Fiber-Coupled Diamond Quantum Nanophotonic Interface

Color centers in diamond provide a promising platform for quantum optics in the solid state, with coherent optical transitions and long-lived electron and nuclear spins. Building upon recent demonstrations of nanophotonic waveguides and optical cavities in single-crystal diamond, we now demonstrate on-chip diamond nanophotonics with a high-efficiency fiber-optical interface achieving $>90\%$ power coupling at visible wavelengths. We use this approach to demonstrate a bright source of narrow-band single photons based on a silicon-vacancy color center embedded within a waveguide-coupled diamond photonic crystal cavity. Our fiber-coupled diamond quantum nanophotonic interface results in a high flux (approximately 38 kHz) of coherent single photons (near Fourier limited at $<1\text{−}\mathrm{GHz}$ bandwidth) into a single-mode fiber, enabling possibilities for realizing quantum networks that interface multiple emitters, both on chip and separated by long distances.

Figure 1

On-chip diamond nanophotonic structures. SEM images of (a) an array of diamond nanophotonic structures fabricated using angled-etching techniques [45]. The four key components in each device are (1) freestanding diamond waveguides, (2) integrated diamond nanobeam photonic crystal cavities [panels (b) with the top-down electric field profile of the fundamental optical-cavity-mode inset], (3) vertical waveguide support structures [panel (c)], and (4) freestanding diamond waveguide tapers [panels (d) and (e)].

Figure 2

Single-mode optical-fiber taper fabrication. (a) Schematic of fiber-waveguide taper adiabatic coupling. The single-ended conical OFT (blue) is physically contacted with a DWT (red). (b) Effective indices ($n_{\mathrm{eff}}$ calculated via an eigenmode solver) of the OFT and TE-polarized DWT modes for taper angle of 1.5° in both cases. Inset: Cross-sectional energy density profile ($|E{|}^2$) obtained from eigenmode simulations at the indicated points along the physically overlapped tapers. (c) Simulated coupling efficiencies for transmission from the fundamental diamond TE-polarized waveguide mode to the optical-fiber ${\mathrm{HE}}_{11}$ mode as a function of fiber-waveguide taper overlap for a $20\text{−}\mu \mathrm{m}$-long DWT. (d) Schematic of the fabrication of single-ended conical OFTs by hydrofluoric acid etching. (e) SEM image of a fabricated OFT tip, with (f) an enlarged image of the chemically etched surface roughness.

Figure 3

Optical characterization of diamond nanophotonic structures. (a) Schematic of the visible-band fiber-optical characterization setup. (b) Optical micrograph of a single-ended OFT in contact with a DWT under white-light illumination. (c) Broadband reflection spectra of diamond PCC (blue curve) and a commercial fiber-coupled retroreflector (red curve). The noise floor of the OSA is shown in gray. (d) Normalized high-resolution spectrum of the fundamental diamond nanobeam PCC mode centered at $\lambda \sim 733.9\mathrm{nm}$. A Lorentzian fit (red curve) yields a $Q$-factor estimate of $Q\sim 1.1\times {10}^4$.

Figure 4

Efficient generation and collection of narrow-band single photons. (a) A diamond SiV color center embedded inside a diamond PCC is excited from free space, and fluorescence in the diamond waveguide is collected into the optical-fiber taper. A FP cavity is used to measure the spectrum of the emitted photons. (b) Spectrum of collected photons. The excitation laser is resonant with the $|u⟩$-to-$|e⟩$ transition. The transmitted Raman fluorescence is recorded as the resonance frequency of the FP cavity is scanned across the $|e⟩$-to-$|c⟩$ transition to obtain an upper bound on the linewidth of detected photons of $1.27\pm 0.2\mathrm{GHz}$. (c) Saturation measurement of Raman photons with the PCC resonant with the $|e⟩$-to-$|c⟩$ transition. We subtract the linear background set by the excitation laser detecting a narrow-band single-photon flux of at least 38 kHz when the PCC is on resonance. Inset: Antibunching of $g^{(2)}(0)=0.21\pm 0.09$ in the Raman fluorescence autocorrelation confirms the generation of single photons.