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Metrology

Metrology

Tiny optical clock is 100 times better than previous chip-based timekeepers

26 May 2019
Clock on a chip
Bean counter: this vapour cell is at the heart of NIST’s next-generation miniature atomic clock. (Courtesy: Hummon/NIST)

A tiny optical clock that is small enough to fit onto three computer chips has been created by physicists at the National Institute of Standards and Technology (NIST) in Boulder, Colorado.

The next-generation device is 100 times more stable than current chip-based atomic clocks of a similar size. With further improvements, the tiny timekeeper could have a diverse range of both scientific and commercial applications.

For over six decades the most accurate timekeeping devices have been atomic clocks – the best of which could run for more than one billion years before being out by just one second. Most commercial atomic clocks operate by monitoring the frequency of microwaves emitted by a certain transition in a caesium atom. However, the best atomic clocks use transitions that emit  light – which has a much higher frequency than microwaves. As a result, these optical clocks are much more accurate that their microwave counterparts. The downside, however, is that optical clocks are bulky, complex, and expensive – limiting their use to labs such as NIST.

Infrared transition

NIST researchers are keen to simplify optical clocks and put them on chips so that they can find wider application. Their design is based around a microfabricated glass vapour cell filled with rubidium atoms, which have a transition at 385 THz. While this is not quite visible light – it is in the infrared – it is much high frequency that the caesium transition at 9 GHz.

An infrared “clock laser” is locked to the rubidium transition. Two interlocking Kerr-microresonator frequency combs are used convert the terahertz clock laser time signal into a gigahertz frequency, which is widely used by standard electronics.

The simplicity of this design meant that the clock could be incredibly compact. It fits onto just three small chips, but it does require supporting electronics and optics. Using just 275 mW of power, the device showed an instability of just one part in 1013 after operating for 4000 s. This makes it around 100 times more stable than current atomic clocks of a similar size. With further improvements, the NIST team believes that their optical clock could become small enough to be handheld, making it highly competitive with current atomic clocks.

If optical clocks become small, portable, and inexpensive enough, they would be suitable for a range of commercial applications, including timing and navigation when GPS is unavailable. It would also open up new opportunities for scientific experiments, including gravitational and remote sensing, long-baseline interferometry, and calibrations of lab instruments; potentially allowing for new tests of fundamental physics.

The clock is described in Optica.

  • One of the NIST researchers, John Kitching, spoke about the lab’s drive to put atomic clocks and other metrology devices on chips in this episode of the Physics World Weekly podcast.

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