Program developed to detect 'leakage' in quantum computers

Researchers from the University of Warwick have developed a quantum computer program to detect the presence of ‘leakage’, where information being processed by a quantum computer escapes from the states of 0 and 1. Published in the journal Physical Review A, their work will give users the ability to check the reliability of a quantum computer without any technical knowledge.

While conventional computers use binary digits, or 0s and 1s, to encode information in transistors, quantum computers use subatomic particles or superconducting circuits known as transmons to encode that information as a qubit. This means that it is in a superposition of both 0 and 1 at the same time, allowing users to compute on different sequences of the same qubits simultaneously. As the number of qubits increases, the number of processes also increases exponentially.

Transmons (and other quantum computer hardware) can exist in a huge number of states: 0, 1, 2, 3, 4 and so on. But an ideal quantum computer only uses states 0 and 1, as well as superpositions of these — otherwise errors will emerge in the quantum computation. When transmons encode information incorrectly into a state other than these, this is known as leakage.

“Quantum computers are ideally made of qubits, but as it turns out in real devices some of the time they are not qubits at all — but in fact are qutrits (three-state systems) or ququarts (four-state systems),” said Dr Animesh Datta from the University of Warwick. “Such a problem can corrupt every subsequent step of your computing operation.”

Even a miniscule leakage accumulating over many millions of hardware components can cause miscalculations and potentially serious errors, nullifying any quantum advantage over conventional computers. As a part of a much wider set of errors, leakage is playing its part in preventing quantum computers from being scaled up towards commercial and industrial application.

“Most quantum computing hardware platforms suffer from this issue — even conventional computer drives experience magnetic leakage, for example,” Dr Datta said. “We need quantum computer engineers to reduce leakage as much as possible through design, but we also need to allow quantum computer users to perform simple diagnostic tests for it.”

The Warwick researchers tested their computer program using the IBM Q Experience quantum devices, through IBM’s publicly accessible cloud service. They used a technique called dimension witnessing: by repeatedly applying the same operation on the IBM Q platform, they obtained a dataset of results that could not be explained by a single quantum bit, and only by a more complicated, higher dimensional quantum system. They have calculated that the probability of this conclusion arising from mere chance is less than 0.05%.

“It is quite something to be able to make this conclusion at a distance of several thousand miles, with very limited access to the IBM chip itself,” said Warwick researcher Dr George Knee. “Although our program only made use of the permitted ‘single qubit’ instructions, the dimension witnessing approach was able to show that unwanted states were being accessed in the transmon circuit components. I see this as a win for any user who wants to investigate the advertised properties of a quantum machine without the need to refer to hardware-specific details.”

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