Despite rumors to the contrary, a mainstay of quantum physics is just as (un)certain as ever.
At first glance, a Nature Physics paper published online July 25 may have appeared to threaten the Heisenberg uncertainty principle, which says that in the quantum world of electrons, photons and other tiny particles it’s impossible to know certain pairs of physical properties, such as a particle’s position and the momentum, at the same time.
The paper reports that in certain quantum scenarios, it’s possible to have a device that stores knowledge of the exact position and exact momentum of a particle. But try to stifle your panicky screams for just a little bit longer. After a reasonable amount of scrutiny (and a few quick checks with study coauthor Roger Colbeck at the Perimeter Institute for Theoretical Physics in Canada), I found out that the result won’t have Heisenberg rolling over in his grave anytime soon.
“It is not the case that Heisenberg has been overturned,” Colbeck says.
Rather, the paper points out that in certain extreme quantum scenarios, the uncertainty principle may not apply in its original form.
In the new paper, Colbeck and his colleagues examine what would happen mathematically if an electron is quantumly linked with a perfect quantum memory, a device that can receive and store all the vast quantum information about a particle. (Such systems, including this one that Alexandra Witze recently wrote about, have been created, but can’t yet store the quantum info long enough to be useful.)
Colbeck and his colleagues studied what would happen if the quantum memory and the particle were entangled, that is, held in a linked existence such that what happens to one seems to instantaneously affect the fate of the other.
An entangled quantum memory could hold information about a particle that could be measured to reveal the particle’s position perfectly, and information that could be measured to reveal the momentum perfectly. “So we can have a memory which stores both the position and the momentum,” Colbeck says.
If we stopped here, Heisenberg might get a little shifty down there. But there’s a caveat, and it’s a big one: “Although the memory stores both the position and the momentum, we can only read out one of the two,” Colbeck says.
As physicists attempt to read out one of these properties exactly, the other gets more and more mysterious, just as Heisenberg thought. So for now, Heisenberg can rest easy, because this uncertainty is still certain.
For more details, check out one of the paper author’s blog posts about the result here.
M. Berta et al. The uncertainty principle in the presence of quantum memory. Nature Physics. Published online July 25, 2010.