University of Bristol researchers, in collaboration with colleagues at the Universities of Osaka and Hokkaido in Japan, have demonstrated a quantum logic gate - a controlled-NOT or CNOT gate - that was first proposed a decade ago.
Back in 2001, this four-photon design opened up the possibility of optical quantum computing. But, the circuit design has to be stable to within a fraction of the wavelength of light, and the difficulties of achieving this in the real world have meant that it has stayed theoretical until now.
Professor Jeremy O’Brien, director of Bristol’s Centre for Quantum Photonics, is quoted in the University’s announcement: "The ability to implement such a logic gate on photons is critical for building up larger scale circuits and even algorithms.
"Using an integrated optics on a chip approach that we have pioneered here at Bristol over the last several years will enable this to proceed far more rapidly, paving the way to quantum technologies that will help us understand the most complex scientific problems."
He says that the focus for further research will be on adding more photons to the circuit. Each additional photon will exponentially increase the processing power of a quantum system, because of the way the photons in their role as qubits, interact. If a one photon system has 10 possible outcomes, a two photon system has 100, three will get you 1,000 and so on.
Initially, the circuits will be used for measurements and simulations in the lab. But longer term, systems based on these chips might be able to shed light on natural processes, such as superconductivity and photosynthesis, that are controlled by quantum mechanics. Much further ahead, they could even be the building blocks of large scale quantum computers.
The work is published here in the Proceedings of the National Academy of Sciences, PNAS.