Google’s quantum computer has just achieved a major breakthrough with “magic states”: ScienceAlert

Quantum computers have a troublesome problem: They are 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 that drive their incredible computing power are also exceptionally fragile, and controlling errors is a monumental task.

In fact, a fully functional quantum computer might end up spending most of its resources not on performing calculations, but simply on making sure those calculations are correct.

Now, the wizards at Google have demonstrated a new way to make quantum computing much less error-prone, using a technique with the delightfully apt name “magic state cultivation.”

“Our results experimentally establish magic state cultivation as a viable solution to one of the most important challenges in quantum computing,” the researchers write in a paper published in Science.

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Quantum computers derive their unusual capabilities from the strange physics that governs particles at the smallest scales.

While the bits in a classical computer represent information in one of two states – a 1 or a 0 – quantum bits, or qubits, can exist in a superposition of the two until they are measured. Measurement collapses this superposition, producing only one precise result.

The sensitivity of this overlay is both a strength and a weakness. The quantum states that make these machines so powerful can be disrupted by the slightest interference from their environment, introducing errors that can quickly derail a calculation.

To control these errors, 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 this protection, another problem remains. Some operations required for complex quantum calculations are much more difficult to perform reliably than others.

To solve this problem, physicists invented so-called “magic states”: pre-prepared quantum states that allow computers to perform difficult quantum operations without compromising their error correction.

The problem is that each magical state can only be used once, so a quantum computer needs a constant supply of new ones.

And creating magical states is itself a difficult task.

Errors can occur when preparing magic states, 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 states.

This is efficient, but can require considerable quantum hardware and processing time.

In 2024, researchers at Google began working on an idea called magic state culture.

It might help to think of it as a greenhouse. In this analogy, distilling into a magical state would be like growing an entire greenhouse full of rose bushes, then using them to produce a smaller number of prize specimens.

Cultivating the magical state, on the other hand, would be like growing a single plant, tending to it, and carefully pruning it along the way to make it the best it can be. If at any point things go wrong, the plant is thrown away and you start again with a new seed.

Eventually, you end up with a truly spectacular plant, and you don’t need an entire greenhouse to make it happen. The idea is not necessarily to produce a better magical state than conventional methods, but to produce one of comparable quality using fewer resources.

Now the Google team has tested this idea on its Willow quantum processor – and the results are promising.

Through cultivation of magic states, researchers reduced errors in the preparation process by a factor of approximately 40, producing magic states with at least 99.99 percent fidelity.

There is a trade-off, however. To achieve this level of reliability, researchers had to abandon most of their attempts, retaining only about 8% of the results in the most rigorous trials.

There was a strange and significant problem during the process that also led to another small breakthrough.

An essential part of demonstrating a quantum technique is proving that it worked. With a cultivated magical state, this is particularly difficult: how can you accurately measure something with so few errors?

When Google quantum physicist Emma Rosenfeld first tested the growing technique, the results seemed to defy the laws of physics, Google said in a blog post.

It took months of investigation to connect the problem to the very method of measuring state loyalty. Small fluctuations in the lab equipment, including temperature variations, introduced noise that made the team’s usual approach unreliable.

The researchers eventually developed a new technique called rebound tomography, which allowed them to assess the reliability of the cultured states without the distortions that had affected their earlier measurements.

Finally, the team demonstrated that their cultivated magical state could be transferred into a larger quantum error-correcting code – a crucial step toward its implementation in real quantum calculations.

It wasn’t a complete victory – the larger code introduced additional errors – but it will be the next piece of the puzzle to tackle.

Nonetheless, the results show that magic state cultivation can work on real quantum hardware, providing a promising alternative to the resource-intensive methods that researchers have had to rely on.

“Ultimately,” the researchers write, “these results transform magic state cultivation from a theoretical study into an experimental reality, thereby opening a practical path to universal logic in superconducting processors.”

The research was published in Science.