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Quantum emitters are an integral component for a broad range of quantum technologies, including quantum communication, quantum repeaters, and linear optical quantum computation. Solid-state color centers are promising candidates for scalable quantum optics due to their long coherence time and small inhomogeneous broadening. However, once excited, color centers often decay through phonon-assisted processes, limiting the efficiency of single-photon generation and photon-mediated entanglement generation. Herein, we demonstrate strong enhancement of spontaneous emission rate of a single silicon-vacancy center in diamond embedded within a monolithic optical cavity, reaching a regime in which the excited-state lifetime is dominated by spontaneous emission into the cavity mode. We observe 10-fold lifetime reduction and 42-fold enhancement in emission intensity when the cavity is tuned into resonance with the optical transition of a single silicon-vacancy center, corresponding to 90% of the excited-state energy decay occurring through spontaneous emission into the cavity mode. We also demonstrate the largest coupling strength (g/2π = 4.9 ± 0.3 GHz) and cooperativity (C = 1.4) to date for color-center-based cavity quantum electrodynamics systems, bringing the system closer to the strong coupling regime.

We investigate the influence of exciton-phonon coupling on the dynamics of a strongly coupled quantum dot-photonic crystal cavity system and explore the effects of this interaction on different schemes for nonclassical light generation. By performing time-resolved measurements, we map out the detuningdependent polariton lifetime and extract the spectrum of the polariton-to-phonon coupling with unprecedented precision. Photon-blockade experiments for different pulse-length and detuning conditions (supported by quantum optical simulations) reveal that achieving high-fidelity photon blockade requires an intricate understanding of the phonons' influence on the system dynamics. Finally, we achieve direct coherent control of the polariton states of a strongly coupled system and demonstrate that their efficient coupling to phonons can be exploited for novel concepts in high-fidelity single-photon generation.

Diamond hosts optically active color centers with great promise in quantum computation, networking, and sensing. Realization of such applications is contingent upon the integration of color centers into photonic circuits. However, current diamond quantum optics experiments are restricted to single devices and few quantum emitters because fabrication constraints limit device functionalities, thus precluding color center integrated photonic circuits. In this work, we utilize inverse design methods to overcome constraints of cutting-edge diamond nanofabrication methods and fabricate compact and robust diamond devices with unique specifications. Our design method leverages advanced optimization techniques to search the full parameter space for fabricable device designs. We experimentally demonstrate inverse-designed photonic free-space interfaces as well as their scalable integration with two vastly different devices: classical photonic crystal cavities and inverse-designed waveguide-splitters. The multi-device integration capability and performance of our inverse-designed diamond platform represents a critical advancement toward integrated diamond quantum optical circuits.

A two-level atom can generate a strong many-body interaction with light under pulsed excitation [1][2][3] . The best known e ect is single-photon generation, where a short Gaussian laser pulse is converted into a Lorentzian single-photon wavepacket 4,5 . However, recent studies suggested that scattering of intense laser fields o a two-level atom may generate oscillations in two-photon emission that come out of phase with the Rabi oscillations, as the power of the pulse increases 6,7 . Here, we provide an intuitive explanation for these oscillations using a quantum trajectory approach 8 and show how they may preferentially result in emission of two-photon pulses. Experimentally, we observe the signatures of these oscillations by measuring the bunching of photon pulses scattered o a two-level quantum system. Our theory and measurements provide insight into the re-excitation process that plagues 5,9 ondemand single-photon sources while suggesting the possibility of producing new multi-photon states.We begin by considering an ideal quantum two-level system that interacts with the outside world only through its electric dipole moment µ (ref. 10). Suppose the system is instantaneously prepared in the superposition of its ground |g and excited |e stateswhere P e is the probability of initializing the system in |e . From this point, spontaneous emission at a rate of Γ governs the remaining system dynamics and a single photon is coherently emitted with probability P e , while no photon is emitted with probability 1 − P e . As detected by an ideal photon counter, this results in the photocount distributionwhere P n is the probability to detect n photons in the emitted pulse. It is on this principle that most indistinguishable single-photon sources based on solid-state quantum emitters operate 4,5 .A popular mechanism for approximately preparing |ψ i is the optically driven Rabi oscillation 4,11 . Here, the system is initialized in its ground state and driven by a short Gaussian pulse from a coherent laser beam (of width τ FWHM ) that is resonant with the |g ↔ |e transition. Short is relative to the lifetime of the excited state τ e = 1/Γ to minimize the number of spontaneous emissions that occur during the system-pulse interaction 5,9 . As a function of the integrated pulse area, that is, A = dtµ · E(t)/ , where E(t) is the pulse's electric field, the system undergoes coherent oscillations between its ground |g and excited |e states. For constant-area pulses of vanishing τ FWHM /τ e , the final state of the system after interaction with the laser field is arbitrarily close to the superpositionwhere φ is a phase set by the laser field. Examining P e (A) (Fig. 1a dotted line), we see Rabi oscillations that are perfectly sinusoidal, with the laser pulse capable of inducing an arbitrary number of rotations between |g and |e . Because |ψ f (A) looks very much like |ψ i for arbitrarily short pulses, it is commonly assumed that the photocount distribution P n always has P 1 P n>1 . However, we will use a quantum trajectory appro...