To make some of the most accurate measurements we can of the world around us, scientists tend to go small – down to the atomic scale, using a technique called atomic interferometry.
Now, for the first time, scientists have made these kinds of measurements in space, using a sounding rocket specifically designed to deliver scientific payloads to low-Earth space.
It is an important step towards being able to perform matter wave interferometry in space, for scientific applications ranging from fundamental physics to navigation.
“We have laid the technological foundation for atomic interferometry aboard a sounding rocket and have shown that such experiments are possible not only on Earth, but also in space,” said physicist Patrick Windpassinger of Johannes Gutenberg University Mainz in Germany.
Interferometry is a relatively simple concept. You take two identical waves, separate them, recombine them, and use the tiny difference between them – called a phase shift – to measure the force that created that distance.
This is called an interference pattern. A famous example is LIGO’s light interferometer that measures gravitational waves: a beam of light is split through two miles of tunnels, reflected by mirrors and recombined. The resulting interference pattern can be used to detect the gravitational waves caused by colliding black holes millions of light years away.
Atomic interferometry, which takes advantage of the wave-like behavior of atoms, is slightly more difficult to achieve, but has the advantage of a much smaller device. It would be very useful in space, where it could be used to measure things like gravity with a high degree of precision; so a team of German researchers has been working for years to get it done.
The first step is to create a state of matter called a Bose-Einstein condensate. These are formed from atoms that have cooled to only a fraction above absolute zero (but do not reach absolute zero, after which atoms stop moving). As a result, they sink to their lowest energy state, move extremely slowly, and overlap in quantum superposition – producing a cloud of high-density atoms that act as one “super atom” or matter wave.
This is an ideal starting point for interferometry, as the atoms all behave identically and the team first created a Bose-Einstein condensate in space in 2017 using their sounding rocket, using a gas of rubidium atoms.
“For us, this ultra-cold ensemble was a promising starting point for atomic interferometry,” said Windpassinger.
For the next phase of their research, they had to separate and recombine the superimposed atoms. Once again, the researchers created their rubidium Bose-Einstein condensate, but this time they used lasers to irradiate the gas, causing the atoms to separate and then rejoin in superposition.
Interference patterns observed in the Bose-Einstein condensate. (Lachmann et al., Nat. Commun., 2021)
The resulting interference pattern showed a clear influence of the microgravity environment of the sounding missile, suggesting that with a little refinement, the technique could be used to measure this environment with high precision.
The next step of the study, scheduled for 2022 and 2023, is to retry the test with separate Bose-Einstein condensates of rubidium and potassium to observe their acceleration during free fall.
Because rubidium and potassium atoms have different masses, this experiment, the researchers said, will provide an interesting test of Einstein’s equivalence principle, which states that gravity accelerates all objects evenly, regardless of their own mass.
The principle has been explored in space before, as can be seen in the famous spring and hammer experiment conducted by Apollo 15 commander David Scott on the moon. The equivalence principle is one of the cornerstones of general relativity, and relativity usually breaks down in the quantum realm, so the planned experiments will get very interesting indeed.
And it will only get more interesting in the future. Probing rockets are going up and down in suborbital flights, but there are plans to conduct even more Bose-Einstein condensate experiments in orbit.
“Conducting experiments like this would be a future objective on satellites or the International Space Station, possibly within BECCAL, the Bose Einstein Condensate and Cold Atom Laboratory, which is currently in the planning phase,” said Johannes Gutenberg physicist André Wenzlawski. University. Mainz in Germany.
“In this case, the achievable accuracy would not be limited by the limited free fall time aboard a missile.”
Within a few years, we could be using atomic interferometry for applications such as quantum testing of general relativity, gravitational wave detection, and even the search for dark matter and dark energy.
We can’t wait to see what happens next.
The team’s research is published in Nature Communications