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...

Diamond attracts considerable attention as a versatile and technologically useful material. For many demanding applications, such as recently emerged quantum optics and sensing, it is important to develop new routes for fabrication of diamond containing defects with specific optical, electronic and magnetic properties. Here we report on successful synthesis of diamond from a germanium-carbon system at conditions of 7 GPa and 1,500–1,800 °C. Both spontaneously nucleated diamond crystals and diamond growth layers on seeds were produced in experiments with reaction time up to 60 h. We found that diamonds synthesized in the Ge-C system contain a new optical centre with a ZPL system at 2.059 eV, which is assigned to germanium impurities. Photoluminescence from this centre is dominated by zero-phonon optical transitions even at room temperature. Our results have widened the family of non-metallic elemental catalysts for diamond synthesis and demonstrated the creation of germanium-related optical centres in diamond.

We present high-resolution, all-optical thermometry based on ensembles of germanium-vacancy (GeV) color center in diamond and implement this method of thermometry in the fiber-optic format. Due to the unique properties of diamond, an all-optical approach using this method opens a way to produce back-action-free temperature measurements with resolution below 0.1 K in a wide range of temperatures.

In this paper, we report on the influence of nitrogen concentration in metal melts on the growth processes, morphology, and defect-and-impurity structure of diamond crystals. In two series of experiments, the concentration of nitrogen in the growth system was varied by adding Fe 3 N and CaCN 2 to the charge; the other parameters and conditions of the growth were constant: FeNiC system, P = 5.5 GPa, T = 1400 °C, and duration of 65 h. It has been found that, with increasing nitrogen concentration (C N ) in the metal melt from 0.005 to 0.6 atom %, the growth of single crystal diamond is followed by formation of aggregates of block twinned crystals and then by crystallization of metastable graphite. At the stage of single crystal growth, an increase in C N results in an increase in nitrogen impurity concentration in diamond crystals from about 200 ppm to approximately 1100 ppm, an increase in density of dislocations, twin lamellae, and internal strains, and a change in crystal morphology. Further increases in C N result in formation of aggregates of block crystals with nitrogen concentration around 120-300 ppm. At nitrogen concentration in the melt higher than a certain critical value, nucleation and growth of diamond are terminated and graphite crystallizes in the diamond stability field.