New Computer Bridges Classical and Quantum Computing

The logo of the Quantum Technology Lab spelt out with the laser beams used in the Boson Sampling device.

A new type of machine could rival quantum computers in exceeding the power of classical computers, researchers say.

Quantum computers rely on the bizarre properties of atoms and the other construction blocks of the universe. The world is a fuzzy place at its very smallest levels — in this realm where quantum physics dominates, things can seemingly exist in two places at once or spin in opposite directions at the same time.

The new computers rely on "boson" particles, and resemble quantum computers, which differ from traditional computers in important ways. Normal computers represent data as ones and zeroes, binary digits known as bits that are expressed by flicking switch-like transistors on or off. Quantum computers, however, use quantum bits, or qubits (pronouced "cue-bits"), that can be on and off at the same time, a state known as "superposition."

This allows the machines to carry out two calculations simultaneously. Quantum physics permits such behavior because it allows for particles that can exist in two places at once or spin in opposite directions at the same time.

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In principle, quantum computers could solve certain problems much faster than can classical computers, because the quantum machines could run through every possible combination at once. A quantum computer with 300 qubits could run more calculations in an instant than there are atoms in the universe.

However, keeping qubits in superposition is challenging, and the problem grows more difficult as more qubits are involved. As such, building quantum computers that are more powerful than classical computers has proven very difficult.

Now, though, two independent teams of scientists have built a novel kind of device known as a boson-sampling computer. Described as a bridge between classical and quantum computers, these machines also make use of the bizarre nature of quantum physics. Although boson-sampling computers theoretically offer less power than quantum computers are capable of producing, the machines should still, in principle, out-perform classical computers in certain problems.

In addition, a boson-sampling computer does not require qubits. As such, "it's technologically far simpler to create than building a full-scale quantum computer," said researcher Matthew Broome, a quantum physicist at the University of Queensland in Australia.

Boson-sampling computers are actually a specialized kind of quantum computer (which is known more formally as a universal quantum computer).

"The main difference between boson-sampling computers and universal quantum computers is that boson-sampling quantum computers can't solve a universal set of problems like universal quantum computers can," Broome said. "But they are still conjectured to be able to solve problems that would be massively intractable for classical computers. Boson sampling computers are an intermediate model of a quantum computer."

Boson-sampling computers are not based on qubits, but on particles called bosons. "In our case, we use photons," said researcher Ian Walmsley, a quantum physicist at the University of Oxford in England. Photons are the packets of energy that make up light, and are one type of boson.

Broome and Walmsley were in separate groups that each devised a boson-sampling computer, based off concepts first described by theoretical computer scientist Scott Aaronson at MIT. The computers involve multiple devices that can each generate single photons. The photons are inserted into a network where they can interact with one another. They emerge from outputs equipped with sensors to analyze the particles.

The task of calculating which outputs these photons will emerge from, an operation known as boson sampling, grows well beyond the capabilities of classical computers the more photons are involved. The new computers accurately resolved what paths the photons would take — three photons with Broome and his colleagues' machine and four in Walmsley and his collaborators' device.

Since boson-sampling computing is in its infancy, it remains uncertain whether these computers can solve problems beyond boson sampling. Still, this research suggests that computers based on quantum physics could indeed tackle problems beyond the reach of classical computers.

Previously, there was nothing to say "that anything you can do on a quantum computer you can't do on a normal computer, which leaves in question the necessity for quantum computers," Broome said. "Now, with boson sampling, we're coming up with machines based on quantum physics that can attack problems strongly believed to be intractable for classical computers."

In the future, "it would be great to push these computers toward more photons to tackle problems that would be challenging to simulate on normal computers," study coauthor Walmsley added. Using about 20 to 30 photons would be a problem beyond the capabilities of classical computers.

Both research teams detailed their findings online Dec. 20 in the journal Science.

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