The nitrogen-vacancy (NV) center in diamond has an optically addressable, highly coherent spin. However, an NV center even in high quality single-crystalline material is a very poor source of single photons: extraction out of the high-index diamond is inefficient, the emission of coherent photons represents just a few per cent of the total emission, and the decay time is large. In principle, all three problems can be addressed with a resonant microcavity. In practice, it has proved difficult to implement this concept: photonic engineering hinges on nano-fabrication yet it is notoriously difficult to process diamond without degrading the NV centers. We present here a microcavity scheme which uses minimally processed diamond, thereby preserving the high quality of the starting material, and a tunable microcavity platform. We demonstrate a clear change in the lifetime for multiple individual NV centers on tuning both the cavity frequency and anti-node position, a Purcell effect. The overall Purcell factor FP = 2.0 translates to a Purcell factor for the zero phonon line (ZPL) of F ZPL P ∼ 30 and an increase in the ZPL emission probability from ∼ 3 % to ∼ 46 %. By making a step-change in the NV's optical properties in a deterministic way, these results pave the way for much enhanced spin-photon and spin-spin entanglement rates.The nitrogen-vacancy (NV) center in diamond constitutes a workhorse in quantum technology on account of its optically addressable, coherent electron spin [1]. The NV stands out for its long spin coherence times [2], robust single photon emission [3] and the possibility of mapping its spin state to nearby nuclear spins [4]. Advances in spin-photon entanglement [5] and two-photon quantum interference protocols [6,7] pave the way for the implementation of quantum teleportation [8] and long-distance spin-spin entanglement [9]. However, the success rate of these protocols and the scaling up to extended networks are both limited by the very small generation rate of indistinguishable photons from individual NV centers [10].There are at least four factors which limit the generation rate of indistinguishable photons. First, the lifetime of NV centers is relatively long, ∼ 12 ns. Secondly, only a small fraction, ∼ 3 − 4 %, of the NV emission goes into the zero phonon line (ZPL) [11,12]. Only ZPL emission is useful for photon-based entanglement-swapping protocols as the phonon involved in non-ZPL emission dephases very rapidly. Thirdly, the photon extraction efficiency out of the diamond is hindered by the large refractive index of diamond itself. Finally, there are random spectral fluctuations in the exact frequency of the NV emission caused by charge noise in the diamond host [6].Coupling the NV center to a high quality factor, low mode volume optical microcavity offers a potential remedy to the first three factors thereby dramatically im- * Electronic address: daniel.riedel@unibas.ch proving the rate of coherent photon generation. These improvements depend on the weak coupling regime of cavity quantum e...

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We report the realization of a spatial and spectrally tunable air-gap Fabry-Pérot type microcavity of high finesse and cubic-wavelength-scale mode volume. These properties are attractive in the fields of opto-mechanics, quantum sensing and foremost cavity quantum electrodymanics. The major design feature is a miniaturized concave mirror with atomically smooth surface and radius of curvature as low as 10 µm produced by CO 2 laser ablation of fused silica. We demonstrate excellent mode-matching of a focussed laser beam to the microcavity mode and confirm from the frequencies of the resonator modes that the effective optical radius matches the physical radius. With these small radii, we demonstrate sub-wavelength beam waists. We also show that the microcavity is sufficiently rigid for practical applications: in a cryostat at 4 K, the root-mean-square microcavity length fluctuations are below 5 pm.

We investigate the strong coupling regime of a self-assembled quantum dot in a tunable microcavity with dark-field laser spectroscopy. The high quality of the spectra allows the lineshapes to be analyzed revealing subtle quantum interferences. Agreement with a model calculation is achieved only by including exciton dephasing which reduces the cooperativity from a bare value of 9.0 to the time-averaged value 5.5. In the pursuit of high cooperativity, besides a high-Q and low modevolume cavity, we demonstrate that equal efforts need to be taken towards lifetime-limited emitter linewidths.Cavity quantum electrodynamics (QED) involves an exchange of energy quanta between a single emitter and a cavity photon. The coupling ratehg = µ 12 E vac , depending on the emitter's dipole moment µ 12 and the vacuum electric field at the location of the emitter E vac , sets the relevant timescale of the coupled dynamics. If g is considerably smaller than the emitter relaxation rate γ or the cavity photon decay rate κ, on resonance the cavity mode acts as an additional decay channel to the emitter giving rise to an enhanced spontaneous emission rate (the Purcell effect of the weak coupling regime). If g is much larger than the energy loss rates, a coherent exchange of energy quanta takes place giving rise to new eigenstates, "polaritons", split in energy by 2hg (the strong coupling regime). The efficacy of the coherent coupling is commonly denoted by the cooperativity parameter C = 2g 2 /(κγ), the figure of merit for this work. The coherent exchange was first realized with single Cs atoms in a high finesse cavity [1].The strong coupling regime is a potentially powerful tool in quantum information processing [2], notably in quantum networks [3], since it enables for instance atomatom entanglement [4] or the distribution of quantum states [5]. Furthermore, strong coupling enables a nonlinear photon-photon interaction and hence the observation of photon blockade [6,7], a prerequisite for the creation of a single photon transistor [8,9].It is clearly desirable to implement cavity-QED in the solid-state as the solid-state host acts as a natural trap for the emitter. Furthermore, on-chip integration of multiple elements is feasible. As emitter, self-assembled quantum dots have desirable properties: high oscillator strength, narrow linewidths and weak phonon coupling [10]. As host, a semiconductor such as GaAs is very versatile: heterostructures can be realized; there is a wide array of post-growth processing techniques. Photoluminescence experiments on single InGaAs SAQD coupled to a photonic crystal cavity or a micropillar cavity revealed an anticrossing, the signature of the strong coupling regime [11][12][13]. For micropillars, recent experiments exhibit cooperativity values of around C 3 [14]. For photonic crystal cavities, a much higher C is achieved [15] but C is skewed by the fact that g γ yet g > ∼ κ.The photon decay rate κ at the emitter wavelength is relatively high in both geometries, limiting the cooperativity. In addition, ...

We propose an extension of optical spin noise spectroscopy that expands the so far accessible frequency range from a few gigahertz to several terahertz employing pairs of ultrafast femtosecond-laser pulses. The method is suitable for probing noise signals with very high bandwidths and signals centered at zero frequency. A time-resolved version of noise spectroscopy for detecting noise after a pump event follows naturally from the scheme. The analytical description of ultrafast spin noise spectroscopy along with numerical simulations proves the method a powerful spectroscopic tool.