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

Nature Photonics volume 18, pages 1285–1292 (2024)Cite this article

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Abstract

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.

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Fig. 1: Spectral translation of ultrahigh-resolution optical frequency combs via Kerr OPO.
Fig. 2: Numerical simulations of CMEs.
Fig. 3: Spectral translation of the pump optical frequency comb with the nanophotonic OPO.
Fig. 4: Frequency-agile pump, signal and idler optical frequency combs.
Fig. 5: Optical parametric oscillator comb spectroscopy of Cs.

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Data availability

All data and supporting materials will be made available at https://data.nist.gov/od/id/mds2-3459 through the National Institute of Standards and Technology.

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Acknowledgements

This work was supported by the NIST-on-a-Chip (D.A.L., J.R.S. and K.S.) and DARPA LUMOS (J.R.S. and K.S.) programmes. Portions of this research were performed in the NIST CNST NanoFab.

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Authors and Affiliations

  1. National Institute of Standards and Technology, Gaithersburg, MD, USA

    David A. Long, Jordan R. Stone, Yi Sun, Daron Westly & Kartik Srinivasan

  2. Joint Quantum Institute, NIST/University of Maryland, College Park, MD, USA

    Jordan R. Stone, Yi Sun & Kartik Srinivasan

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  1. David A. Long
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Contributions

Conceptualization was provided by D.A.L., J.R.S. and K.S. Investigations were carried out by D.A.L., J.R.S., Y.S. and D.W. Funding acquisition was performed by D.A.L., J.R.S. and K.S. The original draft was written by D.A.L. and J.R.S. Review and editing was performed by D.A.L., J.R.S. and K.S.

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Correspondence to David A. Long or Kartik Srinivasan.

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Competing interests

A provisional patent has been filed on some of these concepts on which several of the authors (D.A.L., J.R.S. and K.S.) are listed as inventors. The other authors declare no competing interests.

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Nature Photonics thanks Alfredo de Rossi and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Extended data

Extended Data Table 1 Optical parametric oscillator filtered output power in µW when the electro-optic frequency comb was both on and off
Full size table
Extended Data Table 2 Distribution of optical power between comb teeth
Full size table

Extended Data Fig. 1 Experimental schematic for generation and spectral translation of electro-optic frequency combs.

An external-cavity diode laser (ECDL) was passed through an electro-optic phase modulator (EOM) which was driven by a frequency chirp produced by a direct digital synthesizer (DDS). This results in an electro-optic comb which was then sent through a tapered amplifier (Amp) and then into the nanophotonic OPO. This leads to spectrally translated signal and idler combs (as well as the depleted pump comb).

Extended Data Fig. 2 Measured beat notes between the electro-optic frequency comb teeth and the titanium-sapphire laser.

(Left panel) Example measured beat notes between the electro-optic frequency comb teeth and the titanium-sapphire laser for both the pump comb before the tapered amplifier (780 nm) and the generated idler comb (877 nm). The shown measurements were the average of ten acquisitions with a total acquisition time of 1 ms. Due to the titanium-sapphire laser’s narrow linewidth (<10 kHz), this beat note provides a measure of linewidth of a given comb tooth. The shown Gaussian fits had standard fit uncertainties on their width of 14 kHz. The detuning is relative to the comb’s carrier. (Right panel) Fitted beat note linewidths (full-width at half-maximum) and corresponding standard fit uncertainties from the shown example fits. For each of the detunings the idler and pump linewidths agree to within their combined standard uncertainties. These linewidths are commensurate with the linewidth of the external-cavity diode laser which was utilized (500 kHz) and represent the limit on the spectral resolution for the system with this laser source. Although we note for measurements such as the hyperfine pumping found in Fig. 5 and Extended Data Fig. 3 (or electromagnetically induced transparency) the resolution is not limited by the laser linewidth given the common mode nature of the pump and probe30.

Extended Data Fig. 3 Simulation and measurement of the power in the photodetected OPO idler wave (at the pump phase modulation frequency) for different pump phase modulation frequencies.

Simulation (black line) and measurement (orange points) of the power in the photodetected OPO idler wave (at the pump phase modulation frequency) for different pump phase modulation frequencies. At low modulation frequencies, the response is low due to the lack of residual amplitude modulation (that is, the idler is purely phase modulated). At higher frequencies, the decrease in response results from the finite microcavity linewidth. In our simulations, we set κ/2π = 375 MHz.

Extended Data Fig. 4 Energy level diagram and optical frequency comb spectrum of the Cs D2 transition.

Energy level diagram and optical frequency comb spectrum for transitions from the F = 4 ground state. (Left panel) Relevant energy levels for the hyperfine pumping of a Cs D2 transition53 from the F = 4 ground state. (Right panel) Optical frequency comb spectrum of the Cs D2 transition which is the average of 10 spectra, each of which was acquired in 1 ms. The six hyperfine pumping transitions are observed at the difference frequencies of the upper-state hyperfine levels. The excess noise near 181 MHz (that is, the AOM frequency for this data set) occurs as those comb teeth are near to DC in the comb FFT.

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Long, D.A., Stone, J.R., Sun, Y. et al. Sub-Doppler spectroscopy of quantum systems through nanophotonic spectral translation of electro-optic light. Nat. Photon. 18, 1285–1292 (2024). https://doi.org/10.1038/s41566-024-01532-w

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