* Astronomy

A clock 1000 times more accurate than any of its predecessors has set another benchmark, and could even be used to create a more precise definition of how long a second is.
The new clock is a variant on the atomic clocks that appeared in the 1950s. Atomic clocks usually work by measuring the frequency at which atoms resonate. For instance, the outer electrons of a caesium-133 atom resonate between two energy states exactly 9,192,631,770 times each second, emitting microwaves of exactly that frequency as they do so. This property has been used since 1967 to define what we mean by 1 second - it is officially the time it takes for a caesium atom to resonate 9,192,631,770 times.
“One way to create a more accurate clock is to increase the rate at which it ticks”
The first atomic clocks could pin this down to an accuracy of 1 part in 10^10. Today's caesium clocks can measure time to an accuracy of 1 in 10^15, or 1 second in about 30 million years. But the search is on for ever more precise timepieces.
“One way to create a more accurate clock is to increase the rate at which it "ticks". A clock has a counter that counts something that is periodic. The shorter that period is, the more accurate the clock. That is why people went from sundials, with one period per day, to pendulum clocks, with one period per second, to quartz clocks with 10,000 oscillations per second. Now we have the caesium clock counting with 9 billion oscillations per second." -Thomas Udem of the Max Planck Institute for Quantum Optics in Garching, Germany.
So what ticks faster than a caesium atom? Elements being scrutinised include ytterbium, mercury and strontium, which resonates 429,228,004,229,952 times each second. But until now it has proved impossible to create a useful strontium atomic clock.
In principle, there are two ways to create a strontium clock: using the oscillations of a single atom, or doing the same with many atoms at the same time. The advantage of using a single atom is that it is relatively easy to shield it from external electromagnetic fields, which interfere with its oscillating frequency. The disadvantage is that it is extremely difficult to accurately measure a single atom vibrating at such a high frequency. A multi-atom clock produces a much clearer signal but is less accurate, because the electromagnetic fields of the atoms interfere with each other.
“A second is officially the time it takes for a caesium atom to resonate 9,192,631,770 times”
Now Hidetoshi Katori and his colleagues at the University of Tokyo have come up with an elegant solution that combines the advantages of both systems. Katori uses six laser beams to create a pattern of standing electromagnetic waves. This creates a series of energy wells, each of which supports one strontium atom, in much the same way as each dimple in an egg box holds an egg.
This prevents the electromagnetic fields of individual atoms interfering with those of their neighbours, and allows the oscillating signals of many atoms to be measured at once.
Previous attempts to make clocks this way failed because the trapping lasers themselves interfered with the atoms' oscillation frequency. Katori's group has got round this by tuning the frequencies of the lasers so they alter the upper and lower transition energy levels of strontium by exactly the same amount, so the oscillation frequency remains unaltered. Katori claims that this "optical lattice clock" will keep time with an accuracy of 1 part in 10^18.


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