Physicists Build First Single-Photon Logic Gate

Bringing quantum computing based on manipulating photons a step closer to reality, physicists at Bristol University in the United Kingdom have demonstrated the first logic gates on a silicon chip that can process individual photons.

Led by professor Jeremy O'Brien, the researchers have successfully miniaturized a high-performance, optical "controlled-NOT gate" -- a type of logic gate considered "the building block of a quantum computer," according to a press release from Bristol.

Using silica wave-guides to measure the states of pairs of photons, each encoding a quantum bit or "qubit" of data, the new chip "is a crucial step towards a future optical quantum computer, as well as other quantum technologies based on photons," O'Brien said in a statement. Previous versions of the gate occupied several square meters of space on an optical bench.

"For those who believe that quantum computing is the next big breakthrough in the computing world, and who see the logic gate as a critical component, this is a critical step forward," wrote technology pundit Mark Anderson in his influential "Strategic News Service" newsletter this week.

Exploiting the unique and often bizarre properties of very small particles under the theories of quantum mechanics, quantum computers will rely on the fact that photons and other extremely tiny particles can inhabit two states at the same time. While single photons can be manipulated easily and extremely rapidly, researchers worked for years to get them to interact -- a crucial achievement in creating workable logic gates.

Previous quantum optical circuits, however, have been built on large optical elements, which rely on photons propagating in air -- making them "hard to build and difficult to scale up," noted Alberto Politi, a graduate student at Bristol under O'Brien.

"For the last several years," said O'Brien, "the Centre for Quantum Photonics has been working towards building controlled-NOT gates and other important quantum circuits on a chip to solve these problems."

In addition to getting the device down to a workable size, the researchers also achieved one of the most bizarre phenomena of quantum physics, known as "quantum entanglement," on the chip. Quantum entanglement occurs when two particles interact in such a way that the state of either individual particle cannot be defined, but their collective state can.

While the new optical chip represents an important breakthrough, it's still rough. O'Brien told Physicsworld.com that his chip has an error rate of nearly 90%. Future work will focus on verification schemes to bring that error rate down, and on ways to integrate photon sources and detectors on-chip.