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.

Tin-vacancy (SnV) color centers were created in diamond by ion implantation and subsequent high temperature annealing up to 2100 °C at 7.7 GPa. The first-principles calculation suggests that the large atom of tin can be incorporated into the diamond lattice with a split-vacancy configuration, in which a tin atom sits on an interstitial site with two neighboring vacancies. The SnV center shows a sharp zero phonon line at 619 nm at room temperature. This line splits into four peaks at cryogenic temperatures with a larger ground state splitting of ~850 GHz than those of color centers based on other IV group elements, silicon-vacancy (SiV) and germanium vacancy (GeV) centers. The excited state lifetime was estimated to be ~5 ns by Hanbury Brown-Twiss interferometry measurements on single SnV quantum emitters. The order of the experimentally obtained optical transition energies comparing with the SiV and GeV centers is good agreement with the theoretical calculations.Point defect-related color centers in solid state materials are a promising approach for quantum information processing [1,2]. Nitrogen-vacancy (NV) centers in diamond have been most intensively studied from the viewpoint of both fundamental and applied sciences [3,4]. However, the zero phonon line (ZPL) of the NV center possesses a fraction of only 4 % in its total fluorescence due to its large phonon sideband (PSB). Also, the NV center suffers from external noise, leading to instability of the optical transition energy. To overcome these drawbacks, color centers based on IV group elements, silicon-vacancy (SiV) [5][6][7] and germanium-vacancy (GeV) [8][9][10][11] centers, has attracted attention owing to their large ZPL, structural symmetry robust against the external noise, and availability of quantum emission by single centers. Recently, spin control and evaluation of spin coherence time have been investigated for this two color centers [12][13][14][15][16], revealing that their spin coherence times were limited to sub-microseconds even at cryogenic temperatures < 5 K, much shorter than that of the NV center [17,18]. This limitation originates from phonon-mediated transitions between the lower and upper branches in the ground state [13][14][15][16]. Further cooling down to the sub-Kelvin regime or strain engineering is considered as a solution [15,16]. Another possibility is creation of a novel color center possessing a larger energy splitting in the ground state.For this purpose, in this study, we investigated the utilization of a IV group atom of tin (Sn, Fig. 1a).The incorporation of such a heavy atom into the diamond lattice as a form of a color center, especially single quantum emitter, has not been demonstrated yet. Here, we fabricated tin-vacancy (SnV) centers in diamond in both ensemble and single states by combination of ion implantation and subsequent high temperature annealing up to 2100 °C under a high pressure of 7.7 GPa. Low temperature optical measurements revealed that the ground state splitting of the SnV center was much larg...

We demonstrate a quantum nanophotonics platform based on germanium-vacancy (GeV) color centers in fiber-coupled diamond nanophotonic waveguides. We show that GeV optical transitions have a high quantum efficiency and are nearly lifetime-broadened in such nanophotonic structures. These properties yield an efficient interface between waveguide photons and a single GeV without the use of a cavity or slow-light waveguide. As a result, a single GeV center reduces waveguide transmission by 18 ± 1% on resonance in a single pass. We use a nanophotonic interferometer to perform homodyne detection of GeV resonance fluorescence. By probing the photon statistics of the output field, we demonstrate that the GeV-waveguide system is nonlinear at the single-photon level.Efficient coupling between single photons and coherent quantum emitters is a central element of quantum nonlinear optical systems and quantum networks [1][2][3]. Several atom-like defects in the solid-state are currently being explored as promising candidates for the realization of such systems [4], including the nitrogen-vacancy (NV) center in diamond, renowned for its long spin coherence at room temperature [5]; and the silicon-vacancy (SiV) center in diamond, which has recently been shown to have strong, coherent optical transitions in nanostructures [6][7][8][9]. The remarkable optical properties of the SiV center arise from its inversion symmetry [10], which results in a vanishing permanent electric dipole moment for SiV orbital states, dramatically reducing their response to charge fluctuations in the local environment.

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...

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.