Physicists are turning time backward on a quantum scale to get a better view of dark matter and improve atomic clocks.
In new research, scientists from MIT have discovered a way to amplify the hard-to-read signals from particles in quantum relationships. Their method could lead the way to better measurements of many tiny phenomena, from atomic clocks to the search for elusive dark matter. Their paper appears now in Nature Physics.
Yes, this research is cool and scientific, but we can all relate to the fundamental question here. How do you make quiet things louder? You amplify them. Maybe you’ve tried to watch a video where even the highest sound setting isn’t loud enough on your phone, and then downloaded a “volume booster” program to somehow turn everything up to 11—who knew that was possible?
Quantum phenomena are in desperate need of a volume boost. Basically, physicists can take atoms of different elements and supercool them until they’re as close to absolute zero as we can reasonably manage. At that point, the atoms’ movements are slowed a great deal, making them parseable to the naked eye. Or, that is, the naked eye of the electron microscope or other extremely fine-tuned instrument.
But there’s still a problem. Yes, slowing the atoms down lets us observe a lot about them, but many other phenomena are too tiny to see, even then, and even with the best tools available. It’s like we’re trying to sift riverbed dirt to find gold, but our screen gauge is just still too wide. The gold is washing through. We’ve done all we can to make a finer screen—now we need to bulk up the gold if we can.
The physicists at MIT have observed that tiny phenomena are still escaping the notice of our instruments. And they’re not alone—the search for dark matter is one of the most pressing questions in physics! So everyone is very interested in finding a way that we can boost these phenomena into the realm of observability. In this case, the solution involves something that sounds outlandish: turning time backward.
To study how to boost the quantum phenomena, the scientists made clouds of between 50 and 400 atoms of the metallic element ytterbium. Ytterbium is the global leader in atomic clocks, and holds the record for the absolute least time lost overall—something like one quintillionth of a second, compared with times when the best clocks all lost 15 minutes or more each day. In an atomic clock, the atoms are trapped by lasers and “provoked” by yet more lasers. Ytterbium is popular for another application, quantum computing, where it is also trapped in place by lasers and then stimulated.
In the MIT experiment, the ytterbium cloud is held by a system of lasers and cooled to near absolute zero. Once the atoms are super slowed down by the extreme cold, the scientists use a blue “entangling” laser that puts the atoms into the state of entanglement: a quantum state where the atoms act together even though they are separated by a distance, sometimes even a large distance. Then—and this is the big news—they use an opposite-acting red laser to pull the atoms back out of entanglement. It’s known as “time reversal,” because it rewinds the quantum phenomenon.
Scientists have theorized for decades that a system can have its energy reversed in this way, and anything that had happened to the entangled system would be magnified by the reversal. By using the red laser and observing the results, the team confirmed that this was the case. In repeating their experimental setup thousands of times, they observed that the reversal magnified the quantum phenomena by up to 15 times.
This work impacts the search for dark matter. Quantum atomic clocks are an integral part of the search for dark matter in a key way. Let’s say there are atomic clocks all over the world, all of them measuring time in the extreme quintillionths of a second. Scientists can observe when and how those clocks are disturbed as a way to track where dark matter may be passing, like a dark cloud covering the sun, but for time.