Stability is a must

An optical clock has three main components. The first is a highly stable “reference” frequency provided by a narrow optical absorption line in an atom or ion. The second element is a feedback system that
“locks” the output of a laser (called the local oscillator) to the
reference frequency. The third component provides a very precise
measurement of the frequency of the laser – usually with a “femtosecond
comb”.

The most important component, however, is the atom or ion itself – which must absorb light over an extremely narrow and extremely stable frequency range. To ensure this, the atom or ion is isolated in
a vacuum chamber and cooled to near absolute zero using laser beams –
whereby light is absorbed and reemitted by the ion in such a way as to
reduce its kinetic energy.

(JILA’s strontium atomic clock is now the world’s most accurate clock based on neutral atoms. Credit: Greg Kuebler/JILA)

Aluminium ions are relatively easy to isolate from their surroundings – particularly from the effects of black-body radiation from the surrounding chamber. Unfortunately, aluminium ions are very
hard to cool with laser light because they re-emit the absorbed
radiation at a much lower frequency. This also makes it tricky to
design a feedback mechanism to keep the laser locked on to the
reference frequency.

James Chin-wen Chou, David Wineland and colleagues at the National Institute of Standards and Technology (NIST) in Colorado have got around this problem by pairing the aluminium with a second ion of magnesium, which is easy to cool. The
team begins by trapping the magnesium ion using electric fields and
then cooling it using a laser. They then introduce the aluminium ion
into the chamber, which interacts with the magnesium via electrical
forces. These interactions allow the team to chill the aluminium ion
using “sympathetic cooling”.

Quantum logic measurement

The local-oscillator laser is then fired at the aluminium ion and the efficiency at which the ion absorbs the light is determined by a technique called quantum logic spectroscopy (QLS). QLS is possible
because the ions are just a few microns apart and therefore they behave
as a quantum system – the result being that the absorption efficiency
of the aluminium ion can be determined from light emitted from the
magnesium ion. The greater the absorption efficiency, the closer the
match between the aluminium ion’s reference frequency and the
local-oscillator laser – and this is used in a feedback loop to tune
the laser to the ion.

The team first applied QLS to an optical clock in 2008, when it created a similar device using aluminium and beryllium (Al-Be) ions that had an uncertainty of about 2.3 × 10^–17. By using magnesium and a new type of ion trap, the team has improved by this nearly a factor of three to 8.6 × 10^–18. This shatters the previous record for precision – 1.9 × 10^–17 – which was held by NIST’s mercury ion clock.

The team was also able to compare the output of the new aluminium-magnesium clock with the older Al-Be clock to understand the differences between the two devices. Such comparisons are important if
several optical clocks are deployed around the world – as is the case
with standard atomic clocks.