How far Does Quantum Interference Reach?

Most recent answer: 09/02/2011

Q:
What are the limits/ranges that matter wave (or probability wave described by wave function) can be spread out in space? For example, if you perform "double slit experiment" with an electron and you change the distance between two slits from 0 (or nearest possible allowed by nature) to infinity, at what distance does interference pattern disappear? (Does this mean this distance is how far matter(probability) wave is spread out in space?) or does interference pattern keep showing up regardless of the distance of two slits no matter how far apart they are? If you repeat the same double slit experiment with heavier or lighter particle, how does the particle mass (and its speed(kinetic energy)) affect slits distance that allow interference pattern?
- Anonymous
A:
That's a great fundamental question. The answer is not fully known. In the basic form of quantum theory as we know it, the interference persists no matter how far apart the slits are and no matter what the properties are of the object passing through them. Nevertheless, there are severe practical limits and may also be some basic limits.

In practice, there are practical limits to our ability to see these interference effects. They tend to be lost when there can be excitations of internal modes of the object, such as vibrations in a C60 buckyball. You can picture the C60 wave going through the two slits as having decreasing probability over time of being in the same internal state. Only components in the same internal state interfere. In principle, cooling the C60's etc. enough forces them into the lowest energy internal state, but this procedure becomes more and more impractical for larger objects.

Even for objects with few internal degrees of freedom (e.g. neutrons) decoherence (loss of interference) occurs due to coupling to a complicated outside world. Every particle interacts some with its neighbors, by gravity if not by anything stronger. As the paths become more separated, the resulting states of the neighboring particles become more distinct depending on the particle's path. This too causes decoherence.
There are very active efforts to find ways to reduce such decoherence effects for various quantum systems, since decoherence limits the ability of a system to function as a quantum computer.

No one knows for sure whether at some point there will be a fundamental source of decoherence, beyond the various practical sources. That doesn't stop people from having very strong opinions on the subject, with varying degrees of justification. I'm just now struggling through a paper by Lenny Susskind, in which he proposes that fundamental  decoherence arises from interactions with things propagating out to cosmological horizons, at which information is irreversibly lost.

Mike W.

(published on 09/02/2011)

Follow-Up #1: Is there macroscopic quantum interference?

Q:
Mike, are you basically saying that we don't yet know wether interference MAY apply even to macro objects (provided they don't interact with anything and are in the lowest energy state possible) or if some fundamental decoherence starts to manifest when an object reaches a certain size regardless that it doesn't interact with anything and is in the lowest energy state?
- Anonymous
A:
Yes, that's something we don't know for sure.

Mike W.

(published on 09/05/2011)