Plans for a matter-wave tractor beam

A team of physicists have outlined a means of making tractor beams to push and pull objects at a distance using “matter waves”, those strange analogues of light waves that underlie quantum mechanics.

Tractor beams, staple tools of science fiction for remotely pulling in space shuttles and yanking away incoming space debris, have been edging into reality in recent years.

The first real-life tractor beams were made of photons. It is easy to imagine a stream of photons carrying a particle of matter along like a river picking up a leaf and carrying it downstream. What is astounding about tractor beams is that by skilfully manipulating the transfer of momentum from the beam, physicists do not have to rely only on pushing particles, but can make light pull particles of matter, like a tractor. Beams made of sound waves have also been demonstrated in the lab.

In a paper published last week in Physical Review Letters, Alexey Gorlach from Belarusian State University and colleagues from St Petersburg’s ITMO University and the Technical University of Denmark make the case for using a more exotic, less tangible kind of wave.

The new research is purely theoretical, but the result is still surprising: “The completely different (probabilistic) interpretation of quantum mechanics does not harm the pulling force phenomenon,” the authors write.

It’s surprising because matter waves are essentially waves of probability – they point the way to predicting where a particle is most likely to be at any point in time. Yet Gorlach’s team’s calculations suggest that these waves of chance can still be harnessed to pull physical nanoparticles as if they were magnets drawing tiny iron filings.

“It is the wave nature that is the uniting principle,” the researchers point out: light, sound, and matter waves all behave in similar ways. All have different wavelengths, however, and wavelength determines the size of the particles that can be towed by the tractor beam. (The particle must be smaller than the wavelength of the beam.)

The wavelength of visible light ranges from about 1000 nanometres to 10 nanometres. In 2014, a team from ANU set the optical record by towing microscopic glass beads for a distance of 20 centimetres. The wavelength of sound is longer than that of light, and in 2015, a team from Bristol used these longer waves to pull larger objects (up to a millimetre in diameter).

At the nanoscale is where matter waves could have an edge. Their wavelengths are much smaller than those of light, and Gorlach’s team are looking at wavelengths of one hundredth of a nanometre.

Optical and acoustic beams are still in the testing phase, and matter-wave tractor beams are yet to make it into the lab, but the possibilities are huge. Gorlach’s team member Andrey Novitsky outlines one such application: “Matter-wave tractor beams could be used in an electron microscope that enables us not only to see atomic-scale objects with unprecedented precision, but also to manipulate them.”