Efficient interfaces between photons and quantum emitters form the basis for quantum networks and enable nonlinear optical devices operating at the single-photon level. We demonstrate an integrated platform for scalable quantum nanophotonics based on silicon-vacancy (SiV) color centers coupled to nanoscale diamond devices. By placing SiV centers inside diamond photonic crystal cavities, we realize a quantum-optical switch controlled by a single color center. We control the switch using SiV metastable orbital states and verify optical switching at the single-photon level by using photon correlation measurements. We use Raman transitions to realize a single-photon source with a tunable frequency and bandwidth in a diamond waveguide. Finally, 1 arXiv:1608.05147v1 [quant-ph]

The ability to communicate quantum information over long distances is of central importance in quantum science and engineering [1]. For example, it enables secure quantum key distribution (QKD) [2, 3] relying on fundamental physical principles that prohibit the "cloning" of unknown quantum states [4,5]. While QKD is already being successfully deployed [6-9], its range is currently limited by photon losses and cannot be extended using straightforward measure-and-repeat strategies without compromising its unconditional security [10]. Alternatively, quantum repeaters [11], which utilize intermediate quantum memory nodes and error correction techniques, can extend the range of quantum channels. However, their implementation remains an outstanding challenge [12][13][14][15][16][17], requiring a combination of efficient and high-fidelity quantum memories, gate operations, and measurements. Here we report the experimental realization of memory-enhanced quantum communication. We use a single solid-state spin memory integrated in a nanophotonic diamond resonator [18][19][20] to implement asynchronous photonic Bell-state measurements. This enables a four-fold increase in the secret key rate of measurement device independent (MDI)-QKD over the loss-equivalent direct-transmission method while operating at megahertz clock rates. Our results represent a significant step towards practical quantum repeaters and large-scale quantum networks [21,22].

The negatively-charged silicon-vacancy (SiV − ) color center in diamond has recently emerged as a promising system for quantum photonics. Its symmetry-protected optical transitions enable creation of indistinguishable emitter arrays and deterministic coupling to nanophotonic devices. Despite this, the longest coherence time associated with its electronic spin achieved to date (∼ 250 ns) has been limited by coupling to acoustic phonons. We demonstrate coherent control and suppression of phonon-induced dephasing of the SiV − electronic spin coherence by five orders of magnitude by operating at temperatures below 500 mK. By aligning the magnetic field along the SiV − symmetry axis, we demonstrate spin-conserving optical transitions and single-shot readout of the SiV − spin with 89% fidelity. Coherent control of the SiV − spin with microwave fields is used to demonstrate a spin coherence time T2 of 13 ms and a spin relaxation time T1 exceeding 1 s at 100 mK. These results establish the SiV − as a promising solid-state candidate for the realization of quantum networks.Quantum networks require the ability to store quantum information in long-lived memories, to efficiently interface these memories with optical photons and to provide quantum nonlinearities required for deterministic quantum gate operations [1,2]. Even though key building blocks of quantum networks have been demonstrated in various systems [3,4], no solid-state platform has satisfied these requirements. Over the past decade, solid-state quantum emitters with stable spin degrees of freedom such as charged quantum dots and nitrogenvacancy (NV) centers in diamond have been investigated for the realization of quantum network nodes [5]. While quantum dots can be deterministically interfaced with optical photons [6], their quantum memory time is limited to the µs scale [7] due to interactions with their surrounding nuclear spin bath. In contrast, NV centers have an exceptionally long-lived quantum memory [8] but suffer from weak, spectrally unstable optical transitions [9]. Despite impressive proof-of-concept experimental demonstrations with these systems [10,11], scaling to a large number of nodes is limited by the challenge of identifying suitable quantum emitters with the combination of strong, homogeneous and coherent optical transitions and long-lived quantum memories.The negatively-charged silicon-vacancy (SiV − ) has recently been shown to have bright, narrowband optical transitions with a small inhomogeneous broadening [12,13]. The optical coherence of the SiV − is protected by its inversion symmetry [14], even in nanostructures [15]. These optical properties were recently used to show strong interactions between single photons and single SiV − centers and to probabilistically entangle two SiV − centers in a single nanophotonic device [16]. At 4 K, however, the SiV − spin coherence is limited to ∼ 100 ns due to coupling to the phonon bath, mediated by the spin-orbit interaction [17][18][19][20][21].In this Letter, we demonstrate high-fidelity coherent ...

We demonstrate a quantum nanophotonics platform based on germanium-vacancy (GeV) color centers in fiber-coupled diamond nanophotonic waveguides. We show that GeV optical transitions have a high quantum efficiency and are nearly lifetime-broadened in such nanophotonic structures. These properties yield an efficient interface between waveguide photons and a single GeV without the use of a cavity or slow-light waveguide. As a result, a single GeV center reduces waveguide transmission by 18 ± 1% on resonance in a single pass. We use a nanophotonic interferometer to perform homodyne detection of GeV resonance fluorescence. By probing the photon statistics of the output field, we demonstrate that the GeV-waveguide system is nonlinear at the single-photon level.Efficient coupling between single photons and coherent quantum emitters is a central element of quantum nonlinear optical systems and quantum networks [1][2][3]. Several atom-like defects in the solid-state are currently being explored as promising candidates for the realization of such systems [4], including the nitrogen-vacancy (NV) center in diamond, renowned for its long spin coherence at room temperature [5]; and the silicon-vacancy (SiV) center in diamond, which has recently been shown to have strong, coherent optical transitions in nanostructures [6][7][8][9]. The remarkable optical properties of the SiV center arise from its inversion symmetry [10], which results in a vanishing permanent electric dipole moment for SiV orbital states, dramatically reducing their response to charge fluctuations in the local environment.

Photon-mediated interactions between quantum systems are essential for realizing quantum networks and scalable quantum information processing. We demonstrate such interactions between pairs of silicon-vacancy (SiV) color centers coupled to a diamond nanophotonic cavity. When the optical transitions of the two color centers are tuned into resonance, the coupling to the common cavity mode results in a coherent interaction between them, leading to spectrally resolved superradiant and subradiant states. We use the electronic spin degrees of freedom of the SiV centers to control these optically mediated interactions. Such controlled interactions will be crucial in developing cavity-mediated quantum gates between spin qubits and for realizing scalable quantum network nodes.