Ultra-fast quantum interface between a solid-state spin and a photon

Strong interactions between single spins and photons are essential for quantum networks 1 and distributed quantum computation 2. They provide the necessary interface for entanglement distribution 3,4 , non-destructive quantum measurements 5-7 , and strong photon-photon interactions 8-10. Achieving a spin-photon interface in a solid-state device could enable compact chip-integrated quantum circuits operating at gigahertz bandwidths. Many theoretical works have suggested using nanophotonic structures to attain this high-speed interface 11-16. These proposals exploit strong light-matter interactions to coherently switch a photon with a single spin embedded in a nanoscale cavity or waveguide. However, to date such an interface has not been experimentally realized using a solid-state spin system. Here, we report an experimental demonstration of a nanophotonic spin-photon quantum interface operating on picosecond timescales, where a single solid-state spin controls the quantum state of a photon and a single photon controls the state of the spin. We utilize an optical nano-cavity strongly coupled to a charged quantum dot containing a single trapped spin. We show that the spin-state strongly modulates the a) cavity reflection coefficient, which conditionally flips the polarization state of a reflected photon. We also demonstrate the complementary effect where a single photon applies a π phase shift on one of the spin-states, thereby coherently rotating the spin. These results demonstrate a spin-photon quantum phase gate that retains phase coherence, an essential requirement for quantum information applications. Our results open up a promising direction for solid-state implementations of quantum networks and quantum computers operating at gigahertz bandwidths. Quantum dots coupled to photonic crystals enable strong light-matter interactions in a compact solid-state device structure 17. These interactions can interface stationary solid-state quantum bits (qubits) with photons 11-16 , as previously demonstrated using optically excited quantum dot states 18. But excited states have very short lifetimes that make them impractical for quantum information processing applications. Solid-state spin provides a more promising qubit system to implement a long-lived quantum memory. In particular, the spin of a singly charged quantum dot has attracted great interest for implementing a solid-state spin-photon interface. Charged quantum dots enable quantum memories with microsecond coherence time 19,20 and picosecond timescale single-qubit gates 21,22. The effort to integrate quantum dot spin with cavities has also recently experienced rapid progress. Several works demonstrated deterministic loading of a spin in a quantum dot coupled to a nanophotonic cavity 23-25 , and more recently coherent control of the loaded spin 26. …