We demonstrate an integrated nanophotonic network in diamond, consisting of a ring resonator coupled to an optical waveguide with grating in- and outcouplers. Using a nitrogen-vacancy color center embedded inside the ring resonator as a source of photons, single photon generation and routing at room temperature is observed. Furthermore, we observe a large overall photon extraction efficiency (10%) and high quality factors of ring resonators (3200 for waveguide-coupled system and 12,600 for a bare ring).

We demonstrate quantum interference between indistinguishable photons emitted by two nitrogenvacancy (NV) centers in distinct diamond samples separated by two meters. Macroscopic solid immersion lenses are used to enhance photon collection efficiency. Quantum interference is verified by measuring a value of the second-order cross-correlation function g (2) (0) = 0.35 ± 0.04 < 0.5. In addition, optical transition frequencies of two separated NV centers are tuned into resonance with each other by applying external electric fields. Extension of the present approach to generate entanglement of remote solid-state qubits is discussed.The interference of two identical photons impinging on a beamsplitter leads to perfect coalescence where both photons leave through the same output port. This fundamental effect, known as Hong-Ou-Mandel (HOM) interference [1], is a consequence of bosonic statistics for indistinguishable particles. HOM interference has been demonstrated using single photon pairs from parametric down-conversion [2] and delayed photons from a single photon source [3][4][5]. HOM interference has recently drawn attention as a resource for entanglement generation between distinct single-photon emitters with many potential applications in quantum information science [6]. The effect has been observed for photons emitted by pairs of atoms [7] and trapped ions [8], and has been used for entanglement generation of remote trapped ions [9]. While isolated atoms and ions, which are nominally identical, are a natural source of indistinguishable photons, extending these ideas to condensed matter systems can be challenging since two solid-state emitters are generally distinguishable because of their different local environments. This Letter demonstrates quantum interference of two photons produced by nitrogen-vacancy (NV) impurities in distinct diamond samples separated by two meters. Complementing the recent work involving other solid-state systems [10][11][12][13], the present solid-state realization is particularly significant, since electronic and nuclear spins associated with NV centers can be used as a robust solid-state qubit memory, yielding potential scalable architectures for quantum networks [14,15]. Specifically, in combination with a recent demonstration of spinphoton entanglement [16], the present work paves the way for entanglement generation between remote solidstate qubits.Unlike those associated with atoms in free space, the optical properties of NV centers embedded in a solid state vary substantially from emitter to emitter, especially in distinct samples. This inhomogeneity is due to variation in the local environments of NV centers and, in particular, to variation in the local strain. Furthermore, coincidence experiments are limited by the collection efficiency for light emitted by the NV center. While a wide variety of approaches are currently being explored to enhance the collection efficiency [17][18][19][20], we here utilize solid immersion lenses (SILs) fabricated from bulk diamond [21] to facili...

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Optical nonlinearities typically require macroscopic media, thereby making their implementation at the quantum level an outstanding challenge. Here, we demonstrate a nonlinearity for one atom enclosed by two highly reflecting mirrors 1 . We send laser light through the input mirror and record the light from the output mirror of the cavity. For weak laser intensity, we find the vacuum-Rabi resonances 2-11 . But for higher intensities, we observe an extra resonance 12 , which originates from the fact that the cavity can accommodate only an integer number of photons and that this photon number determines the characteristic frequencies of the coupled atom-cavity system 13-15 . We selectively excite such a frequency by depositing two photons at once into the system and find a transmission that increases with the laser intensity squared. The nonlinearity differs from classical saturation nonlinearities [16][17][18][19] and is direct spectroscopic proof of the quantum nature of the atom-cavity system. It provides a photon-photon interaction by means of one atom, and constitutes a step towards a two-photon gateway or a singlephoton transistor 20 . The quantum nonlinearity has its origin in the fact that under the condition of strong coupling, a system composed of a single atom and a single cavity mode has properties that are distinctively different from those of the bare atom (without the cavity), or the bare cavity (without the atom), or just the sum of the two (Fig. 1a). In fact, the composite system forms a new quantum entity, the so-called atom-cavity molecule, made of matter and light, with its own characteristic energy spectrum. This spectrum consists of an infinite ladder of pairs of states, the dressed states (Fig. 1b). The first doublet contains one quantum of energy and can be probed by laser spectroscopy. For weak probing, the resulting spectrum is independent of the laser intensity and has been dubbed the vacuum-Rabi or normal-mode spectrum, consisting of a pair of resonances symmetrically split around the bare atomic and cavity resonances. This spectrum was first observed with atomic beams 1 , and has been explored recently with single dipole-trapped atoms 2-4 . It constitutes a benchmark for strong atom-cavity coupling and is central to most cavity quantum electrodynamics (QED) experiments, including those outside atomic physics [5][6][7][8][9][10][11] . Note that the normal-mode spectrum on its own can equally well be described classically, by linear dispersion theory or a coupled oscillator model (atomic dipole and cavity field).The next-higher-lying doublet contains two quanta of energy and lacks a classical explanation 12,21,22 . The corresponding dressed states have been observed (together with a few higher-order states) in microwave cavity QED [13][14][15] and even ion trapping, where phonons play the role of photons 23 . At optical frequencies, evidence for these states has indirectly been obtained in two-photon correlation experiments where the conditional response of the system on detection of an ...

Single atoms absorb and emit light from a resonant laser beam photon by photon. We show that a single atom strongly coupled to an optical cavity can absorb and emit resonant photons in pairs. The effect is observed in a photon correlation experiment on the light transmitted through the cavity. We find that the atom-cavity system transforms a random stream of input photons into a correlated stream of output photons, thereby acting as a two-photon gateway. The phenomenon has its origin in the quantum anharmonicity of the energy structure of the atom-cavity system. Future applications could include the controlled interaction of two photons by means of one atom.

The intersystem crossing (ISC) is an important process in many solid-state atomlike impurities. For example, it allows the electronic spin state of the nitrogen-vacancy (NV) center in diamond to be initialized and read out using optical fields at ambient temperatures. This capability has enabled a wide array of applications in metrology and quantum information science. Here, we develop a microscopic model of the state-selective ISC from the optical excited state manifold of the NV center. By correlating the electron-phonon interactions that mediate the ISC with those that induce population dynamics within the NV center's excited state manifold and those that produce the phonon sidebands of its optical transitions, we quantitatively demonstrate that our model is consistent with recent ISC measurements. Furthermore, our model constrains the unknown energy spacings between the center's spin-singlet and spin-triplet levels. Finally, we discuss prospects to engineer the ISC in order to improve the spin initialization and readout fidelities of NV centers.

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Feedback is one of the most powerful techniques for the control of classical systems. An extension into the quantum domain is desirable as it could allow the production of non-trivial quantum states and protection against decoherence. The difficulties associated with quantum, as opposed to classical, feedback arise from the quantum measurement process-in particular the quantum projection noise and the limited measurement rate-as well as from quantum fluctuations perturbing the evolution in a driven open system. Here we demonstrate real-time feedback control of the motion of a single atom trapped in an optical cavity. Individual probe photons carrying information about the atomic position activate a dipole laser that steers the atom on timescales 70 times shorter than the atom's oscillation period in the trap. Depending on the specific implementation, the trapping time is increased by a factor of more than four owing to feedback cooling, which can remove almost all the kinetic energy of the atom in a quarter of an oscillation period. Our results show that the detected photon flux reflects the atomic motion, and thus mark a step towards the exploration of the quantum trajectory of a single atom at the standard quantum limit.