An artist’s rendering of the quantum Fredkin (controlled-SWAP) gate, powered by entanglement, operating on photonic qubits. Credit: Raj Patel and Geoff Pryde, Center for Quantum Dynamics, Griffith University.
The quantum circuit Fredkin gate has been experimentally realised for the first time. “Similar to building a huge wall out lots of small bricks, large quantum circuits require very many logic gates to function. However, if larger bricks are used the same wall could be built with far fewer bricks,” said Dr Patel. “We demonstrate in our experiment how one can build larger quantum circuits in a more direct way without using small logic gates.”
At present, even small and medium scale quantum computer circuits cannot be produced because of the requirement to integrate so many of these gates into the circuits. One example is the Fredkin (controlled- SWAP) gate. This is a gate where 2 qubits are swapped depending on the value of the third. Usually the Fredkin gate requires implementing a circuit of 5 logic operations. The research team used the quantum entanglement of photons to implement the controlled-SWAP operation directly.
“There are quantum computing algorithms, such as Shor’s algorithm for factorising prime numbers, that require the controlled-SWAP operation. The quantum Fredkin gate can also be used to perform a direct comparison of 2 sets of qubits (quantum bits) to determine whether they are the same or not. This is not only useful in computing but is an essential feature of some secure quantum communication protocols where the goal is to verify that 2 strings, or digital signatures, are the same,” said Professor Tim Ralph from the University of Queensland.
Professor Geoff Pryde, from Griffith’s Centre for Quantum Dynamics, is the project’s chief investigator. “What is exciting about our scheme is that it is not limited to just controlling whether qubits are swapped, but can be applied to a variety of different operations opening up ways to control larger circuits efficiently,” said Professor Pryde. “This could unleash applications that have so far been out of reach.” http://www.eurekalert.org/pub_releases/2016-03/gu-utg032116.php
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