Anyone dealing with the smaller world cannot escape quantum mechanics. One of the pillars of this physical theory is Heisenberg’s uncertainty principle. This means that you cannot know exactly where a particular particle is and how fast it is moving. Instead, you have to make a choice: the more precisely you measure the location, the more ambiguous the speed remains – and vice versa.
But a new study seems to suggest that even if you don’t measure a particle’s speed at all, you still won’t be able to pinpoint its location as accurately as you’d like. This can have consequences on how small advanced equipment you can make.
The result, published by Japanese physicists Yui Kuramochi and Hiroyasu Tajima, is based on an idea from 1960: the so-called Wigner-Araki-Yanes (Road) theory. This has already imposed limits on how accurately certain properties can be measured.
There is no absolutely conclusive evidence
There were so many conditions attached to it at the time that it didn’t bother you much in the real world. For example, both properties can only take certain values if the road theory is preserved, while position and speed can have any conceivable value.
The scientists who developed the road theory have hinted that their theory could be extended to position and speed, but they have never taken this step themselves. Other scientists later made further progress, but were never able to prove their evidence conclusively.
And now it appears that Kuramochi and Tajima have taken the final step. “As far as I can see, their evidence is correct,” says physicist Leon Loveridge, who self-published on the issue in 2011. “It seems they have finally finished this beautiful historical journey.”
Do you notice something too?
This means that there appears to be a fundamental limit to how accurately a particle’s position can be measured. Is this something that can be tested experimentally? “This is a very important question,” Loveridge says. “As far as I’m concerned, this is what the physics community needs to focus on next, now that we know that the road theory also applies to this situation.”
Now suppose that quantum mechanics does not actually allow the location of particles to be determined very precisely. Will this have consequences for upcoming technology that makes grateful use of the principles of quantum mechanics, such as quantum sensors, quantum computers, and the quantum internet?
Loveridge does not rule out this possibility. You can overcome that, he adds. He points to a second result from the article that introduced the road theory in 1960. This means that you can go a long way towards measuring with “forbidden” accuracy – if you make your measuring device a lot larger. But for quantum devices, which work best if you make them very small, that’s not exactly the direction you want to go.
Determine the relative position
However, Stephanie Weiner, professor of quantum information at TU Delft, does not expect the extended road theory to cause any problems. “I don’t want to rule out that this principle will come into effect one day, but it is not relevant to the things that concern us at the moment on the practical side.”
It may also be the case that the road theory cannot put a knob in the wheel of quantum technology at all. Perhaps, as Loveridge says, in practice we never measure the position of a particle, but always the position of such a particle relative to something else. The theory of the road says nothing about determining such a relative position. The question then is actually what exactly physicists mean when they talk about “particle position.”
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