In August 2004 the National Institute for Standards and Technology (NIST), in Boulder, Colorado, USA, unveiled a prototype atomic clock the size of a grain of rice. With a volume of less than 10 mm³ (0.0006 in³), and drawing just 75 milliwatts of power, the chip-scale vapour atomic clock is accurate to one second in 3,000 years.

http://physicsweb.org/articles/news/8/9/2/1