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Quantum simulators: Researchers test quantum computers on desktop PCs

Computer scientists from Linz were able to simulate a certain problem, for which a high-end quantum simulator from Microsoft calculated 30 days, in less than a second on a conventional desktop PC.

Quantum computers are supposed to solve certain problems faster than conventional computers. Although the technology is only just beginning to exist, researchers in Upper Austria are already practicing with it - with different approaches: While the University of Applied Sciences Upper Austria has bought a commercial quantum simulator, at the University of Linz the quantum computer is simulated on the desktop PC using clever algorithms.

Computers currently rely on binary operations, i.e. power on or power off or 0 or 1. Quantum computers, on the other hand, use quantum physical systems, for example photons or ions. These can also have two base states, i.e. 0 and 1 - with a certain probability as soon as the state is measured. But because the systems obey the laws of quantum physics, they can - as long as nobody is looking - adopt both states at the same time. Physicists call this superposition. This phenomenon is to be used with problems that can be solved so much faster.

IBM offers access to three quantum computers

There are already the first realizations of the quantum computer and the industry has jumped on this development. IBM, for example, now offers free access to three quantum computers via the cloud, one of which has 16 quantum bits (qubits), as the basic unit of information is called here.

In view of these advances, the field is also becoming the focus of other areas. "Because it is to be expected that the quantum computer will play an essential role for future computer scientists on their career path, we are already showing our students this technology today, but we are also researching how we can test quantum algorithms now," explained Robert Wille from the Institute for Integrated circuits of the University of Linz in conversation with the APA.

50,000 euros for Google researchers in Linz

With success, as the "Google Award", worth the equivalent of around 50,000 euros, shows that Wille and his team recently received for the quantum simulator they developed. They are not relying on upgrading the hardware like their colleagues in Hagenberg, "we are trying to do it another way on conventional machines," says Wille. The key to this is called "clever data structures and clever algorithms".

Quantum operations can be represented as mathematical formulas, which, however, become enormously large and - in the conventional way - can therefore only be mastered by supercomputers. "On the other hand, we use certain redundancies by chopping up the formulas into small pieces and only saving identical parts once," said the computer scientist. In many cases, billions of descriptions can be compactly reduced to a few hundred formulas and efficiently calculated on conventional computers.

The computer scientists in Linz were able to simulate a specific problem, for which a high-end quantum simulator from Microsoft calculated 30 days, in less than a second on a conventional desktop PC. Google was this achievement a "Research Award" value.

"That will never lead to us being faster than the quantum computer at some point, but we can already recreate some algorithms for a quantum computer that does not yet exist that take too long on conventional simulators," said Wille. 31 qubits were simulated for the specific problem in which the Linzers measured themselves with the Microsoft simulator. But there are also algorithms with which hundreds of qubits can be simulated on the desktop PC, "these are artificial test cases to show what works".

The scientists at the Upper Austria University of Applied Sciences also want to find out what is possible: In July, the Hagenberg campus will receive the Quantum Learning Machine (QLM) from Atos, the world's most powerful commercial quantum simulator, according to the manufacturer. In the purchased version, this can simulate 30 qubits and can be expanded to up to 40 qubits.

Sensitive to interference

The lack of access to quantum computers has so far been the main reason for such simulators, but Wille expects this to change quickly, as the example of IBM shows. The simulators could not be dispensed with so quickly, "because at the moment simulators can reproduce even more qubits than previous implementations of quantum computers". But simulators will also be needed in the future because they "simulate very cleanly and calculate perfectly". In contrast, the quantum physical phenomena on which quantum computers are based are extremely sensitive to external interference, "which is why you will have to work a lot with several runs, mean values ​​and probabilities".

In addition, the superiority of the quantum computer has so far only been a theoretical prediction, "the practical proof is still missing," said Wille. There is therefore a separate field of research that tries to delay the "quantum superiority" through ever better simulators. Wille: "If we manage to efficiently simulate any quantum algorithm with 100 qubits, the bar for the physicists in realizing such a powerful quantum computer would be correspondingly high."