A quantum memory may be all scientists need to beat the limit of Heisenberg's uncertainty principle, according to a paper published in Nature Physics. According to a group of researchers, maximally entangling a particle with a quantum memory and measuring one of the particle's variables, like its position, should snap the quantum memory in a corresponding state, which could then be measured. This would allow them to do something long thought verboten by the laws of physics: figure out the state of certain pairs of variables at the exact same time with an unprecedented amount of certainty.

Our ability to observe particles at the quantum level is currently limited by Heisenberg's uncertainty principle. Heisenberg noticed that when someone measured one variable of a particle, such as its position, there were some other variables, like momentum, that could not be simultaneously measured with as much precision—there was a small amount of uncertainty applied to one or both of the measurements.

The physical reasoning behind this is hard to follow. But Paul Dirac, another physicist, made up a scenario to illustrate why some variables have this contentious relationship.

Dirac pointed out that one of the only ways to measure a particle's position is by bouncing a photon off of it, and seeing where and how that photon lands on a detector. How the photon lands completely describes the particle's position, but by hitting it, the measurement changes the particle's momentum.

Likewise, a measure of momentum would change the particle's position. Because of this quirk, scientists thought it was impossible to know certain pairs of variables that affect one another at the same exact time with a very high degree of precision.

Then along came entanglement. When two particles are entangled, reading even one variable of one of the particles collapses the wavefunction of both particles, giving finite values to all related variables.

The cadre of scientists behind the current paper realized that, by using the process of entanglement, it would be possible to essentially use two particles to figure out the complete state of one. They might even be able to measure incompatible variables like position and momentum. The measurements might not be perfectly precise, but the process could allow them to beat the limit of the uncertainty principle.

The system the researchers worked out involves maximally entangling a particle with a quantum memory, meaning all states and all degrees of freedom in the particle would be tied to all of the quantum memory's states. Once they were entangled and separated, an observer would make a measurement of one of the particle's properties, and then tell the keeper of the quantum memory which variable they measured.

In theory, there should be a measurement of the quantum memory that would yield the same result as the measurement done on the particle. The uncertainty relation between the measurement and any other incompatible variables wouldn't be present in the quantum memory, however, allowing observers to see exact measurements for two incompatible variables at the exact same instant in time.

Of course, the operative phrase here is "in theory." The research paper, thought a bit esoteric and lacking in detail, supports its argument with math involving Hilbert systems and entropy. But no experiment has yet taken place because our equipment isn't advanced enough yet.

While quantum computing appears in the news regularly, it's still only taking baby steps. Researchers are currently working with, at most, a handful of qubits at once, so it will be a while until they create a quantum memory that could contain knowledge of all the particle's possible states and variables. Entangling all of this memory with a particle may prove even more difficult, as entangled states are notoriously fragile.

Still, a process that could make Heisenberg's uncertainty principle a distant memory has the potential to shake the core of our particle knowledge. The authors of the paper particularly look forward to using the process to study the nature of entanglement itself, a process we still don't fully understand. Even when the memory and the particle are less than maximally entangled, measuring the two pieces could give scientists some solid numbers on the upper bound of how disordered the states of particles can be.

Nature Physics, 2010. DOI: 10.1038/NPHYS1734 (About DOIs).

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