Phase-continuous comparison of three all-optical time scales over 20 days

The Race for the Perfect Tick

Imagine time as a river. For centuries, we've measured its flow using a simple, reliable, but slightly wobbly raft: the atomic clock based on microwave signals (like the ones in your GPS). These clocks are good, but they have a limit to how steady they can be. Recently, scientists have built "optical" clocks that use light instead of microwaves. These new clocks are like high-speed racing boats; they are incredibly fast and precise, ticking hundreds of thousands of times more often than the old ones. However, there's a catch. To keep a perfect time scale (like the official time for the whole world), you need a "flywheel"—a steady engine that keeps the boat moving smoothly even when the steering wheel (the ultra-precise clock) takes a break to recharge or calibrate.

Currently, the best flywheels we have are still the old microwave rafts (hydrogen masers). They are sturdy, but they wobble too much in the short term. This means that even if you have a super-precise optical clock, you can't fully use its power because the old flywheel drags the whole system down, creating a "noise floor" that hides the clock's true perfection. The big question in the world of timekeeping is: Can we build a new kind of flywheel made of light itself? If we can, we could create a time scale that is as steady as the best optical clocks, potentially leading to a redefinition of the second itself. This is the challenge the researchers in this paper set out to tackle.

The All-Optical Time Machine

In this study, a team of scientists from JILA, NIST, and Vector Atomic decided to stop using the old, wobbly microwave flywheels and try something radical: they built three separate time scales that run entirely on light. Instead of relying on hydrogen masers to keep time between checks, they used three different "optical flywheels": two super-cold silicon cavities (think of them as mirrors made of silicon that are chilled to near absolute zero to stop them from jiggling) and one commercial iodine optical clock.

To keep these light-based engines on track, the team used a highly reliable strontium optical lattice clock as a "steering wheel." This master clock checked the speed of the three flywheels and made tiny adjustments to keep them in sync. The team ran this experiment for over 20 days, watching how well these three all-light time scales compared to each other.

The results were a massive step forward. When the three time scales were compared, they stayed incredibly close together. After just a few days of averaging, their relative instability dropped below $10^{-16}$, a level of precision that would take a traditional maser-based system weeks to achieve. Even more impressive, the total time difference between the three scales over the entire 20-day period was less than 100 picoseconds (that's 0.0000000001 seconds). To put that in perspective, if you were watching a movie, this error is so small you wouldn't even notice a single frame was missing.

The researchers also showed that they could convert these light-based signals back into radio signals (the kind used by our current electronics) without losing accuracy. They found that the process of turning light into radio waves added less than 2 picoseconds of error, proving that this new system is ready to talk to our existing technology.

However, the paper is careful to note that this isn't a magic wand that fixes everything overnight. The system still relies on the strontium clock being available to do the steering. When the master clock took a break (which happened for about 6 hours at a time), the time scales did drift apart slightly, accumulating about 20 picoseconds of difference per gap. But even with these gaps, the performance was far superior to what is possible with current technology. The authors suggest that as optical flywheels become more reliable and the master clocks run for longer periods, these all-optical time scales could become the future standard for keeping time, eventually replacing the old microwave-based systems entirely.

In short, the paper demonstrates that it is possible to build a time scale that lives entirely in the world of light, offering a level of stability that the old microwave methods simply cannot match. It's a proof-of-concept that the future of timekeeping might just be shining a little brighter than we thought.