A fundamental building block for quantum computing that could soon be employed in a range of quantum technologies has been demonstrated. This quantum logic gate, acting on four photons, could lead to secure communications, precision measurement and ultimately a quantum computer — a powerful type of computer that uses quantum bits (qubits) rather than the conventional bits used in today's computers. Unlike conventional bits or transistors, which can be in one of only two states at any one time (1 or 0), a qubit can be in several states at the same time and can therefore be used to hold and process a much larger amount of information at a greater rate. "We have realized a fundamental element for processing quantum information — a controlled-NOT or CNOT gate — based on a recipe that was theoretically proposed 10 years ago," said Jeremy O'Brien, professor and director of the University of Bristol’s Centre for Quantum Photonics, who worked closely with colleagues at the universities of Osaka and Hokkaido in Japan. "The reason it has taken so long to achieve this milestone is that, even for such a relatively simple circuit, we require complete control over four single photons whizzing around at the speed of light." The KLM CNOT gate. (A) The gate is constructed of two NS gates; the output is accepted only if the correct heralding signal is observed for each NS gate. Gray indicates the surface of the BS from which a sign change occurs upon reflection. (B) The KLM CNOT gate with simplified NS gate. (C) The same circuit as (B) but using polarization encoding and PPBSs. (D) The stable optical quantum circuit used here to implement the KLM CNOT gate using PPBSs and a displaced-Sagnac architecture. The target MZ, formed by BS11 and BS12 in Fig. 2B, can be conveniently incorporated into the state preparation and measurement, corresponding to a change of basis, as described in the caption to Fig. 3. The blue line indicates optical paths for vertically polarized components, and the red line indicates optical paths for horizontally polarized components. (Image: PNAS) The approach taken by O'Brien and his colleagues combined several methods for making optical circuits that must be stable to within a fraction of the wavelength of light, that is, nanometers. In 2001, optical quantum computing became possible when a theoretical recipe for realizing this CNOT gate, as well as the other necessary components, was developed. However, the technological challenges associated with making the optical circuits have prevented its realization until now. The implications for this new approach are far-reaching. "The ability to implement such a logic gate on photons is critical for building up larger-scale circuits and even algorithms," said O'Brien. "Using an integrated-optics-on-a-chip approach that we have pioneered here at Bristol over the last several years will enable this to proceed far more rapidly, paving the way to quantum technologies that will help us understand the most complex scientific problems." In the short term, the team expects to apply its new results immediately for developing approaches to quantum communication and measurement and then for simulation tools in their lab. In the longer term, a small-scale quantum simulator based on a multiphoton optical circuit could be used to recreate processes that themselves are governed by quantum mechanics, such as superconductivity and photosynthesis. "Our technique could improve our understanding of such important processes and help, for example, in the development of more efficient solar cells," O'Brien said. Other applications include developing ultrafast and efficient search engines, and designing high-tech materials and new pharmaceuticals. The leap from using one photon to two photons is not trivial because the two particles must be identical in every way and because of how these particles interfere, or interact, with each other. There is no direct analogue of this interaction outside of quantum physics. "Now that we can implement the fundamental building blocks for quantum circuits, the move to larger-scale devices will become our focus. Because of the increasing complexity, the results will be just as exciting," he said. "Each time we add a photon, the complexity of the problem we are able to investigate increases exponentially, so if a one-photon quantum circuit has 10 outcomes, a two-photon system can give 100 outcomes and a three-photon system 1000 solutions, and so on." The Centre for Quantum Photonics now plans to use its chip-based approach to increase the complexity of its experiment: Not only will the researchers add more photons but they will also use larger circuits. The research is published in Proceedings of the National Academy of Sciences. For more information, visit: www.bristol.ac.uk