UCLA

Efficient quantum networking implementations are required for the metropolitan-scale adoption of secure quantum communications and cryptography. Furthermore, quantum networking could enable modular quantum processor topologies for distributed quantum computing. Such a quantum network would require high-rate, high-fidelity entanglement distribution among multiple parties and high-fidelity spin-photon interfaces to implement quantum repeating and transduction protocols for connecting distant quantum processing nodes. For this purpose, in the scope of the thesis, we utilized energy-time entanglement for high-dimensional encoding and entanglement distribution and developed the necessary building blocks for quantum memory networks. First, we report the design of a chip-scale hybrid SixNy and thin film periodically-poled lithium niobate waveguide for generating high purity type-II spontaneous parametric down-conversion (SPDC) photons in the telecommunication band, yielding intrinsic purity with up to 95.17%. and an estimated 2.87x10^7 pairs/s/mW. Second, we demonstrate the assumption-free and measurement-efficient certification of high-dimensional entanglement with trusted measurement Einstein-Podolsky-Rosen steering using time-frequency bases at each receiver node. We show the efficient generation of a 31 × 31-dimensional time-frequency basis to certify high-dimensional entanglement. At the qudit source, we certify a lower bound of the maximum quantum state fidelity of 96.2 ± 0.2%, an entanglement-of-formation of 3.0 ± 0.1 ebits, an entanglement dimensionality of 24, and steering robustness of 8.9. Third, we demonstrate a four-node 1024-dimensional wavelength-multiplexed quantum network testbed with high noise resilience, dense information efficiency, and delivering record secure key rates at least one to two orders of magnitude higher than prior state-of-the-art. A dense photon information efficiency of 2.458 per entangled photon pair is obtained with sufficient resilience to tolerate a quantum bit error rate of up to 28.2%, owing to the noise robustness of high-dimensional entanglement and including error correction coding tailored for our quantum channels. We exemplify d-dimensional arrival-time encoding to encode multiple bits per photon in the entangled energy-time basis with a resulting key rate of up to 26.6 kbits/s per channel at the source and 5 kbits/s after 21 km distribution. Finally, we explored T-centers and *Cu centers as a potential spin-photon interface at the O-band for stationary nodes. We explored the process of developing T centers and transition-metal color center defects for high-fidelity spin-photon interfaces with less host-related decoherence pathways while examining the photoluminescence dynamics. Our process resulted in a T-center ensemble with minimum lattice disturbance with photophysical properties closer to ab initio predictions and host lattice. *Cu center-related doublet emission around 1312 nm close to the zero-dispersion wavelength is examined. A magnetic-field-induced broadening by 25% under 0.5 T is observed, which can be related to spin degeneracy, suggesting an alternative to T centers as a high-fidelity spin-photon interface.