The negatively charged silicon-vacancy (SiV^{-}) color center in diamond has recently emerged as a promising system for quantum photonics. Its symmetry-protected optical transitions enable the creation of indistinguishable emitter arrays and deterministic coupling to nanophotonic devices. Despite this, the longest coherence time associated with its electronic spin achieved to date (∼250 ns) has been limited by coupling to acoustic phonons. We demonstrate coherent control and suppression of phonon-induced dephasing of the SiV^{-} electronic spin coherence by 5 orders of magnitude by operating at temperatures below 500 mK. By aligning the magnetic field along the SiV^{-} symmetry axis, we demonstrate spin-conserving optical transitions and single-shot readout of the SiV^{-} spin with 89% fidelity. Coherent control of the SiV^{-} spin with microwave fields is used to demonstrate a spin coherence time T_{2} of 13 ms and a spin relaxation time T_{1} exceeding 1 s at 100 mK. These results establish the SiV^{-} as a promising solid-state candidate for the realization of quantum networks.
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.
Quantum memories are a key technology enabling arbitrarily-scalable optical quantum information processing architectures via active synchronisation and multiplexing of quantum information. Optically active confined impurities, called colour centres, in a spin-free and highly transparent diamond host material are promising systems to realise such devices in the solid-state, suitable for integration in a scalable guided-wave on-chip photonic platform. In this thesis, the transition from optical Raman quantum memories in atomic vapours to solid-state systems is prepared, exploring two diamond-based systems: On the basis of ensembles of neutrally charged nitrogen vacancy centres, desirable properties and criteria of solid-state systems used for optical quantum memories are developed. These include non-vanishing off-resonant Raman coupling, variable acceptance bandwidth for broadband pulses and cross-linear polarised optical transitions to prevent the generation of noise during the Raman memory protocol, which have been realised experimentally by applying external uniaxial stress to the system. However, attempts to measure the very short orbital ground state lifetime affirmed unfavourable theoretical estimations, rendering this centre unsuitable for memory applications. As a potential alternative, the negatively charged silicon vacancy centre has been investigated, which offers a large ground state splitting with extended life and coherence times and narrow inhomogeneous spectral lines due to its inversion symmetry. In an ensemble of silicon vacancy centres, the ground state lifetime is measured and coherent manipulation via resonant Ramsey interference and Hahn echo is demonstrated. In a second step, off-resonant two-photon transitions are achieved implementing stimulated Raman adiabatic passage between the orbital ground states. Finally, strong light-matter interaction of a weak signal field with the ensemble via phase-sensitive four-wave mixing is shown forming the basis for integrated Raman-based memories and single photon nonlinearities in ensembles of silicon vacancy centres and closely related defects.
Abstract Complete control of the state of a quantum bit (qubit) is a fundamental requirement for any quantum information processing (QIP) system. In this context, all-optical control techniques offer the advantage of a well-localized and potentially ultrafast manipulation of individual qubits in multi-qubit systems. Recently, the negatively charged silicon vacancy centre (SiV − ) in diamond has emerged as a novel promising system for QIP due to its superior spectral properties and advantageous electronic structure, offering an optically accessible Λ -type level system with large orbital splittings. Here, we report on all-optical resonant as well as Raman-based coherent control of a single SiV − using ultrafast pulses as short as 1 ps, significantly faster than the centre’s phonon-limited ground state coherence time of about 40 ns. These measurements prove the accessibility of a complete set of single-qubit operations relying solely on optical fields and pave the way for high-speed QIP applications using SiV − centres.
Ultrasmall (about 10 nm), low-strain, artificially produced diamonds with an internal, active color center have substantial potential for quantum information processing and biomedical applications. Thus, it is of great importance to be able to artificially produce such diamonds. Here, we report on the high-pressure, high-temperature synthesis of such nanodiamonds about 10 nm in size and containing an optically active, single silicon-vacancy color center. Using special sample preparation technique, we were able to prepare samples containing single nanodiamonds on the surface. By correlating atomic-force microscope images and confocal optical images, we verified the presence of optically active color centers in single nanocrystals, and using second-order correlation measurements, we proved the single-photon emission statistics of these nanodiamonds. These color centers have nonblinking, spectrally narrow emission with narrow distribution of spectral width and positions of zero-phonon line, thus proving the high quality of the nanodiamonds produced.