German boffins' clock drops 10 seconds in a billion-and-a-half years

Optical timepieces stretch lifespan of the standard second

Twilight Zone, 'Time Enough At Last'

Optical clocks are already so accurate that you can expect them to be out by a second every 15 billion years, but they suffer from frequent downtime.

Now a group of boffins from The National Metrology Institute of Germany (PTB, which stands for Physikalisch-Technische Bundesanstalt) reckon they've got that problem licked, by using a microwave source to mark time if the optical source isn't available for some reason.

The new work sacrifices some of the accuracy of a purely optical clock – 100 seconds over the age of the universe instead of one second – but that's still an improvement over today's caesium-based atomic clocks.

Their work is published in full in the journal Optica here.

As explained by the Optical Society in its media release, the atomic clocks now in use employ a maser (microwave amplification by the stimulated emission of radiation) as their “pendulum”.

Optical clock schematic

The hydrogen maser resolves the optical clock (left) and the cesium clock (right), keeping time when the optical clock fails.
Image: the Optical Society

It's the use of microwave frequencies that limits their already-impressive accuracy (the error is about a nanosecond a month). A more accurate standard second (useful in GPS, for timestamping financial transactions, and in science) remains a Holy Grail for chrono-boffins.

Light is around 100,000 times the frequency of microwaves, giving optical clocks much greater accuracy and stability over time: “a projected clock uncertainty of ≲1×10−17 corresponds to a time error ≲25 ps after a month”, the paper says.

But as the Optical Society release notes, an optical clock is more complex and therefore they “experience significant downtimes”.

So the PTB paired its strontium lattice clock with a hydrogen maser “flywheel” to cover the 46 per cent of their 25-day-long test that the optical clock was down.

The result, it said, was a 200 picosecond accumulated error, making this experiment more accurate than a caesium clock over the same period.

“The researchers spanned the large spectral gap between the optical clock’s optical frequency and the maser’s microwave frequency with an optical frequency comb, which effectively divides the slower microwave-based 'ticks' to match the faster 'ticks' of the optical clock,” the Optical Society writes.

The researchers believe the fast pace of optical clock development means a redefinition of the standard second can probably wait another 10 years, while boffins work out just how more accurate and reliable they can become, and assess which optical clock design proposals turns out to be the winner. ®

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