Time to redefine time?

A new generation of "optical" clock promises to make time-keeping even more accurate.
27 May 2016


A new generation of "optical" clock promises to make time-keeping even more accurate.

Most people would probably settle for the accuracy offered by the current Clock Facegeneration of atomic clocks, which are correct to within a billionth of a second each month. 

But true to their reputation for precision, a team of German scientists have come up with something at least five times better!

Writing in the journal Optica, National Metrology Institute of Germany researcher Christian Grebing and his colleagues have road tested an "optical" clock that, they say, is accurate to within 1/5th of a billionth of a second over a 25 day period.

This is at least 5 times better than a caesium clock alone, which is the current gold standard definition of the second.

The "second" as we know it today actually dates from 1967 when, rather than being based on the Earth's movement around the Sun as it had been previously, a caesium-based atomic clock, invented by Englishman Louis Essen, was used instead.

In one of these devices, microwaves are used to excite a cloud of very cold caesium atoms.

These behave a bit like a nanoscale pendulum, but rather than one long swing back and forth, the caesium atoms become excited and relax again roughly 9.2 billion times per second, and this number is used to measure "time" in a device called a microwave fountain.

But interactions between the atoms themselves, and the movements of the atoms, contribute uncertainty to the timer, meaning that it's out by about the equivalent of 30 seconds every 4 and a half billion years.

To iron out these inaccuracies, scientists average the timings of some 500 such "clocks" around the world to produce what's called the Universal Coordinated Timescale (UTC).

Systems like GPS rely on this to achieve accurate predictions of position, but the drift in the clock ultimately limits how good the fix is that can be achieved.

One way to solve this is to move to a new generation of timepieces referred to as "optical" clocks; these work in a similar way to the atomic clock but rather than using microwaves to excite atoms instead they use visible light.

This allows optical clocks to "tick" about 100,000 times faster than their atomic counterparts, making them potentially much more accurate.

They use a laser beam to trap a few thousand atoms of an element like strontium into a lattice pattern. Then another light source is used to excite them.

The problem is that the reliability of optical clocks is still quite poor and they suffer from significant down-time when time measurements cannot be made.

The German team have got around the problem by incorporating the light equivalent of a "flywheel"; this is linked to both an existing caesium clock and a newer strontium optical clock.

This design means that the optical clock, when it is running, can be used to correct for the inaccuracy of the caesium clock.

But if the optical clock goes down temporarily, the caesium-clock-flywheel combo keeps the system running until the more-accurate optical clock comes back online.

This rig, they say, is so accurate that were it to have begun ticking at the dawn of the Universe, by now it would have lost less than two minutes in total.

So now there's no excuse for being even a nanosecond late for school physics lessons...


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