We study single silicon vacancy (SiV) centres in chemical vapour deposition (CVD) nanodiamonds on iridium as well as an ensemble of SiV centres in a high-quality, low-stress CVD diamond film by using temperature-dependent luminescence spectroscopy in the temperature range 5–295 K. We investigate in detail the temperature-dependent fine structure of the zero-phonon line (ZPL) of the SiV centres. The ZPL transition is affected by inhomogeneous as well as temperature-dependent homogeneous broadening and blue shifts by about 20 cm −1 upon cooling from room temperature to 5 K. We employ excitation power-dependent g (2) measurements to explore the temperature-dependent internal population dynamics of single SiV centres and infer mostly temperature-independent dynamics.
We investigate phonon induced electronic dynamics in the ground and excited states of the negatively charged silicon-vacancy ( ) centre in diamond. Optical transition line widths, transition wavelength and excited state lifetimes are measured for the temperature range 4 K–350 K. The ground state orbital relaxation rates are measured using time-resolved fluorescence techniques. A microscopic model of the thermal broadening in the excited and ground states of the centre is developed. A vibronic process involving single-phonon transitions is found to determine orbital relaxation rates for both the ground and the excited states at cryogenic temperatures. We discuss the implications of our findings for coherence of qubits in the ground states and propose methods to extend coherence times of qubits.
In the last decade atom-sized point defects in diamond, so called colour centres, have proven to be suitable systems for quantum information processing as well as quantum communication and sensing applications. Besides the wellknown nitrogen vacancy centre, the negatively charged silicon vacancy centre has recently proven to be a promising candidate due to its superior spectral properties such as narrow zero-phonon line widths and small phonon sidebands. In this feature article (article No.201700586) the authors summarize the center's electronic and optical properties as well as recent advances in controlling its quantum state using microwave as well as ultrafast all-optical control techniques which make it accessible for future quantum information processing applications. Moreover, the authors outline potential future experimental directions to improve the center's coherence time scale which so far is limited by phonon-induced decoherence. The cover picture shows an artistic representation of the silicon vacancy centre with its inversion-symmetric structure in a diamond crystal being addressed by an ultrashort laser pulse.
Atomic defects in solids offer access to atom-like quantum properties without complex trapping methods while displaying a rich physics due to interactions with their solid-state environment. Such properties have made them an advantageous building block for quantum information processing, in particular to construct a quantum network, where information would be encoded in spins and transferred between nodes through photons. Among defects in solids, the negatively charged silicon-vacancy centre in diamond (SiV
−) has attracted attention for its very promising optical properties for such a network. In this thesis, we investigate the spin properties of the silicon-vacancy centre as a potential spin-photon interface. First, we use resonant excitation of an SiV
− centre in an external magnetic field to selectively address different electronic states and analyse the resulting fluorescence. We find evidence of selection rules in the optical transitions revealing that the centre possesses an electronic spin S = 1/2. Making use of the dependence of such selection rules on the applied magnetic field orientation, we resonantly drive two optical transitions forming a
Λ-scheme. In the double resonance condition, we achieve coherent population trapping, whereby the SiV
− is pumped into a dark state corresponding to a superposition of the two addressed ground states of opposite spin. This technique allows us to evaluate the coherence time of the dark state and hence of the spin, while demonstrating the possibility of all-optical control of the spin when a
Λ-scheme is available. We then use resonant optical pulses to initialise and read out the spin state of a single SiV
−. By tuning a microwave pulse into resonance between two ground states of opposite spin, we demonstrate optically detected magnetic resonance. Subsequently, by varying the duration of a resonant microwave pulse, we achieve coherent control of a single SiV
− electronic spin. Through Ramsey interferometry, we measure a spin dephasing time of 115
± 9 ns. We then investigate interactions of the SiV
− with its environment. We analyse the hyperfine interaction of the SiV
− spin with the nuclear spin of
29Si, with a view to taking advantage of the long-lived nuclear spin in the future. We show that single-phonon-mediated excitations between electronic states of the SiV
− are the dominant spin dephasing and population decay mechanism and evaluate how external strain alters optical selection rules and can be used to improve the coherence time of the spin.
This thesis reports research conducted on point defects in single crystal diamond utilising the complementary techniques of electron paramagnetic resonance (EPR), optical absorption and photoluminescence (PL) spectroscopy. Intentional silicon-doping of chemical vapour deposition (CVD) diamond allowed the production of samples grown on differently oriented substrates and containing distinct silicon isotopic abundances. The EPR spectrum of the neutral charge state of the silicon split-vacancy centre in diamond, (Si-V)0, has been characterised in the literature. Evidence for the assignment of the 1.31 eV zero phonon line (ZPL) seen in absorption and PL to the 3A2g -> 3A1u transition arising at (Si-V)0 is presented. Reversible charge transfer between the negatively charged centre, (Si-V)− (ZPL at 1.68 eV), and (Si-V)0 enabled the determination of calibration factors relating defect concentrations to their respective ZPL intensities. Preferential alignment of trigonal centres, such as (Si-V), in CVD material grown on {110}-substrates has been observed. The formation of (Si-V) centres during CVD synthesis and via irradiation and annealing of silicon-doped diamond was studied. Variable temperature EPR spectroscopy under illumination was used to investigate the optical spin polarisation (SP) of the (Si-V)0 3A2g ground state. Two different mechanisms for the SP are considered; selective intersystem crossing of (Si-V)0, and photoionisation of (Si-V)−. The properties of (Si-V)0 are compared to those of the extensively studied negatively charged nitrogen vacancy centre, (N-V)−. The effective spin-lattice relaxation and spin polarisation rates for both centres during continuous illumination are explored using pulsed EPR methods. A new defect, labelledWAR3, with spin S = 1 2 was observed in silicon-doped diamond and characterised using multifrequency EPR. Analysis of the data revealed that WAR3 is the neutral charge state of a silicon divacancy complex decorated by a hydrogen atom, (Si-V2:H)0. The experimentally derived 29Si and 1H hyperfine parameters are in good agreement with the values calculated using the spindensity- functional technique, confirming this model and ruling out a non-planar structure.
The negative Nitrogen-Vacancy (NV) colour centre in diamond is an atomic-level impurity that is remarkably well suited for quantum information processing and quantum metrology applications. Underlying these properties is the fact that optical illumination causes the NV centre to preferentially populate one of its three spin levels. The mechanism that gives rise to this spin polarisation has not been well understood, despite intense research interest in the NV centre and its applications. This thesis arrives at an account of the spin polarisation mechanism, although a detailed theoretical treatment remains outstanding. The energy level structure of the NV centre is investigated using techniques of solid state spectroscopy. These include absorption and photoluminescence measurements made on diamonds at cryogenic and room temperatures in the presence of uniaxial stress, magnetic fields, and microwaves. Pulsed light experiments allow the dynamic processes within the NV centre to be measured. It is shown that at room temperature the orbital doublet excited state behaves like an orbital singlet, and this description is elegantly confirmed by single-site observations. This picture of the excited state is inconsistent with cryogenic measurements, and its behaviour is examined across a range of temperatures in order to resolve this tension. It is shown that electron-vibration interaction accounts for the quenching of orbital properties in the excited state with temperature. A new optical transition within the NV centre is reported at a wavelength of 1042 nm. It is established that this transition occurs between the intermediate singlet levels which are thought to provide the decay pathway that produces spin polarisation. The results are consistent with this understanding, and the order and properties of these singlet levels are established. Electron-vibration interaction is observed to be important within the singlet system. It is shown that electron-vibration interaction is an important detail in the NV optical pumping cycle. Taking it into consideration leads to the first plausible physical description of the spin polarisation mechanism. -- provided by Candidate.