Low-noise quantum frequency down-conversion of indistinguishable photons.

We present experimental results on quantum frequency down-conversion of indistinguishable single photons emitted by an InAs/GaAs quantum dot at 904 nm to the telecom C-band at 1557 nm. Hong-Ou-Mandel (HOM) interference measurements are shown prior to and after the down-conversion step. We perform Monte-Carlo simulations of the HOM experiments taking into account the time delays of the different interferometers used and the signal-to-background ratio and further estimate the impact of spectral diffusion on the degree of indistinguishability. By that we conclude that the down-conversion step does not introduce any loss of HOM interference visibility. A noise-free conversion-process along with a high conversion-efficiency (> 30 %) emphasize that our scheme is a promising candidate for an efficient source of indistinguishable single photons at telecom wavelengths.

Deterministic coupling of a single silicon-vacancy color center to a photonic crystal cavity in diamond.

Deterministic coupling of single solid-state emitters to nanocavities is the key for integrated quantum information devices. We here fabricate a photonic crystal cavity around a preselected single silicon-vacancy color center in diamond and demonstrate modification of the emitters internal population dynamics and radiative quantum efficiency. The controlled, room-temperature cavity coupling gives rise to a resonant Purcell enhancement of the zero-phonon transition by a factor of 19, coming along with a 2.5-fold reduction of the emitter's lifetime.

All-optical formation of coherent dark states of silicon-vacancy spins in diamond.

Spin impurities in diamond can be versatile tools for a wide range of solid-state-based quantum technologies, but finding spin impurities that offer sufficient quality in both photonic and spin properties remains a challenge for this pursuit. The silicon-vacancy center has recently attracted much interest because of its spin-accessible optical transitions and the quality of its optical spectrum. Complementing these properties, spin coherence is essential for the suitability of this center as a spin-photon quantum interface. Here, we report all-optical generation of coherent superpositions of spin states in the ground state of a negatively charged silicon-vacancy center using coherent population trapping. Our measurements reveal a characteristic spin coherence time, T2*, exceeding 45 nanoseconds at 4 K. We further investigate the role of phonon-mediated coupling between orbital states as a source of irreversible decoherence. Our results indicate the feasibility of all-optical coherent control of silicon-vacancy spins using ultrafast laser pulses.

Visible-to-telecom quantum frequency conversion of light from a single quantum emitter.

We demonstrate efficient (>30%) quantum frequency conversion of visible single photons (711 nm) emitted by a quantum dot to a telecom wavelength (1313 nm). Analysis of the first- and second-order coherence before and after wavelength conversion clearly proves that pivotal properties, such as the coherence time and photon antibunching, are fully conserved during the frequency translation process. Our findings underline the great potential of single photon sources on demand in combination with quantum frequency conversion as a promising technique that may pave the way for a number of new applications in quantum technology.

Ultrafast all-optical coherent control of single silicon vacancy colour centres in diamond.

Complete control of the state of a quantum bit (qubit) is a fundamental requirement for any quantum information processing (QIP) system. In this context, all-optical control techniques offer the advantage of a well-localized and potentially ultrafast manipulation of individual qubits in multi-qubit systems. Recently, the negatively charged silicon vacancy centre (SiV-) in diamond has emerged as a novel promising system for QIP due to its superior spectral properties and advantageous electronic structure, offering an optically accessible Λ-type level system with large orbital splittings. Here, we report on all-optical resonant as well as Raman-based coherent control of a single SiV- using ultrafast pulses as short as 1 ps, significantly faster than the centre's phonon-limited ground state coherence time of about 40 ns. These measurements prove the accessibility of a complete set of single-qubit operations relying solely on optical fields and pave the way for high-speed QIP applications using SiV- centres.