Quantum optics and sensing with diamond defects

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Single spins embedded in solid-state materials represent a quintessential quantum system, exhibiting quantum mechanical behaviour over seconds, minutes, or even hours at cryogenic temperatures. This remarkable coherence is a resource for a new generation of technologies that exploit the features of quantum mechanics, such as superposition and entanglement, to solve important problems in computing and information security. In contrast to superconducting qubits or trapped ions, these solid-state systems are robust, rigidly held within their crystal host, operate at room temperature, and can be easily integrated with nano-fabricated circuitry. While remarkable advances have been made in controlling individual spins, an outstanding challenge is to develop quantum-coherent connections between spins that are scalable, enabling one to link them into a larger network. The proposed research program will pursue a key building block to meet this challenge: an efficient and coherent quantum interface between spins and photons, which will enable fast, photon-mediated interactions between distant spins. Our approach uses spins associated with optically-active defect centres in diamond, and enhances their interactions with light using microscopic cavities formed on the tip of optical fibres. This spin-photon interface could facilitate creation of so-called "quantum repeaters" to secure communication over global distances, or, ultimately, even a quantum version of the internet. In addition to their future applications in quantum information science, the quantum states of spins have applications in metrology: they are exquisitely sensitive to local magnetic fields. Each spin thus represents an atomic-scale sensor. The second focus of our research will use spins associated with the nitrogen-vacancy defect in diamond to study the dynamics of nanoscale magnetic devices. We will examine how these devices can be controlled with currents of spin-polarized electrons, sense the spin-wave excitations produced, and ultimately explore whether the quantized version of these magnetic excitations ("magnons") could offer a new paradigm for quantum-coherent connections between spins. Beyond the potential applications for magnons in quantum technologies, the techniques we develop will offer new insight into nanoscale magnetic phenomena, with direct application to modern (classical) computation and communication hardware. The outcomes of this research program could thereby impact information technologies for both long- and near-term applications.