03 Sep 2004
An atomic clock that is the size of a grain of rice could bring unprecedented timekeeping to portable electronics.
The precision of atomic clock timekeeping could soon be coming to handheld devices such as cell phones, radios and GPS receivers thanks to a design breakthrough by scientists from the National Institute of Standards and Technology (NIST) in the US (Applied Physics Letters 85 1460).
By exploiting MEMS fabrication technology a team at NIST in Colorado says that it has made the world's smallest atomic clock. Measuring about the same size as a grain of rice, the inner workings of the clock are about 100 times smaller than current designs and consume less than 75 mW of electrical power.
"The real power of our technique is that we're able to run the clock on so little electrical power that it could be battery operated and that it's small enough to be easily incorporated into a cell phone or some other kind of handheld device," said John Kitching, a physicist from NIST. "And nothing else like it even comes close as far as being mass producible."
For more than 50 years, atomic clocks have set the gold standard for time and frequency measurement but have been limited to laboratory use due to their complexity, size and expense.
The scale and ease in which the NIST design could be made potentially opens the door to low-cost mass-production of miniature atomic clocks that can easily be integrated with electronics.
The chip-scale clock contains a 852 nm vertical-cavity surface-emitting laser (VCSEL), a lens, an optical attenuator, a polarizing waveplate, a cell containing cesium vapor and a photodiode.
The VCSEL emits two light signals that are separated by just a few gigahertz. These are focused onto the cesium atoms and tuned until they exactly match the hyperfine split of the cesium's D2 transition. This gives an incredibly precise measure of frequency and thus time.
As for its performance, the clock is stable to one part in 10 billion, equivalent to 1 second in 300 years -- a long-term stability which is several orders of magnitude better than competing portable units such as temperature-compensated quartz crystal oscillators. However, this precision is still a long way from that achieved by large atomic clocks. For example, NIST's F1 clock boasts a stability of 1 part in 1015, equivalent to 1 second in 30 million years.
Oliver Graydon is editor of Optics.org and Opto & Laser Europe magazine.
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