The trick to creating a better quantum sensor? Just give it a little squeeze.
For the first time ever, scientists have used a technique called "quantum squeezing" to improve the gas sensing performance of devices known as optical frequency comb lasers. These ultra-precise sensors are like fingerprint scanners for molecules of gas. Scientists have used them to spot methane leaks in the air above oil and gas operations and signs of COVID-19 infections in breath samples from humans.
Now, in a series of lab experiments, researchers have laid out a path for making those kinds of measurements even more sensitive and faster—doubling the speed of frequency comb detectors. The work is a collaboration between Scott Diddams at CU Boulder Boulder and Jérôme Genest at Université Laval in Canada.
"Say you were in a situation where you needed to detect minute quantities of a dangerous gas leak in a factory setting," said Diddams, professor in the Department of Electrical, Computer and Energy Engineering . "Requiring only 10 minutes versus 20 minutes can make a big difference in keeping people safe."
He and his colleagues published their findings Jan. 16 in the journal Science. Daniel Herman, a postdoctoral researcher in ECEE, led the study.
While normal lasers emit light in just one color, frequency comb lasers send out pulses of thousands to millions of colors—all at the same time. In the new study, the researchers used common optical fibers to precisely manipulate the pulses coming from those lasers. They were able to "squeeze" that light, making some of its properties more precise and others a little more random.
The research, in other words, represents a victory over some of the natural randomness and fluctuations that exist in the universe 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."
Photon wrangling
The results represent the latest step in the evolution of frequency combs, a technology born at JILA, a joint research institute between CU Boulder and the National Institute of Standards and Technology (NIST). Diddams was part of a team led by JILA's Jan Hall that first pioneered frequency comb lasers in the late 1990s. Hall would go on to win a Nobel Prize in Physics for this work in 2005.
As these laser pulses travel through the atmosphere, for example, molecules in the way will absorb certain colors of light, but not others. Scientists can identify what's in the air based on what colors go missing from their laser light. Picture it a bit like a hair comb that's lost a few of its teeth—hence, the name.
But those measurements also come with intrinsic uncertainties, Diddams said.
Light, he noted, is made up of tiny packets called photons. While lasers may look orderly from the outside, their individual photons are anything but.
"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."
Which, in turn, creates what he calls "fuzziness" in the data coming back from a frequency comb sensor.
Enter quantum squeezing.
Giving the squeeze
In quantum physics, many properties are coupled so that measuring one precisely will make your measurements of the other less precise. A classic example is the speed and location of a small particle like an electron—you can know where an electron is or how fast it's moving, but never both at the same time. Squeezing is a technique that maximizes one type of measurement at the expense of the other.
In a series of lab experiments, Diddams and his colleagues achieved that feat in a surprisingly simple way: They sent their pulses of frequency comb light through a normal optical fiber, not so different from what delivers internet to your home.
The structure of the fiber altered the light in just the right way so that photons from the lasers now arrived at a more regular interval. But that increase in orderliness came at a price. It became a little harder to measure the frequency of the light, or how the photons oscillated to produce specific colors.
That trade-off, however, allowed the researchers to detect molecules of gas with a lot fewer errors than before.
They tested the approach out in the lab using samples of hydrogen sulfide, a molecule that is common in volcanic eruptions and smells like rotten eggs. The team reported that it could detect those molecules around twice as fast with its squeezed frequency comb than with a traditional device. The researchers were also able to achieve this effect over a range of infrared light around 1,000 times greater than what scientists had previously accomplished.
The group still has work to do before it can bring its new sensor out into the field.
"But our findings show that we are closer than ever to applying quantum frequency combs in real-world scenarios," Herman said.
Diddams agreed: "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."
Other CU Boulder co-authors of the new study included Professor Joshua Combes; graduate students Molly Kate Kreider, Noah Lordi, Eugene Tsao and Matthew Heyrich; and postdoctoral researcher Alexander Lind. Mathieu Walsh, a graduate student at Université Laval, was also a co-author.
The work at CU Boulder was supported by the U.S. National Science Foundation through the Quantum Systems through Entangled Science and Engineering (Q-SEnSE) Quantum Leap Challenge Institute and by the Office of Naval Research.
