About

Leo Hollberg is a Professor of Physics and Geophysics at Stanford University, based in the Hansen Experimental Physics Laboratory (HEPL). He holds a B.S. in Physics from Stanford University (1976) and a Ph.D. in Physics from the University of Colorado, Boulder (1984). His research focuses on making optimal use of quantum systems—such as atoms, lasers, and electronics—to test fundamental physics principles, enable precision measurements of space-time, and develop useful devices, sensors, and instruments. His experimental program in laser and atomic physics includes high-resolution spectroscopy of laser-cooled and trapped atoms, non-linear optical coherence effects, optical frequency combs, optical/microwave atomic clocks, and high sensitivity trace gas detection. Hollberg's work often involves studying laser noise and methods to circumvent measurement limitations, utilizing technologies like frequency-stabilized lasers and chip-scale atomic devices. His research aims to deepen understanding of fundamental science, address real-world problems in environment, energy, and navigation, and develop advanced technologies and sensors. His work has led to experiments that test fundamental principles with high precision, sometimes resulting in new discoveries, surprises, and new questions, while also contributing to the development of better instruments and sensors.

Research topics

• Computer Science
• Physics
• Geometry
• Aerospace engineering
• Engineering
• Optics
• Computational physics
• Astronomy
• Electronic engineering
• Statistics
• Theoretical physics
• Statistical physics
• Mathematics
• Mechanics

Selected publications

• Enhancing the Monitoring of Dispersed Methane Sources with Open Path Optical Sensors

  2025-01-01

  articleSenior author

  We demonstrate a cost-effective, robust sensor for precise methane detection using nIR laser spectroscopy. Designed for widespread field deployment over large areas, the system enables accurate monitoring of methane emissions at ambient levels.

  PublisherDOI

• New approach for localizing disturbances on optical fiber networks

  2025-03-19

  article1st authorCorresponding

  In this talk, we are presenting a new method for localizing disturbances along an optical fiber cable. In contrast to Distributed Acoustic Sensing (DAS) techniques, our approach uses coherent optical phase measurements of stable low-power CW lasers that are recorded with an accurate time-stamp at each end of an optical fiber. This method is compatible with most types of deployed optical amplifiers, and it should be feasible to make localized strain/disturbance measurements over thousands of kilometers and spanning ocean basins, with around 200 m spatial resolution. When a fiber optic cable experiences vibrations, temperature changes, or other strains, the light passing through it is subject to a phase shift. Since the magnitude of the phase shift is proportional to that of the disturbance, it is possible to reconstruct the original disturbance strain or temperature change based on the phase of the light we observe. The comparison of the arrival time of the phase disturbance at each end relative to the local clock then gives the distance along the fiber of the disturbance relative to the clocks at each end. The accuracy of this localization step is critical for most of the technology’s promising sensing applications, such as undersea seismometry and strain measurements. Using GPS-steered quartz oscillators, absolute timing uncertainties of 10 ns (equivalent to around 5 m in spatial terms) are feasible relative to GPS/UTC time. The limiting factor is instead the ability to accurately extract and compare the arrival times of the two signals, since a temporal resolution on the order of microseconds is required and seismic signals tend to be low-frequency – typically below 10 Hz. Sophisticated time delay estimation (TDE) signal processing techniques are therefore required to determine the difference in time between the onset of each signal. The difficulty is further amplified by the fact that the light going to both detectors is subject to the same background vibrations and broadband noise. Traditional TDE techniques, typically based on a cross correlation, either struggle or falter completely in such environments, where significant common noise exists. We propose instead a new TDE technique, “derivative matching,” which is based on a geometric comparison between the difference of the two time-shifted signals and their averaged derivative. This novel technique requires significantly less time and computational power to perform, and yet we find that it provides at least an order of magnitude of improvement in localization accuracy, while also enabling us for the first time to analyze multiple simultaneous disturbances along a single cable. In a comparison test of 39 samples taken from a noisy 16 km fiber routed through several buildings on the Stanford campus, we were able to localize the vibrational signal produced by a piezoelectric fiber stretcher to within 0.14 ± 0.39 km, while the best traditional technique (“Compensated Time-Shifting Deviation”) had an error of 4.8 ± 8.7 km. These results indicate that derivative matching has the potential to enable CW fiber optic interferometry with disturbance/strain localization at the few-hundred-meter level over fiber links of thousands of kilometers undersea and elsewhere.

  PublisherDOI

• Calibrating Strain Measurements: A Comparative Study of DAS, Strainmeter, and Seismic Data

  Earth and Space Science · 2025-02-01 · 7 citations

  articleOpen access

  Abstract Significant interest has developed in using optical fibers for seismology through Distributed Acoustic Sensing (DAS). However, converting DAS strain measurements to actual ground motions can result in errors and uncertainties due to imperfect coupling of the fiber to the earth and instrument response functions. To address this, we conducted a comparative analysis of strain data recorded by DAS, Optical Fiber Strainmeters (OFSs), and estimates derived from seismic data. This study used dark fibers in a commercial cable connecting two islands in Puget Sound, Washington, USA. The cable extends from a telecommunication substation on Whidbey Island, through an underground conduit, and across Saratoga Passage to Camano Island. The strain along the cable was recorded using OFS Michelson interferometers and a DAS interrogator, with a broadband seismometer positioned at one end. Comparing a teleseismic earthquake recording showed that summed DAS channels agreed well with OFS recordings. The amplitude discrepancies between the measurements and the seismometer's estimated strain indicated poor coupling between the cable and the earth. We also evaluated DAS amplitude response using a piezoelectric cylinder (PZT) to generate ground truth strain. The findings revealed a notable amplitude decrease in DAS recordings at lower frequencies, highlighting the need for amplitude calibration. Moreover, some underwater signals in the study area were strongly correlated with the velocity of the tidal current. These signals can be localized through coherence calculations between the DAS and OFS recordings.

  PublisherOA PDFDOI

• Quantum States Imaging of Magnetic Field Contours Based on Autler-Townes Effect in Ytterbium Atoms

  Physical Review Letters · 2025-05-12 · 2 citations

  articleOpen accessSenior author

  An intercombination transition in Yb enables a novel approach for rapidly imaging magnetic field variations with excellent spatial and temporal resolution and accuracy. This quantum imaging magnetometer reveals "dark stripes" that are contours of constant magnetic field visible by eye or capturable by standard cameras. These dark lines result from a combination of Autler-Townes splitting and the spatial Hanle effect in the ^{1}S_{0}-^{3}P_{1} transition of Yb when driven by multiple strong coherent laser fields (carrier and AM/FM modulation sidebands of a single-mode 556 nm laser). We show good agreement between experimental data and our theoretical model for the closed, 4-level Zeeman shifted V system and demonstrate scalar and vector magnetic field measurements at video frame rates over spatial dimensions of 5 cm with 0.1 mm resolution. Additionally, the ^{1}S_{0}-^{3}P_{1} transition allows for ∼μs response time and a large dynamic range (from microtesla to many tesla).

  PublisherOA PDFDOI

• Multi-Offset Imaging of Bed Topography Using Radio Frequency over Fiber Radar Arrays: Modelling and Initial Field Results

  2025-03-15

  preprintOpen accessSenior author

  Radio-echo sounding (RES) is a widely used tool in glaciology, providing insight into englacial and subglacial environments. Conventional high-spatial resolution RES surveys typically employ zero- or small-offset configurations with a single transmitter-receiver pair. Such surveys often prioritize spatial coverage over monitoring temporal changes in englacial and subglacial conditions. Stationary radar arrays aimed at providing time series data have been previously deployed in glaciated regions to provide estimates of basal melt rates, infer vertical strain within ice sheets, and image englacial layers in 3D. However, these stationary arrays are unable to image the ice-bed interface with sufficiently high resolution to infer changes in bed geometry over time. This is largely due to hardware limitations in the radar systems used in glaciology which typically support an inadequate number of antenna elements. Unlike in towed or airborne radar systems, where spatial resolution can be improved through synthetic aperture processing techniques, the spatial resolution achieved by a stationary array is proportional to the number of real antenna elements deployed. We overcome limitations in the number of supported antennas by integrating radio-frequency over fiber (RFoF) hardware, typically used in the communications industry, into existing radar systems such as the autonomous phase-sensitive radio-echo sounder (ApRES), as well as software-defined radios (SDRs). By converting RF signals to optical signals, lossy copper-based coaxial cables is replaced by low-loss fiber optic cables, permitting large separations between receive and transmit elements without significant signal attenuation during transmission. Further, the low cost, high switching speeds, and large number of output channels provided by fiber optic switches allows for a cost-effective way to rapidly cycle through 100s of antenna elements using a single radar unit RF input or output port. These modifications allow an ApRES, which traditionally supports up to 8 receive and 8 transmit antennas, to handle 100s of antennas on both the receive and transmit side, offering significant improvements in imaging capabilities. Such a system could support advanced imaging geometries capable of 3D time-lapse monitoring of englacial and subglacial processes, such as seasonal hydrology, subglacial erosion, isostatic rebound, and the evolution of sub-ice shelf features. We demonstrate these imaging capabilities through modelling and initial field results using our modified ApRES and SDR systems.

  PublisherDOI

• Quantum States Imaging of Magnetic Field Contours based on Autler-Townes Effect in Yb Atoms

  arXiv (Cornell University) · 2024-11-21

  preprintOpen accessSenior author

  An inter-combination transition in Yb enables a novel approach for rapidly imaging magnetic field variations with excellent spatial and temporal resolution and accuracy. This quantum imaging magnetometer reveals "dark stripes" that are contours of constant magnetic field visible by eye or capturable by standard cameras. These dark lines result from a combination of Autler-Townes splitting and the spatial Hanle effect in the $^{1}S_{0} - ^{3}P_{1}$ transition of Yb when driven by multiple strong coherent laser fields (carrier and AM/FM modulation sidebands of a single-mode 556 nm laser). We show good agreement between experimental data and our theoretical model for the closed, 4-level Zeeman shifted V-system and demonstrate scalar and vector magnetic fields measurements at video frame rates over spatial dimensions of 5 cm with 0.1 mm resolution. Additionally, the $^{1}S_{0} - ^{3}P_{1}$ transition allows for $\simμ$s response time and a large dynamic range (from microtesla to many tesla).

  PublisherOA PDFDOI

• Rapid, accurate imaging of magnetic fields with Yb atomic fluorescence

  2023-03-08

  article1st authorCorresponding

  We describe a fast imaging atomic vector magnetometer based on laser spectroscopy of the Zeeman splitting in the (6s6p) 3P1 state of Yb. In the presence of a magnetic field gradient, we observed prominent dark stripes (visible by eye or camera) in the fluorescence (556 nm) when a thermal Yb atomic beam is driven by square-wave amplitude-modulated light at RF frequencies. The 1S0-3P1 transition forms a “V” system where two laser sidebands interact simultaneously with two 3P1 Zeeman sublevels. The dark lines are contours of constant magnetic field strength, and are consistent with theoretical models dominated by Autler-Townes Splitting. The estimated magnetic field sensitivity is ≈ 10 μG (1 nT) for 100k image pixels recorded in 3 ms.

  PublisherDOI

• Fast and accurate magnetic vector tomograph with Yb atoms

  Optical and Quantum Sensing and Precision Metrology II · 2022-03-09

  article1st authorCorresponding
  PublisherDOI

• Fundamental physics with a state-of-the-art optical clock in space

  Quantum Science and Technology · 2022 · 82 citations

  • Physics
  • Theoretical physics
  • Aerospace engineering

  Abstract Recent advances in optical atomic clocks and optical time transfer have enabled new possibilities in precision metrology for both tests of fundamental physics and timing applications. Here we describe a space mission concept that would place a state-of-the-art optical atomic clock in an eccentric orbit around Earth. A high stability laser link would connect the relative time, range, and velocity of the orbiting spacecraft to earthbound stations. The primary goal for this mission would be to test the gravitational redshift, a classical test of general relativity, with a sensitivity 30 000 times beyond current limits. Additional science objectives include other tests of relativity, enhanced searches for dark matter and drifts in fundamental constants, and establishing a high accuracy international time/geodesic reference.

  PublisherDOI

• Optical atomic clock aboard an Earth-orbiting space station (OACESS): enhancing searches for physics beyond the standard model in space

  Quantum Science and Technology · 2022 · 39 citations

  • Computer Science
  • Physics
  • Astronomy

  Abstract We present a concept for a high-precision optical atomic clock (OAC) operating on an Earth-orbiting space station. This pathfinder science mission will compare the space-based OAC with one or more ultra-stable terrestrial OACs to search for space-time-dependent signatures of dark scalar fields that manifest as anomalies in the relative frequencies of station-based and ground-based clocks. This opens the possibility of probing models of new physics that are inaccessible to purely ground-based OAC experiments where a dark scalar field may potentially be strongly screened near Earth’s surface. This unique enhancement of sensitivity to potential dark matter candidates harnesses the potential of space-based OACs.

  PublisherOA PDFDOI

Frequent coauthors

• Scott A. Diddams

  University of Colorado Boulder

  177 shared

• John Kitching

  National Institute of Standards

  116 shared

• C. W. Oates

  National Institute of Standards and Technology

  103 shared

• H. G. Robinson

  90 shared

• Svenja Knappe

  84 shared

• A. Bartels

  60 shared

• Vishal Shah

  Peking University

  59 shared

• Vladislav Gerginov

  National Institute of Standards and Technology

  55 shared

Education

• Ph.D., Physics

  Stanford University

  1990

• B.S., Physics

  University of California, Berkeley

  1985