报告题目：A quantum Fredkin gate报告人：Dr. Raj Patel(Centre for Quantum Dynamics, Griffith University)时间：5月11日下午3：00摘要： One of the greatest challenges in modern science is the realization of full-scaled quantum computers which, for certain problems, provide up to an exponential speedup over current technology. These computers are constructed from quantum logic gates which provide a means for manipulating quantum states and harnessing phenomena such as superposition and entanglement to process information. While the salient features of a quantum computer have been shown in proof-of-principle experiments, difficulties in scaling quantum systems to encode multiple quantum bits or ‘qubits’ have made demonstrations of more complex operations intractable. This is exemplified in the classical Fredkin (or controlled-swap) gate for which, despite many theoretical proposals, a truly quantum analogue has yet to be realized. Here by exploiting additional degrees of freedom of the information carrier we demonstrate the first quantum Fredkin gate. We show that our scheme improves the overall probability of success by almost two orders of magnitude compared to previous proposals. In addition, our gate is capable of generating genuine tripartite entanglement in the form of GHZ states with fidelities up to 93% which are the highest reported to date for photonic qubits. Finally we demonstrate the use of our gate in performing direct characterisation of a quantum state without recourse to quantum state tomography. We believe our approach should prove valuable in realising current and future linear optical quantum information protocols.