Laser-written nitrogen-vacancy centres in diamond membranes for quantum computing
Joe Gore
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.
•Sep 17, 2026•0 citations