Knowledge of the superposition of states

[Gerinski]

Is it completely impossible, even in principle, that eventually there can be a device by which we could know about the superposition of states without collapsing it?
For example, being able to know that an atom is in a 30% probability of being unexcited and a 70% probability of being excited, without actually collapsing it in either of both states?

 

[ZapperZ]

But there ARE already!

All you need to do is measure a non-commuting observable. That's what the Delft and Stony Brook experiment did, by measuring the energy gap instead of the "current directions" in a superconducting junction.

From a previous thread:

The SQUID experiments conducted recently at Stony Brook[1] and at Delft[2] CLEARLY showed a case where the supercurrent was flowing in BOTH DIRECTIONS simultaneously. It is the ONLY means to attain the two separate states. If the supercurrent is flowing either in one direction OR the other, the effect will NOT be detected.

The same can be said about a 2-slit experiment. It is a superposition of the path through BOTH slits simultaneously. This is the only means to get the interference pattern, implicating that a single photon is interfering with itself. A photon that goes through EITHER one slit or the other will NOT produce the identical interference pattern.

I have mentioned this reference before, but I highly recommend a topical review on this issue by Tony Leggett.[3] There is a huge, substantial but subtle issue that is involved here when discussing the Schrödinger Cat-type states. One cannot fully understand and appreciate QM without knowing and understanding the issues surrounding quantum superposition. When Feynman stated that when you understand the double-slit, you'll understand QM, this is exactly the principle he was referring to!

Zz.

[1] J.R. Friedman et al., Nature v.406, p.43 (2000).
[2] C.H. van der Wal et al., Science v.290, p.773 (2000).
[3] A.J. Leggett J. Phys: Cond. Matt. v.14, p.415 (2002).