Between a clock and outer space

We used a telescope and a high-precision laser system to beam an atomic clock signal through thin air. While we only sent the signal between buildings, this is the first step in developing a system able to beam these signals to satellites in orbit, with the ultimate aim of pushing our theories of the universe to the breaking point.

More than one hundred years ago, Albert Einstein formulated his theory of General Relativity which explained gravity not as a force in the sense we are all used to, but as a warping of the very fabric of spacetime. The theory predicted that, due to this warping of space and time, a clock should tick faster in space than it does on Earth, where it feels the warping due to Earth’s gravity more strongly.

For 99 years, General Relativity has been spectacularly successful and has survived every test physicists have thrown at it. It predicted gravitational lensing, black holes, and gravitational waves, all of which we have found. The GPS satellites that guide us around the world have to take General Relativity into account, because they orbit high above the Earth where the warping of spacetime is smaller. But we know that General Relativity, and Einstein, is wrong, or at least incomplete. When trying to predict what goes on inside a black hole, or at the moment of the Big Bang, General Relativity produces nonsense. General Relativity is also incompatible with quantum mechanics, the rules our universe runs to on atomic scales.

Physicists are still trying to overcome these problems with General Relativity. One way we are doing this is by testing the theory to very, very high precision, because if there is a disagreement between theory and experiment, even at the 19^th decimal place, then that represents a chink in the armour of modern cosmology that might give us the clues we need to make progress.

The ACES mission will do this by comparing state-of-the-art atomic clocks on the ground to atomic clocks in space aboard the International Space Station. However, to compare the ground-based and space-based clocks with the highest precision possible, we need to use lasers to transmit the clock signals through the air from the ground to the space station. One of challenges in doing this is that air turbulence scrambles the beam, destroying the high precision of the link. We built and tested a system that is able to undo the scrambling caused by the air, and took the first step toward developing a laser link that is able to connect clocks on the space station to clocks on the ground.

We mounted a small telescope in an office in the physics building, and a fist-sized mirror in another building 300 m away on the campus. A laser transmitter connected to the telescope sent the laser beam through the air to the mirror, which bounced it back along the same path back into the telescope. The returning beam was directed to our optical receiver. A portion of the signal was reflected out of the receiver, travelled back through the telescope-air-mirror-air-telescope loop again, before arriving back at the transmitter. Our transmitter compared the returning laser signal to the signal it sent to work out in what way the turbulent air had scrambled the laser signal. The transmitter then applied a correction to the outgoing signal which accounted for the turbulence and delivered an un-scrambled laser beam to the receiver. Our system succeeded in removing more than 99.9% of the scrambling caused by the turbulent air.

This is not the first time something like this has been done. Groups in the U. S. and Germany are also working on similar technology, and the U.S. group have managed to beam their clock signals over 12 km, significantly longer than our 600 m. However, our transmission method has some key advantages. The other groups use complex equipment at both the transmitter and receiver ends to transfer both the tick of the atomic clock and actual time determined by the clock. Our system is much simpler. It is only able to transmit the tick of the clock, and not the actual time, but for experiments like the ACES mission, we only need the tick. By focussing on the tick, rather than the time, our system is able to transmit the tick 10 times more precisely than the systems that transmit both tick and time. Also, because our system is much simpler, it can be made smaller and more robust, and so is better suited to being launched on a spacecraft.

This was the first step in developing a system that is able to connect atomic clocks in space to ones on the ground. Next, we need to go to longer distances. We are setting up a new 2.2 km link, and are planning to do a 12 km link in the future. Because the atmosphere gets thinner at higher altitude, going through 12 km of air horizontally gives you the same amount of turbulence as if you were aiming at an orbiting spacecraft. To do that, we will need “adaptive optics”, a system that keeps our laser beam on target despite the atmospheric turbulence trying to deflect it away. That’s the next challenge.

We are also experimenting with transmitting signals from microwave atomic clocks, which have less stringent precision requirements and are more immediately useful to commercial and industrial applications. Systems like the one we are building will contribute to a wide range of high-precision scientific experiments, and might also be part of a future generation of super-accurate GPS navigation systems.

The published paper is available here.
You can read a freely available version here.