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Physics World

NIST builds smallest atomic clock

Peter Rodgers, Oliver Graydon

Published Oct 1, 2004• 0 citations
Publisher

Abstract

The precision of atomic clocks could soon be available in hand-held devices such as mobile phones, radios and GPS receivers thanks to a breakthrough by physicists in the US. John Kitching and colleagues at the National Institute of Standards and Technology (NIST) in Boulder, Colorado, have built a physics package – the components that lie at the heart of an atomic clock – with a volume of just 9.5 mm3. This is about 100 times smaller than those of existing atomic clocks.

DOI
10.1088/2058-7058/17/10/6

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The precision of atomic clocks could soon be available in hand-held devices such as mobile phones, radios and GPS receivers thanks to a breakthrough by physicists in the US. John Kitching and colleagues at the National Institute of Standards and Technology (NIST) in Boulder, Colorado, have built a physics package – the components that lie at the heart of an atomic clock – with a volume of just 9.5 mm3. This is about 100 times smaller than those of existing atomic clocks.
WALKING INTO Jun Ye's lab at the University of Colorado Boulder is a bit like walking into an electronic jungle. There are wires strung across the ceiling that hang down to the floor. Right in the middle of the room are four hefty steel tables with metal panels above them extending all the way to the ceiling. Slide one of the panels to the side and you'll see a dense mesh of vacuum chambers, mirrors, magnetic coils, and laser light bouncing around in precisely orchestrated patterns. ¶ This is one of the world's most precise and accurate clocks, and it's so accurate that you'd have to wait 40 billion years-or three times the age of the universe-for it to be off by one second. ¶ What's interesting about Ye's atomic clock, part of a joint endeavor between the University of Colorado Boulder and the National Institute of Standards and Technology (NIST), is that it is optical, not microwave like most atomic clocks. The ticking heart of the clock is the strontium atom, and it beats at a frequency of 429 terahertz, or 429 trillion ticks per second. It's the same frequency as light in the lower part of the red region of the visible spectrum, and that relatively high frequency is a pillar of the clock's incredible precision. Commonly available atomic clocks beat at frequencies in the gigahertz range, or about 10 billion ticks per second. Going from the microwave to the optical makes it possible for Ye's clock to be tens of thousands of times as precise.
Physics Atomic clocks housed in national laboratories around the world provide a precise time signal on which our present high-technology lives depend. Precise synchronization allows for colossal amounts of data to be reliably transferred around networks of optic fibers at ever-increasing rates, as well as providing accurate navigation systems. With increasing demand for mobile devices, there is a requirement for these atom-based time pieces to shrink. Present chip-scale atomic-clock (CSAC) devices based on alkali atoms in a buffer gas are lightweight (<50 g) and consume little power (<150 mW) but tend to show frequency drift over time that requires frequent recalibration. Clocks based on trapped single atoms or ions should be more stable because the atoms are vacuum-packed and do not interact. However, these tend to be relatively large and power-hungry. Jau et al. have developed a miniature atomic clock based on trapped Yb ions that has size and power requirements similar to those of existing CSAC technology but also offers to match the long-term stability expected of the much larger trapping systems. Appl. Phys. Lett. 101 , 253518 (2012).
This paper presents the latest progress in the development, fabrication, and characterization of microfabricated atomic frequency references at NIST. With volumes below 10 mm3 the physics packages contain the complete integrated assembly for probing the ground-state hyperfine splitting frequency of the alkali atoms by coherent population trapping (CPT). This technique allows for a simple and compact device containing a vertical-cavity surface-emitting laser (VCSEL), optics to shape the laser beam, a vapor cell containing the atoms, and a detector. We present an improved technique for microfabricating the miniature alkali vapor cells. When integrated into a CPT clock, a clear reduction of long-term frequency drifts is observed. This leads to a fractional frequency instability of less than 10-11 at one hour of integration, a more than one order of magnitude improvement over previous results. We identify the remaining sensitivities of the clock frequency to environmental influences and propose ways to reduce them.