To make some of the most accurate measurements of the world around us, scientists tend to go small – even at the atomic scale, using a technique called atomic interferometry.
Now, for the first time, scientists have performed this type of measurement in space, using a sounding rocket specially designed to carry scientific payloads into space on Earth.
It is a significant step towards the ability to perform wave-matter interferometry in space, for scientific applications ranging from basic physics to navigation.
“We established the technological basis for atomic interferometry aboard a sound rocket and demonstrated that such experiments are possible not only on Earth but also in space,” said physicist Patrick Windpassinger of Johannes Gutenberg University in Mainz, Germany.
Interferometry is a relatively simple concept. Take two identical waves, separate them, recombine them, and use the small difference between – called phase change – to measure the force that caused that distance.
This is called an interference pattern. A famous example is the LIGO light interferometer that measures gravitational waves: a beam of light is divided into two mile-long tunnels, jumped off the mirrors and recombined. The resulting interference model can be used to detect gravitational waves caused by the collision of black holes millions of light years away.
Atom interferometry, which capitalizes on the behavior similar to atomic waves, is a little 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 at a high level of accuracy; so a team of German researchers has been working for years to try to achieve this.
The first step is to create a state of matter called the Bose-Einstein condensate. They consist of atoms cooled to just a fraction above absolute zero (but do not reach absolute zero, at which point the atoms stop moving). This causes them to sink to their lowest energy state, moving extremely slowly and overlapping in quantum overlap – producing a high-density cloud of atoms that acts as a “super atom” or wave of matter. .
This is an ideal starting point for interferometry, because the atoms all behave identically, and the team created a Bose-Einstein condensate in space for the first time using their sound rocket in 2017, with a gas of rubidium atoms.
“For us, this ultracold assembly was a very promising starting point for atomic interferometry,” Windpassinger said.
For the next stage of their research, they had to separate and recombine the superimposed atoms. Once again, the researchers created their Bose-Einstein rubidium condensate, but this time they used lasers to irradiate the gas, causing the atoms to separate, then reunite in overlap.
Interference patterns observed in Bose-Einstein condensate. (Lachmann et al., Nat. Commun., 2021)
The resulting interference model showed a clear influence of the sound rocket microgravity medium, suggesting that, with a little refinement, the technique could be used to measure this medium with high accuracy.
The next step of the research, planned for 2022 and 2023, is to retry the test using separate condensates of rubidium and potassium Bose-Einstein to observe their acceleration in case of free fall.
Because rubidium and potassium atoms have different masses, this experiment will be, the researchers said, an interesting test of Einstein’s principle of equivalence, which states that gravity accelerates all objects equally, regardless of their own mass.
The principle was previously investigated in space, as can be seen in the famous experiment with feathers and hammers performed by Apollo 15 commander David Scott on the Moon. The principle of equivalence is one of the cornerstones of general relativity, and relativity tends to decompose in the quantum realm, so the planned experiments are considered to be very interesting indeed.
And it will only become more interesting in the future. Sound rockets rise and fall in suborbital flights, but there are plans to conduct even more Bose-Einstein condensation experiments in Earth orbit.
“Carrying out this type of experiment would be a future goal on satellites or on the ISS International Space Station, possibly at BECCAL, the Bose Einstein Condensing and Cold Atom Laboratory, which is currently in the planning phase,” said physicist André Wenzlawski from Johannes Gutenberg Mainz University in Germany.
“In this case, the achievable accuracy would not be limited by the limited time of free fall on board a missile.”
In just a few years, we could use atomic interferometry for applications such as quantum tests 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 was published in Communications about nature.