Google Willow quantum processor achieves 99.99% logical fidelity with magic state cultivation

Google researchers have shown a new error reduction method on their Willow quantum processor. This method drastically increases the fidelity of the quantum state and is important as it tackles one of the most challenging bottlenecks of having a working quantum computer: preparation errors occurring at very high rates. With a logical fidelity of 99.99%, the team has shown that practical error correction is getting closer and closer. And while a full-fledged fault-tolerant quantum computer is still far away, at least we are getting closer.

Magic state cultivation cuts preparation errors 40 times on Willow quantum chip

The method used is called "magic state cultivation" but was specifically demonstrated on Google's Willow quantum processor. Instead of the normal distillation techniques which require a lot of resources in order to purify the states, cultivation is a method which involves simply checking the states. If a state fails the test it is thrown away. This allows for a better fidelity state without having to spend a lot of resources which is usually what quantum error correction involves.

On the Willow quantum processor the researchers saw a reduction of 40 times in the number of preparation errors when compared to a state that was prepared without the cultivation technique. They managed to reach a logical fidelity of 99.99% (a measure of how well the quantum information is kept). Of course this comes at a cost. For this method to work, around 92% of the attempts had to be thrown away so that only high quality states are left for computation.

An interesting limitation mentioned with the experiment was that when moving these cultivated states to higher quantum error correction codes, more errors happen. This shows that while the magic state cultivation technique is solving the preparation bottleneck, the error correction puzzle is still not fully resolved. This particular experiment is solving an engineering issue but not yet giving us the full fault-tolerant quantum computer.

Source: NotebookCheck