Fundamental building blocks for fault-tolerant quantum computers demonstrated
High-quality manufacturing has made errors during the processing and storage of information a rarity in modern computers. However, for critical applications, where even a few errors can have serious consequences, error correction mechanisms based on the redundancy of the processed data are still used.
Quantum computers are by nature much more prone to failure and therefore error correction mechanisms will almost always be required. Otherwise, errors would propagate unchecked in the system and information would be lost. Since the fundamental laws of quantum mechanics prohibit the copying of quantum information, redundancy can be achieved by distributing logical quantum information in an entangled state of different physical systems, e.g. 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 the RWTH University of Aachen and Forschungszentrum Jülich in Germany, has now succeeded for the first time in a series of computational operations on two logical quantum bits that can be used to perform any possible operation. “For a real 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 processing realized
The team of researchers implemented this universal gate set on an ion trap quantum computer with 16 trapped atoms. The quantum information was stored in two logical quantum bits, each spread over seven atoms.
Now, for the first time, it is 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 logic T gate, which is particularly difficult to implement. on fault-tolerant quantum bits.
“T-gates are very fundamental operations,” explains theoretical physicist Markus Müller. “They are especially interesting because quantum algorithms without T-gates can be simulated relatively easily on classical computers, negating any possible acceleration. This is no longer possible for algorithms with T-gates.” The physicists demonstrated the T-gate by preparing a special state in a logic quantum bit and teleporting it to another quantum bit via an entangled gate operation.
The complexity increases, but so does the accuracy
In encoded logical quantum bits, the stored quantum information is protected from errors. But this is useless without arithmetic operations, and these operations are themselves error-prone.
The researchers performed operations on the logical qubits in such a way that errors caused by the underlying physical operations can also be detected and corrected. For example, they have implemented the first fault-tolerant implementation of a universal set of gates on encoded logical quantum bits.
“The fault-tolerant implementation requires more operations than non-fault-tolerant operations. This will introduce more errors at the single-atom scale, but nevertheless the experimental operations on the logic qubits are better than non-fault-tolerant logic operations,” Thomas Monz is pleased to report. “The effort and complexity increases, but the resulting quality is better.” The researchers also checked and confirmed their experimental results using numerical simulations on classical computers.
The physicists have now demonstrated all the building blocks for fault-tolerant computing on a quantum computer. The task now is to implement these methods on larger and thus useful quantum computers. The methods demonstrated in Innsbruck on an ion trap quantum computer can also be used on other quantum computer architectures.
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 , 25 May 2022, Nature†
Financial support for the research was provided, among others, by the European Union under 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 Tyrol.