Integrated quantum optics promises to enhance the scale and functionality of quantum technologies, and has become a leading platform for the development of complex and stable quantum photonic circuits. Here, we report path-entangled photon-pair generation from two distinct waveguide sources, the manipulation of these pairs, and their resulting high-visibility quantum interference, all on a single photonic chip. Degenerate and non-degenerate photon pairs were created via the spontaneous four-wave mixing process in the silicon-on-insulator waveguides of the device. We manipulated these pairs to exhibit on-chip quantum interference with visibility as high as 100.0 ± 0.4%. Additionally, the device can serve as a two-spatial-mode source of photon-pairs: we measured Hong-Ou-Mandel interference, off-chip, with visibility up to 95 ± 4%. Our results herald the next generation of monolithic quantum photonic circuits with integrated sources, and the new levels of complexity they will offer.

Photon sources are fundamental components for any quantum photonic technology. The ability to generate high count-rate and low-noise correlated photon pairs via spontaneous parametric down-conversion using bulk crystals has been the cornerstone of modern quantum optics. However, future practical quantum technologies will require a scalable integration approach, and waveguide-based photon sources with high-count rate and low-noise characteristics will be an essential part of chip-based quantum technologies. Here, we demonstrate photon pair generation through spontaneous four-wave mixing in a silicon micro-ring resonator, reporting separately a maximum coincidence-to-accidental (CAR) ratio of 602 ± 37 (for a generation rate of 827kHz), and a maximum photon pair generation rate of 123 MHz ± 11 kHz (with a CAR value of 37). To overcome freecarrier related performance degradations we have investigated reverse biased p-i-n structures, demonstrating an improvement in the pair generation rate by a factor of up to 2 with negligible impact on CAR.

The ultrafast intersubband relaxation in GaN quantum wells has been verified. Al0.65Ga0.35N/GaN multiple quantum wells, with as many as 200 wells, were grown by optimizing the barrier thickness and introducing GaN intermediate layers. The intersubband absorption is sufficiently strong for the relaxation time to be measured. A pump–probe measurement is performed to investigate the relaxation. An ultrashort relaxation time of less than 150 fs is obtained at a wavelength of 4.5 μm. The transient time is shorter than that of InGaAs quantum wells by approximately an order of magnitude. This result is promising for realizing ultrafast optical switches.

GaN/AlN multiple-quantum-well structures were grown by molecular beam epitaxy. Abrupt interfaces and good periodicity were confirmed. Absorption measurements indicated that intersubband absorptions occurred at wavelengths of 1.3–2.2 μm. Spectral fits by Lorentzians suggested that the well thicknesses fluctuated by two monolayers. The linewidths of the individual fits were as narrow as 80–120 meV. The characteristics of the absorption saturation were investigated at a wavelength of 1.46 μm. A relaxation time of 400 fs and saturation energy density of 0.5 pJ/μm2 were obtained. These results are promising for realizing ultrafast optical switches with energy consumption of the picojoule order.

The feasibility of the intersubband transition (ISBT) in Al(Ga)N/GaN quantum wells (QWs) as a device mechanism for ultrafast optical switches is theoretically investigated. The 1.55-µ m ISBT is shown to be feasible because of its large conduction band discontinuity. The intersubband relaxation time at 1.55-µ m is estimated to be 80 fs, which is about 30 times shorter than that in InGaAs QWs. The third order nonlinear susceptibility is estimated to be 1.6×10-15 m2· V-2 for N=1×1018 cm-3. These characteristics suggest that the ISBT in nitride QWs is a promising mechanism for multi-terabit/s optical switches.