The pulse of an atom is one of the few things in the Universe that always keeps time.
Yet, even the most advanced “atomic” clocks based on variations of these quantum timekeepers stop working when pushed to their limits.
Scientists have known for a long time that tying atoms together can slow down particles enough to squeeze a little more time out of each tick. However, most experiments have only been able to show this on the smallest scales.
A group of scientists from the University of Oxford in the UK have pushed that limit to a distance of two metres, about six feet. This shows that math still works over larger distances.
This could improve the accuracy of optical atomic clocks as a whole and make it possible to compare the split-second timing of multiple clocks in a way that could reveal signals in a wide range of physical phenomena that had not been seen before.
Optical atomic clocks use light to track the movement of atoms, as the name suggests.
Like a child on a swing, the parts of an atom move back and forth under a set of rules that never change. To get the swinging going, you need a reliable kick, like a photon from a laser.
Over the years, different techniques and materials have been tried to improve the technology. Now, the differences in their frequencies are so small that they barely add up to a second’s worth of error over the 13 billion or so years of the Universe. This accuracy level means we might need to rethink how we measure time.
No matter how well-tuned this technology is, there comes the point when the rules of keeping time become a little fuzzy. This is because the quantum landscape is uncertain, leading to many catch-22 situations.
For example, higher frequencies of light can make measurements more accurate, but this comes at the cost of making small differences between the photon’s kick and the atom’s response matter more.
These, in turn, can be fixed by reading the atom more than once, which is not a perfect solution.
The ideal would be a “single shot” reading with the right kind of laser pulse. Scientists know that the uncertainty of this method can be reduced if the fate of the atom being measured is already tied to that of another.
Entanglement is a strange idea that makes sense at the same time. Quantum mechanics says an object can’t be said to have a value or state until it is looked at.
If they’re already a part of a bigger system like by exchanging photons with other atoms, the system as a whole will likely end up in a way that’s easy to predict.
It’s like flipping two coins from the same wallet and knowing that if one comes up heads, the other will come up tails, even as the coins spin in the air.
In this case, the two “coins” were two strontium ions linked to a photon that was sent down a short length of optical fibre.
Even though it wasn’t the goal of the test, it didn’t make optical atomic clocks more accurate in a way that would change the world.
Instead, the team showed that by tying up the charged atoms of strontium, they could reduce the uncertainty of the measurement under conditions that should allow them to improve precision in the future.
Knowing that distances of a few metres are not hard to measure, it is now theoretically possible to connect optical atomic clocks worldwide to make them more accurate.
Raghavendra Srinivas, a physicist, says, “Our result is very much a proof-of-principle, and the absolute precision we get is a few orders of magnitude below state of the art. However, we hope that the techniques we show here could one day be used to improve state of the art.”
“At some point, entanglement will be needed because it is the only way to get to the highest level of precision that quantum theory can allow.”
Getting a little more confidence out of each tick-tock of an atomic clock could be just what we need to measure the tiny differences in time caused by masses over the smallest distances. This could be a tool that leads to quantum theories of gravity.
Even outside of research, using entanglement to reduce uncertainty in quantum measurements could help with things like quantum computing, encryption, and communication.