Quantum reality just got stranger. A particle鈥檚 angular momentum 鈥� its spin around an axis or rotation around a point 鈥� may be able to travel between two places by itself, disembodied from the particle. This finding may require a rethink of some of the most fundamental laws of physics.
at Tel Aviv University in Israel likens it to the way the Cheshire Cat from Alice in Wonderland can move its body to one place while its smile remains elsewhere. 鈥淎nd here now we showed that physics actually realises this possibility,鈥� he says.
This quantum Cheshire Cat effect has been confirmed before. In 2014, an experiment first showed that a neutron鈥檚 spin can be disembodied from the neutron itself. Similar findings followed, corroborating Aharonov鈥檚 view of the quantum realm being its own sort of Wonderland. Now, along with and at the University of Bristol in the UK, Aharonov has turned to investigating angular momentum more broadly at the quantum scale.
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An object鈥檚 angular momentum depends on its mass, velocity and size, and has traditionally been thought to follow a conservation law: it can never be created or destroyed, only redistributed. For聽angular momentum to move somewhere, it must be carried by聽an object. But now it seems that聽may not always be the case.
The researchers imagined a scenario where a particle with angular momentum is at the left聽edge of a long box that has a聽reflective wall in the middle. The聽particle bounces around the left half of the box, but in rare instances, it can pass through the聽wall via quantum tunnelling. They calculated how the properties of a particle starting on聽the left and moving right would change after a long time.
They found that even when the particle is extremely unlikely to leave the left edge of the box 鈥� for instance, when it is very massive 鈥� its angular momentum could still change. The law of conservation of angular momentum suggests this must be accounted for by an equal and opposite change, transferred elsewhere by the particle. But the team found that despite the particle never being in contact with the box鈥檚 right edge, that is where its momentum ended up.
Checking for signatures of possible contact between the two sides, such as a transfer of velocity from the particle to the wall, left the researchers empty-handed. They concluded that the angular momentum made it to the wall without anything carrying it.
鈥淪uddenly, we had a situation in which a conserved quantity, which means more than some arbitrary property of the particle, can be disembodied from the particles to which it belongs, and can propagate from one place to another without any material support,鈥� says Popescu.
Popescu says this has important implications for conservation laws, the bedrock of physics. Studying these in quantum theory is not at all straightforward, he says 鈥� at that scale, there are fundamental limits to how precise measurements of things like the properties of a particle can be.
But even without perfect measurements, the finding may reveal something fundamental about the world. Aharonov says it聽signals that there are laws of physics that persist even when what we can learn about objects is聽uncertain. Indeed, the conventional idea of a particle as聽a聽knowable object doesn鈥檛 mesh with quantum theory. 鈥淚 think the idea is we have shown now that conservation laws are a deeper issue than the issue of particles,鈥� he says.
at Hiroshima University in Japan says the idea that conservation laws are more complex in the quantum realm is compelling, but a more detailed and nuanced analysis of the set-up presented by the researchers is necessary. 鈥淔or this kind of research, it is important to understand that the conventional ideas of particles as physical 鈥榗arriers鈥� of information does not work at all in quantum mechanics, where the concept of a 鈥榩article鈥� clearly means something more elusive and less tangible,鈥� he says.
To uncover the effect, the researchers used a process called 鈥減ost-selection鈥� in which they infer what must have happened to the particle in the past based on its state at a later time. They argue that the validity of their discovery is independent of this method. at the University of Toronto in Canada says some physicists still object to it, even though Aharonov has an extensive body of work on how such later actions can affect what we can know about a particle.
鈥淭hese 鈥榩ost-selected鈥� experiments often lead to strange聽new effects and bear being聽thought about cautiously,鈥� says Steinberg.
He says the model makes definitive enough predictions to be tested experimentally, even though those would probably be technically challenging. Popescu and Aharonov say they are confident that experiments with quantum light or ultracold atoms could put the phenomenon to the test. Once that happens, we will have more certainty on how conservation laws work in the quantum Wonderland.
Reference: Physical Review A,
