How To Extract Information From A Qubit Without Collapsing It

The Strange Case Of Phase Kickback

In quantum mechanics, there is a rule that is taught early and often: Measurement disturbs.

If you observe a qubit, the wave function collapses. Superpositions disappear. The entanglement breaks. You get an answer. But the system you asked about can never be the same again.

And yet... quantum algorithms routinely extract precise information from sensitive quantum systems without measuring them directly.

They do this cleanly, repeatedly, and even reversibly. So what's going on?

This is not a loophole. It's a feature enabled by phase kickback.