In a breakthrough that could help address a major bottleneck in practical quantum computing, Google has announced that its Willow processor has demonstrated a 40-fold reduction in errors while preparing specialized quantum states.
The technique, known as ‘magic state cultivation,’ achieved 99.99 percent logical fidelity by producing highly accurate quantum states needed to perform complex calculations. Such states are essential for building powerful, error-corrected quantum computers.
However, producing these magic states has long been a major challenge. They are essential for performing complex quantum calculations, but creating them with sufficiently low error rates requires enormous computing resources. Some estimates suggest that up to 95 percent of a quantum computer’s capacity could be devoted to producing these states, leaving little room for actual calculations.
The problem is further complicated by the fact that each magic state can be used only once, meaning quantum computers must continuously produce new ones to carry out operations.
Google’s latest experiment demonstrates a method that could help reduce this resource burden.
Filtering out quantum errors
In 2024, Google Quantum AI researchers Craig Gidney, Noah Shutty and Cody Jones developed the magic state cultivation technique by refining an earlier theoretical approach.
The method works like a filter. Researchers introduce a quantum state containing errors into a small region of a processor and perform a series of carefully coordinated measurements to identify problems.
Attempts that fail these checks are discarded, while those that pass produce much cleaner quantum states. These states can then be expanded into a larger form suitable for more complex quantum operations.
Although the approach showed promise in theory, demonstrating it on actual quantum hardware required several engineering improvements.
During early experiments, Google researcher Emma Rosenfeld encountered unexpected measurement results. After months of testing and simulations, the team traced the problem to noise caused by electrical equipment drift and temperature changes in the laboratory.
This interference was affecting the accuracy of the team’s existing measurement method. Researchers responded by improving the processor’s controls and developing a new benchmarking technique that used the cultivation process itself to evaluate the results.
These changes helped the Willow processor achieve a 40-fold reduction in errors compared with quantum states prepared without cultivation.
The experiment also produced logical T-states, a particular type of magic state, with a reported fidelity of 99.99%.
Scaling remains major challenge
Beyond reducing errors, Google’s researchers demonstrated that the purified quantum states could be transferred between two different error-correction methods.
The team moved states from a distance-3 color code to a distance-5 surface code. These are two ways of organizing quantum information to detect and correct errors, with the distance indicating the code’s level of protection against certain errors.
This process, known as “grafting,” showed that the carefully prepared states could survive the transition between the two methods on actual hardware.
The result is important because future quantum computers may need to combine different error-correction techniques to perform calculations reliably while limiting hardware requirements.
However, the demonstration does not mean Google has eliminated the magic-state bottleneck or built a practical, fault-tolerant quantum computer.
The company acknowledged that cultivation must still be demonstrated at a much larger scale before it can support useful quantum applications.
The study appears in the journal Science.