Will the quantum computer be made of silicon?

Turning pirouettes, left and right simultaneously. The almost magical limbos that electrons in quantum bits do with their magnetic moments – the ‘spins’ – hold much promise. By exploiting quantum mechanical caprioles you could use quantum bits – also known as qubits – to make powerful computers that can solve complicated calculations that are too hard even for the best supercomputers. Read more about the principle behind it here.

But at the moment, qubits aren’t of much help. They are unstable as they are extremely sensitive to external interference, and change state rapidly. This means that errors continuously occur. Errors need to be repaired more quickly than they arise. This is the holy grail in the research into quantum calculations.

That this is possible – at least theoretically – has been demonstrated by Professor of Nanosciences Lieven Vandersypen and his colleagues at QuTech (the alliance between TU Delft and TNO in the area of quantum technology) this month in Nature. The team succeeded in getting two qubits in silicon to exchange information at a completely new level of accuracy.

Last year, Vandersypen was awarded one of the four Spinoza Prizes – also known as ‘the Dutch Nobel Prizes’. At the time, NWO (Dutch Research Council) wrote that it is convinced that Vandersypen could achieve the ‘big scientific and technological breakthrough needed to turn the potential of quantum computers into reality’.

No angry space creatures
The research was featured on the cover. The front page of the journal shows two bright red dots surrounded by four petal shaped turquoise lit up protrusions. The reader is not looking at angry space creatures but at an artist’s impression of entangled qubits. They jointly form a two qubit circuit.

These two qubits can calculate virtually error free; with a fidelity of more than 99%. This means that for every 100 operations, less than one error is made.

Two other teams, one from Australia (UNSW Sydney) and one from Japan (Institute of Physical and Chemical Research/Riken) achieved similar results and published their findings in the same issue.

‘The error correction can beat the error creation from here’

“A 99% fidelity is a milestone,” says the first author of the TU Delft study, Xiao Xue. “It is generally seen as a key threshold. This percentage means that the error correction can beat the error creation.”

In a covering analysis in Nature, experts, who were not involved in the research, say that it could still be challenging to maintain that degree of fidelity as more qubits are linked to each other.

Nevertheless, Nature is impressed. According to the journal, the three studies have taken a giant leap, as reflected in the caption that it placed below the artist’s impression: Silicon qubits cross key error-correction threshold for quantum computing.

Comparable accuracies were obtained a few years ago by other research groups, but they did not work with silicon, the material that the current chip industry is based on. Quantum technology based on silicon can continue to build on the knowledge and experience that already exists in the chip industry. So the breakthrough is also good news for the chip manufacturer Intel, with which the TU Delft scientists regularly work, that hopes to outdo its competitors with the silicon qubits.

Google, IBM and Microsoft are also investing billions in being the first to develop a well functioning quantum computer. Google and IBM have pinned their hopes on qubits in the form of electrical currents in superconducting circuits. They have already managed to thread dozens of qubits to each other. But the rings are relatively large and need to be cooled to just above the absolute zero mark of 273 degrees below zero. The combination of size and extreme cooling makes further scaling up hard.

Microsoft, that also works with QuTech, hopes to make a quantum computer with the illustrious Majorana particles. But these particles are not letting themselves be caught easily. In 2018, TU Delft researchers believed that they had observed Majorana particles in their lab. This turned out to be wrong. And a Nature article needed to be retracted.

‘Many of the researchers involved from different countries know each other well’

That the three research groups came up with almost the same findings at the same time may not have been planned, but it was not really a surprise. “Many of the researchers involved in different countries know each other well,” says Xue.

People in the silicon qubit world work closely together. The silicon and the silicon germanium material used by the TU Delft and Japanese groups was produced at TU Delft and shared between the two groups. The isotopic purified silicon materials that the Australians experimented with came from Japan. Important software that made the breakthrough possible was written by Sandia National Laboratories in the USA and was made available to everyone.

The people too went back and forth. Mateusz Mądzik, the first author of the Australian study, is now working as a post-doc in Vandersypen’s lab. Another author of the Australian article, Serwan Asaad, studied at TU Delft. And in 2016, Lieven Vandersypen spent five months in the lab in Sydney during a sabbatical.

To Xue, the publications in Nature show how fertile the free sharing of ideas, people and materials can be.