Nitrogen-vacancy (NV) centres in diamond show great promise for quantum computing, where photonic entanglement can be generated using long-lived optically-active spins [1]. A quantum processor would require a large number of interconnected NV centres, and an integrated platform becomes necessary for control and routing. Benefits of integration include nanophotonic cavities with ultra-small mode volumes [2], multilayer electronics, and use of existing integrated quantum photonic components and architectures. Pick-and-place methods have been adopted to combine diamond microchiplets with aluminium nitride [3]. Photonic and spin properties has also been measured in nanodiamond encapsulated in optimised silicon nitride (SiNx) [4]. However, challenges remain to interface diamond with photonics in a scalable and manufacturable process. In particular, the need to identify and manipulate stochastically located emitters requires precise and time-intensive confocal microscopy.
Nitrogen-vacancy (NV) centres in diamond show great promise for quantum computing, where photonic entanglement can be generated using long-lived optically-active spins [1]. A quantum processor would require a large number of interconnected NV centres, and an integrated platform becomes necessary for control and routing. Benefits of integration include nanophotonic cavities with ultra-small mode volumes [2], multilayer electronics, and use of existing integrated quantum photonic components and architectures. Pick-and-place methods have been adopted to combine diamond microchiplets with aluminium nitride [3]. Photonic and spin properties has also been measured in nanodiamond encapsulated in optimised silicon nitride (SiNx) [4]. However, challenges remain to interface diamond with photonics in a scalable and manufacturable process. In particular, the need to identify and manipulate stochastically located emitters requires precise and time-intensive confocal microscopy.
Finding the right material platform for engineering efficient photonic interfaces to solid state emitters has been a long-standing bottleneck for scaling up solid state quantum systems. In this work, we demonstrate that nitrogen rich silicon nitride, with its low auto-fluorescence at visible wavelengths, is a viable quantum photonics platform by showing that nitrogen vacancy centers embedded in nanodiamonds preserve both their quantum optical and spin properties post-encapsulation. Given the variety of high-performance photonic components already demonstrated in silicon nitride, our work opens up a promising avenue for building integrated photonic platforms using solid state emitters.
Abstract The nitrogen-vacancy (NV) centre in diamond is a premier solid-state defect for quantum information processing and metrology. An integrated diamond quantum device harnesses the collective properties of multiple NV centres, enabling room-temperature quantum computing and sensing. While large-scale devices are poised to fill an important gap in the burgeoning quantum technology landscape, their practical realisation has not been achieved using current top-down fabrication techniques such as ion implantation. Consequently, this necessitates the development of a bottom-up fabrication technique, which is scalable, deterministic, and possesses atomic-scale precision. Informed by existing methods for fabricating phosphorous defect qubits in silicon, we envision a hydrogen depassivation lithography technique for atomically-precise manufacturing of nitrogen-vacancy centres in diamond. This perspective article outlines a viable multi-step procedure for realising scalable fabrication of diamond quantum devices and identifies the key challenges in its development.
Efficient interfaces between optical photons and quantum bits are fundamental building blocks for quantum networks and large-scale quantum computers. We demonstrate an integrated platform for scalable quantum optics based on color centers coupled to diamond nanodevices. First, we incorporate nitrogen-vacancy (NV) centers in diamond into hybrid nanophotonic crystal cavities. Despite our progress towards coupling the NV to cavity photons, we find that the NV-cavity interface is limited by the imperfections of the color center's properties and its local environment. We approach this challenge first by characterizing and modifying the diamond surface to reduce the environmental noise affecting the NV optical transition. Subsequently, we develop a new approach that makes use of a quantum emitter with intrinsic protection from this noise: the silicon-vacancy (SiV) center in diamond. We demonstrate that SiV centers introduced through ion implantation feature highly coherent optical transitions, even inside nanostructures. By placing SiV centers inside all-diamond photonic crystal cavities, we achieve strong SiV-photon coupling with a cooperativity of greater than 20. Using this platform, we realize a quantum-optical switch controlled by the spin degree of freedom of a single SiV. By measuring intensity correlations of indistinguishable Raman photons emitted into a single waveguide, we observe a quantum interference effect resulting from the superradiant emission of two probabilistically entangled SiV centers. Finally, we demonstrate controllable photon-mediated interactions between the spin degrees of freedom of two cavity-coupled SiV centers. When the optical transitions of the two SiV centers are tuned into resonance, the coupling of each SiV to the common cavity mode results in a coherent interaction between the two SiVs, leading to the observation of super- and sub-radiant two-SiV states.
Manipulation of quantum effects for computing is an important step in the creation of faster and more powerful computational tools. The nitrogen vacancy centre (NVC) in diamond, when in the negative charge state, has shown significant promise towards being used as a qubit, such as optical polarization and readout, long coherence times (T∗ 2 and T2), and a long relaxation time T1. Such times are desired for producing good quantum memory, utilising the spin state of the sites. Cryogenic conditions are required for facilitating the optical entanglement of two NVC, and can serve to significantly extend these times. These sites have been created using a laser writing technique, capable of producing a large number of NVC with good coherence characteristics at arbitrary locations within diamond samples. This thesis reports on methods to reliably create NVC with good coherence characteristics for quantum computing applications by altering the annealing regime used. This annealing regime has been improved by the design and construction of a new vacuum furnace with better temperature control and thermometry. Steps towards the ideal combination of annealing conditions for a range of desired NV concentrations and characteristics have been undertaken, showing good conversion of laser written sites to NVC. NVC have been systematically tracked using a confocal fluorescence microscope apparatus throughout multiple annealing steps, with the collection of coherence data including T1, T2 and T∗ 2 on the same NV centre at multiple annealing temperatures, for the first time. This data allows for tracking of the influence of annealing temperature on the coherence properties of sites without the caveat of comparing data from either different sites, or different diamonds altogether. Such centres show coherence times above 800 μs, with production rates at and above 40% for single emitters. NVC are limited by their weak emission into the zero phonon line. These issues can be alleviated by increasing the collection efficiency of photons, which requires thin diamonds capable of being used with cavity systems. Steps towards producing such diamonds by a thin-region design have been undertaken and show promising results. Further efforts include the use of nanostructures on the diamond surface, which has been implemented and detailed. Magnetic control of NVC within diamond, especially at cryogenic temperatures, introduces some operational complexity to potential quantum operations. By implementing the use of diamond cut at the (111) crystal plane, this alignment is likely to become more straightforward, enabling future work with diamond at this crystal plane. Diamonds have been produced, with NVC laser written in, for the first time.
Candidate platforms in the global endeavour to build long distance quantum networks need to satisfy a variety of properties to act as a network node. These include - interfacing between stationary spins and photonic flying qubits, long quantum memory times, indistinguishable photons and a large photon flux. Cavities operating in the weak coupling regime offer the advantages that the spontaneous emission rate of the emitter is enhanced while the loss rate from the cavity is also high. With optimized output couplers, this results in an increased photon extraction efficiency and the proportion of coherent photons is also improved. Cavity-coupled Nitrogen-vacancy centers in diamond (NV) fulfill most of these criteria with one weakness remaining - sensitivity to electronic noise [1]–[3].
Abstract A common theme in the implementation of quantum technologies involves addressing the seemingly contradictory needs for controllability and isolation from external effects. Undesirable effects of the environment must be minimized, while at the same time techniques and tools must be developed that enable interaction with the system in a controllable and well-defined manner. This chapter addresses several aspects of this theme with regard to a particularly promising candidate for developing applications in both metrology and quantum information, namely the nitrogen-vacancy (NV) centre in diamond. The chapter describes how the quantum states of NV centres can be manipulated, probed, and efficiently coupled with optical photons. It also discusses ways of tackling the challenges of controlling the optical properties of these emitters inside a complex solid state environment.