Is optical cesium the holy grail of atomic clocks?

Ulrich Kohn
Golden chalice

Synchronization in the national interest

Our societies depend on critical infrastructures such as communication networks, power utilities and financial institutions, among many others, and any outage can have tremendous impact. It’s interesting to note, that the operation of those infrastructures requires precise synchronization. 

Today, GNSS receivers are frequently applied to provide precise timing. However, concern is growing about how vulnerable those weak signals from space are to unintentional disturbances or malicious attacks. GNSS outages can have significant negative impact and operators of critical infrastructures are now taking action to assure access to highly precise timing even under the most unfavorable conditions. 

Atomic clocks are a key tool for mitigating GNSS vulnerabilities and they are increasingly being applied to back-up GNSS delivered synchronization. If high accuracy and ultimate hold-over capabilities are required, magnetic cesium clocks are used. Those highly stable oscillators deliver accurate frequency signals at the core of synchronization networks over long periods of time even without access to GNSS. 

This magnetic cesium technology was initially developed more than 50 years ago. It’s served us well since then and will continue to be useful in the foreseeable future. However, the recent emergence of new accuracy requirements demands even higher stability. To address specific applications such as 5G mobile networks, synchronization networks need to go beyond what’s possible with magnetic cesium clock technology.

Improving a well-established technology

Cesium atomic clocks leverage an ultra-thin absorption line in the microwave range caused by electron transitions between ground states. As cesium atoms have both states populated with electrons, there is a balance between absorption and emission. If you want to observe this absorption line, you need to make sure that electrons in cesium atoms only populate one state while the other state remains empty.

Model of Cesium (CS) atoms

You need a way to either filter out the cesium atoms with electrons in the wrong state or you need to push all electrons from the wrong state into the correct one. Magnetic cesium technology follows the first approach. There’s a selection process to separate cesium atoms with the desired energy state from those without. Only those cesium atoms with electrons at the correct energy level can be used for detecting the absorption line; all others are removed. 

However, with optical cesium technology, things are different. An optical pump signal pushes all electrons into the correct state. This means that all cesium atoms can be used for detecting the absorption line.

Obviously, the optical cesium approach doesn’t waste cesium atoms. This results in a better detection of the absorption line without the need for highly sophisticated electronic detection devices. In consequence, optical cesium clocks operate with a much longer lifetime and a significantly higher accuracy. 

Besides this selection process, optical and magnetic cesium atomic clocks share the same proven and well-established atomic clock technology. But due to its improved selection process, optical cesium is the technology that will lead us into the future of ultra-precise timing.   

Paving the way for unprecedented sync excellence with optical cesium clocks

ADVA, with its Oscilloquartz business unit of timing specialists, is a leader and innovator in network synchronization. Building on many years of experience with magnetic cesium technology, the company has mastered this breakthrough technology and managed to productize superior – yet technically very challenging – optical cesium technology. 

OSA 3350

Our OSA 3350 optical cesium clock addresses the urgent need of telecom networks for better timing accuracy and improved lifetime. With this major innovation in atomic clock technology, our customers are well prepared to meet emerging timing requirements, while lowering the cost of network synchronization with extended product lifetime cesium atomic clocks. What’s more, this product will also play a key role in protecting our critical infrastructures from loss of synchronization caused by GNSS outages.

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