We demonstrate that silicon-vacancy (SiV) centers in diamond can be used to efficiently generate coherent optical photons with excellent spectral properties. We show that these features are due to the inversion symmetry associated with SiV centers. The generation of indistinguishable single photons from separated emitters at 5 K is demonstrated in a Hong-Ou-Mandel interference experiment. Prospects for realizing efficient quantum network nodes using SiV centers are discussed.

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.

Emitters of indistinguishable single photons are crucial for the growing field of quantum technologies. To realize scalability and increase the complexity of quantum optics technologies, multiple independent yet identical single-photon emitters are required. However, typical solid-state single-photon sources are inherently dissimilar, necessitating the use of electrical feedback or optical cavities to improve spectral overlap between distinct emitters. Here we demonstrate bright silicon vacancy (SiV À ) centres in low-strain bulk diamond, which show spectral overlap of up to 91% and nearly transform-limited excitation linewidths. This is the first time that distinct single-photon emitters in the solid state have shown intrinsically identical spectral properties. Our results have impact on the application of single-photon sources for quantum optics and cryptography.

Atomic-sized fluorescent defects in diamond are widely recognized as a promising solid state platform for quantum cryptography and quantum information processing. For these applications, single photon sources with a high intensity and reproducible fabrication methods are required. In this study, we report a novel color center in diamond, composed of a germanium (Ge) and a vacancy (V) and named the GeV center, which has a sharp and strong photoluminescence band with a zero-phonon line at 602 nm at room temperature. We demonstrate this new color center works as a single photon source. Both ion implantation and chemical vapor deposition techniques enabled fabrication of GeV centers in diamond. A first-principles calculation revealed the atomic crystal structure and energy levels of the GeV center.

The loss of coherence is one of the main obstacles for the implementation of quantum information processing. The efficiency of dynamical decoupling schemes, which have been introduced to address this problem, is limited itself by the fluctuations in the driving fields which will themselves introduce noise. We address this challenge by introducing the concept of concatenated continuous dynamical decoupling, which can overcome not only external magnetic noise but also noise due to fluctuations in driving fields. We show theoretically that this approach can achieve relaxation limited coherence times, and demonstrate experimentally that already the most basic implementation of this concept yields an order of magnitude improvement to the decoherence time for the electron spin of nitrogen vacancy centers in diamond. The proposed scheme can be applied to a wide variety of other physical systems, including trapped atoms and ions and quantum dots, and may be combined with other quantum technologies challenges such as quantum sensing and quantum information processing.