Pioneering measurements in space

17 February 2017

The MAIUS-1 sounding rocket mission has enabled physicists to generate a Bose-Einstein condensate in space for the first time. This will allow them to measure the Earth's gravitational field more precisely in the future and, crucially, to test Einstein's equivalence principle more accurately than ever before. The research group Experimental Quantum Optics and Quantum Information at Johannes Gutenberg University Mainz (JGU) is closely involved in the project.

Everyone breathed a huge sigh of relief when the sounding rocket of the MAIUS-1 mission finally got off the ground. On January 23, 2017 at 3:30 a.m. CET, the countdown ended and the twelve-meter long rocket roared into the sky. André Wenzlawski was one of twelve scientists who witnessed the launch on site at the control center of the Esrange Space Center in Sweden. He is now back at the JGU Institute of Physics and willing to share his experiences. And he is visibly relieved: "Everything is fine, we flew in the end." We will hear more about that a little later.

There were just 15 minutes between the takeoff and landing of MAIUS-1. That may not seem very long. But it was enough time for the researchers to achieve something groundbreaking. "We were able to demonstrate the practicality of our basic concept when it comes to measuring the Earth's gravitational field from space using ultracold quantum gases and to more accurately testing Einstein's equivalence principle." And, indeed, this was what they achieved, short as the research rocket's flight was. "We were able to show that our scientific technique works. We couldn't be happier."

Joint project involving many specialists

Wenzlawski is a member of Professor Patrick Windpassinger's research group on Experimental Quantum Optics and Quantum Information (QOQI) at the JGU Institute of Physics. Windpassinger came to Mainz from the University of Hamburg in 2013, with Wenzlawski following two years later. "In a way, Professor Windpassinger brought MAIUS-1 with him to Mainz University."

MAIUS-1 is a joint project involving a number of partners. In addition to JGU, also on board are the universities in Hannover, Bremen, Hamburg, and UIm, Humboldt-Universität zu Berlin, the Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik (FBH) in Berlin, Technische Universität Darmstadt, and the German Aerospace Center (DLR).

"We needed specialists in a broad range of fields," explains Wenzlawski. In other projects, the standard solution is to use what already exists and add an individual module to perform a specific experiment. "Our project was different. We developed everything from scratch: the vacuum chamber, the instrument we employ to generate a magnetic field, the laser system, the control electronics and so on."

In atom interferometry, scientists use the wave properties of atoms to perform measurements. For the MAIUS-1 mission, they generated a Bose-Einstein condensate of rubidium atoms. The atoms in the condensate are at a temperature only just above absolute zero. This makes them relatively easy to control and observe.

Measurements a hundred times more precise

"In our experiment, we generate a cloud of about 100,000 atoms. We expose this cloud to a laser beam that is absorbed by the atoms. Then we take a picture and look at the shadows cast by the atoms."

On Earth, scientists have performed such measurements on numerous occasions, but, up to now, no one has ever been able to undertake them in space. "The crucial question for us is how long we can observe the atoms," says Wenzlawski. "On the ground, they are influenced by gravity, which causes the atoms simply to drop. This limits us to a few hundred milliseconds. In space, in contrast, we have several seconds of time. The accuracy with which we can take measurements here increases with the square of the time we have to make them. Thus, an observation time ten times longer allows us to take measurements that are a hundred times more accurate."

This, for example, would mean it would be possible to determine the Earth's gravitational field a hundred times more accurately. And it creates the opportunity to verify Einstein's equivalence principle, which states that all objects in the same gravitational field fall at the same speed, more accurately. "The equivalence principle is integral to the general theory of relativity, one of the standard theories we currently use to describe our universe." This Standard Model of particle physics describes three of the four known fundamental forces that scientists strive to reconcile in one mathematical formula. This has eluded them up to now. The MAIUS-1 measurements may bring them a step closer.

"The goal of MAIUS-1 was to prove that such measurements are fundamentally feasible in space." On Earth, such measurements usually are performed in laboratories. "They are typically the size of this room," states Wenzlawski looking out over the seminar room, which can comfortably accommodate some 50 students. The rocket, however, is much less spacious. The payload could only occupy 2.8 meters of the rocket's length, with a diameter of 50 centimeters.

Reindeer and other challenges

Thus, everything had to be smaller and more robust for the flight. Windpassinger's group focused on the laser system. "The light is normally guided through optical fibers. To successfully introduce the light into the fibers, the beam needs to be positioned to an accuracy of within one micrometer, a range in which even minute temperature changes play a role," explains Wenzlawski. "Even just one or two degrees of warming typically require everything to be recalibrated." Warming was unavoidable in the rocket, which is why the researchers needed a special material. "We used Zerodur, a glass ceramic that does not expand when the temperature changes."

Starting in 2006, the scientists made steady progress towards their goal of eventually performing scientifically viable measurements in space. First, they tested their instruments in a drop tower in Bremen, then on the TEXUS 51 and TEXUS 53 flights, and finally in space on the MAIUS-1 mission.

"We met in Sweden in October 2016 to prepare for the MAIUS launch," recalls Wenzlawski. The teams labored for two months to coordinate all the components. "We spent an entire week in the polar night, which really put a strain on our nerves." And then the launch was postponed. In the designated landing zone shepherds showed up with their reindeer. Naturally they could not be placed at risk. As a result, the researchers had to abort.

Smooth running of the MAIUS-1 mission

At the beginning of 2017, the collaborating teams of researchers met up again – and on January 23, finally, they at last got their wish and MAIUS-1 started.  After 15 minutes of flight with approximately 100 experimental runs, the container with the payload floated on parachutes back down to the ground in Sweden. "We succeeded in performing one of the most technically complex experiments ever carried out on a rocket," emphasizes Wenzlawski.

But that's only a start. The MAIUS-2 and MAIUS-3 missions will be launched in 2020 and 2021 to perform further tests. The scientists also hope that their findings will be used for an experiment using ultracold quantum gases on the International Space Station ISS.

"This would be a great opportunity for us to conduct experiments for the first time in a permanent zero-gravity environment," explains Wenzlawski. "But even that wouldn't be the ideal environment. There are vibrations on the ISS that can affect our measurements. Our best option would be a satellite." But that is way off in the future. For the moment, Windpassinger and his team are delighted that the MAIUS-1 mission went so well. They are just about to get stuck into the detailed analysis. "We know that our technology works," says Wenzlawski.