That's a straw man right there. The only empirical proof we have of consciousness is our personal experience. Cogito ergo sum
I don't experience consciousness in your body or in my father's body.
According to user John Cuthbert, consciousness is only the product of matter structured in certain way. That would mean that if you structure protons, electrons, neutrons, quarks in certain way you will obtain consciousness, while if your structure protons, electrons, neutrons, quarks in other way you will obtain a handkerchief or a rock, and not consciousness. It doesn't solve the problem because they are still the same things. According to that, consciousness would be the fifth state of matter: Solid, Liquid, Gas, Plasma, and Conscious.
If you both don't agree to this, you are implicitly admitting consciousness doesn't exist.
The relation with quantum decoherence is quite understandable. Physics in certain scales work one way and in bigger scales work in a completely different way, with different laws of nature. This is parallel to my former statements about protons, neutrons, quarks, etc.
You haven't studied the nature of our universe and the 4 physical forces who govern it, then it's easy to understand your misperception.
In pure science (physics), there is no such thing as "separate universes" between the outputs and inputs of a system. A system works with the universe, physicists regard the Laws of Nature as Universal, thanks to the confirmations made by the experiments.
Empirically, unequivocally, unquestionably, scientifically, a system works with the Laws of Nature of our universe, and not separated from those Laws of Nature. This means the light/information (one of the 4 forces of Nature) the system uses, is as basic for the functioning of the system as it is the brain or the neuron. Laws of nature are universal for all the systems involved.
I invite you to know more about the electromagnetic field. QED (Quantum Electrodynamics) is the quantum theory for the electromagnetic force. It is the MOST ACCURATE THEORY ever made by the human being. 12 decimal numbers of accuracy: