The Nitrogen Vacancy (NV) center is a defect formed in diamond by one substitutional nitrogen atom and an adjacent vacancy. The NV forms a ground state spin triplet that can be controlled coherently at room temperature using electromagnetic fields. Due to its energy level structure, NV fluorescence is spin-state dependent, allowing simple routes for optical initialization and readout. For these reasons, the NV center is one of the most prominent candidates for room temperature quantum information processing.
We use a confocal microscope (a) to perform fluorescence imaging of diamond samples. NV centers are excited by an off-resonant 532 nm laser and subsequently emit fluorescence with a 637 nm zero-phonon line (b), resulting in bright fluorescence spots in the confocal image (c). To create a qubit, we isolate the states ms = 0, -1 from the state ms = +1 by applying an external magnetic field to induce Zeeman splitting (b). Because of the spin-selective transitions and an extra shelving state 1A1 associated with the ms = -1, +1 states, the fluorescence is suppressed when the NV center is in ms = -1. It is precisely this mechanism that allows for optical initialization and readout of the spin state. Rabi oscillations can be observed by applying microwave radiation to the NV (d). Inhomogeneous dephasing of the qubit due to coupling with proximal electronic and nuclear spins can be overcome using spin-echo, which allows for refocusing of the qubit state, thereby extending the coherence time of the qubit.
Hyperfine coupling of the electronic spin with the intrinsic 14N and proximal 13C nuclear spins has been demonstrated at room temperature  with a coherence time exceeding one second . This electronic-nuclear spin coupling can serve as a basis for quantum registers and quantum memory.
We are also investigating NV centers at cryogenic temperatures where the transitions between the triplet ground state and each of the excited states can be individually addressed via resonant optical excitation (e). With resonant excitation it is possible to perform single-shot readout of a qubit state , which will be crucial for the realization of quantum error correction protocols. Furthermore, we can tune these resonances using on-chip electrodes (f).
While we can easily manipulate and read out NV centers, the host material, diamond, imposes a fundamental constraint on the readout efficiency. Given the high refractive index of diamond, the collection efficiency is limited by a very shallow critical angle of total internal reflection. We are working to overcome this limitation by fabricating solid immersion lenses (SILs) on the surface of bulk diamond (g). These hemispherical SILs are milled with high-energy gallium ions and positioned such that the NV center of interest is at the origin of the sphere. This eliminates dispersion due to refraction, thereby improving the overall collection efficiency.
In addition, we have begun utilizing ion implantation as a method of nanopositioning NV centers. Deterministic defect placement is crucial to the integration of NV centers with quantum photonic networks. Recent progress has been made using e-beam resist as a scalable implantation mask  and with implantation into high-optical-flux nanostructures . An array of implanted NV centers, imaged with our confocal scope, is visible in (h).References