Spin impurities in diamond can be versatile tools for a wide range of solid-state-based quantum technologies, but finding spin impurities that offer sufficient quality in both photonic and spin properties remains a challenge for this pursuit. The silicon-vacancy center has recently attracted much interest because of its spin-accessible optical transitions and the quality of its optical spectrum. Complementing these properties, spin coherence is essential for the suitability of this center as a spin-photon quantum interface. Here, we report all-optical generation of coherent superpositions of spin states in the ground state of a negatively charged silicon-vacancy center using coherent population trapping. Our measurements reveal a characteristic spin coherence time, T2*, exceeding 45 nanoseconds at 4 K. We further investigate the role of phonon-mediated coupling between orbital states as a source of irreversible decoherence. Our results indicate the feasibility of all-optical coherent control of silicon-vacancy spins using ultrafast laser pulses.

Resonance fluorescence in the Heitler regime provides access to single photons with coherence well beyond the Fourier transform limit of the transition, and holds the promise to circumvent environment-induced dephasing common to all solid-state systems. Here we demonstrate that the coherently generated single photons from a single self-assembled InAs quantum dot display mutual coherence with the excitation laser on a timescale exceeding 3 s. Exploiting this degree of mutual coherence, we synthesize near-arbitrary coherent photon waveforms by shaping the excitation laser field. In contrast to post-emission filtering, our technique avoids both photon loss and degradation of the single-photon nature for all synthesized waveforms. By engineering pulsed waveforms of single photons, we further demonstrate that separate photons generated coherently by the same laser field are fundamentally indistinguishable, lending themselves to the creation of distant entanglement through quantum interference.

Resonance fluorescence arises from the interaction of an optical field with a two-level system, and has played a fundamental role in the development of quantum optics and its applications. Despite its conceptual simplicity, it entails a wide range of intriguing phenomena, such as the Mollow-triplet emission spectrum, photon antibunching and coherent photon emission. One fundamental aspect of resonance fluorescence--squeezing in the form of reduced quantum fluctuations in the single photon stream from an atom in free space--was predicted more than 30 years ago. However, the requirement to operate in the weak excitation regime, together with the combination of modest oscillator strength of atoms and low collection efficiencies, has continued to necessitate stringent experimental conditions for the observation of squeezing with atoms. Attempts to circumvent these issues had to sacrifice antibunching, owing to either stimulated forward scattering from atomic ensembles or multi-photon transitions inside optical cavities. Here, we use an artificial atom with a large optical dipole enabling 100-fold improvement of the photon detection rate over the natural atom counterpart and reach the necessary conditions for the observation of quadrature squeezing in single resonance-fluorescence photons. By implementing phase-dependent homodyne intensity-correlation detection, we demonstrate that the electric field quadrature variance of resonance fluorescence is three per cent below the fundamental limit set by vacuum fluctuations, while the photon statistics remain antibunched. The presence of squeezing and antibunching simultaneously is a fully non-classical outcome of the wave-particle duality of photons.

* These authors contributed equally to this work.Understanding the interplay between a quantum system and its environment lies at the heart of quantum science and its applications. To-date most efforts have focused on circumventing decoherence induced by the environment by either protecting the system from the associated noise 1-5 or by manipulating the environment directly 6-9 . Recently, parallel efforts using the environment as a resource have emerged, which could enable dissipation-driven quantum computation 10,11 and coupling of distant quantum bits 12,13 .Here, we realize the optical control of a semiconductor quantum-dot spin by relying on its interaction with an adiabatically evolving spin environment. The emergence of hyperfineinduced, quasi-static optical selection rules enables the optical generation of coherent spin dark states without an external magnetic field. We show that the phase and amplitude of 1 the lasers implement multi-axis manipulation of the basis spanned by the dark and bright states, enabling control via projection into a spin-superposition state. Our approach can be extended, within the scope of quantum control and feedback 14,15 , to other systems interacting with an adiabatically evolving environment.Techniques for controlling spins often rely upon a well-defined Zeeman splitting due to a static external magnetic field. This field also controls the selection rules of the optical transitions enabling, for example, optical single-shot spin readout [16][17][18] and fast spin manipulation 2,19 in selfassembled quantum dots (QDs). Unfortunately, these two capabilities require different alignments of the external field with respect to the growth direction of the QD: a field along the growth direction of a QD (Faraday configuration) provides cycling transitions for spin readout, while a magnetic field applied perpendicular to the growth direction (Voigt configuration) enables optically-driven spin control. In an effort to achieve both readout and control on a QD spin qubit, a large, rapidly switchable, multi-axis magnetic field would be the ideal toolbox.While the quantisation axis due to a static field in Voigt configuration can in principle be converted to that in Faraday configuration via the optical Stark effect, the prohibitively large laser power required to achieve this renders the scheme impractical. We instead consider the smallest effective field we can devise: that due to the local fluctuating nuclear spins within the QD. They give rise to an effective magnetic field for an electron spin in the QD, known as the Overhauser (OH) field (Fig. 1a), with a dispersion of 10-30 mT and a many-microsecond correlation time [20][21][22] . The hyperfine interaction therefore lifts the Kramers degeneracy and provides a quantisation axis for the electron spin, which in turn leads to temporally quasi-stable optical selection rules due to the quasi-static nature of the nuclei (Fig. 1b). This OH field is small 2 enough to allow for Stark tilting the quantisation axis for optical read-out feasibl...

We report on the feedback stabilization of the zero-phonon emission frequency of a single InAs quantum dot. The spectral separation of the phonon-assisted component of the resonance fluorescence provides a probe of the detuning between the zero-phonon transition and the resonant driving laser. Using this probe in combination with active feedback, we stabilize the zero-phonon transition frequency against environmental fluctuations. This protocol reduces the zero-phonon fluorescence intensity noise by a factor of 22 by correcting for environmental noise with a bandwidth of 191 Hz, limited by the experimental collection efficiency. The associated sub-Hz fluctuations in the zerophonon central frequency are reduced by a factor of 7. This technique provides a means of stabilizing the quantum dot emission frequency without requiring access to the zero-phonon emission.A robust single photon source is an integral component for the implementation of many quantum technologies including linear optical quantum computing [1,2], quantum relays [3], and quantum networks [4]. Indium Arsenide self-assembled quantum dots (QDs) offer one of the most promising platforms [5] for such applications due to the large tuneability of their emission frequency [6], fast triggered emission rates [7,8], and the possibility of integration into nanostructures [9]. However, charge carriers confined to a semiconductor QD interact with the solid-state environment, in which disparate noise sources cause fluctuations of the central frequency of the optical transitions [10][11][12]. These fluctuations lead to both spectral distinguishability of the fluorescence from independent QDs as well as a decreased emission intensity in resonant excitation and represent a challenge to be overcome in order to achieve a scalable architecture using QDs. Here, we develop an active feedback stabilization scheme which makes use of the phonon-assisted fluorescence intensity as a probe for fluctuations of the zero-phonon line (ZPL) frequency. Our scheme allows for the detection and correction of environment-induced frequency fluctuations in a manner compatible with broadband photon collection strategies. Ideally, the frequency of the QD transition is stabilized without sacrificing any of the photon emission with a high bandwidth and in resonant excitation. Indeed, proposals for efficient linear optical computing with a polarization-entangled photon source require photon collection and detection efficiencies above 0.5 [13,14]. Previously demonstrated frequency stabilization protocols with solid state emitters have a bandwidth limited by periodic measurements of ZPL peak position [15,16], or rely on continuously detecting a fraction of the ZPL [17]. In this letter, we present a technique for simultaneous and discriminatory detection of both the zero-phonon and the phonon-assisted components of the resonance fluorescence (RF) from a single QD. We use the latter to generate an error signal in order to stabilize the intensity of the ZPL emission with a high bandwidth. We sho...