Qubits, the quantum analogues of ordinary bits, are extremely fragile and prone to errors, which makes building reliable quantum computers an extremely difficult task. One way to combat this is traditional magic state distillation, which filters out defects but requires enormous resources. The alternative is magic state cultivation: cleaning a noisy state directly inside a protected qubit. Until now, however, this method had not been successfully verified on real hardware.
For the first time in the history of quantum computing, researchers from Google Quantum AI have demonstrated magic state cultivation on the superconducting Willow processor, and this radically reduced the number of errors. The work was published in the journal Science.
In the experiment, the scientists first encoded a noisy magic state (a specially prepared quantum state needed for complex computations) in several physical qubits, distributing the quantum information across a small lattice for error protection. Then, using auxiliary qubits, they scanned the information for errors without destroying the data itself. If the checks detected defects, the attempt was discarded and the process started over. If an attempt passed all the tests, the cleaned state was retained for further processing.
The results were impressive: cultivation reduces error by a factor of 40, the authors write. The accuracy of the cleaned states was at least about 99.99 percent, an indicator of how closely a prepared quantum state matches the ideal. To verify such small errors, the scientists developed a new method called kickback tomography, which requires significantly fewer measurements compared with traditional approaches. The filtering was strict: only about 8 percent of the initial attempts passed all the checks.
In addition, the researchers transferred the cleaned states into a larger protective structure compatible with the surface code, a system for protecting quantum information from errors. This shows that cleaned states can be transferred into larger codes for storage and use. Our results on transplantation demonstrate the fundamental possibility of moving a magic state into a favorable code, the authors note.
The work addresses one of the key problems of quantum computing: the need for an enormous amount of additional equipment to create the special states required for universal calculations. Cultivation makes it possible to refine noisy states directly inside a protected qubit, potentially reducing resource costs. The next step is scaling the technology for larger quantum computers.