The fuzziness of the quantum world has been demonstrated on its largest-ever scale, probing the limits of quantum mechanics. More than a billion atoms inside a glass bead acted as a single quantum wave, a crucial step in making macroscopic matter interfere with itself and testing theories of quantum gravity.
In the early 20th century, physicists realised that, at tiny scales, matter appeared to be fuzzy. Although previous experiments had shown that particles like electrons or atomic nuclei were solid, new experiments demonstrated that they could also act as waves, interfering with each other like ripples on a pond. This wave-like nature meant that particles鈥� positions couldn鈥檛 be precisely pinned down, but could instead only be described as a cloud of probabilities until this cloud was disturbed by an outside interaction, like a measurement.
This phenomenon, known as delocalisation, is a fundamental principle of quantum mechanics and appears to be universal for minuscule things. Most physicists assume that delocalisation also exists at much larger scales, but that we don鈥檛 see it because the fragile, wave-like cloud of probabilities is destroyed by myriad interactions with other particles. However, it is unclear how far the fuzzy nature of the quantum world extends for large objects and whether they can be observed if these interactions are removed.
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Now, at ETH Z眉rich in Switzerland and his colleagues have measured the quantum wave nature of a 100-nanometre glass bead, a thousandth of the width of a human hair, containing billions of atoms.
To measure the bead鈥檚 fuzzy nature, Rossi and his team first had to localise its position with extreme precision. This was to ensure that the measured delocalisation wasn鈥檛 just from the non-quantum uncertainty of not knowing where the particle exists or from random jiggling caused by heat. To do this, they trapped the bead using an infrared laser, which could precisely measure the particle鈥檚 position and record its random jiggling. They also placed the bead and laser inside an extreme vacuum, so that once the particle was delocalised, its delicate quantum nature wouldn鈥檛 be disturbed by passing molecules.
Once the bead鈥檚 position had been measured, the researchers momentarily turned the laser off, which made the bead鈥檚 wave nature take over. 鈥淚f we switch off the laser, what happens is similar to when you throw a rock in the lake,鈥� says Rossi. 鈥淎t first, the surface is perturbed only at the rock location, it鈥檚 only the water around that rock that starts to move. But if you then wait some time, this wave starts to expand and propagate.鈥� They then switched the laser back on and recorded the bead鈥檚 position.
By repeating this experiment hundreds of times, they gained a picture of the bead鈥檚 fuzzy nature over a scale of picometres, around 100,000 times smaller than the bead itself. The next step will be to record the bead鈥檚 wave nature over distances around the same length as the bead itself, which will make it possible to do interference studies, similar to the famous double-slit experiments, but with macroscopic matter, says Rossi.
Experiments like these will also allow us to test possible theories of quantum gravity. Physicists still don鈥檛 know whether gravity at its most fundamental level is made up of discrete levels and energies, like light is, or whether it is smooth and indivisible on the same length scales as quantum phenomena. It is hard to test gravity鈥檚 nature at tiny lengths because it is comparatively much weaker than other forces, but it would have an effect on the evolution of the wave that Rossi and his team measured, says at the University of Southampton, UK.
In particular, the evolution of the bead鈥檚 wave would look different according to whether gravity was quantised or continuous, says Ulbricht. 鈥淚f you are able to generate [interference with] another particle like the one which they have, then you could study how it then evolves and if gravity plays a role in its evolution,鈥� says Ulbricht. 鈥淭hen you can answer these kinds of questions of quantum gravity. It鈥檚 exciting stuff.鈥�
arXiv
Article amended on 2 September 2024
The size of the glass bead relative to a human hair was corrected
