Quantum sensors could detect space debris from its gravitational pull

Devices based on quantum properties of very cold and very small crystals could be mounted on satellites and sense space debris that could collide with them.

Sensors based on the quantum behaviour of tiny crystals could detect pieces of space debris hurtling towards satellites.

There are thousands of small pieces of junk orbiting Earth (as well as thousands of satellites)
Shutterstock/Dima Zel

The US government estimates that there are around half a million pieces of debris between 1 and 10 centimetres across, like chips from defunct satellites and rocket fragments, orbiting Earth. Some bits of space junk are tracked from the ground with radar, but as the number of satellites in orbit grows, so does the need for devices that can alert them to possible collisions with orbital debris.

Marko Toroš at the University of Glasgow in the UK and his colleagues calculated that modifying a device called an interferometer could make it so sensitive it would be able to detect space debris from its gravitational pull.

Interferometers are used in many physics experiments, like for detecting gravitational waves. Inside, they contain waves, such as light waves, which travel through the device, then crash together and create patterns of ripples called interference patterns. Researchers then use the details of these patterns to determine what forces or fields influenced the waves as they were moving.

Instead of an interferometer filled with light, Toroš and his team built a mathematical model for one filled with a very cold nano-sized crystal. They assumed that it would be cold enough to have quantum properties that don’t exist at room temperature, like behaving like a wave of matter rather than a finite chunk of crystal.

The device would be configured so that there are two paths for the nanocrystal to move along. The nanocrystal would then be put in a quantum superposition state, which means that it would act like a wave that travels through both paths at once then crashes into itself, creating an interference pattern.

The researchers found that this process was so sensitive that if an object lighter than a kilogram moved towards the device at a few kilometres per hour, its gravitational force on the nanocrystal would noticeably disturb this pattern from a kilometre away. Monitoring changes in the device’s interference pattern would then be a plausible way to see such objects coming before they impact the device.

Toroš says that his team started thinking about these sensors as part of searching for signatures of the poorly understood physics of quantum gravity, but their potential turned out to be broader. “We thought, these are such exceptional sensing devices, could we use them to do something useful for space technology? And one clear thing we could do if we built these interferometers is sense tiny masses around satellites,” he says. In this way, researchers could map out threats to satellites more precisely, and even help them avoid collisions.

The team included some extreme cases in its calculations, to illustrate the range these sensors could theoretically have. For instance, very advanced versions of the device could sense the gravitational pull of a primordial black hole if one exists in the outer solar system, or pick up signs of a rogue asteroid moving towards Earth like the Tunguska even in 1908. In either case, however, the nanocrystal would have to be dramatically bigger than any object that has ever been put into a quantum superposition state.

Andrew Geraci at Northwestern University in Illinois says that the size of the nanocrystals in the current model is already a challenge – the current mass record for matter-wave interferometry is held by ultracold molecules, which are about a million times lighter. “But there is a road map for how experiments will get there. In the next few years, we will probably see some experiments demonstrating matter-wave effects in relatively large objects, maybe even at the nanoscale,” says Geraci. After that, he says, researchers will have to work out how to pack such quantum systems into devices robust enough to be sent into space.

Journal reference

Physical Review DDOI: 10.1103/PhysRevD.107.104053

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