Decoherence: Why Everyday Things Don't Act Quantum
Decoherence happens when a system's surroundings, such as air, light and heat, keep picking up records of its state. Each record acts like a tiny which-path detector, so the amplitudes can no longer cancel and the system behaves like an ordinary object. Big things decohere almost instantly, which is why a cat is never seen in superposition, and slowing decoherence is the central engineering problem of building qubits.
If everything is quantum, why does your coffee cup act normal?
You are made of atoms. So is your coffee cup. Atoms follow quantum rules. So why don't cups show interference stripes, or tunnel through tables?
Part of the answer you have already seen. Big things have tiny wavelengths, as in the baseball example of chapter 3. But the bigger reason is decoherence. That is the loss of interference caused by the surroundings. It is also the main reason quantum computers are so hard to build.
- How does the environment act like a which-path detector?
- Run it: a faint record from the environmentINTERACTIVE
- How much interference is left? Compute it.
- Try this: a perfect recordINTERACTIVE
- Try this: a very faint recordINTERACTIVE
- Worked example: why big things decohere so fast
- How does this become the engineering problem of building qubits?
- Where should you go after this course?
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