On-demand generation of multi-photon linear cluster state

Photonic quantum devices promise a scalable way to realize real-world applications of quantum communication and computation. An integral part of this vision comprises utilizing cluster states, i.e., a type of highly entangled multi-qubit state which provides the fault tolerance needed in almost all quantum protocols. I plan to demonstrate the first on-demand multi-photon cluster state source. Although the realization of an efficient cluster state source has been a long-standing challenge, recent improvements in quantum dot spin control and the collection efficiency in photonic crystal waveguides have made this ambitious goal feasible. I will use a recently established technique to prepare and cool the nuclear spins surrounding the quantum dot. Cooling down the nuclear spins will improve the electron spin dephasing time by 20 folds to 50 ns, which corresponds to above 99% spin rotation fidelity. To improve the photon collection efficiency, I will utilize a one-sided photonic crystal waveguide which provides a near-unity coupling to a single waveguide mode. Moreover, I will use an inverse design method to improve the chip-to-fiber coupling efficiency to above 90%. The cluster state generation will be implemented with a time-bin protocol, which entangles the quantum dot spin states with the emission time of single photons. Using realistic experimental parameters, the estimated three-photon cluster state fidelity in this protocol is above 80%. The project will achieve a cluster state production rate at 5 orders of magnitude higher than the current state-of-the-art devices. The successful demonstration of an on-demand cluster state source is a critical step towards scalable and practical photonic quantum architecture.