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.
Integrated quantum photonics leverages the on-chip generation of nonclassical states of light to realize key functionalities of quantum devices. Typically, the generation of such nonclassical states relies on whispering gallery mode resonators, such as integrated optical micro-rings, which enhance the efficiency of the underlying spontaneous nonlinear processes. While these kinds of resonators excel in maximizing either the temporal confinement or the spatial overlap between different resonant modes, they are usually associated with large mode volumes, imposing an intrinsic limitation on the efficiency and footprint of the device. Here, we engineer a source of time-energy entangled photon pairs based on a silicon photonic crystal cavity, implemented in a fully CMOS-compatible platform. In this device, resonantly enhanced spontaneous four-wave mixing converts pump photon pairs into signal/idler photon pairs under the energy-conserving condition in the telecommunication C-band. The design of the resonator is based on an effective bichromatic confinement potential, allowing it to achieve up to nine close-to-equally spaced modes in frequency, while preserving small mode volumes, and the whole chip, including grating couplers and access waveguides, is fabricated in a single run on a silicon-on-insulator platform. Besides demonstrating efficient photon pair generation, we also implement a Franson-type interference experiment, demonstrating entanglement between signal and idler photons with a Bell inequality violation exceeding five standard deviations. The high generation efficiency combined with the small device footprint in a CMOS-compatible integrated structure opens a pathway toward the implementation of compact quantum light sources in all-silicon photonic platforms.
Advances in quantum photonics have shown that chip-scale quantum devices are translating from the realm of basic research to applied technologies. Recent developments in integrated photonic circuits and single photon detectors indicate that the bottleneck for fidelity in quantum photonic processes will ultimately lie with the photon sources. We present and demonstrate a silicon nanophotonic chip capable of emitting telecommunication band photon pairs that exhibit the highest raw degree of time-energy entanglement from a micro/nanoscale source, to date. Biphotons are generated through cavity-enhanced spontaneous four-wave mixing (SFWM) in a high-Q silicon microdisk resonator, wherein the nature of the triply resonant generation process leads to a dramatic Purcell enhancement, resulting in highly efficient pair creation rates as well as extreme suppression of the photon noise background. The combination of the excellent photon source and a new phase locking technique allow for the observation of a nearly perfect coincidence visibility of (96.6 ± 1.1)%, without any background subtraction, at a large pair generation rate of (4.40 ± 0.07) × 10 5 pairs/s.
We demonstrate and characterize a heralded single photon source which is based on the signal and idler photon pairs generated from pulse pumped spontaneous four wave mixing in photonic crystal fibre. The heralded single photons are approximately in a pure temporal mode, achieved by eliminating the frequency correlation of the photon pairs. Since the influence of Raman scattering is negligible and no narrow bandpass filter is employed, the heralding efficiency of the source is up to 85%. When the production rate is 0.002 pair/pulse, the measured second-order intensity correlation function of the heralded single photons is , which is suppressed by a factor of more than 80 relative to the classical Poissonian light limit. In addition, the demonstrated source is pumped by a photonic crystal fibre laser, which makes the source more compact and low-cost.
Cavity-assisted spontaneous parametric down-conversion (SPDC) and spontaneous four-wave mixing (SFWM) in nonlinear optical materials are practical and versatile methods to generate narrowband time-energy entangled photon pairs. Time-energy entangled photons with tailored spectro-temporal properties are particularly useful for efficient quantum optical interfaces. In this work we study the generation of photon pairs in cavity-assisted SPDC and SFWM for the general case of off-resonant conversion, namely, when the frequencies of the generated photons do not match the cavity resonances. Such a frequency mismatch in particular depends on temperature and requires an additional control in the experiment. First, we propose a generic model, for description of cavity-assisted SPDC and SFWM. We show that in both processes the mismatch reduces the generation rate of photons, distorts the spectrum and the auto-correlation function of the generated fields, as well as affects the photon generation dynamics. Second, we verify the results experimentally using parametric generation of photon pairs in a nonlinear whispering gallery mode resonator (WGMR) as an experimental platform with controlled frequency mismatch. Our work reveals the role of the frequency mismatch in the photon generation process and shows a way to control it. Obtained results constitute one more step in the direction of full control over the spectro-temporal properties of entangled photon pairs and the heralded generation of single-photon pulses with a tailored temporal mode.