Collaborative Research: A Fast, Scalable, and High-Fidelity Spin Entangling Gate On-A-Chip

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By harnessing uniquely quantum mechanical effects such as quantum superposition and entanglement, it becomes possible to create exponentially fast quantum computers, unconditionally secure quantum networks, and ultraprecise quantum sensors. However, to achieve these quantum advantages requires controlled interactions among a large number of quantum bits, which is extremely difficult to realize. One way to scale up a quantum system is to interconnect multiple small-scale quantum modules using a bus formed by optical photons. This program aims to develop a chip-integrated quantum photonic circuit that can optically interconnect electron spins with unprecedented entanglement rate and fidelity. To deterministically couple two or multiple electron spins with the photonic circuit, the principle investigators will explore a new hybrid photonics platform by merging bottom-up material synthesis with top-down device fabrication. This capability will pave the way towards scalable manufacture of quantum circuit in an integrated photonics chip and open new opportunities in both solid-state spin and optical photon based quantum information processing. In addition to the research component, this program will include the training of the next generation of scientists and engineers in quantum science and technology, as well as a strong outreach effort to educate K-12 students and broaden participation in STEM fields. Technical Description: Among the many qubit platforms for solid-state quantum technologies, defect centers in diamond exhibit some of the best spin coherence properties. Both the electron and nuclear spins of the defect centers can be used as qubits, and they can interact with each other through direct dipolar coupling. However, there is a fundamental limit in scaling up this system, due to the short range of the dipolar interactions. On the other side, photons are ideal carriers to mediate remote entanglement. They are highly versatile interconnects and can bridge quantum interactions over multiple distance scales from micrometers to kilometers. To attain the full potential of spin-based quantum technologies requires photon-mediated entanglement with sufficient rate and fidelity, which is difficult to achieve with traditional entanglement schemes based on spontaneous emission. The proposed research aims to develop a new entanglement scheme based on cavity scattering, which will significantly boost the achievable entanglement rate and fidelity. To deterministically couple two or multiple spins with different cavities, the principal investigators will explore a new device engineering approach by merging bottom-up material synthesis with top-down nanofabrication. Specifically, they will develop a novel technique to grow nanodiamonds on a mature photonic material, silicon nitride. Following the material growth, the researchers will use top-down nanophotonic engineering to develop a coherent spin-photon interface by coupling single electron spins of silicon-vacancy centers in nanodiamonds with silicon nitride nanocavities. Combining both capabilities, the researchers will develop an integrated quantum photonic circuit to generate photon-mediated spin entanglement with an unprecedented entanglement rate and fidelity.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.