Improving the Measurement of Electron Spin Qubits to Develop Fault-Tolerant Quantum Computers

April 08, 2020 by Alessandro Mascellino

Scientists from the RIKEN Center for Emergent Matter Science in Japan have successfully taken repeated "non-demolition" measurements of the spin of an electron in a silicon quantum dot (QD), without changing its spin in the process.

The findings are particularly relevant for electronic engineers interested in building quantum computers that are fault-tolerant.

This is because quantum machines using single electron spins in silicon QDs have extended potential for scalability and because the material is already widely used in electronics technology.


Measuring Electron Spin Qubits for Electronics Development

Quantum computers are particularly sensitive to external noise, so measuring the spin of an electron in a QD has been a problem scientists have been trying to solve for some time.

This being said, calculation involves measuring a quantum value that, unlike traditional transistors, is never in a single state but in a "superimposed state" instead.

In these conditions, the detection process itself, which normally would see single electron spins in silicon to be converted into measurable charges, would affect the electron spin, causing what is referred to as a “demolition” type of measurement.

In other words, the process of measuring itself is what causes these calculations to go awry.


Schematic of a device where a qubit spin (blue) and the ancilla spin (red) in a silicon quantum well layer.

Schematic of a device showcasing the qubit spin (blue) and the ancilla spin (red) hosted in two singly-occupied dots in a silicon quantum well layer. Image credited to Nature Communications

An Ising-Type Interaction Model for "Non-Demolition" Measurements

To overcome these limitations, the researchers from the RIKEN Center for Emergent Matter Science used the Ising type interaction model. This is a model looking at how ferromagnetism on a lattice surface is formed through the alignments of electron spins of neighboring atoms.

The team led by Seigo Tarucha was able to transfer the spin information of an electron in a QD to another electron in the adjoining QD utilizing the Ising type interaction in a magnetic field. After doing so, the scientists were then able to measure the spin of the second electron using the conventional method, so that they could leave the original spin unaffected.

They then repeated this process several times to obtain repeated and rapid measurements of the neighbor.


Promising Results for Electronics

The results obtained by Tarucha’s team are promising for the creation of future electronics components that are traditionally made of silicon. "We were able to achieve a non-demolition fidelity rate of 99%, and by using repeated measurements would get a readout accuracy of 95%,” Tarucha explained. “We have also shown that theoretically, this could be increased to out 99.6%, and plan to continue work toward reaching that level,” he added.

Moving forward, the researcher said these findings could also be combined with existing quantum technologies.

"This is very exciting, because if we can combine our work with high-fidelity single- and two-qubit gates, which are currently being developed, we could potentially build a variety of fault-tolerant quantum information processing systems using a silicon quantum-dot platform," Tarucha concluded.