RT Journal Article SR Electronic T1 A quantum Fredkin gate JF Science Advances JO Sci Adv FD American Association for the Advancement of Science SP e1501531 DO 10.1126/sciadv.1501531 VO 2 IS 3 A1 Patel, Raj B. A1 Ho, Joseph A1 Ferreyrol, Franck A1 Ralph, Timothy C. A1 Pryde, Geoff J. YR 2016 UL http://advances.sciencemag.org/content/2/3/e1501531.abstract AB Minimizing the resources required to build logic gates into useful processing circuits is key to realizing quantum computers. Although the salient features of a quantum computer have been shown in proof-of-principle experiments, difficulties in scaling quantum systems have made more complex operations intractable. This is exemplified in the classical Fredkin (controlled-SWAP) gate for which, despite theoretical proposals, no quantum analog has been realized. By adding control to the SWAP unitary, we use photonic qubit logic to demonstrate the first quantum Fredkin gate, which promises many applications in quantum information and measurement. We implement example algorithms and generate the highest-fidelity three-photon Greenberger-Horne-Zeilinger states to date. The technique we use allows one to add a control operation to a black-box unitary, something that is impossible in the standard circuit model. Our experiment represents the first use of this technique to control a two-qubit operation and paves the way for larger controlled circuits to be realized efficiently.