UC Santa Barbara

The exploration of quantum technologies presents a promising avenue to achieve new functionalities and capabilities not possible with classical systems, which could revolutionize computing, information transfer and storage, encryption and security, sensing, and enhance our understanding of many natural phenomena. Among many quantum platforms, quantum photonics---the science and engineering of encoding and manipulating quantum information with photons---offers advantages due to the ability to maintain robust quantum coherence at room temperature, achieve scalability through semiconductor processing, minimize undesirable environmental interactions, and encode quantum states in many different degrees of freedom of a photon.

State-of-the-art quantum photonic platforms are based on silicon waveguides, which benefit from the decades of research on silicon semiconductor processing but are limited by the weak optical nonlinearity, small electronic bandgap, and lack of a strong Pockels effect. Here, integrated quantum photonics with nonlinear III-V semiconductors, namely InGaP and AlGaAs, is explored to demonstrate ultrabright sources of quantum light, efficient entanglement distribution, and two-photon interference from nominally indistinguishable photon-pair sources. We demonstrate a novel, ultra-low-loss AlGaAs-on-insulator platform capable of generating time-energy entangled photons through spontaneous four wave mixing in a Q >1 million microring resonator with nearly 1,000-fold improvement in brightness compared to existing sources. The waveguide-integrated sources exhibit internal generation rates greater than 10 million pairs per second below 50 uW pump power, emit in a wavelength ranges spanning 1400 nm to 1700 nm, produce heralded single photons with >99% purity, and violate Bell’s inequality by more than 40 standard deviations with visibility >97%. After developing an efficient source of quantum light, we demonstrate the fundamental building blocks required for chip-scale quantum photonic circuitry including chip-to-fiber couplers, waveguide crossers, optical filters, interferometers, and spectral pulse shapers all with comparable performance to the silicon and silicon nitride quantum photonic platforms.

Using the bright source of quantum light, we demonstrate the most efficient time-bin quantum key distribution protocol to date with 8 kbps sifted key rates using less than 110 uW of input power while maintaining error rates below 10% and sufficient two-photon visibility to ensure security of the channel. As a proof of principle, a quantum key is distributed across 12 km of deployed fiber on the UCSB campus and used to transmit a 21 kB image with <9% error. Finally, preliminary chip-scale quantum photonic circuits are developed to demultiplex qubits and perform the Hong-Ou-Mandel two-photon interference experiment on an integrated photonic chip. This circuit utilizes beamsplitters, optical filters, interferometers, and efficient ring-based quantum light sources as a foundation for larger circuits and experiments in the near future. Additionally, the III-V semiconductor material platforms offer exciting potential for engineering other quantum states of light, including squeezed vacuum states and broadband entangled-photon pairs spanning visible to telecommunications wavelengths. This work demonstrates the benefits of III-V semiconductor materials for nonlinear quantum photonic circuits that can surpass the capabilities possible with state-of-the-art silicon photonics.