How to Build a Quantum Computer
It’s all a matter of scale. When most of us flip open our laptops, we don’t consider bits and bytes. A sunny desktop wallpaper display means we’re booted up.
Most don’t break it all down to 0s and 1s.
Physics professor Nathan Harshman is far more thoughtful about scale. He remembers at age five, first seeing Carl Sagan’s Cosmos on television. He learned about stars, the staggering expanse of the universe.
“I think it made me feel wonder . . . ecstatic wonder,” he recalls. “From then on, I said, ‘I want to be a physicist.’”
While the magnitude of astronomy first caught Harshman’s attention, it’s the universe’s smallest components that are central to his life’s work. With a notebook, a computer, chalk, and chalkboard, he studies the mathematics of quantum mechanics. In proofs that run like an alphabet soup of variables, he tests probabilities and describes the building blocks of our universe.
Harshman’s particular interest is in quantum information technology (QIT). If traditional IT was invented to understand how to digitize information and run data along transistors processing a binary code of 0s and 1s, QIT is a study of how atoms and photos can be used to store and process information.
To imagine what a quantum computer might be like, consider that instead of having bits in your computer acting as either a 0 or a 1, there might be a system of particles. The particles could be 0s or 1s. They might be 0s AND 1s simultaneously and only collapse into a 0 or 1 when observed or measured. (They might also collapse into a 0 or 1 if the particles were inadvertently bumped into — a trouble that makes construction of a quantum computer tricky, to say the least.)
Quantum computers would be ideal for solving certain quantum problems: folding proteins for drugs or efficiently factoring prime numbers. In order to build a quantum computer — and there are prototypes — one would need to know how to predict and influence the movement of subatomic particles. Armchair physicists will remember from their college science classes that doing that is no easy task.
One resource for building a quantum computer might depend upon entangled particles. Albert Einstein, and fellow physicists Boris Podolsky and Nathan Rosen, hypothesized in 1935 that if quantum mechanics was right, there would be something called entangled particles that, once separated, mirror each other’s properties no matter how far apart they are pulled. Einstein called it “spooky action at a distance,” and — to put this in simplest terms — that would be just crazy. He reasoned that because quantum mechanics implied entanglement, quantum mechanics couldn’t be right.
It seems Einstein was wrong on this count. Fast-forward and decades later, Harshman, a student of quantum mechanics, has been tracking the behavior of entangled particles. The controversy surrounding entanglement makes sense. It’s hard to believe anything in our physical universe could behave like these particles.
Imagine Alice and Bob are scientists working with a pair of entangled particles. Alice and Bob separate their particles, and Alice names her particle A. Bob calls his B. They each take their particle and move to labs a kilometer apart. The space between the two particles doesn’t matter; they are connected on a quantum level.
Alice measures A, and instantly B is changed. If Bob had measured B first, A would have changed. Or, instead imagine Alice entangles particle A with a new particle C, then Alice measures A. Instantly, B would take on the properties of C. That is, when Bob measures his particle, he finds C — all with Alice a kilometer across town.
This is the process that makes quantum teleportation possible. Yes, teleportation.
The state of C transfers from A to B like the character Sam Beckett, in the old television show Quantum Leap, moved from body to body. Like in the television series, a core bit of information, here the character Sam’s identity, switched from one person’s body to another. Also like in the TV series, Sam (the state of C) can’t go back to a prior body (or particle). The entanglement, at this point, has been broken.
Using the same process, Alice could have sent other bits of information from A to B, or Bob from B to A. Qubits (the quantum equivalent of a bit) can transfer from A to B without actually traveling through space. Instead of 0s and 1s, qubits might transmit A, B, C, D, E, etc.
This sort of thing is not theoretical, but has been replicated through experiments since the 1990s, and holds a great deal of promise for quantum encryption — code passing between A and B. To apply the use of entangled particles in something like a quantum computer, or a quantum network, there’s likely more than a decade’s work ahead for theoretical physicists.
A Recipe for Entangled Particles
Recently, Harshman has been working to understand the best way to isolate packets thick with these entangled particles. One might call it the quantum equivalent of packing a computer processor with the most memory. You can do more with more.
In a proof that will appear in Physical Review A, Harshman and Kendar Ranade — member at the Institute of Quantum Physics at the University of Ulm where Harshman recently conducted research during a sabbatical — give a usable procedure for experimentalists wanting to control the entanglement in their work.
In a given system of particles, there are various ways of breaking the whole into pieces. Some pieces will be very entangled, some not so much. It all depends on which way you slice it. Harshman showed that for any system, there is a way to cut it to maximize entanglement or select pieces with no entanglement.
As Harshman explains, “We wrote a recipe for splitting systems into smaller systems for any kind of entanglement you want.”
The recipe brings those hoping to utilize entangled particles one step closer to application. Tailored Observables Theorem, Harshman and Ranade’s proof, refines a delicate process, giving more entanglement bang for your buck.
It’s a step along a long thread of discovery.
As Harshman puts it, “When we think of geometry, we think of points, lines, and angles. The first person who studied shapes had to come up with all those definitions. I’m still just defining all the important features if you’re looking at entanglement in particular systems . . . the fundamental conceptual units that the rest of the system can be built out of.”
It’s a matter of scale. Science and mathematics, much like the rest of the universe, is a construct of smaller parts. Discovery is a sweeping term for a broad effort of experiment, testing, and realization. Harshman’s recipe, for now, has application to a very specialized realm of science, but one that is developing and deepening, and one that could revolutionize our lives. It shows logic in systems that appear strange, spooky, or illogical. And there’s quite a bit of wonder to be had in that.