These ‘chip-scale’ atomic clocks extend precise timing beyond GPS limits

Now, researchers at the National Institute of Standards and Technology (NIST), in collaboration with researchers from Georgia Tech, have developed a new type of miniature atomic clock called a chip-scale beam clock.

The chip-scale beam clock offers improved timing over weeks and months compared to existing systems.

Atomic beams for timekeeping

Since the 1950s, NIST has utilized atomic beam clocks to keep time—these work by sending a beam of atoms through a vacuum chamber.

Beam clocks are precise, stable, and accurate. They work by sending a beam of prepared atoms, most commonly cesium, through a vacuum chamber, measuring their ticking rate, and using it as a reference for timekeeping and synchronization.

The vacuum chambers where the atoms travel are the key to the success of these clocks, but the size of the microwave cavity used to probe the atomic ticking is an issue. NIST-7, the final beam clock used for the principal frequency standard in the United States, had a vacuum chamber that was more than 2.5 meters (8 feet) long.

Making atomic clocks portable is necessary for precise timekeeping and synchronization in navigation systems, telecommunication, remote environments, scientific research, and defense applications. 

Atomic clocks of the size of a briefcase exist but require a significant amount of power of around 50 Watts, three times the power needed for a smartphone to run a task. 

Chip-scale atomic clocks (CSACs)

Chip-scale atomic clocks, or CSACs, were first developed in 2001 at NIST to create a portable, low-power atomic clock.  

CSACs have been used for various applications, such as gas explosions, underwater, military navigation, and telecommunication, where traditional GPS fails. However, a significant problem with CSACs is that the clocks’ timekeeping drifts when there is a change in temperature or the gas surrounding the atoms degrades. 

They have the same principle as the traditional atomic clock but achieve miniaturization using innovative microfabrication techniques while aiming to overcome these limitations. CSACs use vapor cells, which are small chambers where the atoms are measured and held onto a chip-scale platform. 

The team used a stack of etched glass and silicon layers to fabricate a chip-scale atomic beam device. The device, the size of a postal stamp, is a highly miniaturized version of the chambers used in the atomic beam clock like NIST-7. 

The device contains one chamber of a small rubidium pill, which heats up, releasing rubidium atoms through microcapillary channels just 100 micrometers wide. Non-evaporable getters, or NEGs for short, pull the rubidium atoms with them and collect them, keeping the vacuum in the microcapillaries clean. 

The stray atoms are collected by tiny rods of graphite.

Currently, the chip-scale beam device is a prototype, with initial tests showing a scarcely worse performance than existing CSACs. However, the team is confident that they can push their precision by a factor of 10 and stability of present CSACs by a factor of 100 over weekly periods. 

NIST has partnered with HRL Laboratories, Virginia Tech, and CU Boulder to investigate the feasibility of building a tiny clock based on this technology. 

By leveraging the stability and accuracy of atomic beam clocks in a compact form factor, this new chip-scale atomic beam clock has the potential to provide stable timing in locations where GPS signals may be unavailable or weak.

The findings of the study are published in the journal Nature Communications.

Study abstract:

Atomic beams are a longstanding technology for atom-based sensors and clocks with widespread use in commercial frequency standards. Here, we report the demonstration of a chip-scale microwave atomic beam clock using coherent population trapping (CPT) interrogation in a passively pumped atomic beam device. The beam device consists of a hermetically sealed vacuum cell fabricated from an anodically bonded stack of glass and Si wafers in which lithographically defined capillaries produce Rb atomic beams and passive pumps maintain the vacuum environment. A prototype chip-scale clock is realized using Ramsey CPT spectroscopy of the atomic beam over a 10 mm distance and demonstrates a fractional frequency stability of ≈1.2 × 10⁻⁹/√τ for integration times, τ, from 1 s to 250 s, limited by detection noise. Optimized atomic beam clocks based on this approach may exceed the long-term stability of existing chip-scale clocks, and leading long-term systematics are predicted to limit the ultimate fractional frequency stability below 10⁻¹².