“Well, remember our example of Kyle coming to work. In one universe, he walks around the east side of Queen’s Park; in the other, he walks around the west side. Now, Kyle, suppose your boss had asked you to solve two problems before you came into work, and—having never overcome your student ways—you’ve left them both to the last moment. There’s time to puzzle out the answer to just one of them in your head as you walk to work. Let’s say that if you went down the west side, you’d spend your time solving problem A, and if you went down the east side, you’d spend your time solving problem B. Is there any way without slowing down or taking the journey around the Parliament Buildings twice that by the time you got to work, you’d have the answers to both problems?”
Kyle was sure he’d had a blank expression.
“Anyone?” asked Papineau, bushy eyebrows raised.
“I’m surprised you think Graves would come up with even one answer,” said D’Annunzio.
Snickers from several students. Papineau smiled.
“Well, there is a way,” said the professor. “You know the old saying, ‘Two heads are better than one’? Well, if our Kyle—the one from this universe who went down the west side and who solved problem A—could join back up with the other Kyle—the one from the parallel universe who went down the east side and solved problem B—then he’d have both answers.”
A hand went up.
“Glenda?”
“But when talking about the photon and the slits, you said the only way the two universes could rejoin is if there was no way to tell which slit the photon had taken in each universe.”
“Exactly. But if we could devise a method by which it made no difference whatsoever which way Kyle went in this universe—indeed, a method by which Kyle himself didn’t know which way he had gone, and no one saw him during his journey—then, at the end of it all, the two universes might stitch back together. But in the rejoined universe, Kyle would know the answer to both problems, even though he’d really only had time to solve one of them.”
Papineau grinned at the class.
“Welcome,” he said, “to the world of quantum computing.” He paused. “Of course, there were really more than two possible universes for Kyle—he could have stayed home, he could have driven to work, he could have taken a cab. Likewise, it’s possible to envision the lightbulb experiment with dozens or even hundreds of slits. Well, suppose each of the photons coming off the lightbulb represented a single bit of information. Remember, all computing is done with glorified abacuses; we actually move things around in order to compute, whether it’s pebbles or atoms or electrons or photons. But if each of those things could simultaneously be in multiple places at once, across parallel universes, extraordinarily complex computing problems could be solved very, very quickly.
“Consider, for instance, the factoring of numbers. How do we do that? Essentially by trial and error, although there are a few tricks that help. If we want to determine the factors of eight, we start dividing numbers into it. We know that one goes evenly into eight—it goes evenly into every whole number. What about two? Yes, it’s a factor: it goes in four times. Three? No—it doesn’t go in evenly. Four? Yes, it goes in twice. That’s how we do it: by brute-force computing, testing every possible factor in turn. But as numbers get bigger, the number of factors they have get bigger. Earlier this year, a network of sixteen hundred computers succeeded in finding all the factors of a 129-digit number—the largest number ever factored. The process took eight months.
“But imagine a quantum computer—one that was in touch with all the possible alternative computers in parallel universes. And imagine a program that factors large numbers by working on all the possible solutions simultaneously. Peter Shor, a mathematician at AT T Bell Laboratories, has worked out a program to do just that; it would try every possible factor of the big number simultaneously by testing just one possible factor in each of many parallel universes. The program outputs its results as interference patterns, sent to a piece of photographic film. Shor’s algorithm would cause those numbers that aren’t factors to cancel out in the interference pattern, leaving darkness. The patterns of light and dark would form a sort of barcode that could be read to indicate which numbers actually are factors of the big number you started with. And since the calculations are performed across parallel universes, in the time it takes for our universe to test any one number, all the other numbers are tested as well, and we have the result. So long as it makes no difference which number our own computer calculated, the result should be achieved almost instantaneously; what normal computers took eight months to do, quantum computers could do in a matter of seconds.”
“But there’s no such thing as a quantum computer,” said Kyle.
Papineau nodded at him. “That’s right, at least not yet. But someday someone is going to build a quantum computer. And then we’ll know for sure.”
6
Kyle and Heather had dinner together every Monday night.
They’d been separated for a year now. It had never been intended to be permanent—and they’d never mentioned the D-word. They’d just needed some time, they both felt, to come to terms with Mary’s death. They’d both been on edge, sniping at each other, little things that shouldn’t have mattered at all escalating into huge fights, unable to console each other, unable to comprehend why it had happened.
They’d never missed a Monday dinner together, and although tensions were high since Becky’s visit four days ago, Kyle assumed that Heather would show up at their usual restaurant, a Swiss Chalet franchise a few blocks from their house.
Kyle stood outside, enjoying the warm evening breeze. He couldn’t bring himself to go in yet; Heather’s car wasn’t in the lot, and if she didn’t show, the embarrassment would be too much.
At about 6:40—ten minutes late—Heather’s powder-blue skimmer floated into the lot.
Still, things were different. For an entire year now, they’d greeted each other on Monday nights with a quick kiss, but this time—this time they both hesitated. They entered the restaurant, Kyle holding the door for Heather.
The server tried to seat them beside another couple, even though there was no one else in the place. Kyle hated that at the best of times, and this evening he did protest. “We’ll sit over there,” he said, pointing to a distant corner.
The server acquiesced, and they were escorted to a booth at the back. Kyle ordered red wine; Heather asked for a glass of the house white.
“I was beginning to think you weren’t coming,” said Kyle.
Heather nodded, but her face was impassive. The lamp hanging above their table made her normally pleasant features look severe. “I’m sorry I was late.”
There was silence for a time.
“I don’t know what we’re going to do about this,” said Kyle.
Heather looked away. “Me neither.”
“I swear to you—”
“Please,” said Heather, cutting him off. “Please.”
Kyle nodded slowly. He was quiet for a moment longer, then: “I went to see Zack on Saturday.”
Heather looked apprehensive. “And?”
“And nothing. I didn’t get into a fight with him, I mean. We talked a bit. I wanted him to agree to come to the forensics lab at the university. I was going to take a lie-detector test, prove that I didn’t do it.”
“And?” said Heather again.
“He refused.” Kyle lowered his eyes, looking at the paperite place mat with the current month’s chicken promotion illustrated on it. He looked up again and sought Heather’s eyes. “I could do the same thing for you,” he said. “I could prove my innocence.”