You might associate much of science with zooming in—using microscopes to look at microbes, for example, or isolating a single chemical reaction—but zooming out is equally as vital. Sometimes, we are simply standing too close to get a good look at a problem. When we back up, the picture comes into clearer focus.
Quantum physicists want to do just that, with one goal being to understand and address climate change. Their plan is to learn more about the Earth using ultracold atoms in outer space to create “the coldest spot in the universe.”
Quantum technologies for Earth observation and sensing are needed in order to more fully understand the vastly complex dynamics that govern our natural world. As one report on quantum applications from the European Space Agency put it: "Given the extreme effects of global warming that mankind is facing, earth observation is maybe the most important scientific endeavour of our times... However, it has become clear that the classical measurements cannot be pushed much further."
The Cold Atom Lab (CAL) is a tiny lab that was developed by NASA’s Jet Propulsion Laboratory and installed on the International Space Station (ISS) in 2018. It’s about the size of a microwave and weighs about as much as a large dog. Inside, scientists plan to conduct experiments on clouds of atoms cooled to near-absolute zero temperatures that all exist in the same quantum state. This June, CAL was able to pass an important proof-of-concept in producing one of these clouds, known as Bose-Einstein condensates. Eventually, physicists want to use them to understand fundamental truths about the universe and our place in it, and they say the possibilities are beyond imagination.
“The first thing you propose is things you can imagine, and then the history of science says you're going to find things you can't imagine,” said Keith Schwab, an applied physics professor at Caltech.
But to be more concrete, one goal of CAL is to serve as the most precise matter-wave atom interferometer, a device that will help us in measuring the Earth in ways we haven’t been able to before.
Why do we need quantum technology to understand climate change?
Matter interferometer in space will be important due to its sensitivity, or how well a device can measure tiny changes. CAL would be unprecedentedly sensitive, which could help scientists understand minute changes to the Earth as they occur.
The primary way CAL would do that is through geodesy, which refers to the study of the planet’s shape, orientation, and gravitational field. As climate change causes ice to melt and the resulting water mass to be redistributed, the geodesic properties of Earth are also changing.
Being able to measure changes in the density of what lies below the Earth’s surface with such precision will complement existing data collected by NASA’s Gravity Recovery and Climate Experiment (GRACE) satellites.
The science of matter-wave interferometry is based on a key concept in quantum mechanics, namely wave-particle duality. To understand what’s going on, it can be useful to first consider interferometry with light, a classic example of a wave.
A single wave of light has a given amplitude, but when you put two waves together, you get a resulting wave where the two add onto one another—called constructive interference—or cancel each other out—called destructive interference. Interference in waves is common, and it’s the scientific principle at work behind noise-canceling headphones.
Here’s where the “quantum” in quantum physics comes in: the physicist Louis-Victor de Broglie theorized in 1924 that all matter, not just light, can behave like waves. Physicists since have built off his work and observed atoms behaving like waves, as de Broglie predicted. The rub is that to observe this effect, clouds of these atoms must be cooled close to 0 degrees Kelvin. Even then, on Earth, these clouds—called Bose-Einstein condensates—last an extremely short period of time, unlike light, which persists in wave form. Gravity is the culprit.
“Light and matter have one important difference: light doesn't fall down,” said Holger Müller, an atomic, molecular, and optical physics professor at UC Berkeley.
That’s where outer space comes in. The ISS experiences microgravity, enabling Bose-Einstein condensates to be observed for a much longer time, relatively. What physicists can do next is kick that cloud of atoms with a laser in a way that causes the two resulting clouds to exhibit quantum superposition, said Joe Murphree, a postdoctoral associate at Bates College and one of the physicists working on CAL. The added longevity of Bose-Einstein condensates in space will allow scientists to do matter-wave interferometry in a way that isn’t possible on Earth, he added.
But the first step toward geodesic sensing is getting CAL up and running. The first iteration of the lab was installed on the ISS in 2018 and did not contain an atom interferometer. CAL was upgraded in early 2020, adding, among other things, an atom interferometer. Now, Murphree and others are working on science module three, the third iteration of CAL, to be installed on the ISS in the near future.
“What [CAL] is doing is essentially testing out, ‘Can an atom interferometer successfully operate in space?’” Müller said. “So far we've only done it on the ground, where people can stand by and fix things when something goes wrong, and that has always been necessary. CAL, if it works, would be the first time that something is in orbit and indeed works.”
What's next for quantum sensing in space?
CAL isn't the only project aiming to harness quantum technology in space. The Chinese Quantum Experiments at Space Scale (QUESS) project and Germany’s Matter-Wave Interferometry in Microgravity (MAIUS) missions are all attempting to bring interferometry to space.
The main challenge the CAL team has faced in putting an atom interferometer in space hasn’t been the science itself, as physicists have fleshed out these theories for decades. Rather, funding for a project grounded in understanding fundamental theory (and not flashy practical applications) has stymied the work.
The cost to conduct an experiment in space is a further complication, said Schwab.
“Sensitive measurements are important because they're the measurements of the way that we are able to make sense of the physical world”
“When it costs $10,000 a kilogram to put something in space, you want to think carefully about which, of the 100 ideas that people are throwing at you, which is the one you could actually do,” he said.
Furthermore, Earth-based interferometers and other equipment used for remote sensing are typically bulky and difficult to calibrate. Murphree said that atomic physicists are used to conducting experiments where they can tinker and tune the devices as they go along, but with CAL, the tinkering has to take place before it’s up in space.
Though the applications of quantum remote sensing in space will be wide-ranging, Murphree said the experiments are just as valuable for science on their own.
“Sensitive measurements are important because they're the measurements of the way that we are able to make sense of the physical world,” he said. “Improving our understanding of the physical world relies upon our measurements of it, and that understanding has inherent worth.”
Addressing climate change will be a global effort, and CAL as well as international projects are a start towards probing fundamental physics in order to understand our planet.