Hooray, she is alive!

As early as 1935, physicist Erwin Schrödinger showed with a thought experiment that the principles of the quantum world cannot be easily transferred to our everyday life. In the scenario, a now quite famous cat squats in a box for an hour together with a barbaric killing device. Whether the killing machinery is activated or not depends on the decay of the unstable atoms of a radioactive substance. That the decay occurs while the poor animal is still squatting in the box is, according to Schrödinger, as probable as it is improbable. If the rules of quantum physics applied here, the cat would be dead and alive at the same time until someone opened the box and clearly determined its state. Does that sound rather contrived? It is, at least as far as cats in boxes and other everyday systems are concerned. On a microscopic level, however, this is exactly what happens, which is why this image is still often invoked today. Quantum objects, electrons for example, can occupy more than one state simultaneously. However, this superposition state dissolves as soon as someone observes it. CISPA faculty member Dr. Nico Döttling explains what this observer effect can be used for and why the potential and dangers posed by quantum computers are often overestimated.

"The observer effect is a fundamental principle of quantum physics that states: you cannot observe a system without affecting it," Döttling explains. While this effect is barely visible, if at all, in macroscopic systems, the impact on quantum systems, which consist of just a few particles, is enormous, he says. "The basic idea of quantum cryptography is - to put it simply - to use the observer effect to determine whether a communication channel has been tapped." If that is the case, the secret key exchanged over that channel is discarded, he said. The so-called quantum key exchange referred to here is one of the simplest research problems in quantum cryptography, according to Döttling. "It is considered very well understood from the theoretical side." And it's an example of how cybersecurity researchers are looking to exploit the properties of quantum systems.

Yet quantum computers in particular are currently considered by many to be one of the biggest threats when it comes to the security of digital communication. "They are relevant from the point of view of cryptography because they can efficiently break some cryptographic procedures that are considered secure against classical attackers. These include the well-known and widely used RSA and ElGamal methods, on which the security of current Internet technology is based," explains Döttling. Since the early 1990s, however, numerous quantum-safe methods have existed that could be used as alternatives. The problem is therefore more a political and bureaucratic one, since new encryption methods have to be standardized in order to be widely used. This, he said, is complex and costly.

Döttling also believes quantum computers' potential is overestimated, just like the risks they pose to secure communications. That's because what makes them special also makes them particularly challenging and difficult to control. What does that mean in concrete terms? Quantum computers are considered to be much more powerful than classical computers because they can perform computing operations not only sequentially, but also in parallel. This is possible because their smallest information and computational units, known as qubits, can not only assume two states like the bits in classical computers, but can also be in superposition states. Moreover, they are able to connect or entangle in a special way, even when they are very far apart. For a quantum computer to perform calculations, as many qubits as possible must remain entangled. However, they lose their superposition state at the slightest external influence.

In recent years, the technology has experienced a tremendous surge in development. However, it is too early to speak of a quantum computer, says Döttling - also with respect to the first practical implementations in this field, with which large corporations such as Google and IBM have made headlines in recent years. "Google's quantum computer 'Sycamore' can only solve the one specific problem it was built around." To speak of a computer, therefore, is not entirely accurate, he said. "When and if a fully functional quantum computer will come is yet to be said."

There is at least one application scenario that makes quantum systems particularly interesting for researchers worldwide: the simulation of other quantum systems. This could play a major role in the development of new drugs, for example. "The hope is that it can be used to simulate how a particular drug is absorbed by the body and whether the desired effect occurs." In this way, the researchers hope that it may be possible to develop the best possible drug for various applications without having to test it on live subjects.

Döttling, who is originally from Heilbronn, has been employed at CISPA since 2018 and teaches at Saarland University. Quantum cryptography is just one of his research fields. "I mainly deal with so-called multiparty computations." This, he says, involves developing secure methods by which multiple parties, who know each other to some extent but do not trust each other, can achieve a common goal. "That could be, for example, 100 competing chip manufacturers who want to compare their statistics to find out what number of microchips will sell. But they don't want to disclose their sales figures." Normally, they would then need a third party to whom they would entrust the data, who would calculate and announce the result for them, Döttling explains. "But it's not clear whether this third party is really incorruptible." In the future, a cryptographic process could take the place of such an intermediary, the 38-year-old explains. Before coming to Saarbrücken, Döttling researched and taught at Friedrich Alexander University in Erlangen-Nuremberg. Before that, he spent two years as a postdoc in Denmark at Aarhus University and at UC Berkeley in California.

translated by Oliver Schedler