Microwave signal stability gets a boost

US researchers say they have taken a giant step towards more accurate time dissemination, improved navigation and more reliable communications by boosting the stability of microwave signals 100-fold.

Their work transferred the impressive stability of laboratory atomic clocks operating at optical frequencies to microwave frequencies, which are currently used to calibrate electronics.

Improving a microwave signal’s consistency over a specific time period helps ensure reliable operation of a device or system using it, they write in a paper in the journal Science.

And it took some serious kit to do it. As well as state-of-the-art atomic clocks, Frank Quinlan from the National Institute of Standards and Technology (NIST )and colleagues made use of advanced light detectors and a measurement tool called a frequency comb.

“Years of research, including important contributions from NIST, have resulted in high-speed photodetectors that can now transfer optical clock stability to the microwave domain,” Quinlan says. “The second major technical improvement was in the direct tracking of the microwaves with high precision, combined with lots of knowhow in signal amplification.”

The researchers used the “ticking” of two ytterbium lattice clocks – which operate at frequencies of 518 terahertz (trillion cycles per second) – to generate light pulses and frequency combs serving as gears to translate the higher-frequency optical pulses accurately into lower-frequency microwave signals.

Advanced photodiodes converted light pulses into electrical currents, which in turn generated a 10 gigahertz microwave signal that tracked the clocks’ ticking exactly, with an error of just one part in a quintillion (1 followed by 18 zeros).

Such a performance level was on par with that of both optical clocks and 100 times more stable than the best microwave sources, the researchers say.

“This is a field where just doubling microwave stability can take years or decades to achieve,” says co-author Chris Oates. “A hundred times better is almost unfathomable.”

Optical waves have shorter, faster cycles than microwaves, and thus different shapes. In converting stable optical waves to microwaves, the team tracked the phase – the exact timing of the waves – with a resolution corresponding to just one millionth of a cycle to ensure they were identical, and not shifted relative to one another.

In a related commentary in Science, E Anne Curtis from the National Physical Laboratory in the UK describes the breakthrough as “a paradigm shift in the field of microwave metrology”.

“The impact will extend to applications in fundamental physics, communication, navigation, and microwave engineering,” she writes.

Image details

In the image above, the black rectangle in the centre is a high-speed, semiconductor photodiode that converts laser pulses to super-stable microwave frequencies. It is surrounded by a gold-coated border in which electrical leads are embedded.

Wires connect the leads to the copper electrical circuit (top) used to extract microwave signals. The entire set-up rests on a brass plate for mechanical stability.

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