Am I dumb? Or can we at least guess when a quantum cryptographic message is read?

Quantum cryptography is based on the fact that measures will destroy what it observes.

In quantum cryptography you normally intricate to states A & B and transmit them over an unsecured layer, but any attempt at measuring the message will destroy the intrication/entanglement.

However, the problem is what if I can :
  1. I can store a message;
  2. I can know when the initial entanglement disappears; 
Then I can know exactly when the person deciphered the messages.

I don't know if it would be useful to an attacker, and I don't even know I my vague souvenir of quantum mechanics are solid enough to be certain of what I suggest.

Here are the experiments that make me wonder:

Storing quantum states without decoherence (losing information)

Here is a way to do it. Anyway, if we want to do usable quantum cryptography, we will have either to store the message or better repeat or regenerate it without measuring it which will be fun because I think the weakness in quantum cryptography will lie there.

Knowing if states are intricated without destroying the intrication

I remember that the initial experiment of Cohen Tannoudji on light decoherence was made using a non destructive way of measuring the number of photon using Ramsay interferometer : you send cold atom in a cavity containing light, and you make them interfere with other atoms using an other path (of same length) in a dual slit kind of montage. At the end, if level of energy varies, your interferometry changes without destroying anything.
Ramsay interferometer not destroying quantum states

Weak points in my theory

Did I understand correctly all the implications of the measures ?

Can we generate another system with a new entanglement C without destroying the initial entanglement, and do we keep the primary information ?

Ridiculing myself and dreaming at the same time :) 

The other point is quantum mechanics is hardly fit for common world intuition. I guess I may have missed quite a few point and that teams are already working on the topic :
it may be one of the point of quantum teleportation. The fun there is we would have to transport anyway quantum states at a speed lesser than light.  Since the source of information will weight quite a lot, it will probably use normal channels (air carrier, boats, human beings), making us going back to pre XXth century spy problematics.

But once both system are safely exchanged, if you synchronize measure on a series of entangled particles on both sides, you have a faster than light one way of exchanging information. Yes, it looks like an ansible.

And no, don't dream : nothing related to energy travels at a speed faster than the speed of light but phase can. That is causality.

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