Factoring Humanity

The departmental assistant stuck his head in. “Sorry to bother you, Professor Davis, but the CBC is on the phone. They want to talk to you about what happened to the aliens.”

10

Kyle’s lab was crowded. The dean leaned against one wall, the department chair had his butt perched on the shelf jutting out of the bottom of Cheetah’s console, a lawyer from the university’s patent unit sat in Kyle’s usual chair, and the five grad students who worked on Kyle’s quantum-computing team were milling around as well.

“Okay,” said Kyle to the group. “As you know, there’s been a technique available since nineteen ninety-six for producing simple quantum-logic gates; that technique was based on using nuclear magnetic resonance to measure atomic spins. But it was hampered by the fact that as you added bits, the output signal got exponentially weaker: a thirty-bit quantum computer based on that principle produces output only one-billionth as strong as that from a one-bit computer based on the same technique.

“Well, the method we’re going to demonstrate today is, we believe, the long-sought-after breakthrough: a quantum computer that, in theory, can employ an unlimited number of bits with no reduction in output quality. For our demo today, we’re going to try to factor a randomly generated three-hundred-digit number. To do that on the department’s ECB-5000 would take approximately one hundred years of constant calculation. If we’re right — if this works — we’ll have an answer about thirty seconds after I commence the experiment.”

He moved across the room.

“Our prototype quantum computer, which we call Democritus, has not just thirty registers, but one thousand, each of which consists of a single atom. The results will be a series of interference patterns, which another computer — that one over there — will analyze and reduce to a numeric readout.” He looked from face to face. “All set? Let’s go.”

Kyle walked over to the simple black console containing the Democritus computer. For the sake of drama, they’d built a large knife switch, worthy of Frankenstein’s lab, into the side of the cabinet. Kyle pulled it down, its blade touching the metal contacts. A bright red LED came on and -

— and everyone held their breath. Kyle kept watching Democritus, which, of course, was operating absolutely silently. Part of him missed the old days of clicking relays. Others were watching the digital clock mounted next to the red EXIT sign on the curving wall.

Ten seconds went by.

Then ten more.

Then a final ten.

And then the LED went dark.

Kyle let out his breath.

“Done,” he said, heart pounding.

He gestured for everyone to follow him across the room. There, another computer was analyzing the output from Democritus.

“It’ll take about five minutes to decode the interference pattern,” said Kyle. He allowed himself a smile. “If you’re thinking that that’s a lot longer than it took to produce the pattern, you’re right — but we’re now dealing with a conventional computer.”

“How many computations would it take to factor a number that big?” asked the dean, her voice clearly intrigued.

“Approximately ten to the five hundredth,” said Kyle.

“And there’s no way to do it in fewer steps?” she asked. “This isn’t a case of Democritus taking a shortcut?”

Kyle shook his head. “No, it really does take ten to the five hundredth steps to factor a number that big.”

“But Democritus didn’t do that many steps.”

“This Democritus didn’t — in fact, it performed only one calculation, using a thousand atoms as the stones in its abacus, so to speak, to do so. But if all went well, 10⁵⁰⁰ other Democrituses in other universes will also each have done one calculation — involving, of course, a total of a thousand times 10⁵⁰⁰ atoms, which is 10⁵⁰³ atoms. And that, my friends, is a very significant number.”

“How so?” asked the department chair.

“Well, the precise value isn’t important. What is important is how it relates to the number of atoms in our entire universe.” Kyle smiled, waiting for the inevitable question.

“And how many atoms are there in our universe?” asked the dean.

“I called up Holtz over in the McLennan Physical Labs and asked her,” said Kyle. “The answer, plus or minus a couple of orders of magnitude, is that there are ten to the eightieth atoms in the universe.”

A few jaws dropped.

“Do you see?” said Kyle. “In that thirty-second period, to factor our test number, Democritus must have accessed many trillions of times more atoms than there are in our entire universe. Other, earlier quantum-computing demonstrations have never involved enough bits to actually exceed the quantity of atoms available to them in our universe, leaving open some doubt as to whether they’d actually accessed parallel worlds, but if this experiment works, the only answer will be that our Democritus worked in tandem with computers in other universes.”