# Can coherence be restored?

1. Jul 15, 2012

### San K

can coherence be restored (after de-coherence)?

let's look at single particle (interference) in a double slit experiment:

when we try to gain which way ....

the ordering of the phase angles between the components of a photon in a quantum superposition is disturbed/changed (?)

can we restore the coherence back?

is this question conceptually similar to - can entropy be restored?

on a separate note:

We can introduce a phase difference, between say the two paths of a photon, say via placing a half/fully silvered mirror on one of the paths. We can zero out the phase difference via another half/fully silvered mirror on the other path.

Thus we are able to introduce a phase difference and again reduce the phase difference back to zero as long as there is no measurement.

Last edited: Jul 15, 2012
2. Jul 16, 2012

### Demystifier

Restoring coherence is like reversing the thermodynamic arrow of time. In principle possible, in practice impossible.

3. Jul 16, 2012

### San K

ok....so let's for a moment go in principle.....

so let's assume coherence can be resorted......so in principle....

could one have which-way and interference pattern (via restoration of coherence)?

4. Jul 16, 2012

### Demystifier

Yes, as long as restoration of coherence is possible in principle, one could, in principle, have both which-way and interference pattern.

5. Jul 16, 2012

### San K

thanks for the information, Demystifier

6. Jul 16, 2012

### Delta Kilo

But restoring coherence would necesserily involve erasing all traces of the which-way information from the environment, would it not?

7. Jul 16, 2012

### Demystifier

It is certainly a possibility, but I don't think that it is strictly necessary. In your case re-coherence applies to the whole universe including the measuring apparatus, while in my case re-coherence applies only to the measured system and not the measuring apparatus. Your case is probably simpler mathematically, but in practice it is even more difficult to achieve than mine. If re-coherence was done by a quantum Maxwell demon, it would be easier for him to affect only the measured system and not the rest of the universe.

8. Jul 16, 2012

### San K

On a separate note:

Do we have two kinds of phase changes

1 phase change without measurement/detection
2 phase change(s) due to measurement/detection. Is this one called decoherence?

We know from experiments that 1 is easily possible. Also interference can be restored but which-way is lost and becomes unrecoverable.

The process and interactions for 2 (i.e. Decoherence) above is much more complex.

9. Jul 16, 2012

### Demystifier

Fundamentally, there is no difference between the two. But from a practical point of view, decoherence is not merely a change of phase. Decoherence is a change of phase in a complex and uncontrollable way, so that you cannot longer predict what the phase will be. For a quantum Maxwell demon who can see the phase of the wave function of the whole universe, there is no such thing as decoherence.

10. Jul 16, 2012

### f95toli

The answer to the questions is actually yes, at least to some extent. So called re-focusing pulse schemes are routinely used in e.g. NMR (and MRI, i.e. at most hospitals) to increase the effective T2 time of atomic systems. Similar schemes are also used for qubit operations.

This is yet another case where one can go wrong if one focuses on the double-slit experiment, simply because it is not a very "typical" system.
NMR or similar system (electron spin in a magnetic field) are much better "toy systems" if one want so learn about coherence.