Neglected Atom Has Top Properties for Atomic Clocks

Like watchmakers choosing superior materials to build a fine timepiece, physicists at the Centre for Quantum Technologies (CQT) at the National University of Singapore have singled out an atom that could allow us to build better atomic clocks.

The CQT team report in Nature Communications that a previously neglected element – lutetium – could improve on today’s best clocks. Lutetium (Lu) is a rare earth element with atomic number 71.

“The ultimate performance of a clock comes down to the properties of the atom – how insensitive the atom is to its environment. I would call lutetium top in its class,” says Murray Barrett, who led the research.

Barrett is so confident because data in the team’s paper, published 25 April, show Lu to have lower sensitivity to temperature than atoms used in clocks today. These measurements add to earlier results showing lutetium could make a high-performance clock.

Atomic clocks have set the global standard for measuring time for over half a century. But since the second was defined with reference to caesium atoms in the 1960s, there has been world-wide competition to improve the accuracy and stability of atomic clocks.

Time signals from caesium clocks still support the Global Positioning System and help to synchronise transport and communication networks, but atoms of many other species, such as ytterbium, aluminium and strontium, now vie to make the most precise measurements of time.

These new generation clocks, with uncertainties around one part in a billion billion, are proving their mettle in testing fundamental physics – from measurements of gravity to looking for drifts in fundamental constants.

The ‘tick’ of an atomic clock comes not directly from the atom, but from the oscillation of a light wave. The oscillation frequency is fixed by locking it to resonant frequency of the atom.

In practice, this means a laser is tuned to make one of atom’s electrons leap from a low energy level to a higher energy level. How much energy this jump takes is a fixed property of the atom. The laser’s frequency is matched to deliver just the right amount of energy in a single light particle (a photon).

Once this sweet spot is found, the clock counts time by measuring the oscillations of the light wave.

Caesium clocks run at microwave frequency – or exactly 9,192,631,770 ticks per second. The most recent generation of atomic clocks run at optical frequencies, which tick some ten thousand times faster. Counting time in smaller increments allows for more precise measurement.

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