Optical atomic clock becomes portable

Sep 03, 2009
This is a view into the ultrahigh vacuum chamber in which strontium atoms are cooled and stored. In the upper third of the window, the blue fluorescent light of a cloud of cold strontium atoms (Sr) is to be seen. Credit: (Image: PTB)

You imagine a clock to be different -- yet the optical table with its many complicated set-ups really is one. Optical clocks like the strontium clock in the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig could be the atomic clocks of the future; some of them though are already ten times more precise and stable than the best primary caesium atomic clocks.

Now they might also become more compact and even portable, maybe in the future even travel to space. PTB scientists have shown how some fundamental difficulties, which a more simple set-up had previously hindered, could be avoided. They have written about this in the current edition of the journal "". In the next step they want to build such a clock. They already have a practical application in mind: the clock could help to determine geographical heights even more exactly than before.

An optical clock is so exact because its "pendulum" swings so quickly. The same effect that makes a quartz clock more precise than a classical grandfather clock is behind this: the periodically swinging element within, the oscillating quartz crystal, moves by far more quickly than the pendulum of the grandfather clock; thus the scale can to some extent be divided more precisely and also be more precisely checked. The "pendulum" of a caesium atomic clock swings even more quickly: i.e. that microwave radiation which can bring about a spin change in each electron of a caesium atom. Precisely the microwave frequency at which this effect is largest defines the second. An optical works with the still higher frequency of optical radiation - that is with an even faster pendulum.

As the movement of the atoms leads to very large frequency shifts through the Doppler effect, in the best of these clocks the atoms are slowed down to a hundredth of the speed of a pedestrian in a first preparation step with the aid of laser cooling. In a lattice clock a further step then follows in which the atoms are held in potential wells. These are created through the intensive light field of a laser. Several tens of thousands of strontium atoms are trapped in this so-called optical lattice. The movement of the atoms is thus limited to the fraction of an optical wavelength, so that shifts through the Doppler effect can be ignored.

A few hundred atoms which can disturb each other are trapped in each potential well. If the isotope strontium-87, a fermion, is used, two of these particles do not come close to each other at very low temperatures due to the Pauli principle. That is the reason why this isotope is used to construct optical clocks. But as it can only be cooled relatively complicatedly with laser light and, moreover, only has a natural abundance of 7 %, it is, in principle, not so well suited for simple, transportable clocks or even for clocks suitable for space.

The isotope strontium-88 with over 80 % natural abundance, which is also easier to cool, is, however, a boson. That means that even at the lowest temperatures many collisions between the atoms occur. They can lead to losses and to a shift and broadening of the reference line. How strongly these collisions influence the accuracy of the clock was, however, not known previously. In an experiment at PTB, these influences have now been measured in detail for the first time. The results of the investigation have shown how the optical lattice has to be dimensioned and how many atoms may be stored in it to operate a very accurate lattice clock also with strontium-88. A clock is now being built on this basis which is more compact and more transportable than the previous lattice clocks.

The gravitational red shift of the earth, amounting to a height difference of 10󈝼 per meter on its surface, is being discussed as a possible first use for the precise determination of the height over the geoid. So the clock could be used to improve, for example, gravitation maps.

More information: Collissional losses, decoherence, and frequency shifts in optical lattice clocks with bosons. Ch. Lisdat, J.S.R. Vellore Winfred, T. Middelmann, F. Riehle, U.Sterr, Phys. Rev. Lett. 103, No.9 (2009), DOI: 10.1103/PhysRevLett.103.090801

Source: Physikalisch-Technische Bundesanstalt

Explore further: Could 'Jedi Putter' be the force golfers need?

add to favorites email to friend print save as pdf

Related Stories

Optical Atomic Clock: A long look at the captured atoms

Feb 05, 2008

Optical clocks might become the atomic clocks of the future. Their "pendulum", i.e. the regular oscillation process which each clock needs, is an oscillation in the range of the visible light. As its frequency is higher than ...

Portable Precision: A New Type of Atomic Clock

Dec 10, 2008

(PhysOrg.com) -- The most accurate atomic clocks in the world are based on the output of cesium atoms. These ultra-precise fountain clocks measure the frequency and time interval of seconds by using a fountain-like movement ...

Portable Precision: A New Type of Atomic Clock

Jun 11, 2009

The most accurate atomic clocks in the world are based on the output of cesium atoms. These ultra-precise fountain clocks measure the frequency and time interval of seconds by using a fountain-like movement of cesium atoms. ...

Experimental atomic clock uses ytterbium 'pancakes'

Mar 06, 2006

Scientists at the National Institute of Standards and Technology working with Russian colleagues have significantly improved the design of optical atomic clocks that hold thousands of atoms in a lattice made ...

Atomic fountain clocks are becoming still more stable

Mar 18, 2009

They are at present the most accurate clocks in the world: Caesium fountain clocks furnish the second accurate to 15 places after the decimal point. Until they reach this accuracy, caesium fountain clocks, however, need a ...

Mercury atomic clock keeps time with record accuracy

Jul 14, 2006

An experimental atomic clock based on a single mercury atom is now at least five times more precise than the national standard clock based on a "fountain" of cesium atoms, according to a paper by physicists ...

Recommended for you

Could 'Jedi Putter' be the force golfers need?

Apr 18, 2014

Putting is arguably the most important skill in golf; in fact, it's been described as a game within a game. Now a team of Rice engineering students has devised a training putter that offers golfers audio, ...

Better thermal-imaging lens from waste sulfur

Apr 17, 2014

Sulfur left over from refining fossil fuels can be transformed into cheap, lightweight, plastic lenses for infrared devices, including night-vision goggles, a University of Arizona-led international team ...

User comments : 2

Adjust slider to filter visible comments by rank

Display comments: newest first

ThomasS
not rated yet Sep 03, 2009
So where's the graph with the allen deviation? :)
Slotin
not rated yet Sep 07, 2009
What does it mean portable? NIST scientists demonstrated a chip-scaled atomic clock in August 2004 already

http://tf.nist.go...ndex.htm

More news stories

UAE reports 12 new cases of MERS

Health authorities in the United Arab Emirates have announced 12 new cases of infection by the MERS coronavirus, but insisted the patients would be cured within two weeks.