Australian and U.S. scientists have made a major breakthrough towards the development of the next generation of computation, the very fast quantum computer.
In a world first, they have reported successfully building and testing a C-NOT gate, an essential component to enable quantum computers to work. The gate was built using single particles of light, "photons".
Researchers from The University of Queensland (UQ) and the University of Illinois made the breakthrough, which is published today in prestigious international journal, Nature.
Project team leader Dr Andrew White of UQ's School of Physical Sciences said in the every day world, objects were either here or there. Current computers were based on this premise, with bits that were either on or off.
However, in quantum mechanics things were different.
"Objects can be in two places or 'states' at once," he said. "This is famously illustrated by Schrödinger`s cat, which can be simultaneously alive and dead (and presumably somewhat confused by it all).
"Similarly, quantum bits - qubits - which carry information - can simultaneously be on and off.
(1933 Nobel Laureate and Austrian physicist Erwin Schrödinger is best known for the thought experiment, known as Schrödinger`s cat. He argued that in a quantum system, particles are in an indeterminate state until they are actually observed.)
Dr White said interest in quantum computers had grown since it was recognised that quantum computers could, in principle, solve certain hard problems that are effectively impossible to solve with conventional computers. Quantum computers, by exploiting properties of quantum information, make many attempts to solve hard problems at the same time, speeding up the process.
In theory, quantum computers using qubits could solve problems impossible with conventional computers, such as cracking codes or weather forecasting.
"The critical component necessary for a quantum computer to work is a Controlled-NOT (C-NOT) gate: a gate that lets one qubit control the state of another," he said.
"If one qubit is simultaneously on and off, then both bits can become entangled — that is, correlated in apparently impossible ways. It is entanglement that makes quantum computing so powerful."
In 1988 UQ`s Professor Gerard Milburn published one of the first proposals for quantum computing. In 2001 Professor Milburn, in collaboration with co-workers Dr Manny Knill and Dr Raymond Laflamme, then at Los Alamos National Laboratory, U.S., proposed a scheme to do quantum computing with light using only simple optical devices; so called "linear optics". A large number of groups worldwide have started major research programs to implement the proposal.
Until recently no-one had fully demonstrated a C-NOT gate. However, the researchers from The University of Queensland reported successfully building and fully testing a C-NOT gate.
At the invitation-only annual review of quantum computing by the US Army Research Office (ARO), Dr White presented results from a C-NOT gate made using single photons — the basic particles of light. The ARO sponsors the world's largest and most diverse research program in quantum computing, with over 89 groups in 11 countries working on a range of technologies.
In a world first, the UQ team demonstrated that their gate reliably makes one qubit control another. Using an automated tomography system developed in UQ`s Quantum Technology Lab, Dr White reported that if one qubit is simultaneously on and off, the UQ gate produces highly entangled qubits.
Dr White said the research team was a "lean team" in terms of numbers of researchers, compared to some international groups.
Research team members include Dr White, Dr Jeremy O`Brien, Dr Geoff Pryde, Associate Professor Timothy Ralph (all from The University of Queensland) and Dr David Branning (University of Illinois).
The UQ team next plans to study gate errors. In a normal logic gate this is simple, as there are a small number of possible input states. It is more difficult in a quantum logic gate, as an infinite number of input states exist.
Dr White said the research team was not planning to be in the lab for an infinite amount of time.
"We have a trick up our sleeves," he said. "However, we`ll need a lot of tricks to make a quantum computer work."
The UQ members are a part of the Australian Research Council Centre of Excellence in Quantum Computing Technology. Their work has received US Army Research Office funding of US$0.75M for a three-year program.
Research website: www.quantinfo.org
Media: Further information, Dr Andrew White, telephone 61 7 3365 7902 (after hours telephone 61 7 3720 8584) email: andrew.white@uq.edu.au or Jan King at UQ Communications 0413 601 248 or Jan King at UQ Communications 0413 601 248.
