Errors must be recognized and fixed for quantum computers to be useful in practice. A team of experimental physicists at the University of Innsbruck, Austria, has now implemented a universal set of computational operations on fault-tolerant quantum bits for the first time, demonstrating how an algorithm may be written on a quantum computer so that mistakes do not ruin the outcome.

Because of high-quality production, mistakes in information processing and storage have become rare in modern computers. However, in important applications where even little mistakes can have major consequences, error correcting systems based on data redundancy are still utilized.

Quantum computers are naturally more vulnerable to disturbances and will almost certainly always require error correcting systems, as faults will spread uncontrollably throughout the system and information will be lost. Because quantum mechanics' fundamental constraints prohibit copying quantum information, redundancy can be obtained by dispersing logical quantum information into an entangled state of different physical systems, such as several individual atoms.

The team led by Thomas Monz of the University of Innsbruck's Department of Experimental Physics and Markus MÃ¼ller of RWTH Aachen University and Forschungszentrum JÃ¼lich in Germany has now realized for the first time a set of computational operations on two logical quantum 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," argues Lukas Postler, an experimental physicist from Innsbruck.

### Fundamental quantum operation realized

The researchers used an ion trap quantum computer with 16 trapped atoms to create this universal gate set. The quantum data was encoded in two logical quantum bits, each spread across seven atoms.

For the first time, two computational gates required for a universal set of gates may now be implemented on fault-tolerant quantum bits: a computational operation on two quantum bits (a CNOT gate) and a logical T gate, which is particularly challenging to execute on fault-tolerant quantum bits.

"T gates are very fundamental operations," theoretical physicist Markus MÃ¼ller says. "They are particularly interesting because quantum algorithms without T gates can be simulated relatively easily on classical computers, negating any possible speed-up. This is no longer possible for algorithms with T gates." The T-gate was shown by establishing a particular state in a logical quantum bit and transporting it to another quantum bit via an entangled gate operation.