What is a
magic state?

The full narration of the video.

Up to ninety-five percent of a quantum computer's effort may go into making magic. That's an estimate Google has cited. Scientists really do call the thing a magic state. Why would a computer spend so much effort making magic?

Start with what a quantum computer fears most. Its tiny parts, called qubits, are incredibly fragile. A little heat, a little shaking, and they slip. So scientists add error correction: several qubits guard the same piece of information, and whichever one drifts gets pushed right back. The catch: error correction can only watch over a few kinds of moves.

Picture a machine whose knobs only turn in whole notches. Every twist is exactly a quarter turn, and it clicks into place. Anything sitting in a notch shows the slightest slip at once, so error correction can keep it in line. But whole notches have a big flaw: an ordinary computer can follow along and get the same answer. The quantum computer is no better.

To truly beat an ordinary computer, it has to turn half a notch: an eighth of a turn. Half a notch doesn't click into anything. If it slips a little while turning, error correction can't tell a deliberate turn from a mistake. The trick: don't turn it on the machine at all.

First turn a separate part to half a notch off to the side, test it, then plug it in. Plugging it in does that half-notch turn for the machine. That pre-turned part is the magic state. It's really just a qubit turned to that eighth-of-a-turn angle. It's called magic because with it, a whole-notch machine can compute anything.

Here's the hard part: each one is used up once it's plugged in. A truly useful problem burns through more magic states than you can count. And turning them off to the side goes wrong too. The classic fix is called distillation: take fifteen slightly-off parts and run them through one test together. Pass, and you get one far more accurate part; fail, and the whole batch is thrown away.

Refine layer after layer like that, and the magic-state workshop takes over most of the machine. This October, Google published a new method in the journal Science, called cultivation. Instead of refining fifteen into one, a single part is checked as it's turned, and tossed and restarted the moment something's off.

The parts that survive have about forty times fewer errors, roughly one mistake in ten thousand. Still, this was a test on a small chip, and a truly useful quantum computer is a long way off. In future quantum computers, will the magic workshop keep shrinking, or always take up most of the machine? Nobody knows yet. Which side are you on?