Decoherex: Interference / Quantum Decoherence
Quantum Foundations arc, page 5 of 193. Back to The Pixelated Physics Universe
Who they are
Why don’t chairs show interference? Because the world keeps watching them. Decoherence: a quantum system entangled with its environment loses interference and looks classical. Decoherex personifies it. Decoherex wears it in pixel art on a unisex tee.
Decoherex is a character of the Quantum Foundations arc: drawn in pixels, ruled by one piece of real physics, and never allowed to break it.
The physics
The off-diagonal terms in a density matrix carry the ability to interfere. Contact with the environment makes them decay away, often extremely fast, so a superposition starts behaving like an ordinary 'either/or' mixture.
$$ \begin{gathered} \rho_{01}(t) = \rho_{01}(0)\,e^{-t/\tau_D} \end{gathered} $$
| Symbol | Meaning | SI unit |
|---|---|---|
| $\rho_{01}$ | off-diagonal (coherence) element of the density matrix | 1 |
| $t$ | time | s |
| $\tau_D$ | decoherence time | s |
Sources
- Zeh (1970) On the interpretation of measurement in quantum theory, Found. Phys. 1, 69
- Zurek (2003) Decoherence, einselection, and the quantum origins of the classical, Rev. Mod. Phys. 75, 715
- Encyclopaedia Britannica — Decoherence
- NobelPrize.org — The Nobel Prize in Physics 2012 (Haroche, Wineland)
- Brune et al. (1996) Observing the Progressive Decoherence of the “Meter” in a Quantum Measurement, PRL 77, 4887
- Arndt et al. (1999) Wave–particle duality of C60 molecules, Nature 401, 680
- Hackermüller, Hornberger, Brezger, Zeilinger, Arndt (2004) Decoherence of matter waves by thermal emission of radiation, Nature 427, 711
Crossovers
Decoherex meets Zeno (Quantum Zeno Effect / The Observer). Zeno's measurements and Decoherex's environment do the same job: each one records which state the system is in. Zeno makes the records on purpose; Decoherex lets the surroundings make them for free. Either way, the interference fades.
Decoherex meets Wignera (Wigner's Friend). Wignera, outside the lab, treats her friend and the qubit as one big superposition. Decoherex warns her that any real lab is constantly entangling with its air and walls, which makes interference between the friend's outcomes practically impossible to see. That explains why the friend looks classical; it does not settle the deeper question of who is right.
Decoherex meets Interfera (Path Unchosen / Quantum Interference). Interfera sends each particle down both paths, and the stripes build up. Decoherex only has to leave a record of which path was taken, even in a stray photon, and the stripes wash out. Path information and interference cannot both be had in full.
Wear the lore
Concept keywords: Interference / Quantum Decoherence, Decoherex, Quantum Foundations, physics lore, Pixelated Physics