A new world record of 57 seconds.
Text: Benedikt Vogel
Basel-based physicists are working on a quantum computer that will hopefully use the electron spin to carry digital information. For this to work, they will need to keep the spin stable for a sufficient length of time. Only recently, a new world record was set in Basel.
We find ourselves in the laboratory on Basel’s Klingelbergstrasse. Where there was once a particle accelerator, scientists have now set up an experiment that allows them to trap electrons within a tiny space. Alongside two helium tanks and a tower of measuring apparatus, a kind of piston rises up into the air. At the tip, there is a 5 × 5 mm chip made of the semiconductor gallium arsenide. Using this setup, researchers led by Professor of Physics Dominik Zumbühl are investigating how an electron can be manipulated so that it is suitable for building a quantum computer.
The scientists recently announced a new world record: they managed to maintain an electron’s spin in one direction, and to prevent it from flipping to the opposite direction, for a period of 57 seconds. By doing so, they broke their own one-second record, dating back to 2008, as well as the 30-second record set by an Australian team in spring 2017 using a silicon chip. “This result is a milestone on the long road towards a quantum computer,” says Zumbühl. “If we want to build a high-speed computer of this kind, we’ll need to be able to keep the electron spin stable and control it in a well-controlled manner.”
Preventing the spin from flipping
To appreciate the significance of the new world record, you have to bear in mind that the Basel physicists are working with an unimaginably small object. Electrons are particles of the minutest dimensions: if a pinhead were the size of the Sun, you could still fit more than 1,000 electrons within the diameter of a human hair. This tiny object carries an electrical charge that produces an extremely weak magnetic field – the spin – as a result of its intrinsic rotation. Since a magnetic field always has a direction, the electron’s spin is usually depicted by an arrow. When an external magnetic field is applied, the spin can adopt one of two directions under its influence: “up” or “down”. In the natural state, the spin tends to flip from the “up” state to the lower-energy “down” state.
The scientists in Zumbühl’s group are attempting to prevent this flipping process – which physicists refer to as relaxation – or to delay it for as long as possible. This is because the electron spin can be used as a reliable information carrier as long as the electron spin can be held in one direction. In one experiment, the researchers have now managed to keep the electron spin pointing “up” for 57 seconds. Creating a stable and effective experimental setup in such a tiny space was quite a challenge. It involves cooling the electron – positioned on the aforementioned chip – to a temperature of 60 millikelvin, or just above absolute zero, using sophisticated techniques. Over the course of the several-day experiment, thousands of relaxations were measured – producing an average value of 57 seconds.
The trick of measuring the tiny spin
The experimental setup included a mechanism for establishing how long the spin points “up” before it flips. Determining the direction of the spin is difficult because the electron’s magnetic field is extremely weak and thus hard to measure. However, the physicists had a trick up their sleeve: instead of determining the spin’s direction using the magnetic field, they developed a measuring setup that works based on the charge and the higher energy level of the “up” state.
The relaxation time of 57 seconds is a scientific triumph. Still, the prospect of developing a quantum computer in the future will depend not on relaxation, but rather on coherence. This is the time during which the spin can be reliably held in one direction (“up” or “down”). Researching the process of relaxation will help the pioneers of the quantum computer utilize the coherence of the electron spin.
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