Nitrogen vacancy centres in diamond have shown strong potential for use as qubits in quantum computing. However, nitrogen vacancy centres have a low percentage of overall emission emitted through the Zero Phonon Line (ZPL). In addition, the fabrication of NV centres with controlled positioning is limited. This thesis presents the ZPL linewidth characterisation of NV centres fabricated using laser writing and laser diffusion. Only one out of the six NV centres characterised showed a <100 MHz average single scan linewidth, which suggests that the laser diffusion process needs to improved. The low ZPL emission can be enhanced by cavity coupling to Fabry-Pérot microcavities. Fabry-Pérot microcavities are susceptible to vibrations, which will decouple the cavity from the NV centre and reduce the photon enhancement. A cavity stabilisation system based on cavity fringe stabilisation is implemented, and the results of the stability are presented. The Fabry-Pérot microcavity can be stabilised with 0.13 nm using fringe stabilisation. High collection efficiency of the emission is also desired for maximising the entanglement rate and is reduced by optical aberrations. For NV centres in bulk diamond, using Solid Immersion Lenses (SILs) removes the spherical aberration that is introduced when collecting NV emission through a planar interface. This thesis presents the results of modelling the aberrations for NV centres cavity coupled to a Fabry-Pérot microcavity and for hemispherical SILs on diamond. The wavefronts of the aberrations introduced by the planar mirror of the microcavity and the aberrations introduced by the positioning error of the SIL are modelled numerically. A deformable mirror is implemented into a confocal microscope to monitor the aberrations of the photonic devices and the results are compared with the numerical model. For Fabry-Pérot microcavities, the intensity loss caused by the aberrations introduced by the planar mirror was predicted to be less than 10 \% due to the thin mirror substrate and low numerical aperture of the cavity mode. This prediction was consistent with the experimental results where an effective 3 \% increase in light collection was observed with aberration correction. For the SILs, the model predicts that the collection efficiency is more sensitive to the lateral positioning of the SIL relative to the NV centre than the axial positioning error.
Nitrogen vacancy centres in diamond have shown strong potential for use as qubits in quantum computing. However, nitrogen vacancy centres have a low percentage of overall emission emitted through the Zero Phonon Line (ZPL). In addition, the fabrication of NV centres with controlled positioning is limited. This thesis presents the ZPL linewidth characterisation of NV centres fabricated using laser writing and laser diffusion. Only one out of the six NV centres characterised showed a <100 MHz average single scan linewidth, which suggests that the laser diffusion process needs to improved. The low ZPL emission can be enhanced by cavity coupling to Fabry-Pérot microcavities. Fabry-Pérot microcavities are susceptible to vibrations, which will decouple the cavity from the NV centre and reduce the photon enhancement. A cavity stabilisation system based on cavity fringe stabilisation is implemented, and the results of the stability are presented. The Fabry-Pérot microcavity can be stabilised with 0.13 nm using fringe stabilisation. High collection efficiency of the emission is also desired for maximising the entanglement rate and is reduced by optical aberrations. For NV centres in bulk diamond, using Solid Immersion Lenses (SILs) removes the spherical aberration that is introduced when collecting NV emission through a planar interface. This thesis presents the results of modelling the aberrations for NV centres cavity coupled to a Fabry-Pérot microcavity and for hemispherical SILs on diamond. The wavefronts of the aberrations introduced by the planar mirror of the microcavity and the aberrations introduced by the positioning error of the SIL are modelled numerically. A deformable mirror is implemented into a confocal microscope to monitor the aberrations of the photonic devices and the results are compared with the numerical model. For Fabry-Pérot microcavities, the intensity loss caused by the aberrations introduced by the planar mirror was predicted to be less than 10 % due to the thin mirror substrate and low numerical aperture of the cavity mode. This prediction was consistent with the experimental results where an effective 3 % increase in light collection was observed with aberration correction. For the SILs, the model predicts that the collection efficiency is more sensitive to the lateral positioning of the SIL relative to the NV centre than the axial positioning error.
The nitrogen-vacancy (NV) centre in diamond is one of the most prominent diamond defect centres studied for quantum information applications. The NV centre’s long coherence times and atom-like spin-optical properties have made it one of the possible candidates for a node in a quantum network. Recent achievements include their use as a ten qubit solid-state quantum register, and the first loophole-free Bell experiment separated at km-scale distances. However, current implementations of the NV centre in cavity quantum electrodynamic (cQED) protocols are restricted due to its low emission rate (3%) into its coherent zero-phonon line (ZPL) transition as well as spectral diffusion originating from electric field noise on nearby surfaces. Coupling NV centres in diamond membranes to open Fabry-Pérot fibre microcavities offers one possible solution by enhancing the emission rate into the (ZPL) without degrading the NV centre’s optical properties.The work in this thesis is concerned with the in-situ characterization of the Fabry-Pérot cavities. By adequately characterizing the membrane-in-cavity system in situ, we hope to pave the way to maximize the achievable enhancement of the zero-phonon line emission rate of an NV centre inside the cavity. We study analytical and numerical models for the membrane-in-cavity system and analyze their regime of validity. By combining these models with transmission and reflection characterization of the system we determine the diamond thickness and length of the cavity with a resolution of 100 nm. The cavity finesse ranges between 16000 to 26000, depending on the presence of the diamond membrane and mirror termination. Finally, we expand upon the previously presented models to include surface losses and achieve a method to measure (sub-nm-rms) surface roughness and (ppm) mirror losses by measuring the cavity linewidth as a function of wavelength
A quantumnetwork would allow the distribution of a quantum state over many spatially separated quantum nodes which individually possess the ability to generate, process and store quantum information. Connecting these nodes through quantum communication channels would enable sending quantum information over arbitrarily long distances, secret key distribution with guaranteed secure communication and distributed quantum computing. An appealing platform for implementation of quantum networks is the nitrogen-vacancy (NV) center in diamond. An NV center is an optically active defect in diamond created by the substitution of two adjacent carbon atoms in the diamond crystal matrix by a nitrogen atom and a neighboring vacancy. The NV center fits remarkably well in the described quantum network framework. Its electronic spin can be optically initialized, read out in a single shot, coherently manipulated and coupled to the nearby carbon-13 nuclear spins. These properties represent necessary ingredients of a multi-qubit quantum node. The possibility of the entanglement between the state of the NV center spin and a photon establishes a photonic quantum link which can enable the entanglement of the quantum nodes, forming a quantum network. First building blocks of the proposed quantum networks based on NV centers were demonstrated by performing entanglement between two distant NV centers separated by more than a kilometer. However, the current success rate of the entangling protocols is greatly reduced due to the low emission probability of the NV center photons into the resonant zero phonon line (ZPL) and the inefficient photon extraction from the diamond. This is the central problem which prevents promoting our experiments beyond proof-of-principle demonstration towards practical implementation of the proposed quantumnetworks. The goal of this thesis is tackling these issues by coupling NV centers to optical cavities which would greatly increase the rate of generation and collection of the ZPL photons through themechanism of Purcell enhancement. The design and the fabrication of the components constituting a diamond-basedFabry-Perot microcavity, as used in this thesis, are described in Chapter 4. For large enhancement of the NV center resonant emission, a low cavity mode volume is necessary. This is achieved by inserting a micrometers thin diamond slab into the cavity architecture; we present the fabrication details of such samples. Chapter 5 describes the fabrication of an integrated platform for microwave signal delivery to the NV centers within a diamond membrane in the cavity architecture. Microwave striplines and arrays of unique markers are embedded in the mirror onto which the diamond is bonded. We investigate the mirror optical properties post fabrication and find that the fabrication method preserves the mirror optical performance. We describe the diamond bonding method and demonstrate addressing of the NV center spin. In Chapter 6 we probe the properties of the cavity with the embedded diamond membrane through measurements of the cavity finesse. We investigate the cavity finesse dependence on length, mode structure and temperature. We further explore the operation at cryogenic temperatures by probing the effect of cryostation vibration on the cavity linewidth. Finally, in Chapter 7 we discuss ways of improving the existing experimental capabilities, outline the first steps for demonstrating enhancement of the NV center resonant emission and suggest future experiments that can be performed with this system. We conclude that coupling of the NV centers to the cavities developed in this research could lead to an increase of remote entanglement success rates by more than three orders of magnitude.
The nitrogen-vacancy (NV) center in diamond has an optically addressable, highly coherent spin. However, an NV center even in high quality single-crystalline material is a very poor source of single photons: extraction out of the high-index diamond is inefficient, the emission of coherent photons represents just a few per cent of the total emission, and the decay time is large. In principle, all three problems can be addressed with a resonant microcavity, which significantly boosts the emission rate of coherent photons into the cavity mode based on the Purcell effect. In practice though, it has proved difficult to implement this concept: photonic engineering hinges on nano-fabrication yet it is notoriously difficult to process diamond without degrading the NV centers. \nIn this thesis, we present a microcavity scheme which employs minimally processed diamond membranes, thereby preserving the high quality of the starting material. The miniaturized plano-concave Fabry-Perot microcavity platform features full in situ spatial and spectral tunability. We demonstrate a clear change in the lifetime for multiple individual NV centers on tuning both the cavity frequency and anti-node position. The overall Purcell factor for the zero-phonon line (ZPL) of ~30 translates to an increase in the ZPL emission probability from ~3% to ~46%. \nFurthermore, we report the creation of a low-loss, broadband optical antenna giving highly directed output from a coherent single spin in the solid state. The device, the first crystalline solid-state realization of a dielectric antenna, is engineered for individual NV electronic spins in diamond. The photonic structure preserves the high spin coherence of single-crystalline diamond (T2 ~100µs). We demonstrate a directionality of close to 10 and single photon count rates approaching one MHz. The analysis of the angular emission pattern of our device suggests that 95% of the broadband NV fluorescence is channeled into a solid angle corresponding to a numerical aperture of 0.8. \nThe abovementioned approaches feature complementary benefits. The narrowband enhancement of the ZPL emission rate provided by the microcavity benefits applications in quantum information processing relying on coherent photons. With the prospect of integrating lifetime-limited emitters and achieving a high ZPL collection efficiency our results pave the way for much enhanced spin-photon and spin-spin entanglement rates. On the other hand, by channeling the major fraction of the broadband NV fluorescence into a narrow solid angle the dielectric optical antenna facilitates efficient spin readout. Our approach enables a near-unity collection efficiency which, upon mitigation of the known photon losses, renders it a potential key technology for quantum sensing applications. \n
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.
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.
Single negatively charged nitrogen-vacancy (NV-) defect centres in diamond are attractive systems for the implementation of solid state quantum information processing (QIP) and quantum key distribution (QKD). In both approaches, the coupling of an individual NV-centre to a resonant microcavity is of interest: it would improve the optical readout of the NV-centre electron spin state in QIP and enhance the collection efficiency of the single photon fluorescence in QKD while increasing the coherence length of the emission. We theoretically demonstrate that, at room temperature, the coupling to a single-mode Fabry-Perot microcavity can be used to obtain a fluorescence spectrum narrowing without loss of total intensity. We also experimentally demonstrate the control of the fluorescence spectrum of a single NV-centre by adjusting the thickness of the microcavity. Finally, we report a doubling of the emission spectral density with respect to the single mirror configuration.