New Technique Improves Frequency Comb Gas Sensors

Researchers at the University of Colorado Boulder have improved the gas sensing capabilities of optical frequency comb lasers by implementing a technique called "quantum squeezing." These highly sensitive sensors operate by identifying molecular fingerprints of gases. They have been utilized to detect methane leaks from oil and gas operations and to analyze human breath samples for markers of COVID-19 infection. The study was published in Science.

Researchers have outlined a method to double the speed of frequency comb detectors, improving the sensitivity and efficiency of such measurements in laboratory tests. Scott Diddams of CU Boulder Boulder and Jérôme Genest of Université Laval collaborated on the study.

Say you were in a situation where you needed to detect minute quantities of a dangerous gas leak in a factory setting. Requiring only 10 minutes versus 20 minutes can make a big difference in keeping people safe.

Scott Diddams, Professor, Department of Electrical, Computer and Energy Engineering, University of Colorado Boulder

The study was led by ECEE Postdoctoral Researcher Daniel Herman.

Frequency comb lasers emit pulses of thousands to millions of colors simultaneously, unlike conventional lasers, which emit a single color. In this study, the researchers used standard optical fibers to precisely control the pulses emitted by these lasers. By applying a technique known as “squeezing,” they were able to refine certain characteristics of the light while allowing others to vary more freely.

The research demonstrates a method to overcome the inherent randomness and fluctuations present in the universe at very small scales.

Beating quantum uncertainty is hard, and it does not come for free. But this is a really important step for a powerful new type of quantum sensors.

Scott Diddams, Professor, Department of Electrical, Computer and Energy Engineering, University of Colorado Boulder

Photon Wrangling

These findings represent a significant advancement in frequency comb technology, originally developed at JILA, a joint research facility of CU Boulder and the National Institute of Standards and Technology (NIST). Diddams was part of the team that pioneered frequency comb lasers in the late 1990s under the leadership of Jan Hall, who was later awarded the 2005 Nobel Prize in Physics for this work.

Frequency comb lasers function by emitting pulses of light containing specific colors. As these pulses pass through the atmosphere, certain colors are absorbed by molecules in their path, while others remain unaffected. The pattern of absorbed colors allows scientists to determine the composition of the air. The name "frequency comb" reflects the resemblance of this pattern to a hair comb with missing teeth.

However, Diddams noted that these measurements are subject to inherent uncertainties due to the nature of light.

Light consists of photons, which, while appearing orderly in laser beams, are highly irregular at the photon level. Diddams explained, “If you are detecting these photons, they do not arrive at a perfectly uniform rate like one per nanosecond. Instead, they arrive at random times.”

This randomness introduces "fuzziness" into the data collected by frequency comb sensors.

Enter quantum squeezing.

Giving the Squeeze

Quantum physics imposes interdependencies on certain properties, meaning that measuring one property more precisely reduces the accuracy of another. A classic example is the trade-off between determining a particle's location and its velocity. Squeezing is a method that prioritizes one type of measurement at the expense of another.

In their experiments, Diddams and his team applied this principle by transmitting frequency comb light pulses through a standard optical fiber, similar to those used in internet connections. The fiber's structure adjusted the light, causing photons to arrive at more regular intervals. This increased order came with a trade-off: measuring the light's frequency and the oscillation patterns that generate specific colors became less precise.

Despite this trade-off, the researchers achieved more accurate detection of gas molecules, reducing measurement errors. Using samples of hydrogen sulfide, a molecule commonly found in volcanic emissions, the team demonstrated that the squeezed frequency comb could detect these molecules roughly twice as quickly as conventional methods.

The team achieved this improvement across an infrared light spectrum approximately 1,000 times broader than previously possible.

More work needs to be done before the team can deploy its new sensor in the field.

But our findings show that we are closer than ever to applying quantum frequency combs in real-world scenarios.

Daniel Herman, Postdoctoral Researcher, University of Colorado Boulder

Diddams agreed: “Scientists call this a ‘quantum speedup. We have been able to manipulate the fundamental uncertainty relationships in quantum mechanics to measure something faster and better.”

Journal Reference:

‌Herman, D. I., et al. (2025) Squeezed dual-comb spectroscopy. Science. doi.org/10.1126/science.ads6292.