Physicists design new type of universal quantum computer

The computing power of quantum machines is low and increasing it can be challenging. Physicists from the University of Innsbruck have presented a new architecture for a universal quantum computer that overcomes such limitations and could be the basis of the next generation of quantum computers.

Quantum bits (qubits) in a quantum computer serve as a computing unit and memory at the same time. Because quantum information cannot be copied, it cannot be stored in a memory as in a classical computer. Due to this limitation, all qubits in a quantum computer must be able to interact with each other; this still presents a challenge for building powerful quantum computers. In 2015, theoretical physicist Wolfgang Lechner, together with Philipp Hauke and Peter Zoller, proposed a new architecture for a quantum computer, now named LHZ architecture after the authors. According to Lechner, the architecture was originally designed for optimisation problems, but in the process, the physicists reduced the architecture to a minimum in order to solve these optimisation problems as efficiently as possible. The physical qubits in this architecture do not represent the individual bits but encode the relative coordination between the bits. “This means that not all qubits have to interact with each other anymore,” Lechner said.

Lechner has now shown that this parity concept is also suitable for a universal quantum computer. Parity computers can perform operations between two or more qubits on a single qubit. “Existing quantum computers already implement such operations very well on a small scale. However, as the number of qubits increases, it becomes more and more complex to implement these gate operations,” said Michael Fellner, one of the scientists on Lechner’s team.

In two publications in Physical Review Letters and Physical Review A, the Innsbruck scientists have shown that parity computers can, for example, perform quantum Fourier transformations — a fundamental building block of many quantum algorithms — with fewer computation steps and thus more quickly. “The high parallelism of our architecture means that, for example, the well-known Shor algorithm for factoring numbers can be executed very efficiently,” Fellner said.

The new concept also offers hardware-efficient error correction; because quantum systems are very sensitive to disturbances, quantum computers must correct errors continuously. Significant resources must be devoted to protecting quantum information, which increases the number of qubits required. Anette Messinger, a member of the Innsbruck team, said that their model operates with a two-stage error correction, with one type of error (bit flip error or phase error) prevented by the hardware used.

The Innsbruck team is in the initial experimental approach for this on different platforms. “The other type of error can be detected and corrected via the software,” Messinger said. This would allow a next generation of universal quantum computers to be realised with manageable effort. The spin-off company ParityQC, co-founded by Lechner and Magdalena Hauser, is also working in Innsbruck with partners from science and industry on possible implementations of the new model.

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