Why it falls apart

The reason quantum computers are hard is not that the physics is exotic. It is that the physics is fragile. A superposition survives only as long as nothing in the outside world learns which way it leans. The world does not have to be a person with a meter. A single air molecule bouncing off, one photon of heat, a magnetic wobble from a passing truck: each one takes a little of the information away, and the qubit slides toward being a plain coin on the table.

This is decoherence. It happens on a clock. For a superconducting qubit today the clock runs out in about a hundred millionths of a second, sometimes a thousandth. For a trapped ion it can be seconds or minutes. The number is called T2, and a machine's whole design is an argument with it.

So the machines are cold, very cold: the superconducting kind sits at about 15 thousandths of a degree above absolute zero, colder than deep space, in a nested set of gold-plated cans that looks like a chandelier. Ions and atoms float in vacuum held by lasers and electric fields. Everything is shielded. And still every gate errs about once in a thousand, and every second of waiting costs a little.

Errors in a quantum computer are worse than a flipped bit. A bit can only flip. A qubit's arrow can drift by any angle in any direction, and drift is continuous, so there is no clean 'right' and 'wrong' to compare against. That is why people thought for a while that error correction was impossible, and why the 1995 discovery that it is possible was the field's second birth.