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.
Photonic quantum information processing has emerged as a powerful platform for realising quantum-enhanced technologies. In order to be scalable, many of these technologies depend on the availability of a suitable quantum memory for the coherent storage and on-demand retrieval of photonic quantum states. In this thesis, I investigate broadband light storage in a room-temperature Raman memory, implemented both in free space and, for the first time, inside a low-finesse optical cavity designed for low-noise operation. The ability of the Raman memory to preserve phase coherence was tested by storing coherent polarisation states in two spatially separate atomic ensembles. Polarisation storage with a fidelity of up to 97 ± 1% was demonstrated by performing full process tomography on the system. The Raman memory was then interfaced for the first time with a spontaneous parametric downconversion (SPDC) source of heralded, GHz-bandwidth single photons. The memory performance was characterised by measuring the second-order autocorrelation of the retrieved fields. While the SPDC input photon statistics showed a clear influence on the statistics of the retrieved field, four-wave mixing (FWM) noise, stimulated by spontaneous Raman scattering, prevented the preservation of non-classical photon statistics during read-out. Suppressing this source of noise represents the last remaining challenge for realising a broadband single-photon Raman memory suitable for quantum information applications. To this end, I demonstrate a novel cavity implementation of the Raman memory which reduces the FWM contribution relative to the signal field by re-distributing the density of states into which the noise photons can be scattered. Cavity-enhanced memory operation was investigated using weak coherent input states, showing a significant improvement of the signal-to-noise ratio compared to the free-space memory implementation. This proof-of-principle demonstration suggests that cavity Raman memories may offer a practical route towards low-noise, high-bandwidth quantum storage at room temperature.
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.
Atomic defects in solids offer access to atom-like quantum properties without complex trapping methods while displaying a rich physics due to interactions with their solid-state environment. Such properties have made them an advantageous building block for quantum information processing, in particular to construct a quantum network, where information would be encoded in spins and transferred between nodes through photons. Among defects in solids, the negatively charged silicon-vacancy centre in diamond (SiV
−) has attracted attention for its very promising optical properties for such a network. In this thesis, we investigate the spin properties of the silicon-vacancy centre as a potential spin-photon interface. First, we use resonant excitation of an SiV
− centre in an external magnetic field to selectively address different electronic states and analyse the resulting fluorescence. We find evidence of selection rules in the optical transitions revealing that the centre possesses an electronic spin S = 1/2. Making use of the dependence of such selection rules on the applied magnetic field orientation, we resonantly drive two optical transitions forming a
Λ-scheme. In the double resonance condition, we achieve coherent population trapping, whereby the SiV
− is pumped into a dark state corresponding to a superposition of the two addressed ground states of opposite spin. This technique allows us to evaluate the coherence time of the dark state and hence of the spin, while demonstrating the possibility of all-optical control of the spin when a
Λ-scheme is available. We then use resonant optical pulses to initialise and read out the spin state of a single SiV
−. By tuning a microwave pulse into resonance between two ground states of opposite spin, we demonstrate optically detected magnetic resonance. Subsequently, by varying the duration of a resonant microwave pulse, we achieve coherent control of a single SiV
− electronic spin. Through Ramsey interferometry, we measure a spin dephasing time of 115
± 9 ns. We then investigate interactions of the SiV
− with its environment. We analyse the hyperfine interaction of the SiV
− spin with the nuclear spin of
29Si, with a view to taking advantage of the long-lived nuclear spin in the future. We show that single-phonon-mediated excitations between electronic states of the SiV
− are the dominant spin dephasing and population decay mechanism and evaluate how external strain alters optical selection rules and can be used to improve the coherence time of the spin.
The properties of the negatively charged silicon-vacancy (SiV) centre in diamond were investigated in this thesis. Polarisation measurements revealed transition dipoles and the symmetry groups of the centre. In a diamond grown by chemical vapour deposition, a high spectral overlap of SiV centres with nearly transform limited optical line widths was found pointing to a low sensitivity of SiV centres to both strain and local electric fields. This low inhomogeneous distribution allowed for a quantum interference leading to a contrast of over 70 % in a Hong-Ou-Mandel experiment. Additionally, an isotopic shift at single site level could be confirmed and than used to identify the 64meV peak in the phonon sideband as a local vibrational mode. A low temperature investigation of optical line width and orbital relaxation time indicated a direct single-phonon process between the orbital states while at higher temperatures a two-phonon dephasing was found to be the main electron-phonon process. The results were consistently described by the microscopic model developed and additional strength was added by consistent studies of spin-orbit splitting and optical lifetime. By applying a magnetic field the spin degeneracy in the ground state was lifted allowing for spin initialisation with over 95 % fidelity with a spin relaxation time of over 2 milliseconds. Two resonant lasers were used to demonstrate coherent population trapping leading to a coherence time in the order of the orbital relaxation time. The coherence was concluded to be limited by orbital dephasing processes and we propose techniques to overcome them. One of them is to use nanodiamonds with a size smaller than half the phonon wavelength to suppress the dephasing process. Investigating the spectral properties at cryogenic temperatures, nearly lifetime limited line widths have been found, making these nanodiamonds good candidates for further spin applications with SiV centres.
The work in this thesis is embedded in the framework of quantum metrology and explores quantum effects in solid state emitters and optical sensing. Specifically, the thesis comprises studies on silicon vacancy centres in nanodiamonds, phase measurements and cavity optomechanics utilising optical squeezed states, and a theoretical study on quantum amplifiers. Due to its similarity to single atoms, colour centres in diamond are ideal objects for exploring and exploiting quantum effects, because they are comparably easy to produce, probe and maintain. While nitrogen vacancy centres are the most renowned colour centres, we studied the silicon vacancy (SiV−) centre. In bulk diamond it features strong zero-phonon-line emission and, at cryogenic temperatures, a linewidth of hundreds of MHz, but it displays a weak spin coherence in the order of ns. To suppress the relaxation process which limits the coherence time, we utilised SiV− centres in nanodiamond. By means of confocal microscopy and resonant excitation at cryogenic temperatures, we measured linewidths in recently developed nanodiamond which were an order of magnitude smaller compared to previous studies on SiV− nanodiamonds. Furthermore, we identified spectral diffusion as the main hindrance in extending spin coherence times. Overcoming this issue will provide a promising candidate as an emitter for quantum information. Next, the question of how squeezed states of light can improve optical sensing was addressed. For this purpose, a squeezed light source was designed and built from scratch, which achieved a noise suppression of −8 dB at an optical pump power of 40mW. The generated squeezed light was first used to demonstrate how Gaussian states and detection can beat the shot noise limit and Rayleigh criterion in phase measurements simultaneously. Compared to quantum phase measurements based on single photon states, this approach is inherently deterministic. In addition, the applied homodyne detection enables close-to-unity detection efficiencies and thereby outperforms single photon state strategies which rely on comparably inefficient or demanding detection techniques. A second experiment combined squeezed light and feedback control to cool an optomechanical system. This proof-of-principle study is the first reported squeezing enhanced optomechanical cooling experiment. Despite losses of more than 50% (a resulting noise suppression of −2 dB), the mechanical resonator was cooled from room temperature to 130K. This represents a 12% improvement compared to the use of a coherent state protocol. Finally, we theoretically investigated the fundamental properties of quantum amplifiers. Such devices can be used in information and sensing technology to amplify signals to overcome e.g. technical detection limitations. Amplified communication channels were characterised by applying the measure of mutual information I, as it offers strict bounds on the maximum achievable performance, which enabled a fair comparison between different applications scenarios. As a result, we identified two peculiar configurations: A configuration where amplification does not affect I, and a configuration where quantum correlations do not always lead to an enhanced I.
Abstract Recently, the possibility of implementing a cavity-enhanced off-resonant Raman quantum memory in an ensemble of silicon-vacancy centers in diamond has been studied (Kalachev et al 2019 Laser Phys. 29 104001). It was shown that the signal-to-noise ratio can significantly exceed unity for short single-photon input pulses if the orbital level splitting in the ground state of the color centers is significantly enhanced by strain. The latter results in decreasing the coherence time between the two available orbital branches of the ground state, though. In the present work, we consider the possibility of increasing the storage time due to the use of nanodiamonds. It is shown that suppression of direct electron-phonon transitions in the diamond nanocrystals makes it possible to increase the coherence time of both orbital and spin qubits by orders of magnitude.