on-insulator) platform benefits from both tight modal confinement, due to high index contrast between core and cladding, and CMOS compatible manufacturing process, making it a suitable platform for such sources. In the present work, we discuss the possibility of using planar resonant silicon devices with bend radii of a few tens of microns as photon pair sources. We discuss that a silicon nanophotonic chip consisting of either a single microring or coupled microrings can be used to generate photon pairs around 1.55 um. We discuss that the photons are generated in multiple pairs of wavelengths and can be tuned over several nanometers, demonstrating the capability to generate wavelength division multiplexed photon pairs at freely chosen telecommunication-band wavelengths. Further on, we discuss the ease of control over the bi-photon spectrum of the generated photon pairs and its implication in controlling their correlation properties. Uncorrelated photon pairs can be used to herald a single photon, and correlated photon pairs can be used in various quantum applications such as communication and sensing. In the end, we discuss the design parameters of a single microring resonator based sources.
A single semiconductor quantum dot that is strongly coupled to a photonic crystal nanocavity yields optical nonlinearities at the single photon level. The promise of quantum information processing is driving developments in several different physical implementations. Photons are attractive as carriers of quantum information be-cause they can travel long distances without losing their infor-mation through interactions with their environment. However, their passive nature can be a problem in applications such as the construction of logic gates, where it is necessary for two or more photons to interact. This interaction requires a nonlinear optical medium. Particularly strong nonlinearities are achieved when photons drive an emitter near its resonance frequency, and this nonlinearity is even greater if the photon is confined near the emitter in an optical cavity.
) crystal to realize a new scalable, ultrathin, and efficient SPDC source. By coinciding fundamental and higher order resonances of the metasurface with the generated signal and idler frequencies, respectively, the electric field in the nonlinear media is significantly boosted. This leads to a substantial enhancement in the SPDC process which, subsequently, by using the quantum-classical correspondence principle, translates to very high photon-pair generation rates. The emitted radiation is highly directional and perpendicular to the metasurface in contrast to relevant dielectric structures. The incorporation of circular polarized excitation further increases the photon-pair generation efficiency. The presented work will lead to the design of new efficient ultrathin SPDC single-photon nanophotonic sources working at room temperature that are expected to be critical components in free-space quantum optical communications. In a more general context, our findings can have various applications in the emerging field of quantum plasmonics.
On-chip generation of non-classical states of light is a fundamental building block for the development of quantum information [1]. A convenient method harnesses resonantly enhanced parametric processes, since they operate at room temperature. We consider here spontaneous Four-Wave-Mixing (FWM). The generation of time correlated, entangled photons pairs and heralded single photons via FWM has been demonstrated on silicon and on different III-V semiconductor ring resonators with very large efficiency [2], [3]. Here, an alternative type of cavity is used: a Photonic Crystal (PhC) cavity. These periodic structures offer the possibility to reduce the mode volume V in all-dielectric structure, with a micrometer-scale footprint, which strongly increases the efficiency of the nonlinear process
(∝Q3/V2)if high quality factor Q is preserved. We achieve it with a bichromatic Photonic Crystal cavity depicted in fig. 1.a). The cavity is located between two lines of holes with reduced period
a′compared to the rest of the holes with period
a; while light propagates in a waveguide created with a missing line of holes ended by a reflector [4]. This design imposes an effective parabolic potential for the optical field, that ultimately leads to equispaced resonances in frequency, which is a strict condition for efficient resonant enhancement of the interaction imposed by the energy conservation of FWM. The cavity is pumped with a CW laser with pump power level of
P0transferred to the cavity. A self thermal tuning technique is used to compensate for any residual frequency mismatch of the resonance triplet [4] after fabrication. After the chip, the generated photon pairs encounter an unbalanced Mach-Zehnder “Franson” interferometer [5]. At the output, they are time-energy entangled. They travel through a filtering stage to suppress any residual pump photons before being detected by superconducting nanowire single-photon detectors. The efficiency of our source is measured by bypassing the Franson interferometer. Taking into account the losses between the cavity and the detectors, it is estimated to be 16 GHz/mW2and is comparable to the state-of-the-art of semiconductor based resonators for photon pair generation (20 GHz/mW2, fig. 1.b)) [2]; while our average Q-factor (2.1 × 105) is five times smaller for this sample but could be larger (up to 7 × 105). Comparable efficiency is achieved because the resonator volume is at least one order of magnitude smaller than in ring resonators. Consequently, the footprint is very small
(<70μm2,≈600and 300 m2in [2] and [3]), which is an important asset for scalability. The entangled nature of the photon pair is quantified with two-photon interferences depicted in fig. 1.c). For pump power of 15
μW, large net visibility is measured up to 96.6 ± 4.2 %. This work shows that PhC cavities are novel and promising sources for integrated quantum optics.