DARPA has announced a new initiative called the Robust Optical Clock Network (ROCkN) program, which will look to develop a practical, super-accurate optical atomic clock that is robust and small enough to fit inside a military aircraft, warship, or field vehicle.
Ours is a world that runs on time, but we often aren't aware just how precisely. In the golden age of the railways, companies would pride themselves on trains arriving to the minute of the schedule. Today, we think a watch is keeping excellent time if it's accurate to the second, but to modern engineers and scientists that's about as acceptable a level of accuracy as doing brain surgery with a pile driver.
Modern systems like the internet, GPS and others require timekeeping with a level of accuracy that is measured not in seconds or milliseconds, but in nanoseconds. In general engineering applications, for example, this is necessary so that digital systems are properly synchronized. If they weren't then it would be impossible to send data from one place to another without most of the packets of information getting lost or ending up at the destination as a pile of gibberish.
For military applications, it's even worse. Modern militaries are increasingly involved in the area of cyberwarfare, which is an obvious reason for the need for precision clocks, but even conventional war-fighting needs nanosecond-level accuracy because military assets have to operate at extreme range, extreme speed, and strike with extreme accuracy. An error of a billionth of a second can cause a munition to miss its target by a meter, which is unacceptable by modern standards, and a similar error can place an aircraft dangerously off course.
Another reason is that the modern military is heavily reliant on GPS for both navigation and time synchronization. However, GPS isn't always available. It can also be jammed or spoofed, so field units need to be able to operate independently of the system for a limited period of time.
To produce the needed accuracy, the go-to solution has been atomic clocks, which are based on measuring the frequency of atoms like cesium as they change from one energy state to another. By using this frequency as a starting point, it is now possible to define a second with an accuracy that shows a gain or loss of one second every 31.71 million years.
Unfortunately, even this isn't accurate enough. What's needed is ones that are accurate to a trillionth of a second, so over the past 30 years there has been a concerted effort to make atomic clocks more accurate. Ignoring a lot of technical details, a conventional atomic clock works by using a beam of microwaves to measure the frequency of the target atoms, but by replacing the microwaves with light, the accuracy is boosted by a factor of 100. In fact, such optical clocks are so accurate that the most advanced wouldn't gain or lose a second through the entire lifespan of the universe.
Such optical atomic clocks have been built, but they're still huge, delicate, room-filling machines that aren't practical for military application. The goal of DARPA's ROCKn program is to study the basic physics of the principle behind the optical clock and find a way to make optical atomic clocks with low size, weight, and power (SWaP). Not only that, they will be more precise and accurate than current state-of-the-art atomic clocks.
To do this, ROCKn will first look to produce a robust, high-precision small portable optical clock that can maintain picosecond accuracy for 100 seconds at a time. This clock would be small enough to install in a fighter jet or satellite and tough enough to withstand the temperatures, acceleration, and vibrational noise of such an environment.
The second stage will aim to create a larger transportable version that can be used in a Navy ship or field unit that is accurate to a nanosecond for up to 30 days without an outside GPS signal.
"The goal is to transition optical atomic clocks from elaborate laboratory configurations to small and robust versions that can operate outside the lab," said Tatjana Curcic, program manager in DARPA’s Defense Sciences Office. "If we’re successful, these optical clocks would provide a 100x increase in precision, or decrease in timing error, over existing microwave atomic clocks, and demonstrate improved holdover of nanosecond timing precision from a few hours to a month. This program could create many of the critical technologies, components, and demonstrations leading to a potential future networked clock architecture."
Source: DARPA