Abstract One ultimate goal of quantum information processing is to construct a quantum network for direct sharing of quantum information between distant parties based on stationary qubits for information storage and flying qubits for information transmission. This requires long‐lived quantum memories, efficient light–matter interface, and deterministic quantum gate operations. Among matter qubits, the electron spin of nitrogen vacancy center in diamond is an appealing option for its long coherence time at room temperature, deterministic microwave control, and optical preparation and readout of the qubit. However, its poor optical properties, including weak zero‐phonon‐line emission and large spectral diffusion, limit its potential for large‐scale deployment in quantum nodes. This has motivated the investigation for alternative quantum emitters that combine long‐time memory and coherent optical properties together. The emerging group‐IV split‐vacancy color centers in diamond, such as silicon‐vacancy, germanium‐vacancy, tin‐vacancy, and lead‐vacancy centers, are promising candidates of this ongoing exploration. These quantum emitters simultaneously possess microsecond spin coherence time and optically bright transitions with narrow linewidths. This review reports recent efforts on extending the spin coherence time of these color centers via temperature, strain, and charge control, which paves the road towards constructing solid‐based matter–photon interface for quantum network applications.
Abstract One ultimate goal of quantum information processing is to construct a quantum network for direct sharing of quantum information between distant parties based on stationary qubits for information storage and flying qubits for information transmission. This requires long‐lived quantum memories, efficient light–matter interface, and deterministic quantum gate operations. Among matter qubits, the electron spin of nitrogen vacancy center in diamond is an appealing option for its long coherence time at room temperature, deterministic microwave control, and optical preparation and readout of the qubit. However, its poor optical properties, including weak zero‐phonon‐line emission and large spectral diffusion, limit its potential for large‐scale deployment in quantum nodes. This has motivated the investigation for alternative quantum emitters that combine long‐time memory and coherent optical properties together. The emerging group‐IV split‐vacancy color centers in diamond, such as silicon‐vacancy, germanium‐vacancy, tin‐vacancy, and lead‐vacancy centers, are promising candidates of this ongoing exploration. These quantum emitters simultaneously possess microsecond spin coherence time and optically bright transitions with narrow linewidths. This review reports recent efforts on extending the spin coherence time of these color centers via temperature, strain, and charge control, which paves the road towards constructing solid‐based matter–photon interface for quantum network applications.
Atomic‐scale impurity spins, also called color centers, in an otherwise spin‐free diamond host lattice have proven to be versatile tools for applications in solid‐state‐based quantum technologies ranging from quantum information processing (QIP) to quantum‐enhanced sensing and metrology. Due to its wide band gap, diamond can host hundreds of different color centers. However, their suitability for QIP or sensing applications has only been tested for a handful of these, with the nitrogen vacancy (NV) strongly dominating this field of research. Due to its limited optical properties, the success of the NV for QIP applications however strongly depends on the development of efficient photonic interfaces. In the past years the negatively charged silicon vacancy (SiV−) center received significant attention due to its highly favourable spectral properties such as narrow zero phonon line transitions and weak phonon sidebands. Here, the recent work investigating the SiV center's orbital and electron spin coherence properties is reviewed as well as techniques to coherently control its quantum state using microwave as well as optical fields. Also, potential future experimental directions to improve the SiV's coherence time scale and to develop it into a valuable tool for QIP applications are outlined.
Quantum emitters in diamond are a promising candidate for quantum network applications. Here, we show basic properties of tin-vacancy (SnV) centers in diamond, which we recently discovered. The SnV center shows a sharp zero phonon line and a high fluorescence intensity. The SnV center has a possibility to have a long spin coherence time in the Kelvin temperature range, in contrast to other group-IV color centers, i.e. silicon-vacancy and germanium-vacancy centers. We discuss important experiments regarding spin and optical properties of the SnV quantum emitter for further development towards quantum network.
Abstract Diamond has emerged as a highly promising platform for quantum network applications. Color centers in diamond fulfill the fundamental requirements for quantum nodes: they constitute optically accessible quantum systems with long‐lived spin qubits. Furthermore, they provide access to a quantum register of electronic and nuclear spin qubits and they mediate entanglement between spins and photons. All these operations require coherent control of the color center's spin state. This review provides a comprehensive overview of the state‐of‐the‐art, challenges, and prospects of such schemes, including high‐fidelity initialization, coherent manipulation, and readout of spin states. Established microwave and optical control techniques are reviewed, and moreover, emerging methods such as cavity‐mediated spin–photon interactions and mechanical control based on spin–phonon interactions are summarized. For different types of color centers, namely, nitrogen–vacancy and group‐IV color centers, distinct challenges persist that are subject of ongoing research. Beyond fundamental coherent spin qubit control techniques, advanced demonstrations in quantum network applications are outlined, for example, the integration of individual color centers for accessing (nuclear) multiqubit registers. Finally, the role of diamond spin qubits in the realization of future quantum information applications is described.
The construction of a quantum internet, an interconnected network which can distribute and store quantum states, is an essential step towards the development of next generation quantum technologies, including unconditionally secure communication, enhanced metrology, and distributed quantum computing. Development of these networks require functional nodes consisting of stationary registers with the capability of high-fidelity quantum information processing and storage, which efficiently interface with photonic qubits. This thesis outlines one emerging platform for building a quantum network node using the silicon-vacancy color center in diamond (SiV). Although previous work has established the SiV as a superior optical emitter capable of strong light-matter coupling, its feasibility as a quantum memory has remained elusive, primarily due to coupling to thermal phonons. This suggests two branching research directions: one where this coupling is investigated and characterized as a nanoscale sensor, and one where this coupling is supressed in order to realize the SiV as a quantum information processor. This thesis begins by highlighting the extreme phonon sensitivity as a metrological tool, where an ensemble of SiVs are used as an all-optical thermometer, capable of \SI{70}{\milli\kelvin} precision at room temperature. When incorporated into nanodiamonds, the sensing properties deviate by less than 1% between nanodiamonds, enabling calibration-free thermometry for sensing and control of complex nanoscale systems. For quantum information tasks, coherent control of the SiV electronic spin qubit is enabled by suppressing phonon-induced dephasing by five orders of magnitude at temperatures below \SI{500}{\milli\kelvin}. By aligning the magnetic field along the silicon-vacancy symmetry axis, optical transitions are highly spin-conserving, allowing for single-shot readout of the qubit with high fidelity. This coherent control is used to demonstrate a spin coherence time T2> \SI{13}{\milli\second} and spin relaxation time T1> \SI{1}{\second} at \SI{100}{\milli\kelvin}. Finally, SiV quantum memories are packaged into a fully integrated quantum network node by incorporating SiV qubits into nanophotonic cavities. The efficient SiV-cavity coupling (with cooperativity C>30) provides a nearly-deterministic interface between photons and the spin qubit, which enables heralded single-photon storage. Coherent coupling to nearby \cnuc\ nuclear spins (with nearly second-long coherence times) enables multi-qubit registers and paves the way towards large-scale quantum networks.
Color centers in diamond are a promising platform for quantum communication, as they can serve as solid state quantum memories with efficient optical transitions. Much recent attention has focused on the negatively charged NV center in diamond, which has a spin triplet ground state electronic configuration that can be measured and initialized optically, exhibits long spin coherence times at room temperature, and has narrow, spin-conserving optical transitions. However, the NV center exhibits a large static and dynamic inhomogeneous linewidth, and over 97% of its emission is in a broad, incoherent phonon side band, severely limiting scalability. Alternatively, the negatively charged SiV center exhibits excellent optical properties, with 70% of its emission in the zero phonon line and a narrow inhomogeneous linewidth. However, SiV− suffers from short electron spin coherence times, limited by an orbital relaxation rate (T1) of around 40 ns at 5 K.
Solid-state quantum emitters that couple coherent optical transitions to long-lived spin qubits are essential for quantum networks. Here we report on the spin and optical properties of individual tin-vacancy (SnV) centers in diamond nanostructures. Through cryogenic magneto-optical and spin spectroscopy, we verify the inversion-symmetric electronic structure of the SnV, identify spin-conserving and spin-flipping transitions, characterize transition linewidths, measure electron spin lifetimes, and evaluate the spin dephasing time. We find that the optical transitions are consistent with the radiative lifetime limit even in nanofabricated structures. The spin lifetime is phonon limited with an exponential temperature scaling leading to T_{1}>10 ms, and the coherence time, T_{2}^{*} reaches the nuclear spin-bath limit upon cooling to 2.9 K. These spin properties exceed those of other inversion-symmetric color centers for which similar values require millikelvin temperatures. With a combination of coherent optical transitions and long spin coherence without dilution refrigeration, the SnV is a promising candidate for feasable and scalable quantum networking applications.