Just when you thought it was safe to assume quantum computing – though distant – would eventually succumb to clever technology, another potentially confounding factor pops up. It’s the Heisenberg Limit (HL), close relative of the Heisenberg Uncertainty Principle, whose value may be three times greater than thought. Broadly, the HL is the uncertainty limit in measuring some quantum quantities; hence smaller (less uncertainty) is better.

Many quantum quantities, including for example gates in a quantum computer, are constrained by this fundamental limit on our ability to measure them. It turns out, reports physicist Wojciech Górecki of the University of Warsaw, Poland, that applying a branch of math known as Bayesian statistics to familiar quantum measurement problems produces this new higher value for HL. This week IEEE Spectrum posted a brief and fascinating article by Mark Anderson summarizing the finding.

Here’s a quick excerpt from the IEEE Spectrum article:

“Górecki notes that canonical Heisenberg isn’t as much help here as is a related concept called the “Heisenberg limit.” The Heisenberg Limit, he says, delineates the smallest possible uncertainty in a measurement, given a set number of times a system is probed. “It is a natural consequence of Heisenberg’s uncertainty principle, interpreted in a slightly broader context,” says Górecki.

“It was long believed that, with a hypothetical technology trying to discover phase as precisely as possible using only n photons, the Heisenberg Limit to the uncertainty in phase was 1/n. But no technology had been devised to prove that 1/n was the ultimate universal “Heisenberg Limit.”

“There’s a good reason why. Górecki and colleagues report in a new paper in the journal Physical Review Letters that the Heisenberg Limit in this case scales as π/n instead of 1/n. In other words, the smallest measurable uncertainty is more than three times as much as previously believed. And so now we know that our observations of the universe are a little bit fuzzier than we imagined.

“To be clear, “n” here is not necessarily just the number of photons used in a measurement. It could also represent a number of other limits on the amount of resources expended in making a precision observation. The variable “n” here could also be, Górecki notes, the number of quantum gates in a measurement or the total time spent interrogating the system.”

As noted in the IEEE article, many quantum scale measurements involve neither position nor momentum: “For instance, some photonics instruments measure quantities like the phase of a wavefront versus the number of photons counted in a given energy range.” It is natural to wonder what the implications are for quantum computing.

Link to IEEE Spectrum article (New Math Makes Scientists More Certain About Quantum Uncertainties): https://spectrum.ieee.org/nanoclast/semiconductors/nanotechnology/new-math-makes-scientists-more-certain-about-quantum-uncertainties