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Boosting the Stability of Microwave Signals by ‘100-Fold’

May 31, 2020 by Luke James

Researchers at the National Institute of Standards and Technology (NIST) have reportedly used state-of-the-art atomic clocks, advanced light detectors, and a frequency comb measurement tool to boost the stability of microwave signals.

Optical atomic clocks are likely to redefine the international standard for measuring time. This is because they are far more accurate and stable than microwave atomic clocks, the current standard. 

Now, a research team at NIST has figured out how to convert high-performance signals produced by optical clocks into a microwave signal that can be applied to modern electronic systems.

The team claims that this stability boost makes microwave signals better by 100-fold, marking a step towards better electronics with more accurate time dissemination, better navigation, and more reliable communications. 

The team’s findings were published in the journal Science on May 22. 


Converting Optical Clock Signals Into a Microwave Signal

Currently, microwave atomic clocks that measure time based on the frequency of cesium atom vibrations are used to synchronize electronic systems. These vibrations occur at microwave frequencies and can easily be used in electronic systems.

However, newer optical atomic clocks, based on atoms like strontium, vibrate much faster at higher frequencies and generate optical signals. Before electronic systems can use these signals, they must be converted to microwave signals. 

“How do we preserve that timing from this optical to electronic interface?” asked Frank Quinlan, a lead researcher at NIST. This, he said, was the piece that made the team’s research work. 

The NIST team used the “ticking” of two of NIST’s ytterbium lattice clocks to create light pulses, as well as frequency combs acting as gears to translate the higher-frequency optical pulses accurately into lower-frequency microwave signals.

The two lattice clocks acted as optical-to-electronic signal generators, creating a 10-gigahertz microwave signal that synchronizes with the ticking of an optical clock. According to the research team, their precise method has an error of just one part in a quintillion. This performance level is on par with that of both optical clocks and 100 times more stable than the best microwave sources.

"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," lead researcher Quinlan said. "The second major technical improvement was in the direct tracking of the microwaves with high precision, combined with lots of knowhow in signal amplification."


Semiconductor photodiode that concerts laser pulses to microwave frequencies.

A view of the high-speed semiconductor photodiode (center) that converts laser pulses to microwave frequencies. Image Credit: F. Quinlan/NIST


Switching to Optical Clocks

This “100-fold” improvement comes as many researchers expect the Système International (SI)—the international standard that defines a second in time—to switch over to optical clocks. Today’s cesium-based atomic clocks require a month-long averaging process to achieve the same stability that an optical clock is able to achieve in a matter of seconds. 

“Because optical clocks have achieved unprecedented levels of accuracy and stability, linking the frequencies provided by these optical standards with distantly located devices would allow direct calibration of microwave clocks to the future optical SI second,” said Anne Curtis, a British senior research scientist at the National Physical Laboratory, when commenting on the research in an article of her own. 


Pending Use in Real Applications 

Optical clocks can already be physically linked using fiber-optic networks, but this approach still limits their use in many applications. The new achievement by the NIST team could remove these limitations by combining optical clock performance with microwave signals that can travel in areas without a fiber network.

Some of the potential applications include ultra-stable electronic signals that could support more sensitive imaging systems such as astronomical imaging systems, higher-accuracy radar systems used in navigation and tracking, and future space telescopes that could benefit from the highly stable microwave signals synchronized with optical clocks.

Although some components of the NIST team’s system are ready to be used in real applications, the team is still working on transferring their state-of-the-art optical clocks to mobile platforms. At present, the ytterbium clocks, which operate at 518 terahertz, are currently housed in large tables under controlled laboratory settings.

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