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U of T scientists make quantum computing leap

Our physicists are part of an international team that has made the first execution of a quantum calculation, a major step towards building the first quantum computers.

“What is difficult for the brain is also difficult for conventional computers,” says Professor Daniel James, the Canadian lead on the project. “Quantum calculations are important: it is the computational difficulty of factoring very large numbers that forms the basis of security in such things as widely-used internet encryption systems.”

By manipulating quantum mechanically entangled photons – the fundamental particles of light – the team was able to calculate the prime factors of the number 15: 3 and 5. “Of course, this is a simple enough calculation for an eight-year old: the difficult part is doing these calculations as the number gets bigger and much more difficult,” explains Professor James.

But calculating the prime factors of 15 is a major step to calculating much larger numbers, which could be used to crack cryptographic codes that are unbreakable using conventional computers. These codes form the basis of all banking and computer security and have implications of how successfully we can keep all data secure in the future, says James.

Classical computers use two-level systems called bits -- binary digits -- while quantum computers use two-level quantum-mechanical systems called qubits -- quantum bits. “A qubit is like a coin that can be heads (on), tails (off) or simultaneously heads and tails (on and off) or any possible combination in-between, James explains. “This is impossible with normal bits but one qubit can be in two possible states, two qubits can be in four; three qubits in eight, and so on: quantum memory sizes grow exponentially with the number of qubits.

“Functional large-scale quantum computers may be as many years away, and it is hard to know how exactly they will change the world – but change our world they will.”

The research was published in the prestigious Physical Review Letters in December 2007.