The nitrogen-vacancy (N-V) defect in ultra-pure diamond shows great promise for the implementation of qubits for quantum computing. The N-V defect is a three level system which behaves as an efficient room temperature source of single photons at a wavelength of 637 nm. The N-V centre ground state spin strongly modulates the rate of spontaneous emission providing a mechanism for spin read out. Recently, it is shown that spin-readout with the minimum number of photons can be achieved by efficiently coupling light to the transition in a sub- wavelength scale micro-cavity. The aim of this paper is to place the N-V centre into a cavity at the centre of a suspended slab photonic crystal structure made from hexagonal array of cylindrical air holes. The light is confined by distributed Bragg reflection in the plane of periodicity (xy) and by total internal reflection in the perpendicular plane (z). The work consists of both modelling the photonic crystal to optimise the parameters and fabricating the structures using focussed ion beam milling.
The nitrogen-vacancy (N-V) defect in ultra-pure diamond shows great promise for the implementation of qubits for quantum computing. The N-V defect is a three level system which behaves as an efficient room temperature source of single photons at a wavelength of 637 nm. The N-V centre ground state spin strongly modulates the rate of spontaneous emission providing a mechanism for spin read out. Recently, it is shown that spin-readout with the minimum number of photons can be achieved by efficiently coupling light to the transition in a sub- wavelength scale micro-cavity. The aim of this paper is to place the N-V centre into a cavity at the centre of a suspended slab photonic crystal structure made from hexagonal array of cylindrical air holes. The light is confined by distributed Bragg reflection in the plane of periodicity (xy) and by total internal reflection in the perpendicular plane (z). The work consists of both modelling the photonic crystal to optimise the parameters and fabricating the structures using focussed ion beam milling.
We use focused ion beam (FIB) etching to create suspended photonic crystal cavities in diamond. Our goal is to enhance and control the spontaneous emission of single N-V colour centres, which act like single `two-level atoms' emitting single photons. The single nitrogen-vacancy (N-V) colour centre in diamond has attracted a lot of interest because it can be optically addressed, is photostable at room temperature and has a paramagnetic ground state whose spin can be initialized and read-out optically. By enclosing the (N-V) colour centre in a resonant cavity we aim to develop rapid readout of this optically addressable single spin. We show a technique using FIB and confocal microscopy to locate single N-V centres and discuss progress towards etching cavities around single centres.
The Nitrogen-Vacancy (NV) centre in diamond possesses an optical read out of its spin state and shows great promise for applications in solid state quantum technologies. Its broad phonon assisted emission spectrum, with only 4% of emission from the zero-phonon line, is a major drawback to this. The optical microcavity will be essential in efficiently interfacing these centres with photonic networks. Here we present investigations into the coupling of NV centres in nanodiamond to tunable open-microcavities, both at room temperature and at cryogenic temperatures. These structures will be shown to achieve good photon confinement, with mode volumes down to 5λ3. Room temperature studies on ensembles of NVs will illustrate the tunable spectral and spatial overlap between the emitter and the cavity mode. It will be shown that small enhancements are possible in this regime. After the preparation and characterisation of single emitters, low temperature coupling to the narrowed zero-phonon line will be the central theme of this thesis. Single photon emission into the cavity mode is verified. We observe the enhancement of the light-matter interaction in this regime, by a 39% increase in the emission rate when cavity-coupled, with the dependence on mode volume also demonstrated. These results are important for the realisation of a spin-photon interface in scalable quantum networks.
The Nitrogen-Vacancy (NV) centre in diamond possesses an optical read out of its spin state and shows great promise for applications in solid state quantum technologies. Its broad phonon assisted emission spectrum, with only 4p of emission from the zero-phonon line, is a major drawback to this. The optical microcavity will be essential in efficiently interfacing these centres with photonic networks. Here we present investigations into the coupling of NV centres in nanodiamond to tunable open-microcavities, both at room temperature and at cryogenic temperatures. These structures will be shown to achieve good photon confinement, with mode volumes down to 5λ3. Room temperature studies on ensembles of NVs will illustrate the tunable spectral and spatial overlap between the emitter and the cavity mode. It will be shown that small enhancements are possible in this regime. After the preparation and characterisation of single emitters, low temperature coupling to the narrowed zero-phonon line will be the central theme of this thesis. Single photon emission into the cavity mode is verified. We observe the enhancement of the light-matter interaction in this regime, by a 39p increase in the emission rate when cavity-coupled, with the dependence on mode volume also demonstrated. These results are important for the realisation of a spin-photon interface in scalable quantum networks.
In this work we aim to exploit one of the most studied defect color centers in diamond , the negatively charged nitrogen vacancy (NV -) color center, a three level system which emits a single photon at a wavelength of 637nm providing a possible deterministic single photon emitter very useful for quantum computing applications.Moreover the possibility of placing aNV -in a photonic crystal cavity will enhance the coupling between photons and NV -center.This could also allow us to address the ground state of the NV -center, whose spin, could be used as qubit.It is also remarkable to notice that for quantum computing purposes it is very useful to increase the light collection from the NV -centers, and in order to do that we performed a study of another structure, the solid immersion lens, which consists of an hemisphere whose center is at the position of an emitter, in this case the NV -center, increasing the collection of the light from it.In order to create these structures we used a method called focused ion beam which allowed us to etch directly into the diamond many different kinds of structures.In order to allow an interaction between these structures and the NV -centers we need to have a method to locate the NV -center precisely under the etched structures.We developed a new technique (Marseglia et al. (2011)) where we show how to mark a single NV -center and how to etch a desired structure over it on demand.This technique gave very good results allowing us to etch a solid immersion lens onto a NV -previously located and characterized, increasing the light collection from the NV -of a factor of 8×.
The negatively charged nitrogen vacancy centre in diamond is known for its coherent spin properties and optical interface, and thus is regarded a promising candidate for quantum information applications [1]. Realisation of an efficient spin-photon interface with the NV centre is made challenging however by the fact that, in bulk diamond, only 3-4% of spontaneously emitted photons occur in the zero phonon line (ZPL). Placing NV centre in an optical cavity is being explored by several groups [2][3][4] as an effective way to selectively enhance the coherent emission of NVs and thereby increase the efficiency of the coherent spin-photon coupling. Previous work reported successful coupling of the NV in nano-diamond to an open access micro-cavity and observed enhanced ZPL emission [5]. However the NV centres in nano-diamond suffer from broadened zero phonon transition and poor spin coherence. By fabricating NV centres in a ~micrometre thick membrane of high purity single crystal material we can take advantage of the tunability of open access cavities, and at the same time, provide close-to-bulk crystal environment to maintain the coherent spin properties of the NV centres. Here we report our work on the tunable cavity coupling of the ZPL of a NV centre in a 1.2micrometre-thick diamond membrane at 4K. The diamond membrane is fabricated from a 0.5mm-thick E6 CVD diamond plate where ion implantation is carried out on both surfaces to create NV centres at the depth of around 70nm. The plate is then machined into 30micrometre-thick slices, and thinned by ICP-RIE with a combination of Ar/Cl[6] and pure oxygen plasma etching recipes. The open cavity consists of a concave mirror (99.99% reflectivity) deposited on a template fabricated using Focused Ion Beam (FIB) milling[7] and a planar mirror (99.8% reflectivity) which supports the membrane. For bare cavities with mirror radii of curvature (RoC) of 12micrometre, we measured a finesse of F~2000 and mode volume as small as 0.75micrometre^3. In-situ tuning of the cavity resonance is achieved with piezoelectric actuators. When mounted in our bath cryostat the cavity modes have dominant Lorentzian line profiles which indicate a passive stability of the cavity length of better than 0.15nm. No active locking is currently deployed. With the presence of a diamond membrane inside the cavities, the measured finesse and mode volume of a cavity with 12micrometre RoC are found to be around 300 and 3 micrometre^3, respectively. We attribute the reduction in finesse to scattering at the membrane-air and membrane-mirror interfaces. On coupling to the ZPL of a target NV centre, we record a factor of 4 increase in the saturated intensity of ZPL fluorescence compared to that measured from the same NV centre in absence of the concave mirror. This result is consistent with the calculated Purcell factor of 16 combined with a relatively low efficiency of light extraction (estimated to be around 19%) from the cavity due to the scattering losses.
Room temperature solid-state emitters provide a promising source of single photons for use in quantum technologies. The emitters studied in this thesis are the nitrogen-vacancy (NV) centre in diamond, and defects in two-dimensional hexagonal boron nitride (hBN). In this thesis, I present investigations into the coupling of these defects to open-access microcavities at room temperature. The optical microcavity serves to confine the electric field down to mode volumes on the order of λ3, increasing the spontaneous emission rate of these emitters and channelling this emission into well defined cavity modes. The general properties of both emitters will be presented, including examples of single photon emission, emission spectra, saturation behaviour and photo-stability. A few examples highlighting the long-term photo-stability of the NV centre under pulsed excitation will be displayed. This will highlight the suitability of the NV centre for a practical device. Following the preparation and characterisation of each emitter, single photon emission into the cavity modes is verified, reaching single photon purities greater than 97%. Enhancements in the spectral density of photons were observed for both the NV centre (~25×) and defect in hBN (~3×), along with decreases in the excited state lifetimes. While the NV centre provides a stable source of single photons, the cavity enhancement is limited by its broad emission spectrum. Defects in hBN present near-ideal optical properties in the absence of the cavity, and were expected to deliver superior results within the cavity. However, the poor photo-stability of these defects did not allow complete characterisation of the cavity coupled emission. FDTD simulations were combined with changes in the excited state lifetime to infer the quantum efficiency of these defects. The results of this thesis form the foundation for the realisation of a robust room temperature single photon source.