Squeezing optical frequency comb lasers makes them more sensitive as gas sensors… in the quantum realm

The trick to creating a better quantum sensor is to give it a little ‘squeeze’

American and Canadian scientists have developed a technical first ─ called ‘quantum squeezing’ ─ to improve the gas sensing performance of devices known as optical frequency comb lasers, in a study led by Daniel Herman, a postdoctoral researcher in Colorado University (CU) at Boulder, US. They already have real world uses such as to detect methane leaks from above oil and gas operations and also signs of COVID-19 infections in human breath samples.

Now, in a series of ‘quantum’ laboratory experiments, researchers have laid out a path for making those kinds of measurements even more sensitive. And have doubled the speed of frequency comb detectors to make them work faster. The research is a collaboration between Professor Scott Diddams, Department of Electrical, Computer and Energy Engineering of CU and Professor Jérôme Genest at the Université Laval in Québec, Canada.

“Let’s say, for example, you were in a dangerous situation where you needed to detect minute quantities of a dangerous gas leak in a factory setting,” said Professor Diddams.

“Only needing 10 minutes versus 20 minutes can make a big, big difference in keeping people safe.”

While lasers normally emit light in only one colour, frequency comb lasers send out pulses of thousands to millions of colours – all at the same time. In the new study, the researchers used common optical fibres 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 therefore has found a method to exert some control 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 towards  a powerful new type of quantum sensors.”

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) in Gaithersburg, Maryland. 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 colours of light, but not others. Scientists can identify what’s in the air based on what colours 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: optical frequency comb lasers

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’.

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 either know where an electron is, or how fast it’s moving, but never know both at the same time. Squeezing is a technique that maximizes one type of measurement at the expense of the other.

In the lab, the research team achieved squeezing in a surprisingly simple way. They sent their pulses of frequency comb laser light through a normal optical fibre, the same type that delivers internet connections into the normal family home.

The structure of the fibre altered the light in just the right way so that photons from the lasers now arrived at a more regular interval. But that increased 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 colours.

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 sulphide – a molecule that smells like rotten eggs and is common in volcanic eruptions – and 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 that which had previously accomplished.

“Our findings show that we are closer than ever to applying quantum frequency combs in real-world scenarios,” Herman said.

“Scientists call this a ‘quantum speedup,’” Diddams said, “in that we’ve been able to manipulate the fundamental uncertainty relationships in quantum mechanics to measure something faster and better.”

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