Low temperature spectroscopy of single colour centres in diamond - the silicon-vacancy centre in diamond

In the first part of the thesis, we present a historical overview of how the negatively charged silicon-vacancy (SiV−) centre in diamond was discovered. In it the contribution of silicon atom, a vacancy and the negative charge state of the centre was established and lead us into a brief discussion of the physical structure of the centre and its D3d symmetry. These details represent a literature review of the ensemble properties of SiV− centres, as no new experimental results are presented. Starting from the D3d symmetry, which at this point will be assumed to be correct, the electronic level structure was derived from the electron wave functions. Following the argument we extracted the properties of the expected transitions, especially polarisation, and compared them to our measurements at room and cryogenic temperatures [1]. Thus we established the electronic properties of the centre and were able identify two different excited states. By focussing on the zero-phonon-line (ZPL) of the transition at cryogenic temperatures we then investigated the inhomogeneous distribution of the photoluminescence excitation (PLE) spectra in the SiV− centre in our diamond samples grown by chemical vapour deposition (CVD). A high spectral overlap in all of the investigated sites pointed to a low sensitivity of SiV− centres to both strain and local electric fields [2]. Interestingly, the shielding due to a large spin-orbit splitting could be distinguished from an even stronger shielding arising from the centre’s inversion symmetry. With these highly uniform centres, the natural next step was to interfere photons from two distinct centres in a Hong-Ou-Mandel (HOM) experiment. By using filtering techniques and selecting two similar sites below a flat surface, an interference contrast of over 70 % was identified [3]. The scheme used was especially favourable since about half of the emitted photons arose from the interfered transition, proving the centre to be a promising candidate as an efficient single-photon source for quantum cryptography applications. Due to the low inhomogeneous distribution of the ZPL we were able to detect an isotopic shift of the optical transitions at the single site level [4]. The individual SiV− sites formed three groups corresponding in frequency of occurrence to the natural abundance of the silicon isotopes and the shift matched the simple harmonic oscillator model. With this knowledge about the isotopes, it was possible to investigate the phonon sideband (PSB) in emission for any isotopic shifts. When using off-resonant excitation, multiple peaks in the PSB could be identified but only the 64 meV peak showed an isotopic shift, identifying it as a local mode of oscillation. Starting from the investigation of the high energy phonons, we switched to temperature dependence measurements of the optical properties of the SiV− centre to capture the low energy phonon processes [5]. A low temperature investigation of optical line width and orbital relaxation time (T Orbit 1 ) showed a linear behaviour indicating a direct single-phonon process between the orbital states in the ground and excited state doublets. At higher temperatures, a cubic dependence for the line width and transition wavelength established a two-phonon dephasing as the main electron-phonon process. All the measured results were consistently described by the microscopic model developed here. Additional strength was added by consistent temperature dependent studies of spin-orbit splitting and optical lifetime. Having a thorough understanding of the SiV− centre and the processes involved, made it possible to investigate its potential as a spin qubit [6]. A magnetic field was used to lift the spin degeneracy in the ground state, and different spin states were selectively addressed using narrow band resonant excitation. As a first step in manipulating the spin, an initialisation into a defined spin state with a fidelity of over 95 % was demonstrated. In a magnetic field well aligned along the centre’s symmetry axis, spin relaxation times (T Spin 1 ) of over 2 ms were reached and an efficient read-out process was established. Two resonant lasers were used to demonstrate coherent population trapping (CPT), which involves the preparation of a coherent dark superposition state. From the width of this dark dip in the spectrum the coherence time of the superposition was calculated to be much shorter than T Spin 1 but of the same order as T Orbit 1 . The coherence was concluded to be limited by orbital dephasing processes and we proposed techniques to overcome these limitations. One way to get longer coherence is to use nanodiamonds with a size smaller than half the phonon wavelength to suppress the dephasing process. Since the nanodiamonds reported in literature so far have not shown a narrow ZPL, we investigated a new kind a high-pressure high-temperature (HPHT) grown nanodiamonds. The silicon has been added by silanoxides in the growth process so that we were able to find SiV− centres in this sample. Investigating the spectral properties at cryogenic temperatures, nearly lifetime limited line widths have been found making these nanodiamonds good candidates for further spin applications with SiV− centres. Another advantage of the nanodiamond was that the relatively easy handling makes it a good candidate to incorporate into microcavities in the future, using the full tool set of modern quantum optics.