DecoherexThe Interference / Quantum Decoherence

Decoherex does not destroy superpositions. They spread them too thin to see.

A quantum system's density matrix holds two kinds of number. On the diagonal sit plain probabilities: the chance of finding it here, the chance of finding it there. Off the diagonal sit the coherences, the terms that produce interference fringes. The formula on this page models one of them dying away over a time \(\tau_D\). Once it is gone, the system behaves like an ordinary coin: odds known, no interference.

Where does it go? Into the surroundings. Every stray photon or air molecule that bounces off the system and carries away even a trace of information about it becomes entangled with it. The superposition is still there, but it now belongs to the system plus a growing crowd of environmental particles that nobody tracks. Look at the system alone and the interference has vanished. H. Dieter Zeh pointed this out in 1970; Wojciech Zurek and others developed it into an account of how a classical world emerges.

It is measured. In 1996 Serge Haroche's group in Paris put a microwave field in a cavity into a superposition of two states and watched the coherence between them fade, faster the more different the two states were. Haroche shared the 2012 Nobel Prize. In Vienna in 1999, buckyballs of sixty carbon atoms sent through a grating produced interference. In 2004 the same group heated similar molecules with lasers until they glowed; each emitted thermal photon carried away a little which-way information, and the fringes washed out as the temperature rose.

What decoherence does not obviously do is select a single outcome. Whether it solves the measurement problem, or only explains why big objects never show interference, is still debated.

Decoherex deletes nothing. They just make sure everyone finds out.