An international team of scientists sent high-frequency sound waves across a modified semiconductor device to direct the behavior of a single electron, with efficiency above 99%.
A quantum computer would be able to solve previously unsolvable computational problems by taking advantage of the strange behavior of particles at the subatomic scale, and quantum phenomena such as entanglement and superposition.
However, precisely controlling the behavior of quantum particles is a mammoth task.
“What would make a quantum computer so powerful is its ability to scale exponentially,” said Hugo Lepage, a PhD candidate in the Cavendish Laboratory at the University of Cambridge.
“In a classical computer, to double the amount of information you have to double the number of bits. But in a quantum computer, you’d only need to add one more quantum bit, or qubit, to double the information.”
Some designs of quantum computers are based on superconducting loops, which are complex circuits and, like all quantum systems, are highly fragile.
“The smallest fluctuation or deviation will corrupt the quantum information contained in the phases and currents of the loops,” Lepage said.
“This is still very new technology and expansion beyond the intermediate scale may require us to go down to the single particle level.”
Instead of superconducting loops, the quantum information in the quantum computer Lepage and colleagues are devising use the ‘spin’ of an electron — its inherent angular momentum, which can be up or down — to store quantum information.
“Harnessing spin to power a functioning quantum computer is a more scalable approach than using superconductivity, and we believe that using spin could lead to a quantum computer which is far more robust, since spin interactions are set by the laws of nature,” Lepage said.
Using spin allows the quantum information to be more easily integrated with existing systems.
The device developed by the team is based on widely-used semiconductors with some minor modifications and measures just a few millionths of a meter long.
The researchers laid metallic gates over a semiconductor and applied a voltage, which generated a complex electric field.
They then directed high-frequency sound waves over the device, causing it to vibrate and distort, like a tiny earthquake.
As the sound waves propagate, they trap the electrons, pushing them through the device in a very precise way, as if the electrons are ‘surfing’ on the sound waves.
Lepage and co-authors were able to control the behavior of a single electron with 99.5% efficiency.
“To control a single electron in this way is already difficult, but to get to a point where we can have a working quantum computer, we need to be able to control multiple electrons, which get exponentially more difficult as the qubits start to interact with each other,” he said.
The team’s work is published in the journal Nature Communications.
Shintaro Takada et al. 2019. Sound-driven single-electron transfer in a circuit of coupled quantum rails. Nature Communications 10, article number: 4557; doi: 10.1038/s41467-019-12514-w