About

Professor Scott Diddams holds the Robert H. Davis Endowed Chair at the University of Colorado Boulder, where he is also a Professor of Electrical Engineering and Physics. His experimental research spans precision spectroscopy and quantum metrology, nonlinear optics, microwave photonics, and ultrafast lasers. Diddams earned his Ph.D. from the University of New Mexico in 1996 and conducted postdoctoral work at JILA, NIST, and the University of Colorado from 1996 to 2000. He subsequently served as a Research Physicist, Group Leader, and Fellow at the National Institute of Standards and Technology (NIST). In 2022, he transitioned to his current position at the University of Colorado Boulder and assumed the role of Faculty Director of the Quantum Engineering Initiative in the College of Engineering and Applied Science. During his postdoctoral tenure, Diddams built the first optical frequency combs in the laboratory of Nobel laureate John Hall. Throughout his career, he has pioneered the use of optical frequency combs for applications including optical clocks, tests of fundamental physics, novel spectroscopy, and astronomy. His research contributions are documented in over 750 peer-reviewed publications, conference papers, and invited talks. Among his many honors, Dr. Diddams has received the Distinguished Presidential Rank Award, the Department of Commerce Gold and Silver Medals for "revolutionizing the way frequency is measured," the Presidential Early Career Award in Science and Engineering (PECASE), the IEEE Photonics Society Laser Instrumentation Award, and the IEEE Rabi Award. He is a Fellow of OPTICA (formerly OSA), the American Physical Society, and IEEE.

Research topics

• Computer Science
• Physics
• Telecommunications
• Optics
• Optoelectronics
• Astronomy
• Astrobiology
• Engineering
• Materials science
• Geology
• Astrophysics
• Electronic engineering
• Algorithm

Selected publications

• Photonic Generation and Free-Space Distribution of Millimeter Waves for Portable Optical Clocks

  Journal of Lightwave Technology · 2026-01-01

  articleOpen accessSenior author

  Robust and portable optical clocks promise to bring sub-picosecond timing instability to smaller form factors, offering possible performance improvements and new scenarios for positioning and navigation, radar technologies, and experiments probing fundamental physics. However, there are currently limited methods suitable for broadly disseminating the sub-picosecond timing signals or performing frequency comparison of these clocks, particularly over open-air paths. Established microwave time-transfer techniques only offer nanosecond level time synchronization, whereas optical techniques have challenging pointing requirements and lack the capability of all-weather operation. In this paper, we explore optically derived millimeter-wave carriers as a time-frequency link for full utilization of the next generation of portable optical clocks. We introduce an architecture that synthesizes 90 GHz millimeter waves with a one-second residual instability of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$2\times 10^{-15}$</tex-math></inline-formula>, averaging into the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$10^{-17}$</tex-math></inline-formula> range. In addition, we demonstrate a first-of-its-kind 110 m phase-stabilized free-space frequency comparison link over a millimeter-wave band with a one second instability in the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$10^{-14}$</tex-math></inline-formula> region. Technical and systematic uncertainties are investigated and characterized, providing a foundation for future time and frequency transfer experiments among distributed portable optical clocks.

  PublisherOA PDFDOI

• Searching for GEMS: Discovery and Characterization of Two Brown Dwarfs Around M Dwarfs*

  The Astronomical Journal · 2025-04-03 · 4 citations

  articleOpen access

  Abstract Brown dwarfs bridge the gap between stars and planets, providing valuable insight into both planetary and stellar-formation mechanisms. Yet the census of transiting brown-dwarf companions, in particular around M-dwarf stars, remains incomplete. We report the discovery of two transiting brown dwarfs around low-mass hosts using a combination of space- and ground-based photometry along with near-infrared radial velocities. We characterize TOI-5389Ab ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>68</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mn>0</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>2.2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>2.2</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:msub> <mml:mi>M</mml:mi> <mml:mi mathvariant="normal">J</mml:mi> </mml:msub> </mml:math> ) and TOI-5610b ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>40</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mn>4</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.0</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.0</mml:mn> </mml:mrow> </mml:msubsup> <mml:mspace width="0.25em"/> <mml:msub> <mml:mi>M</mml:mi> <mml:mi mathvariant="normal">J</mml:mi> </mml:msub> </mml:math> ), two moderately massive brown dwarfs orbiting early M-dwarf hosts ( T eff = 3569 ± 59 K and 3618 ± 59 K, respectively). For TOI-5389Ab, the best fitting parameters are period P = 10.40046 ± 0.00002 days, radius <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">BD</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0.82</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.031</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.033</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> R J , and low eccentricity <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>e</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.096</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.0046</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.0027</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> . In particular, this constitutes one of the most extreme substellar-stellar companion-to-host mass ratios of q = 0.150. For TOI-5610b, the best-fitting parameters are period P = 7.95346 ± 0.00002 days, radius <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>R</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">BD</mml:mi> </mml:mrow> </mml:msub> <mml:mo>=</mml:mo> <mml:mn>0.88</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>7</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.031</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.031</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> R J , and moderate eccentricity <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>e</mml:mi> <mml:mo>=</mml:mo> <mml:mn>0.35</mml:mn> <mml:msubsup> <mml:mrow> <mml:mn>4</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.012</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.011</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> . Both targets are expected to have shallow, but potentially observable, occultations: ≲500 ppm in the Johnson K band. A statistical analysis of M-dwarf/BD systems reveals for the first time that those at short orbital periods ( P &lt; 13 days) exhibit a dearth of 13 M J &lt; M BD &lt; 40 M J companions ( q &lt; 0.1) compared to those at slightly wider separations.

  PublisherDOI

• Searching for GEMS: The Occurrence of Giant Planets orbiting M-dwarfs within 100 pc

  ArXiv.org · 2025-09-10 · 1 citations

  preprintOpen access

  We present results from a systematic search for transiting short-period Giant Exoplanets around M-dwarf Stars (GEMS; $P &lt; 10$ days, $R_p \gtrsim 8~R_\oplus$) within a distance-limited 100 pc sample of $149,316$ M-dwarfs using TESS-Gaia Light Curve (TGLC) data. We describe the development and application of the \textit{TESS-miner} package and associated vetting procedures used in this analysis. To assess detection completeness, we conducted $\sim$72 million injection-recovery tests across $\sim$26,000 stars with an average of $\sim$3 sectors of data per star, subdivided into early-type (M0--M2.5), mid-type (M2.5--M4), and late-type (M4 or later) M-dwarfs. Our pipeline demonstrates high sensitivity across all subtypes within the injection bounds. We estimate the occurrence rates of short-period GEMS as a function of stellar mass, and combine our measured rates with those derived for FGK stars, fitting an exponential trend with stellar mass, consistent with core-accretion theory predictions. We find GEMS occurrence rates of $0.118\% \pm 0.068\%$ for early-type M-dwarfs, $0.153\% \pm 0.069\%$ for mid-type, and $0.036\% \pm 0.024\%$ for late-type M-dwarfs, with a mean rate of $0.068\%\pm0.024\%$ across the full sample. While our search spanned $1.0~\mathrm{days} &lt; P &lt; 10.0$ days, these rates were calculated using planets orbiting with $1.0~\mathrm{days} &lt; P &lt; 5.0$ days. This work establishes the basis for future occurrence rate studies of transiting GEMS.

  PublisherOA PDFDOI

• Harnessing micro-Fabry–Pérot reference cavities in photonic integrated circuits

  Nature Photonics · 2025-06-06 · 11 citations

  articleOpen access

  Compact photonic systems that offer high frequency stability and low noise are of increasing importance to applications in precision metrology, quantum computing, communication and advanced sensing technologies. However, on-chip resonators comprising dielectrics cannot match the frequency stability and noise characteristics of Fabry–Pérot cavities, whose electromagnetic modes live almost entirely in vacuum. Here we present a novel strategy to interface microfabricated Fabry–Pérot cavities with photonic integrated circuits to realize compact, high-performance integrated systems. Using this new integration approach, we demonstrate the self-injection locking of an on-chip laser to a millimetre-scale vacuum-gap Fabry–Pérot cavity using a circuit interface that transforms the reflected cavity response to enable efficient feedback to the laser. This system achieves a phase noise of –97 dBc Hz–1 at 10-kHz offset frequency, a fractional frequency stability of 5 × 10−13 at 10 ms, a 150-Hz 1/π integral linewidth and a 35-mHz fundamental linewidth. We also present a complementary integration strategy that utilizes a vertical-emission grating coupler and a back-reflection cancellation circuit to realize a fully co-integrated module that effectively redirects the reflected signals and isolates back-reflections with a 10-dB suppression ratio, serving as a key for on-chip Pound–Drever–Hall locking. Together, these results highlight how vacuum-gap Fabry–Pérot reference cavities can be harnessed for ultrastable, low-noise photonic systems. Self-injection locking of an on-chip laser to a milimetre-scale vacuum-gap Fabry–Pérot cavity is demonstrated, with a phase noise of –97 dBc Hz–1 at a 10-kHz offset frequency and a fractional frequency stability of 5 × 10−13 at 10 ms, enabling next-generation high-performance integrated systems.

  PublisherOA PDFDOI

• Discovery of a Nearby Habitable Zone Super-Earth Candidate Amenable to Direct Imaging

  The Astronomical Journal · 2025-10-23 · 1 citations

  articleOpen access

  Abstract We present the discovery of GJ 251 c, a candidate super-Earth orbiting in the habitable zone (HZ) of its M dwarf host star. Using high-precision Habitable-zone Planet Finder and NEID RVs, in conjunction with archival RVs from the Keck I High Resolution Echelle Spectrometer, the Calar Alto High-resolution Search for M dwarfs with Exoearths with Near-infrared and optical Echelle Spectrograph, and the Spectropolarimétre Infrarouge, we improve the measured parameters of the known planet, GJ 251 b ( P b = 14.2370 days; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>m</mml:mi> <mml:mi>sin</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>i</mml:mi> <mml:mo stretchy="false">)</mml:mo> </mml:math> = 3.85 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mrow/> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.33</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.35</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> M ⊕ ), and we significantly constrain the minimum mass of GJ 251 c, placing it in a plausibly terrestrial regime ( P c = 53.647 ± 0.044 days; <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>m</mml:mi> <mml:mi>sin</mml:mi> <mml:msub> <mml:mrow> <mml:mi>i</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>c</mml:mi> </mml:mrow> </mml:msub> </mml:math> = 3.84 ± 0.75 M ⊕ ). Using activity mitigation techniques that leverage chromatic information content, we perform a color-dependent analysis of the system and a detailed comparison of more than 50 models that describe the nature of the planets and stellar activity in the system. Due to GJ 251’s proximity to Earth (5.5 pc), next generation, 30 meter class telescopes will likely be able to image terrestrial planets in GJ 251’s HZ. In fact, GJ 251 c is currently the best candidate for terrestrial, HZ planet imaging in the northern sky.

  PublisherDOI

• Searching for GEMS: TOI-7149 b, an Inflated Giant Planet Causing a 12% Transit of a Fully Convective M-dwarf

  The Astronomical Journal · 2025-09-03 · 3 citations

  articleOpen accessCorresponding

  Abstract We describe the discovery and characterization of TOI-7149 b, a 0.705 ± 0.075 M J , 1.18 ± 0.045 R J gas giant on a ∼2.65 days period orbit transiting an M4V star with a mass of 0.344 ± 0.030 M ⊙ and an effective temperature of 3363 ± 59 K. The planet was first discovered using NASA’s TESS mission, which we confirmed using a combination of ground-based photometry, radial velocities, and speckle imaging. The planet has one of the deepest transits of all known main-sequence planet hosts at ∼12% ( R p / R ⋆ ∼ 0.33). Pushing the bounds of previous discoveries of giant exoplanets around M-dwarf stars (GEMS), TOI-7149 is one of the lowest mass M-dwarfs to host a transiting giant planet. We compare the sample of transiting GEMS to stars within 200 pc with a Gaia color–magnitude diagram and find that the GEMS hosts are likely to be high metallicity stars. We also analyze the sample of transiting giant planets using the nonparametric MRExo framework to compare the bulk density of warm Jupiters across stellar masses. We confirm our previous result that transiting Jupiters around early M-dwarfs have similar masses and densities to warm Jupiters around FGK stars, and extend this to mid M-dwarfs, thereby suggesting a potential commonality in their formation mechanisms.

  PublisherDOI

• Spectroscopic characterization of LOFAR radio-emitting M dwarfs

  Astronomy and Astrophysics · 2025-12-20

  articleOpen access

  Recent observations with the LOw Frequency ARray (LOFAR) have revealed 19 nearby M dwarfs showing bright circularly polarized radio emission. One of the possible sources of such emission is through magnetic star-planet interactions (MSPIs) with unseen close-in planets. We present the initial results from a spectroscopic survey with the Habitable-zone Planet Finder (HPF) and NEID spectrographs designed to characterize this sample and further investigate the origin of the radio emission. We provide four new insights into the sample. I) We uniformly characterized the stellar properties, constraining their effective temperatures ( T eff ), surface gravities (log g ), metallicities ([Fe/H]), projected rotational velocities ( v sin i ⋆ ), rotation periods ( P rot ), stellar radii ( R ⋆ ), and stellar inclinations ( i ⋆ ) where possible. Further, from a homogeneous analysis of the HPF spectra, we inferred their chromospheric activity and spectroscopic multiplicity states. From this, we identified GJ 625, GJ 1151, and LHS 2395 as single, quiescent stars amenable to precise radial velocity follow-up, making them strong MSPI candidates. II) We show that the distribution of stellar inclinations are compatible with an isotropic distribution, providing no evidence for a preference to pole-on configurations. III) We refined the radial velocity solution for GJ 625 b, the only currently known close-in planet in the sample, reducing the uncertainty in its orbital period by a factor of three, to facilitate future phase-dependent radio analysis. IV) Finally, we identified GJ 3861 as a spectroscopic binary with an orbital period of P = 14.841181 −0.00010 +0.00011 d and with a mass ratio of q = 0.7663 −0.0018 +0.0020 , making it the only confirmed binary with a relatively short orbit in the sample, where we surmise the radio emission is likely related to magnetospheric interactions between the two stars. These results advance our understanding of radio-emitting M dwarfs and establish an observational foundation for identifying MSPIs.

  PublisherDOI

• Dynamic spectral tailoring of a 10 GHz laser frequency comb for enhanced calibration of astronomical spectrographs

  Optics Express · 2025-03-24 · 4 citations

  articleOpen accessSenior author

  level, as required to identify and characterize Earth-like exoplanets. However, large intensity variations across the LFC spectrum that arise in the nonlinear broadening limit the range of comb modes that can be used for optimal wavelength calibration with sufficient signal-to-noise ratio. Furthermore, temporal spectral-intensity fluctuations of the LFC, that are coupled to flux-dependent detector defects, alter the instrumental point spread function (PSF) and result in spurious RV shifts. To address these issues and improve calibration precision, spectral flattening is crucial for LFCs to maintain a constant photon flux per comb mode. In this work, we demonstrate a dynamic spectral shaping setup using a spatial light modulator (SLM) over the wavelength range of 800-1300 nm. The custom shaping compensates for amplitude fluctuations in real time and can also correct for wavelength-dependent spectrograph transmission, achieving a spectral profile that delivers the constant readout necessary for maximizing precision. Importantly, we characterize the out-of-loop properties of the spectral flattener to verify a twofold improvement in spectral stability. This technique, combined with our approach of pumping the waveguide spectral broadener out-of-band at 1550 nm, reduces the required dynamic range. While this spectral region is tailored for the LFC employed at the Habitable-zone Planet Finder (HPF) spectrograph, the method is broadly applicable to any LFC used for astronomical spectrograph calibration.

  PublisherDOI

• Breaking the standard phase noise model for optical frequency combs

  2025-08-26

  preprintOpen accessSenior author

  The standard phase noise model is the leading theory that describes noise sources in a frequency comb. We demonstrate that the standard model can be broken for a resonant electro-optic comb.

  PublisherOA PDFDOI

• Quantification of broadband chromatic drifts in Fabry–Pérot resonators for exoplanet science

  Nature Astronomy · 2025-02-26 · 6 citations

  article
  PublisherDOI

Recent grants

• Collaborative Research: Turnkey Laser Frequency Comb for the Calibrator for the Habitable Zone Planet Finder

  NSF · $639k · 2013–2018

• Collaborative Research: An Agile Electro-Optic Frequency Comb for Precision Near-Infrared Radial Velocity Spectroscopy with the Habitable Zone Planet Finder

  NSF · $495k · 2020–2025

Frequent coauthors

• Franklyn Quinlan

  National Institute of Standards and Technology

  272 shared

• Connor Fredrick

  National Institute of Standards

  271 shared

• Tara M. Fortier

  238 shared

• Scott B. Papp

  National Institute of Standards and Technology

  238 shared

• Andrew J. Metcalf

  National Institute of Standards and Technology

  187 shared

• L. Hollberg

  177 shared

• Suvrath Mahadevan

  155 shared

• Charles A. McLemore

  142 shared

Labs

• Frequency Combs & Quantum MetrologyPI

  Precision spectroscopy and quantum metrology, nonlinear optics, microwave photonics and ultrafast lasers

Education

• Ph.D.

  University of New Mexico

  1996

• Other

  JILA, NIST and the University of Colorado

Awards & honors

• Distinguished Presidential Rank Award
• Department of Commerce Gold Medal
• Department of Commerce Silver Medal
• Presidential Early Career Award in Science and Engineering (…
• C.E.K. Mees Medal from OPTICA