Atomic defects in wide-bandgap materials, such as the nitrogen-vacancy (NV) color center in diamond, show considerable promise for the development of a new generation of quantum information technologies, but progress has been hampered by the inability to produce and engineer the defects in a controlled manner. Here, we demonstrate an all-optical method for the deterministic writing of individual NV centers at selected locations with high positioning accuracy. Ultrashort pulse laser processing is used to both create and diffuse defects inside the crystal through local annealing. During the laser-annealing process, online fluorescence feedback provides a trigger to stop processing once the NV formation is detected. This method provides a new tool for the optical fabrication of engineered materials and devices for quantum technologies.

Engineering single atomic defects into wide-band-gap materials has become an attractive field in recent years due to emerging applications such as solid-state quantum bits and sensors. The simplest atomic-scale defect is the lattice vacancy, which is often a constituent part of more complex defects such as the nitrogen-vacancy (NV) center in diamond, therefore an understanding of the formation mechanisms and precision engineering of vacancies is desirable. We present a theoretical and experimental study into the ultrafast laser generation of vacancy-interstitial pairs (Frenkel defects) in diamond. In a range of other materials, Frenkel defect formation has previously been linked to the recombination of laser generated excitonic states, however the mechanism in diamond is currently unknown and to date no quantitative agreement has been found between experiment and theory. Here, we find that a model for Frenkel defect generation via the recombination of a bound biexciton as the electron plasma cools provides good agreement with experimental data. The process is described by a set of coupled rate equations of the pulsed laser interaction with the material and of the non-equilibrium dynamics of charge carriers during and in the wake of the pulse.

The ability to create phase-controlled pulses of light with wavelength tunability has applications spanning quantum and classical communications networks. Traditionally, optical transmitters are able to either produce phase-controlled pulses at a fixed wavelength or require a chain of bulky and expensive external modulators to convert wavelength tunable continuous-wave light into optical pulses. One technology of great interest is quantum key distribution (QKD), a technology for generating perfectly random keys at remote nodes to ensure secure communications. Environments such as data centers, where the user needs change regularly, will require adaptability in the deployment of QKD to integrate into classical optical networks. Here we propose and demonstrate an alternative quantum transmitter design consisting of a multimodal Fabry-Perot laser optically injection locked by a wavelength tunable laser. The transmitter is able to produce phase-controlled optical pulses at GHz speeds with a tunable wavelength range of $>65\mathrm{nm}$ centered at 1550 nm. With this transmitter, we perform proof-of-principle QKD with secure bit rates of order Mb/s.