Quantum circuits based on optical multimode interference (MMI) are being used to advance quantum computers. Building a quantum computer will require a large number of interconnected components (gates), whose function is similar to that of a microprocessor in a personal computer. Currently, most quantum gates are large structures that are too bulky to be scaled to the large, complex circuits required for practical applications. Representation of a circuit for quantum information processing that makes use of multiport MMI devices. These circuits will be more compact and robust to fabrication tolerances compared to the current 2 x 2 port devices. (Image: Alberto Peruzzo) Researchers from the University of Bristol's Center for Quantum Photonics showed that quantum information can be manipulated with integrated photonic circuits. Such circuits are compact (enabling scalability) and stable (with low noise) and could lead in the near future to mass production of chips for quantum computers. This is a simulation of classical light propagating in a multimode interference device. The multimode propagation results in equal intensity in each of the four output waveguides. (Image: Alberto Peruzzo) Now the team, in collaboration with Dr. Terry Rudolph at Imperial College London, has shown a new class of integrated divides that promise further reduction in the number of components that will be used for building future quantum circuits. These devices, based on optical MMIs, have been widely employed in classical optics, as they are compact and very robust to fabrication tolerances. "While building a complex quantum network requires a large number of basic components, MMIs can often enable the implementation with much fewer resources," said Alberto Peruzzo, a PhD student working on the experiment. Research physicists Alberto Peruzzo (left), Jeremy O'Brien and Alberto Politi in front of the optical table. (Image: Yameng Cao) Until now, it was not clear how these devices would work in the quantum regime. Bristol researchers have demonstrated that MMIs can perform quantum interference at the high fidelity required. Scientists will now be able to implement more compact photonic circuits for quantum computing. MMIs can generate large entangled states, at the heart of the exponential speedup promised by quantum computing. "Applications will range from new circuits for quantum computation to ultraprecise measurement and secure quantum communication," said Jeremy O'Brien, professor and director of the Center for Quantum Photonics. For more information, visit: www.bristol.ac.uk