Proceedings of SPIE, the International Society for Optical Engineering/Proceedings of SPIE
Cross-absorption as a limit to heralded silicon photon pair sources
Chad A. Husko, Alex S. Clark, Matthew J. Collins, Alfredo De Rossi, Sylvain Combrié, Gaëlle Lehoucq, Isabella H. Rey, Thomas F. Krauss, Chunle Xiong, Benjamin J. Eggleton
In recent years integrated waveguide devices have emerged as an attractive platform for scalable quantum tech- nologies. In contrast to earlier free-space investigations, one must consider additional effects induced by the media. In amorphous materials, spontaneous Raman scattered photons act as a noise source. In crystalline materials two-photon absorption (TPA) and free carrier absorption (FCA) are present at large intensities. While initial observations noted TPA affected experiments in integrated semiconductor devices, at present the nuanced roles of these processes in the quantum regime is unclear. Here, using single photons generated via spontaneous four-wave mixing (SFWM) in silicon, we experimentally demonstrate that cross-TPA (XTPA) between a classical pump beam and generated single photons imposes an intrinsic limit on heralded single photon generation, even in the single pair regime. Our newly developed model is in excellent agreement with experimental results.
In recent years integrated waveguide devices have emerged as an attractive platform for scalable quantum tech- nologies. In contrast to earlier free-space investigations, one must consider additional effects induced by the media. In amorphous materials, spontaneous Raman scattered photons act as a noise source. In crystalline materials two-photon absorption (TPA) and free carrier absorption (FCA) are present at large intensities. While initial observations noted TPA affected experiments in integrated semiconductor devices, at present the nuanced roles of these processes in the quantum regime is unclear. Here, using single photons generated via spontaneous four-wave mixing (SFWM) in silicon, we experimentally demonstrate that cross-TPA (XTPA) between a classical pump beam and generated single photons imposes an intrinsic limit on heralded single photon generation, even in the single pair regime. Our newly developed model is in excellent agreement with experimental results.
Single photons are of paramount importance to future quantum technologies, including quantum communication and computation. Nonlinear photonic devices using parametric processes offer a straightforward route to generating photons, however additional nonlinear processes may come into play and interfere with these sources. Here we analyse spontaneous four-wave mixing (SFWM) sources in the presence of multi-photon processes. We conduct experiments in silicon and gallium indium phosphide photonic crystal waveguides which display inherently different nonlinear absorption processes, namely two-photon (TPA) and three-photon absorption (ThPA), respectively. We develop a novel model capturing these diverse effects which is in excellent quantitative agreement with measurements of brightness, coincidence-to-accidental ratio (CAR) and second-order correlation function g((2))(0), showing that TPA imposes an intrinsic limit on heralded single photon sources. We build on these observations to devise a new metric, the quantum utility (QMU), enabling further optimisation of single photon sources.
Integrated quantum signal sources play a key role in quantum communications in space, where compactness is of utmost importance. InP is an excellent candidate that can provide an on-chip single photon source as a device, where an excitation laser and waveguide for photon generation can be integrated on a single chip, because of its direct bandgap and high nonlinearity. However, one of the significant challenges that remains with InP is the elevated magnitude of the imaginary component of the third-order susceptibility ℑ[ χ (3) ] at 1550 nm wavelength. The higher value of ℑ[ χ (3) ] leads to two-photon absorption (TPA) and free carrier absorption (FCA) at very low powers, which drastically impacts the photon generation rate. In this paper, HHI and IMOS-based channel waveguides are modelled for the first time as an integrated platform for single photon generation, exploring spontaneous four-wave mixing (SFWM), considering TPA and FCA effects. The IMOS-based waveguide is more dynamic in generating entangled photons over a broader wavelength range, which would benefit photon demultiplexing at later stages. Nonlinear losses become dominant at power levels of 11 mW and 2 mW in HHI and IMOS waveguides, respectively, saturating the generation of single photons. TPA-free photon generation rates at these respective power levels are 4.1 MHz and 4.6 MHz , with generation bandwidths of 12.6 nm and 20.1 nm in the HHI channel and waveguide based on the IMOS platform, respectively. The saturated generation rates obtained are close to 0.15 GHz for both platforms. The on-chip monolithic integration of lasers with the proposed devices will potentially lead to more compact and stable single photon sources without relying on external lasers, in the telecommunications regime.
We report a comprehensive characterization of photon-pair generation by spontaneous four-wave mixing in a partially deuterated amorphous silicon (a-Si:D) waveguide for C-band heralded single-photon generation. In a 3.3-mm-long strip waveguide pumped with a continuous-wave laser at telecom wavelengths, we measure an on-chip pair-generation rate of 3.1 × 10 6 pairs/s, a maximum coincidence-to-accidental ratio of 66, and a heralded second-order correlation g H (2) (0)=0.058±0.002, with these values obtained at different pump powers. We also demonstrate, to the best of our knowledge, the first observation of time-energy entanglement from an amorphous silicon waveguide, with a Franson interference visibility of 77±5%. The power scaling of the singles and accidental coincidences indicates a significant noise contribution that currently limits the source performance. Our results identify a-Si:D as a promising nonlinear platform for bright, integrated photon-pair sources and future hybrid quantum photonic circuits.
On-chip heralded single photon sources are of key importance in the development of chip-scale devices exploiting the quantum properties of light. Single photon states can be produced as single photons heralded from correlated photon pairs generated through spontaneous four wave mixing. On-chip heralded single photon sources based on spontaneous four wave mixing have been already demonstrated. However, the heralded and herald photons are usually generated with wavelengths very close to the pump one, limiting the pump rejection efficiency and the application to the mid infrared. Moreover, the common sources of heralded photons based on spontaneous four wave mixing require spectral post filtering to achieve high purity, limiting the brightness and the integration of these sources. A solution to these problems can be provided by intermodal four wave mixing. In this work, we demonstrate the generation of photon pairs through intermodal four wave mixing in silicon waveguides, measuring the coincidences between the idler at 1.281 μm and the signal at 1.952 μm. We then discuss the application of intermodal four wave mixing to on-chip heralded single photon sources.
Quantum technologies that harness the properties of superposition and entanglement promise to revolutionise many fields, from information processing to metrology. Silicon-integrated quantum photonics operating in the telecom band suffers from deleterious two-photon absorption (TPA) [1,2]. Cross-TPA reduces the brightness and ultimately the heralding efficiency [3] of spontaneous four-wave mixing (SFWM) photon-pair sources, where single photons necessarily co-propagate with a bright pump. Nonlinear losses can be mitigated by pumping and generating photons with energies close to and beyond silicon's two-photon band edge [4,5]. We report the first quantum-correlated photon pairs in the 2.1 μm short-wave infrared (SWIR) band, produced on-chip from a silicon waveguide spiral.
We investigate theoretically the spontaneous four-wave mixing (FWM) process that occurs in optical waveguides, as a source of quantum correlated photon-pairs. We consider that the waveguide used to implement the spontaneous FWM process presents a high value of nonlinear parameter, γ = 93.4 W-1m-1, and a non-negligible value of loss coefficient, α = 133.3 dB/m. Moreover, the theoretical model also consider the Raman scattering that inevitably accompanies the FWM process, and generates time-uncorrelated (noise) photon pairs. We use the coincident-to-accidental ratio (CAR) as a figure of merit of the photon pair source, and we were able to observe a CAR of the order of 65 in a high loss regime. After, we use the time-correlated photon pairs generated by the spontaneous FWM process to implement a heralded single photon source at waveguide output. In this scenario, the detection of one photon of the pair heralds the presence of the other photon. The quality of the source was studied by the evaluation of the second order coherence function for one photon of the pair conditioned by the detection of its twin photon. We observe that the presence of a high loss coefficient tends to improve the quality of the photon source, when compared with the lossless regime, even considering the Raman noise photons. We obtain a value for the conditional second order coherence function of the order of 0.11 in absence of loss, and a value of 0.03 for a loss coefficient of 133.3 dB/m.