Quantum computers have an inconvenient problem: they’re magnificently efficient at making mistakes.
These marvelous machines could one day solve problems that would bring even the most powerful conventional supercomputers to their knees.
But the quantum states behind their incredible computing power are also exceptionally fragile, and keeping errors under control is a monumental task.
In fact, a fully functional quantum computer could end up devoting most of its resources not to performing calculations, but simply to making sure those calculations are correct.
Now, the wizards at Google have demonstrated a new way to make quantum computing significantly less error-prone, using a technique with the delightfully appropriate name “magic state cultivation”.
“Our results experimentally establish magic state cultivation as a viable solution to one of quantum computing’s most important challenges,” the researchers write in a paper published in Science.
Quantum computers derive their unusual abilities from the strange physics that governs particles on the smallest scales.
Where the bits in a classical computer represent information as one of two states – a 1 or a 0 – quantum bits, or qubits, can exist in a superposition of both until they are measured. Measurement collapses that superposition, yielding just one definite outcome.
The sensitivity of this superposition is both a strength and a weakness. The quantum states that make these machines so powerful can be disturbed by the slightest interference from their surroundings, introducing errors that can rapidly derail a calculation.
To keep those errors in check, quantum computers need elaborate error-correction systems, which use multiple physical qubits to protect the information encoded in a single logical qubit.
But even with that protection, another problem remains. Some of the operations needed for complex quantum calculations are much harder to perform reliably than others.
To address this problem, physicists invented something called “magic states” – quantum states prepared in advance that allow computers to perform difficult quantum operations without compromising their error correction.
The problem with this is that each magic state can only be used once, so a quantum computer needs a constant supply of fresh ones.
And making magic states is in itself a difficult job.
Errors can creep in during magic state preparation, so researchers have traditionally relied on magic state distillation, a technique that takes a batch of imperfect magic states and processes them to produce a smaller number of more reliable ones.
It’s effective, but it can require extensive quantum hardware and processing time.
In 2024, Google researchers started working on an idea called magic state cultivation.
It might help to think of it like a greenhouse. In this analogy, magic state distillation would be like growing a whole greenhouse full of rose bushes, then using them to produce a smaller number of prize-winning specimens.
Magic state cultivation, by contrast, would be like growing just a single plant, carefully tending and pruning it along the way to make it the best it can be. If at any point it goes wrong, the plant is discarded, and you start again with a new seed.
Eventually, you end up with one truly spectacular plant, and you didn’t need an entire greenhouse to get there. The idea isn’t necessarily to produce a better magic state than conventional methods can, but to produce one of comparable quality using fewer resources.
Now, the Google team has put the idea to the test on its Willow quantum processor – and the results are promising.
Using magic state cultivation, the researchers reduced errors in the preparation process by a factor of around 40, producing magic states with a fidelity of at least 99.99 percent.
There’s a trade-off, though. To achieve that level of reliability, the researchers had to discard most of their attempts, retaining only around 8 percent of the results in the most stringent runs.
There was a significant weird hiccup during the process that led to another small breakthrough, too.
A critical part of demonstrating a quantum technique is proving that it worked. With a cultivated magic state, that is especially challenging – how do you accurately measure something with so few errors?
When Google quantum physicist Emma Rosenfeld first tested the cultivation technique, the results appeared to defy the laws of physics, Google said in a blog post.
It took months of investigation to trace the problem to the very method of measuring the state’s fidelity. Small fluctuations in the laboratory equipment, including temperature variations, were introducing noise that made the team’s usual approach unreliable.
The researchers eventually developed a new technique called kickback tomography, which allowed them to assess the reliability of the cultivated states without the distortions that had plagued their earlier measurements.
Finally, the team demonstrated that their cultivated magic state could be transferred into a larger quantum error-correcting code – a crucial step towards putting it to work in actual quantum computations.
It wasn’t a complete slam-dunk – the larger code introduced additional errors – but that will be the next piece of the puzzle to tackle.
Still, the results show that magic state cultivation can work on real quantum hardware, offering a promising alternative to the resource-intensive methods on which researchers have had to rely.
“Ultimately,” the researchers write, “these findings transform magic state cultivation from a theoretical study into an experimental reality, providing a practical route to universal logic in superconducting processors.”
The research has been published in Science.
This article was fact-checked by Peter Dockrill and edited by Peter Dockrill. While we pride ourselves on our process, we are only human. If you spot a mistake, please let us know.
