A breakthrough by University of Sydney physicists could quadruple the error margin quantum computers can tolerate without becoming dysfunctional. The researchers achieved the feat by “hacking” the software used to remove spontaneous errors in quantum computing hardware.

While quantum pro­cessors promise to solve problems far beyond the capacity of today’s most powerful computers, there are significant technical hurdles before we can reach a general purpose quantum computer.

The Challenge of Maintaining Coherence

One of those technical hurdles is that the building blocks of quantum computers – quantum bits, or qubits – are extremely fragile and are prone interference from tiny electrical and thermal fluctuations and even wayward light particles. This means that qubits can only remain in a quantum state of coherence for a tiny amount of time, and are hard to read without interfering with the results.

IBM quantum scientist, Jerry Chow explained the challenge challenge. “One of the main difficulties of making a quantum computer is keeping quantum information alive. Trying to preserve that quantum information right now is like doing something impossible”.

“Like trying to balance an egg on the tip of a pencil”

“How you accurately keep it in its place and preserve it is one of the aspects we work on in terms of improving the technology to make this happen,” he explained.

Microsoft recently revealed their attempts to answer this problem with a type of qubit that fragments electrons – so that the same piece of information is held in various places at the same time and the information contained is not lost. The news today from the University of Sydney suggests that there are multiple approaches to this and there will soon be a breaking point.

Study leader: David Tuckett, centre, with team members Stephen Bartlett, left, and Steven Flammia. Picture: Louise Cooper

Quantum ‘Hack’

The theoretical breakthrough from the team of David TuckettProfessor Stephen Bartlett and Associate Professor Steven Flammia allows for a 400 percent gain in the amount of interference noise a quantum computing system can theoretically sustain while retaining its fidelity.

“This is achieved by tailoring our quantum decoder to match the properties of the noise experienced by the qubits,” claims Associate Professor Flammia.

“We are hacking the generally accepted coding for error correction,” revealed co-author Stephen Bartlett.

The research is published this week in the top-tier journal Physical Review Letters. It forms part of Mr Tuckett’s work as a PhD candidate at the University.

At present the rule-of-thumb threshold for fidelity in a qubit architecture is about 1 percent. This means at least 99 percent of a system’s qubits need to retain information and coherence for relevant periods of time in order to do any useful computations. This real-world threshold of 1 percent comes from a theoretical approach where ideal hardware should allow for 10.9 percent error threshold. The drop in tolerance comes from ‘noise’ in using real-world machines.

Fewer Qubits Required

Assuming ideal hardware, the work of the Sydney quantum team, which is based at the University of Sydney Nano Institute, has an error correction threshold of up to 43.7% – a fourfold improvement on the current theoretical basis for error correction. This means fewer physical qubits could be required for a single quantum logic gate – or basic quantum circuit – that can perform a useful calculation.

This new approach should be applicable in any quantum system – whether the qubits rely on superconductors, trapped ions, semiconductors, or topological structures (should they need them).

Experimental scientists now need to apply this ‘quantum hack’ to real-world systems to see how it flows through using ‘noisy’ hardware.

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Reference: EurekAlert