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Deterministic spin and photon control with a symmetry protected colour centre



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Parker, Ryan 


Quantum networking, whereby quantum mechanical entanglement is distributed through a network and used as a vector for information transfer, is an ambitious emerging technology. It requires a stationary, compute, qubit at a node in the network to interface with a flying, photonic, qubit for information distribution where these two qubits have different requirements to be technologically relevant. The stationary qubit needs long-lived internal degrees of freedom that can be coherently manipulated and optically interfaced. Whereas, the flying photonic qubit should interact minimally with the environment, to prevent decoherence from disrupting information distribution throughout the network. These two sets of independent requirements for the stationary and flying qubits impose strong engineering constraints on the underlying physical system underpinning the quantum network, and no one physical system has demonstrated a scalable solution that meets both sets of requirements.

In this thesis, the negatively charged tin vacancy centre (SnV) in diamond is presented as a viable, symmetry protected, platform for quantum networking applications. The stationary qubit in the network is formed by the SnV centre's intrinsic optically addressable spin-1/2 qubit. This thesis accesses the coherence of the SnV centre's spin-manifold for the first time and is subsequently leveraged to achieve multi-axis coherent control of the SnV centre's spin at Ω/2π = 4.5(1) MHz Rabi frequencies and with 82(5)% π/2-gate fidelities. Leveraging this control reveals long-lived internal degrees of freedom yielding an inhomogeneous dephasing time of T2 = 1.4(3) µs. Access to an ancillary quantum register, formed of proximate nuclear spins, is also shown, which further provides a resource for quantum networking by enabling quantum memory operations and quantum state storage to be available during network activity.

The flying photonic qubit interaction channel takes the form of the optically addressable, spin-selective, transitions of the SnV centre. This thesis demonstrates, for the first time, isolation of coherent photons from the SnV centre with 99.7−2.5+0.3% purity and 63(9)% indistinguishability. The symmetry protected nature of the SnV centre's spin manifold enables up to 106 identical photons to be generated per entanglement attempt before optical coherence is lost. Full quantum control of the optical transition is further demonstrated, yielding a 77.1(8)% fidelity optical π-pulse in 1.71(1) ns. Thus, the presence of both a robust photon-photon interaction and controllable optical channel is demonstrated for quantum networking applications leveraging the SnV centre.

The stationary SnV spin qubit and the flying photonic qubit are combined in a single high-efficiency packaged platform, yielding a 57(6)% collection efficiency and the observation of 5-photon states. A giant optical non-linearity conditional on the spin qubit's state is used for information transmission in a two-node directional network. Further, the presence of an ancillary quantum memory register is extended through the use of the intrinsic spin-1/2 117Sn register of the 117SnV centre. This novel resource, discovered in this thesis, enables a high-efficiency photonic interface to interact directly with the 117Sn nuclear spin degrees of freedom. Thus, nuclear initialisation to 98.6(3)% fidelity and single-shot optical nuclear spin readout with 80(1)% fidelity are achieved in an all-optical control protocol, significantly reducing the overhead per qubit needed for quantum repeater nodes.

Therefore, this thesis presents the SnV centre in diamond as a novel resource for quantum networking. The symmetry of the centre enables robust coherence of both the stationary spin-qubit and the flying photonic qubit that is insensitive to nano-photonic integration. This high intrinsic coherence positions the SnV centre for class-leading integration into Purcell enhanced cavity systems. Such integration would then facilitate near unity collection efficiencies and control fidelities, thereby enabling fault-tolerant quantum networking with a single, low overhead, platform.





Atature, Mete


Networking, Photonics, Physics, Quantum, Solid-state


Doctor of Philosophy (PhD)

Awarding Institution

University of Cambridge
General Sir John Monash Foundation ND Goldsworth Trust NanoDTC, G-Research Grant