Observatoire de Paris
Pierre Lemonde, Observatoire de Paris, SYRTE
Tél: 33 1 40 51 22 24
Fax: 33 1 43 25 55 42
28 February 2006
Still more accurate clocks
Optical lattice clocks are based on the measurement of the frequency of
an atomic transition, and are quite promising to be in the future among
the most precise clocks. A team from Paris Observatory (SYRTE) has just
eliminated an obstacle in the race to the precision, which lets hope to
reach in the future an accuracy of 10**-18. In an article to Physical
Review Letters, the team presents in details an experimental study on
the most serious limitation for this new type of clocks.
Optical lattice clocks have recently opened the most promising route
towards more accurate and stable frequency standards. But to improve the
present accuracy of a few 10**-16 of the best atomic clocks, it was
necessary to lift the most serious limitation.
These clocks consist in measuring the optical frequency corresponding to
the transition between two atomic levels of 87Sr (Figure 1). The
objective is to be able to control the transition frequency of 500 THz
(5x10**14 Hz) to within one millihertz (10**-3 Hz), or 10**-18 in
fractional units. To reach this level, the motions of the atoms must be
totally under control.
This is done by tightly confining the atoms in an optical lattice which
freezes the atomic motion (Figure 2). Such a trap however can
significantly shift the transition frequency and thus severely affect
the clock accuracy.
Previous studies have shown that this frequency shift can be cancelled
to first order by adjusting the wavelength of the trapping laser. Given
the required level of control however, effects of higher order must be
considered and so far nobody had been able to conclude regarding these
The experimental results published by A. Brusch and collaborators are
the first study of these higher orders effects, which was achieved by
operating the clock at an unprecedented laser intensity. This study
shows that these effects are not a limitation for a future accuracy at
the 10**-18 level.
Home page of the team Horloge optique à atomes froids de strontium Sr,
Anders Brusch, Rodolphe Le Targat, Xavier Baillard, Mathilde Fouché,
Pierre Lemonde: 2006, Hyperpolarizability effects in a Sr optical
lattice clock Physical Review Letters, in press.
The cloud of cold atoms of Srontium (Sr)
Optical lattice trapping the atoms