Optical frequency comb lasers are used to detect toxic airborne substances ranging from methane leaks to the COVID-19 virus. Rapid detection can protect everyone in the vicinity of the leak from harm. “Say you were in a situation where you needed to detect minute quantities of a dangerous gas leak in a factory setting,” Scott Diddams, professor at the University of Colorado Boulder (CU Boulder), said. “Requiring only 10 minutes versus 20 minutes can make a big difference in keeping people safe.” (From left) Professor Scott Diddams and graduate students Pooja Sekhar and Mary Kate Kreider in their quantum engineering lab on campus. Courtesy of CU Boulder. Researchers at CU Boulder and Université Laval demonstrated a way to double the speed of frequency comb detectors, increasing their sensitivity even further. The researchers used optical fibers to manipulate the pulses emitted by frequency comb lasers and quantum squeezing to make some properties of the light more precise. Frequency comb lasers can emit pulses of thousands, and even millions, of colors simultaneously. As these pulses travel through the atmosphere, molecules in the air absorb some colors, but not others. Scientists can identify what molecules are present in the atmosphere by identifying which colors are missing from the laser light. However, these measurements are not exact. Due to the inherent nature of light, there is some uncertainty as to when the individual photons that comprise the laser light will arrive at the sensor. This results in what Diddams refers to as “fuzziness” in the data that comes back from the frequency comb sensor. “If you’re detecting these photons, they don’t arrive at a perfectly uniform rate like one per nanosecond,” Diddams said. “Instead, they arrive at random times.” Quantum mechanics ultimately limits the metrological precision that can be achieved with laser frequency combs. But, using the technique of quantum squeezing, Diddams and his colleagues were able to make the measurements taken by frequency combs considerably more precise. Illustration depicting how frequency comb gas sensors work: Lasers emit pulses of light in many different colors (left), and molecules in the air absorb some of those colors (right). Scientists can then identify what molecules are present based on what colors are missing. Courtesy of Scott Diddams. In quantum physics, many properties are coupled, and measuring one property precisely will cause the measurement of the other property to be less precise. A classic example is measuring the speed and location of an electron. It is possible to measure where an electron is or how fast it is moving, but not both properties at the same time. Quantum squeezing is a way to maximize the precision of one measurement at the expense of the other. The researchers sent pulses of frequency light through an optical fiber. Using the Kerr effect in the nonlinear optical fiber, a 1 GHz frequency comb, centered at 1560 nm, was amplitude-squeezed by more than 3 dB over a 2.5 THz bandwidth. The structure of the fiber altered the light in a way that caused individual photons from the frequency comb laser to arrive at more regular intervals. At the same time, quantum squeezing of the light from the comb made it harder to measure the frequency of the light. Despite this tradeoff, the researchers found that squeezing the light enabled them to detect gas molecules more accurately. The team tested its approach on samples of hydrogen sulfide (H2S; a molecule that is common in volcanic eruptions. Dual-comb interferometry yielded mode-resolved spectroscopy of H2S gas with a signal to noise ratio nearly 3 dB beyond the shot-noise limit. The quantum noise reduction led to a two-fold quantum speedup in the determination of gas concentration, compared with a traditional device. The researchers were able to achieve this effect over a range of IR light about 1000 times greater than the range that was previously reported. The laser emitter for a frequency comb gas sensor developed by LongPath Technologies, a company founded by researchers at CU Boulder. The company’s detectors can spot methane leaking from oil and gas facilities in real time. Courtesy of Casey Cass/CU Boulder. The team has more work to do before it can bring the new sensor into the field. “But our findings show that we are closer than ever to applying quantum frequency combs in real-world scenarios,” researcher Daniel Herman said. The team’s approach could help speed the measurements of multiple species taken in dynamic chemical environments. For the researchers, it also represents a victory over some of the natural randomness and fluctuations that exist at very small scales. “Beating quantum uncertainty is hard, and it doesn’t come for free,” Diddams said. “But this is a really important step for a powerful new type of quantum sensors.” “Scientists call this a ‘quantum speedup,’” he said. “We’ve been able to manipulate the fundamental uncertainty relationships in quantum mechanics to measure something faster and better.” The research was published in Science (www.doi.org/10.1126/science.ads6292).