Physicists at the Oxford Clarendon Laboratory have developed a highly precise quantum ‘Fredkin gate’, a building-block required to make a quantum computer, bringing us closer to making this theoretical super-computer a reality.

A quantum computer is a hypothesised, incredibly powerful machine capable of performing many large calculations simultaneously, in contrast to the desktops we are familiar with which compute far fewer at a time. It would make use of ‘qubits’, the quantum analogue of the digital bits we use today.

In traditional computing, ‘bits’ are units of information which can take the value 0 (‘off’) or 1 (‘on’) and which can be strung together to encode ‘words’. Performing a calculation involves electrical signals passing through ‘logic gates’ which change bits from 1 to 0 and from 0 to 1, acting as ‘on-off’ switches. With a quantum computer, however, the qubits make use of the quantum states of sub-atomic particles to store information. The strange quantum behaviour of such particles means that, as well as taking the value 0 and 1, the qubit can also exist in a third, superposed state – effectively existing as both 0 and 1.

The Oxford researchers have increased the precision of a quantum version of a three-bit logic gate known as a Fredkin gate to 99.9%, exceeding the theoretical threshold required for the manufacture of a quantum computer. The work makes use of a quantum-phenomenon known as ‘entanglement’: if something happens to one of a pair of entangled particles, the other particle is instantaneously and simultaneously affected, no matter how far away it is. Einstein called this intertwining of fates ‘spooky action at a distance’.

There remains much work to be done before quantum computers become a genuine possibility, but the viability of this crucial building-block is an encouraging step.

“To put this in context,”, comments co-author Prof David Lucas, “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.”

Nonetheless this achievement is “another important milestone on the road to developing a quantum computer.”

Research paper: High Fidelity Quantum Logic Gates Using Trapped-Ion Hyperfine Qubits, C. J. Ballance, T. P. Harty, N. M. Linke, M. A. Sepiol, and D. M. Lucas, Phys. Rev. Lett. 117, 060504, 4 August 2016

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