Intel-backed boffins demo long-lived silicon qubit

0.4 milliseconds? An eternity in quantum dog-years

Bit by bit, the world gets closer to creating the quantum equivalent of a storage gate – a silicon-based qubit that can last long enough for general-purpose computing.

If you want to use the superposition – the quantum property in which a gate can exist as 0 and 1 at the same time – for computation, it needs a usable “coherence time” (the period of time for which a superposition persists).

That's what's got the University of Delft's QuTech research group excited: a coherence time of 0.4 milliseconds, which the researchers describe as “nearly an eternity” for computing purposes.

As the university's Lieven Vandersypen says here, silicon also offers between 10 times and 100 times the gate fidelity of gallium arsenide qubits.

In their paper at PNAS, the Delft group explain their qubit was represented by the spin on one electron in a silicon/silicon-germanium quantum dot, controlled “electrically via an artificial spin-orbit field from a micromagnet”.

Gate fidelity was measured at around 99 per cent, the paper says, “which is consistent with dephasing from the slowly evolving nuclear spins in the substrate.”

The Delft device does, however, still need the other bugbear of quantum device: the paper notes that the experiment was carried out at around 25 milliKelvin. Aka about -270 celsius.

While this used “ordinary” silicon, Delft notes that even better results could be obtained if the isotope silicon-29 is removed from the mix. Work at the University of New South Wales has shown that Si-29 degrades both coherence and gate fidelity.

Intel began partnering with QuTech in 2015, and American researchers from the University of Wisconsin and Ames Laboratory worked with Delft on the current project. ®

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