We investigate phonon induced electronic dynamics in the ground and excited states of the negatively charged silicon-vacancy ( − SiV ) 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 − SiV 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 − SiV qubits. IntroductionColour centres in diamond have emerged as attractive systems for applications in quantum metrology, quantum communication, and quantum information processing [1][2][3]. Diamond has a large band gap which allows for optical control, and it can be synthesized with high purity, enabling long coherence times as was demonstrated for nitrogen-vacancy ( − NV ) spin qubits [4]. Among many colour centres in diamond [5,6], the negatively charged silicon-vacancy ( − SiV ) centre stands out due to its desirable optical properties. In particular, near transform-limited photons can be created with high efficiency due to the strong zero-phonon line emission that constitutes ∼70% of the total emission. − SiV centres can also be created with a narrow inhomogeneous distribution that is comparable to the transform limited optical line width [7]. These optical properties, due to the inversion symmetry of the system which suppresses effects of spectral diffusion, recently enabled demonstration of two-photon interference from separated emitters [8] that is a key requirement for many quantum information processing protocols [9][10][11][12].Interfacing coherent optical transitions with long-lived spin qubits is a key challenge for quantum optics with solid state emitters [13][14][15][16][17]. This challenge may be addressed using optically accessible electronic spins in − SiV centres [18]. It has recently been demonstrated that coherent spin states can be prepared and read out optically [19,20], although the spin coherence time was found to be limited by phonon-induced relaxation in the ground states [19]. Here we present the first systematic study of the electron-phonon interactions that are responsible for relaxation within the ground and excited states of the − SiV centre. This is achieved by measuring the temperature dependence of numerous processes within the centre. A comprehensive microscopic model is then developed to account for the observations. In section 4.1 we discuss the implications of these phonon processes for spin coherences in the − SiV ground state, and identify approaches that could extend the spin coherences.

The silicon-vacancy (SiV − ) color center in diamond has attracted attention because of its unique optical properties. It exhibits spectral stability and indistinguishability that facilitate efficient generation of photons capable of demonstrating quantum interference. Here we show optical initialization and readout of electronic spin in a single SiV − center with a spin relaxation time of T 1 ¼ 2.4 AE 0.2 ms. Coherent population trapping (CPT) is used to demonstrate coherent preparation of dark superposition states with a spin coherence time of T ⋆ 2 ¼ 35 AE 3 ns. This is fundamentally limited by orbital relaxation, and an understanding of this process opens the way to extend coherence by engineering interactions with phonons. Hyperfine structure is observed in CPT measurements with the 29 Si isotope which allows access to nuclear spin. These results establish the SiV − center as a solid-state spin-photon interface. Coherent quantum systems which efficiently couple long-lived quantum memories to optical photons are a key resource for realizing quantum networks [1]. Color centers in diamond [2] are attractive candidates owing to unique properties of diamond, which include optical transparency and a high lattice quality that allows spin to function as long-lived quantum memory [3]. The negative silicon-vacancy (SiV − ) defect in diamond [4-6] has exceptional optical properties that facilitate efficient generation of indistinguishable photons from multiple distinct emitters [7]. Here we show optical initialization and readout of electronic spin in a single SiV − center with a spin relaxation time of T 1 ¼ 2.4 AE 0.2 ms. Two-photon resonance [8] is used to demonstrate coherent preparation of dark superposition states with a spin coherence time of T ⋆ 2 ¼ 35 AE 3 ns. This is shown to be limited by orbital relaxation that may be suppressed by engineering interactions with phonons. We present the first evidence of hyperfine interaction with a 29 Si nuclear spin in SiV − which can potentially be used as a memory qubit [9]. Quantum information processing efforts in diamond have mainly focused on the nitrogen-vacancy (NV − ) center because of its excellent spin properties at ambient conditions [10]. All-optical access to NV − spin is possible [11][12][13][14]; however, its large phonon sideband and spectral diffusion reduce coherent photon generation rates and limit the development of NV − quantum networks [15][16][17]. The main optical advantage provided by the SiV − center is that 70% of its fluorescence is concentrated in a sharp zero-phonon line (ZPL), making it ideal for single photon source applications [18,19]. It is spectrally stable at 737 nm, exhibits line widths limited by the excited state lifetime [20], and can be coupled to optical cavities [21,22]. Physically, the SiV − center consists of a single silicon atom replacing two carbon atoms in the diamond lattice, forming D 3d symmetry as illustrated in Fig. 1(a) [4][5][6]23]. This geometry makes the SiV − center insensitive to small electric fields [7] and th...

A solid-state system combining a stable spin degree of freedom with an efficient optical interface is highly desirable as an element for integrated quantum optical and quantum information systems. We demonstrate a bright color center in diamond with excellent optical properties and controllable electronic spin states. Specifically, we carry out detailed optical spectroscopy of a Germanium Vacancy (GeV) color center demonstrating optical spectral stability. Using an external magnetic field to lift the electronic spin degeneracy, we explore the spin degree of freedom as a controllable qubit. Spin polarization is achieved using optical pumping, and a spin relaxation time in excess of 20 µs is demonstrated. Optically detected magnetic resonance (ODMR) is observed in the presence of a resonant microwave field. ODMR is used as a probe to measure the Autler-Townes effect in a microwave-optical double resonance experiment. Superposition spin states were prepared using coherent population trapping, and a pure dephasing time of about 19 ns was observed. Prospects for realizing coherent quantum registers based on optically controlled GeV centers are discussed.Over the last few decades significant effort has been directed towards the exploration of solid-state atom-like systems such as quantum dots or color centers in diamond owing to their potential application in quantum information processing [1][2][3][4]. The nitrogen vacancy (NV) center in diamond has become prominent due to its optical spin initialization and readout [5], and the ease of spin control by microwave fields [1]. However the small Debye-Waller factor of this defect [6] and its spectral instability [7] hinder the realization of an efficient quantumoptical interface [8], motivating an ongoing search for new candidates. Here we investigate the recently discovered germanium vacancy (GeV) center in diamond [9-11], demonstrating its outstanding spectral properties devoid of measurable spectral diffusion. We show spin-1 2 Zeeman splitting which confirms this is the negative charge state of this defect. We use two-photon resonance to optically prepare coherent dark spin superposition states, and show microwave spin manipulation via optically-detected magnetic resonance (ODMR). The spin coherence time is found to be T 2 = 19 ± 1 ns, which is concluded to be limited by phonon-mediated orbital relaxation as in the closely-related silicon-vacancy (SiV) center [12,13]. Optical and microwave control of GeV spin, combined with the possibility of GeV centers in nanophotonic devices [14], make it a promising platfrom for quantum op- * petr.siyushev@uni-ulm.de † These two authors contributed equally ‡ lachlan.j.rogers@quantum.diamonds tics and quantum information science applications.The GeV center can be produced in diamond during crystal growth and by ion implantation, and it fluoresces strongly with a zero-phonon line at 602 nm accompanied by a weak phonon sideband (PSB) containing about 40% of the fluorescence [9,10]. Isotopic shifts of the fluorescence spectrum establis...

The silicon-vacancy centre ( − SiV ) in diamond has exceptional spectral properties for single-emitter quantum information applications. Most of the fluorescence is concentrated in a strong zero phonon line (ZPL), with a weak phonon sideband extending for 100 nm that contains several clear features. We demonstrate that the ZPL position can be used to reliably identify the silicon isotope present in a single − SiV centre. This is of interest for quantum information applications since only the 29 Si isotope has nuclear spin. In addition, we show that the sharp 64 meV phonon peak is due to a local vibrational mode of the silicon atom. The presence of a local mode suggests a plausible origin of the measured isotopic shift of the ZPL.

Qudi is a general, modular, multi-operating system suite written in Python 3 for controlling laboratory experiments. It provides a structured environment by separating functionality into hardware abstraction, experiment logic and user interface layers. The core feature set comprises a graphical user interface, live data visualization, distributed execution over networks, rapid prototyping via Jupyter notebooks, configuration management, and data recording. Currently, the included modules are focused on confocal microscopy, quantum optics and quantum information experiments, but an expansion into other fields is possible and encouraged. Qudi is available from https://github.com/Ulm-IQO/ qudi and is freely useable under the GNU General Public Licence.