A smart accelerator for qubits: Spin-orbit approach boosts both speed and stability

There are high hopes for quantum computers: they are supposed to perform specific calculations much faster than current supercomputers and, therefore, solve scientific and practical problems that are insurmountable for ordinary computers. The centerpiece of a quantum computer is the quantum bit, qubit for short, which can be realized in different ways—for instance, using the energy levels of atoms or the spins of electrons.

When making such qubits, however, researchers face a dilemma. On the one hand, a qubit needs to be isolated from its environment as much as possible. Otherwise, its quantum superpositions decay in a short time and the quantum calculations are disturbed. On the other hand, one would like to drive qubits as fast as possible in analogy with the clocking of classical bits, which requires a strong interaction with the environment.

Normally, these two conditions cannot be fulfilled at the same time, as a higher driving speed automatically entails a faster decay of the superpositions and, therefore, a shorter coherence time.

A team led by Professor Dominik Zumbühl at the University of Basel has now succeeded in adjusting a spin qubit in a way that allowed them to increase both its speed and its coherence time simultaneously. These results, recently published in Nature Communications, could also make other qubits faster and more robust in the future.

Stepping on the accelerator in a smart way

"Initially, we asked ourselves what would happen if we simply 'stepped on the accelerator' of our qubit—however, not just in any way, but the smart way," says Dr. Miguel J. Carballido, first author of the study.

He and his colleagues built a tiny device consisting of a wire made of the semiconductor material germanium, measuring only 20 nanometers in diameter and featuring a thin silicon coating. They then removed a single electron from a low or higher energy level of the wire, which resulted in a "hole." "This hole behaves similarly to an air bubble," says Carballido.

For such a system, a team of theoretical physicists led by Prof. Dr. Daniel Loss at the University of Basel had predicted a few years ago a mechanism that could achieve the impossible: a faster drive and, at the same time, a longer coherence time. "We exploit a certain kind of spin-orbit coupling for this," Carballido explains.

In spin-orbit coupling, a moving electrically charged particle—an electron or a hole—creates a magnetic field. This magnetic field, in turn, "couples" to the particle's spin and hence influences its energy through a magnetic interaction. For holes in a solid-state material, this effect is very strong and also electrically controllable.

It's all in the mix

By applying an electric voltage to the nanowire, the Basel researchers can, therefore, influence whether the hole comes mainly from a low or from a higher energy level, or a mixture of both levels. This mixture has a crucial influence on how the "accelerator pedal" for driving the qubit reacts: for a particular mix, a so-called plateau appears, which means that stepping harder on the accelerator does not make the drive faster, but instead slows it down.

Another consequence of this plateau is that fluctuations, for instance, due to electric fields in the environment, influence the qubit much less than they would for a standard spin-orbit coupling. As a result, the quantum states are disturbed less, and the coherence time increases.

"In this way, we were able to increase the coherence time of our qubit fourfold while also making the drive three times faster," says Carballido. He also stresses a further peculiarity: instead of the extremely low temperatures of less than 100 millikelvin typically needed to operate a qubit, he can get away with a relatively warm 1.5 kelvin. "That requires less energy and works without the rare helium-3," he says.

For now, the plateau trick only works in the nanowires made in Basel, in which holes can only move in one spatial dimension. However, Zumbühl and his collaborators hope that their method can soon also be applied to two-dimensional semiconductors and to other kinds of qubits. That would be an essential contribution towards more powerful quantum computers.