Australian computer scientists have made a major breakthrough in quantum computing research by managing to build a quantum version of a key universal gate in computing.

Researchers from Griffith University and the University of Queensland have managed to build a Fredkin gate for the first time ever that operates on photonic qubits rather than bits, which is a key component needed to make a quantum circuit possible. Until now, it has been so complex to create such a gate that no one has yet succeeded in building one.

Computers today are coded using traditional bits, which is the small unit of data that usually has a single binary value of 0 or 1. When put together, the bits create code words such as 00, 01, 10 or 11 that can be used to programme the computer to perform specific commands.

However, in quantum computing, bits can be in superposition known as "qubits", so they can have the value of 1 and 0 at the same time, so code words could be much wider, for example, code words like 00+11, 00-11, 01+10 or 01-10.

### A quantum version of the Fredkin gate

The Fredkin gate (also known as a "CSWAP gate") is a reversible controlled swap three-bit gate that swaps three inputs on to three outputs. So if the first of the three bits is 1, then the last two bits are swapped from either 0 to 1 or 1 to 0, but if the first bit is already 0, then the last two bits won't be swapped.

The Fredkin gate typically requires a circuit of five logic operations, and to make the quantum version work, the researchers used the quantum entanglement of light particles to directly implement larger quantum circuits without needing to use small logic gates.

"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 Raj Patel, a research fellow with Griffith University's Centre for Quantum Dynamics.

Being able to build a Fredkin gate means that it will now be possible for quantum computing algorithms such as Shor's factoring algorithm that require controlled swaps to be carried out to factor impossibly large numbers.

### Groundwork laid for secure quantum communications

The quantum Fredkin gate can also be used to perform a direct comparison of two sets of qubits to determine whether they are the same or not, which would be crucial in order to make secure quantum communication protocols work, whereby the computer needs to figure out whether two digital signatures are the same.

This is important as security agencies like the NSA in the US are now worrying that quantum computing will be able to foil the RSA cryptography protecting all data to date, as quantum computers would make it possible to factor huge numbers in a drastically short amount of time.

"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 Geoff Pryde, the project's chief investigator and deputy director of the Centre for Quantum Dynamics at Gryffith University. "This could unleash applications that have so far been out of reach."

The research, entitled "A quantum Fredkin gate" has been published in the journal Science Advances.