Invented 50 years ago this month, atomic clocks have revolutionized how we measure time. But optical clocks, which use light
rather than
microwaves, promise to be even more accurate and could lead to the second being redefined.
It is 50 years this month since Louis Essen demonstrated the first caesium atomic clock at the National Physical Laboratory and set in motion the shift to atomic timekeeping. Back then, the second was defined in terms of the period of the Earth's rotation, but this was found to fluctuate as our ability to measure time improved. Essen showed that atoms, which have a set of discrete energy levels, could provide a much more stable reference time interval. In 1967, some 12 years later, the second was officially redefined by the Comité International des Poids et Mesures in terms of the gap between two specific energy levels in a caesium-133 atom. Since Essen's pioneering work, the accuracy of caesium clocks has steadily improved by a factor of 10 or so every decade, such that today's best atomic clocks are accurate to better than one part in 1015. These improvements have led to many scientific advances as well as to technologies such as the Global Positioning System (GPS) and the Internet, which depend critically on time and
frequency standards. Although this performance is impressive, a new type of device known as an optical clock could lead to even greater improvements.
In a standard atomic clock, a beam of caesium-133 atoms is probed by microwaves that have a frequency of about 9.2 x 109 Hz. When the microwave frequency is adjusted to a value of exactly 9192 631 770 Hz, the photons have an energy that is equal to the energy difference between the two very closely spaced energy levels that make up the ground state of the caesium atoms. The atoms absorb the microwaves and a signal generated from the absorption is fed back to the microwave source, which stops it drifting from this specific frequency. The stability imposed on the microwave source by the atoms is what allows us to define the second as the duration of 9192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.Optical clocks, in contrast, use light rather than microwaves. All other things being equal, the stability of an atomic clock is proportional to its operating frequency and inversely proportional to the width of the electronic transition. Since light has a frequency of about 1015 Hz - roughly 100,000 times higher than that of microwaves - clocks based on narrow transitions at optical, rather than microwave, frequencies should be much more stable. Clocks need to be both stable and accurate, with greater stability making it much easier and quicker to judge how accurate a clock really is. Optical clocks have many potential applications - from improved GPS measurements and better tracking of deep-space probes to fundamental tests of general relativity and measurements of the physical constants. They could even lead to the second being redefined once again.
For more detailed description on Atomic clocks: from past to present,Enter the optical clock,Inside an optical clock,Clocks from clouds of atoms,Counting optical frequencies,Future times visit http://physicsweb.org/articles/world/18/5/8
Defining moments in atomic timekeeping
1949 Ramsey's separated oscillatory field technique
1955 First caesium atomic clock
1960 Hydrogen maser
1967 Redefinition of the second in terms of caesium
1975 Proposals for laser cooling of atoms and ions
1978 Laser cooling of trapped ions
1980s GPS satellite navigation introduced
1985 Laser cooling of atoms
1993 First caesium-fountain clock
1999 First optical-frequency measurement with femtosecond combs
2001 Concept of an optical clock demonstrated