New demonstrations show it is possible to recycle the photons inside a quantum computer so that quantum factoring can be achieved using only one-third of the particles originally required.

University of Bristol scientists recycled one of the photons in a quantum optical circuit and achieved a record by factoring 21 with a quantum algorithm – previous demonstrations have factored 15.

“Quantum computers promise to harness the counterintuitive laws of quantum mechanics to perform calculations that are forever out of reach of conventional classical computers,” said Dr. Anthony Laing, who led the study, which appeared in Nature Photonics (doi: 10.1038/nphoton.2012.259). “Realizing such a device is one of the great technological challenges of the century.”

The full range and capabilities of quantum computers are still under investigation by mathematicians and scientists, but the application currently motivating them is the hard problem of factoring large numbers. The best classical computers can run for the lifetime of the universe, searching for the factors of a large number, yet still be unsuccessful.

Internet cryptographic protocols are based on this exponential overhead in computational time: If a third party wants to hack into your emails, he must solve difficult factoring problems first. Quantum computers, on the other hand, can efficiently factor large numbers, but the physical resources required make such devices difficult to construct.

“While it will clearly be some time before emails can be hacked with a quantum computer, this proof-of-principle experiment paves the way for larger implementations of quantum algorithms by using particle recycling,” said Enrique Martín-López, a doctoral candidate at the university’s Centre for Quantum Photonics.