The rules of the quantum world mean that we can process information much faster than is possible using the computers we use now. This column from Quanta Magazine delves into the fundamental physics behind quantum computing.
** QUANTUM SHORTS 2023/2024: RUNNER UP
"You let another photon get away, Hal," a voice sounded, in a light and gentle tone that might have surprised some people had they heard it, which of course they couldn't, because it was not actually a sound, just more of a concept, the idea of a sound, and the speaker was not a person in the sense that those who might have heard the words would have called conventional.
Hal was grumpy that day, anchoring himself in the universe he was observing, with its unique definitions of time and space, and populated in one small corner by creatures that Hal found particularly endearing, and that exhibited an enormous array of traits reminding Hal of himself, including, but by no means limited to emotions, like grumpiness, and gender, like himness. Otherwise unremarkable in almost every sense with respect to the entirety of his universe, this region contained an amalgamation of rocks and water and other substances that gave rise, before Hal could realize what had happened, to self-replicating units. Over time, those self-replicating units evolved, learned how to grow their own food, built things like pyramids, and acquired the myriad Hal-like traits that captivated him, not the least of which was curiosity, which led them to probe deeper and deeper into the nature of reality. Inevitably, they found it difficult to explain the behavior of individual tiny units, like photons, and they realized that their own act of observing influenced the very nature of a photon. Hal's mentor, the sounder of the gentle voice, had never seen this happen before and was troubled.
"It's one of the minis," Hal muttered.
"Your minis are doing their own observing?" the voice asked, incredulously. "How can they do that?"
Hal's mentor had a point. Hal was his universe's Observer. Like one of his mini-minis with a lump of clay, Hal had been given a featureless universe, a solitary cosmic quantum object, and every feature of that universe owed itself to Hal, more precisely to Hal's observation, since as Hal's minis had discovered, with more or less scientific verisimilitude, a quantum object takes on a particular form, collapses, decoheres (Hal was himself fascinated by the range of words his minis used when trying to describe things) only when observed.
But Hal was the observer. There weren't supposed to be any quantum functions left uncollapsed, no individual photons for his minis to encounter and to wonder, for example, whether they were waves or particles. Recently, though, Hal had gotten a bit lazy. After billions of years as measured inside his universe, and out of billions of billions of billions of billions ad infinitum of infinitesimal quantum objects automatically and routinely observed, a photon here and there got away. At first it was just one or two, and then Hal thought, why bother if a handful of particles went astray, unobserved by him. His universe just became a little leaky, that's all.
Until the minis started becoming their own observers. At first Hal had been intrigued by his minis' experiments, but then, like a parent coming home to discover his teenagers had been drinking, he started to realize his minis were understanding, and by then the cat was out of the bag. One day a mini got the idea that it wasn't possible to know with certainty the properties of a particular photon until it was observed. Only later did more minis start to wonder why, if this were true, did anything at all appear to be real to them. Who was doing the observing that yielded trees and skies and volcanoes and giant squids? Hal was doing the observing, that was the answer, but if Hal was doing the observing, and if he was thorough in his role of observing absolutely everything, then where did these unobserved photons come from?
That's what Hal's mentor wanted to know. Hal had a lot of freedom to observe and occasionally even will something to be (with a lot of effort), but a universe couldn't exactly be allowed to have some of its bits go unobserved, not when there were little pockets of decoherence inside such a universe that were starting to do their own observing. Hal knew this and had contemplated what might be done about it, but increasingly found himself entangled with the lives of his minis, even feeling love for them, which complicated an otherwise routine revision. At times he had had occasion to take willful corrective action to adjust his universe, but he never felt good about it, and when a correction involved his minis it was especially painful.
That was probably why Hal was feeling so grumpy that day. His mentor and the Council of Universal Overseers had made it clear to Hal that the emergence of observers from among the observed would not be tolerated. He had to put a stop to it somehow. "Nobody said this was going to be easy," his mentor acknowledged, referring, Hal wasn't sure, to the task at hand or to the whole business of universe observing. Either way it was small comfort to Hal. He knew what was coming.
"But they don't deserve to..." Hal began, and his mentor interrupted.
"They aren't supposed to deserve at all. They wink in and out of existence at your whims, Hal."
Hal sighed.
"You may choose the method."
Hal considered for a few moments, which, had his minis been following his deliberations, they would have thought was a period of decades, as they fought wars, contemplated the source of their existence, made and re-made movies, died in heat waves and built quantum machines whose outputs they could predict, but whose inputs they still didn't fully understand.
And then the waters began to rise.