“We need millions of quantum bits”
Quantum computers currently have only a few dozen quantum bis. This means they can basically prove their functionality. But for practical tasks, many more Qbits are needed. A German-British physicist explains how this could be achieved.
BAlready during his doctoral thesis in Australia, Winfried Hensinger set himself a challenging goal that he wanted to achieve in his life as a physicist: “I want to build a quantum computer”. Sussex when he finally met all the requirements in 2018 to start his own company.
A scientific breakthrough achieved in 2016 was an important aspect. Hensinger now comes to Hamburg with “Universal Quantum” because the physicist with a British and German passport can benefit from generous funding for innovative quantum computer companies in this country.
There are dozens of research groups and companies around the world that research and develop quantum computers. Very different technologies are used to realize so-called quantum bits (Qbits). Which one will ultimately come out on top and be best suited for commercial quantum computers is an open question. Naturally, every scientist places the greatest hope in the path of development that he himself follows.
In fact, there are already the first quantum computers that have fundamentally proven their functionality. They were able to solve specific problems using special quantum algorithms. The number of Qbits in these systems is usually just two digits.
“To be able to solve real problems with quantum computers in practice, we need millions of Qbits,” says Hensinger. For this reason, some of the technologies pursued so far would have no chance in the long term because they are not “scalable”.
“A technology is called scalable if it can be used not only to build systems with, say, 100 Qbits, but also through modular expansion to those with 1,000, 10,000 or a million Qbits,” explains Hensinger. And that’s exactly what should be possible with the technology he prefers.
Hensinger quantum bits are represented by ions, that is, electrically charged ions that are found in an ion trap on microchips. “I presented the first ion trap microchip in 2005”, says the Anglo-German researcher. In the same year he received his professorship in England.
“The main advantage of this technology is its scalability,” says Hensinger. In Hamburg, we want to build a quantum computer in which we connect four chips with electric fields using a new technology we invented. So we built a truly modular system. And of course you could build a quantum computer with millions of Qbits in exactly the same way; you would just connect a lot more chips.”
Because if this works, other modules must be coupled together using the same principle and a scalable quantum computer can be built. In similar approaches, researchers use laser light to communicate with Qbits. But it seems pointless to try to do this with millions of Qbits at the same time.
Microwaves operate with Qbits
“It is not possible to direct millions of laser beams at ion microchips,” says Hensinger, “but in 2016 we achieved a decisive breakthrough. We have developed chips that produce inhomogeneous magnetic money within the processor zone. You can then change the ion’s resonant frequency by simply shifting the ion in the inhomogeneous magnetic field using an applied voltage. This means that by applying a voltage, you can determine whether the ion can absorb microwaves. The microwaves then change the state of the ion and so you can perform calculations simply by applying voltages.”
A big advantage of this quantum computer technology is that it basically works at room temperature. The quantum computer developed by Google, on the other hand, needs to be cooled to temperatures in the millikelvin range – that is, just above absolute zero of minus 273 degrees Celsius. This is complex and expensive and another obstacle to scaling. Taking millions of Qbits to such low temperatures is an almost impossible challenge to overcome.
And it’s this scalability that will ultimately require cooling of Hensinger’s chips. If millions of ions are active there as Qbits, the residual heat that will inevitably arise will simply be too great. “We want to cool our chips to minus 200 degrees Celsius with helium gas,” says the physicist. This can be done with little effort and is very cost-effective. “