Computer scientists have made a breakthrough that brings quantum computers one step closer to reality – they have succeeded building a quantum version of a Fredkin gate which is incredibly accurate.

Researchers from Oxford University have demonstrated that single-qubit and two-qubit logic gates are possible with a record-breaking 99.9% precision, which is the benchmark that is required theoretically in order to make a quantum computer possible.

The research, entitled 'High-Fidelity Quantum Logic Gates Using Trapped-Ion Hyperfine Qubits' is published in the journal Physical Review Letters.

Fredkin gate and quantum entanglement

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.

The Fredkin gate (also known as a 'CSWAP gate') is a reversible controlled swap three-bit gate that switches 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, quantum entanglement of light particles is required. This is a process whereby two particles are able to stay connected and a single action on one particle affects the other, even if the two particles are located far apart from each other.

The future looks bright

This then enables larger quantum circuits to be directly implemented without needing to use small logic gates.

"'Achieving a logic gate with 99.9% precision is another important milestone on the road to developing a quantum computer. A quantum logic gate on its own does not constitute a quantum computer, but you can't build the computer without them," said David Lucas, a professor in Oxford University's Department of Physics and Balliol College and a co-author of the paper.

"The precision of the gate is a measure of how well this works: in our case, 99.9% precision means that, on average, 999 times out of 1,000 we will have generated the entangled state correctly, and one time out of 1,000 something went wrong.

"To put this in context, quantum theory says that – as far as anyone has found so far – you simply can't build a quantum computer at all if the precision drops below about 99%. At the 99.9% level you can build a quantum computer in theory, but in practice it could very difficult and thus enormously expensive. If, in the future, a precision of 99.99% can be attained, the prospects look a lot more favourable."

The method used by the Oxford team was invented at the US National Institute for Standards and Technology (Nist) in Boulder, US, which has reported the achievement in a paper published alongside Oxford's in Physical Review Letters.