Errorless quantum computing becomes real

Artist’s impression of gate operations on logical quantum bits, which are protected from failure by quantum error correction. Credit: Johannes Knünz

Demonstration of the building blocks of fault-tolerant quantum computing

Due to high-quality manufacturing, errors during information processing and storage have become a rarity in modern computers. However, for critical applications, where even individual errors can have serious effects, error correction mechanisms based on the redundancy of the processed data are still used.

Quantum computers are inherently much more susceptible to disturbances, and thus error correction mechanisms will almost always be required. Otherwise, errors would propagate uncontrollably in the system and information would be lost. Because the fundamental laws of quantum mechanics forbid copying quantum information, redundancy can be achieved by distributing logical quantum information in an entangled state to various physical systems, for example, multiple individual atoms.

The research team, led by Thomas Monz from the Department of Experimental Physics at the University of Innsbruck and Markus Müller from RWTH Aachen University and Forschungszentrum Jülich in Germany, has succeeded for the first time in performing a set of computational operations on two quantum logics. bits that can be used to implement any possible operation. “For a real-world quantum computer, we need a universal set of gates with which we can program all algorithms,” explains Lukas Postler, an experimental physicist from Innsbruck.

Fundamental quantum operation performed

The team of researchers implemented this set of universal gates in an ion trap quantum computer with 16 trapped atoms. Quantum information was stored in two logical quantum bits, each distributed over seven atoms.

Now, for the first time, it has been possible to implement two computational gates on these fault-tolerant quantum bits, which are necessary for a universal set of gates: a computational operation on two quantum bits (a CNOT gate) and a logical T gate, which it is particularly difficult to implement in fault-tolerant quantum bits.

Fundamental Building Blocks for Fault-Tolerant Quantum Computing

Demonstration of the fundamental building blocks for fault-tolerant quantum computing. Credit: Uni Innsbruck/Harald Ritsch

“T gates are very fundamental operations,” explains theoretical physicist Markus Müller. “They are particularly interesting because quantum algorithms without T gates can be simulated relatively easily on classical computers, nullifying any possible speedup. This is no longer possible for algorithms with T gates.” Physicists demonstrated the T gate by preparing a special state in a logical quantum bit and teleporting it to another quantum bit via an interleaved gate operation.

The complexity increases, but the precision also

In encoded logical quantum bits, the stored quantum information is protected against errors. But this is useless without computational operations, and these operations are themselves error-prone.

The researchers have implemented operations on the logical qubits in such a way that errors caused by the underlying physical operations can also be detected and corrected. Therefore, they have implemented the first fault-tolerant implementation of a universal set of gates on encoded logic quantum bits.

“Fault-tolerant implementation requires more operations than non-fault-tolerant operations. This will introduce more errors at the scale of individual atoms, but nonetheless experimental operations on logical qubits are better than non-fault-tolerant logical operations,” Thomas Monz is pleased to report. “The effort and complexity increase, but the resulting quality is better.” The researchers also verified and confirmed their experimental results using numerical simulations on classical computers.

Physicists have now demonstrated all the building blocks for fault-tolerant computing in a quantum computer. The task now is to implement these methods in larger and thus more useful quantum computers. The methods demonstrated in Innsbruck on an ion trap quantum computer can also be used on other architectures for quantum computers.

Reference: “Demonstration of Fault-Tolerant Universal Quantum Gate Operations” by Lukas Postler, Sascha Heuβen, Ivan Pogorelov, Manuel Rispler, Thomas Feldker, Michael Meth, Christian D. Marciniak, Roman Stricker, Martin Ringbauer, Rainer Blatt, Philipp Schindler, Markus Müller and Thomas Monz, May 25, 2022, Nature.
DOI: 10.1038/s41586-022-04721-1

Financial support for the research was provided, among others, by the European Union in the framework of the Quantum Flagship Initiative, as well as by the Austrian Research Promotion Agency FFG, the Austrian Science Fund FWF and the Federation of Austrian Industries. of the Tyrol.

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