Physicists have conducted successful "comparative tests" of three-level quantum computing devices based on ions and superconducting contacts and proved that they can successfully simulate complex physical processes and require fewer logical elements than quantum devices based on "traditional" two-level qubits. The results of the study are published in the journal Physical Review A (Q1).

In quantum computing, qubits – quantum bits - are used as logical elements. If classical bits can take only one of two values – 0 or 1, then quantum bits can be in a superposition of several states, each of which is realized with a given probability when measuring a qubit. This property of qubits gives quantum machines the ability to solve many problems that are practically inaccessible to the most powerful classical computers, for example, factorization of large numbers.

Usually, quantum computers use two–level qubits, which can be in a superposition of two states, however, there are also multi-level qubits - they are called qudits, in which three (these are qubits) or more states can be encoded. Their capabilities were tested by the authors of the study.

"The use of cutrites allows not only to encode quantum information more tightly, but also to solve some tasks more efficiently using fewer resources. This is exactly what we demonstrated in our experiment. We have experimentally shown that the dynamics of the simplest PT-symmetric system can be calculated using only one cut, while in this case more qubits would be needed," says Nikolai Kolachevsky, lead author of the study, director of the Lebedev Physical Institute of the Russian Academy of Sciences (FIAN).

Qubits can be created on the basis of different objects – on the basis of ions, cold atoms, defects in diamond crystal lattices, superconducting contacts, which play the role of logical elements on which calculation algorithms can be run. FIAN has been developing quantum devices based on ytterbium ions since 2020. One of these devices and the second – based on Josephson superconducting contacts (transmons) – were used in the experiment.

The authors of the article, physicists from FIAN, MIPT, the Russian Quantum Center and MISIS, using both devices, modeled the process of violation of space-time symmetry in a physical system. Symmetry is one of the fundamental properties of the physical world, many of the physical laws remain true in the "mirror world" if, for example, we reverse the charges of all particles, mirror their location in space or run time in the opposite direction. However, experimenters have discovered processes that violate symmetry, in particular, it is one of these violations that underlies the Higgs mechanism, which provides the mass of elementary particles.

Physicists modeled a system with PT-symmetry, the symmetry of space and time, where two levels of couture "worked" as the system itself, and the third simulated the environment external to it.

"As a result of the experiment, both calculators showed very similar results that have a good match with the theoretical model. This is how we demonstrated the advantages of the digital approach in quantum computing. The calculators we use – ion and superconducting – are arranged in completely different ways. But each of them supports its own set of commands, just like the usual processor in our computers. We can turn any task into a sequence of commands understandable to each of the available calculators," explains Ilya Zalivako, a researcher at the Optics of Complex Quantum Systems laboratory at FIAN.

According to the scientists, they expect, using the advantages of qubit systems over qubit ones, to demonstrate the operation of a number of algorithms, where the presence of additional levels significantly reduces the resources required for calculations. For example, in an application to Grover's algorithm, which helps to search through disordered databases or calculate inverse functions.

Information and photos provided by the Public Relations Department of FIAN

Photo source: FIAN Information taken from the portal "Scientific Russia" (https://scientificrussia.ru /)

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