Sub-Doppler spectroscopy of quantum systems through nanophotonic spectral translation of electro-optic light

An outstanding challenge for deployable quantum technologies is high-resolution laser spectroscopy at the specific wavelengths of ultranarrow transitions in atomic and solid-state quantum systems. Here we demonstrate a highly flexible approach to high-resolution spectroscopy for quantum technologies across a broad range of wavelengths, through the synergistic combination of fine-tooth electro-optic frequency combs and efficient Kerr nonlinear nanophotonics. We show that such fine-tooth combs, which provide simultaneous high spectral and temporal resolution in atomic spectroscopy, undergo coherent spectral translation with essentially no efficiency loss through third-order optical parametric oscillation (OPO) in a silicon-nitride microring. This enables nearly a million comb pump teeth, separated by a 1 kHz spacing, to be translated onto signal and idler beams that can be located across a broad range of wavelengths in the visible and short near-infrared. The generated wavelengths are subject to OPO phase and frequency-matching conditions that are highly controllable through nanophotonic dispersion engineering, and in the current implementation span between 589 and 1,150 nm, with both the electro-optic comb generation process and its spectral translation not introducing appreciable broadening to the pump laser linewidth. We further demonstrate the application of this approach to quantum systems by performing sub-Doppler spectroscopy of the hyperfine transitions of Cs atomic vapour with our electro-optically driven Kerr nonlinear light source. The generality, robustness and agility of our approach, as well as its compatibility with photonic integration, are expected to lead to its widespread applications in areas such as quantum sensing, telecommunications and atomic clocks. A nonlinear nanophotonic resonator is used to spectrally translate an electro-optic frequency comb to a controllable set of wavelengths between 600 nm and 1,050 nm, with comb properties that are advantageous for high-resolution spectroscopy preserved.