A three-decades-old formula for phase shifting has been used to accurately control quantum interference between atoms, paving the way for fault-tolerant circuits in photonic quantum simulators. Photonic quantum simulators, systems of photons designed to simulate other quantum systems, are set to be physically realized much sooner than universal quantum computers. The geometric phase effect, known as Berry’s phase, was first described by professor Sir Michael Berry of the University of Bristol in 1984. He described the phenomenon as a quantum particle that returns to its start point after a cyclic journey and is found to be subtly changed, or phase-shifted. Berry’s way of phase shifting is reliable for robust methods needed to implement the circuits of a universal quantum computer. In 1987, Hong Ou and Mandel (HOM) demonstrated how two photons interfere in a very peculiar way, preferring to stick together. The HOM quantum interference effect is at the core of anticipated photonic quantum simulators that, by their very nature, are impossible to run on a conventional computer. Now, scientists at the university’s Centre for Quantum Photonics have brought these two phenomena together to show how HOM interference can be exquisitely controlled with Berry’s phase. The study appeared in Physical Review Letters. For more information, visit: http://www.bristol.ac.uk/news/2012/8617.html