AN ATOMIC clock could soon be shrunk to the size of a sugar cube, promising an era in which highly accurate clocks will become available in consumer devices, including computers, watches, cellphones and GPS satellite navigation receivers.
Until now, the smallest portable atomic clock was the size of a shoebox, including its battery. At the heart of such clocks is a large glass chamber containing caesium vapour. When excited by a beam of microwaves, the caesium atoms oscillate at a precisely known rate, close to 9.2 billion times per second. By counting these “ticks”, an atomic clock is able to measure time to an accuracy of 1 second in 300 years.
Until now, no one had worked out a way to make the delicate glass apparatus at the heart of an atomic clock any smaller. But John Kitching and his colleagues at the US National Institute of Standards and Technology in Boulder, Colorado, have managed to do so by harnessing microchip manufacturing techniques.
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Instead of sealing the caesium inside a glass vessel, they trap a small amount of it in a cavity about the size of a rice grain etched in a silicon wafer. Laser light enters at one end, while the 9.2-gigahertz oscillations of the clock are detected on the other side. The wafer is sandwiched between two glass sheets in order to seal the cavity.
The microwave beams used in large atomic clocks have a wavelength of 1 centimetre. This wavelength would be diffracted in a millimetre-scale cavity, so Kitching used an infrared laser with a wavelength of 1000 nanometres. It’s easy to etch many atomic clock mechanisms on a single silicon wafer, which can then be cut up to produce individual clocks (Applied Physics Letters, vol 85, p 1460). This, alongside miniaturisation of the frequency-counting electronics, should allow the devices to be mass-produced.
“This development is pretty significant,” says Michael Garvey of Symmetricom, a Massachusetts firm that builds atomic clocks. “They have shown you can put it all together and it works.”
Giving portable devices atomic-clock accuracy would bring significant benefits. Cellphones, military radios and Wi-Fi enabled laptops, for instance, could synchronise faster hops between communication frequencies, leading to better security.
Kitching says that his clocks could also make GPS receivers more reliable in cities, where the view of the sky is restricted. Normally, a GPS receiver has to take signals from three satellites to triangulate its position, and then has to find a fourth to get the time. Receivers that are fitted with one of the new miniature atomic clocks would not need the fourth satellite, which may be hidden by buildings in cities.
