So this “logical possibility’ (blood sugar measurement with actual result) must simulate not only the blood, but the heart, the person, the planet on which he resides and the universe which houses the planet, in order for the combined quantum state to appropriately render a universe to calculate the correct results of a blood sugar measurement (or any other wanted measurement) that is made on this merely “possible” universe.

Slow down there. In order to "simulate" the behavior of an entity X using counterfactual measurement, you need X (both actually and counterfactually) to be isolated from the rest of the universe (interactions must be weak enough to not decohere the superposition). To say that we must be able to simulate the rest of the universe because we could instead be measuring Y, Z, etc is confusing the matter.

The basic claim of the counterfactualists is: We can find out about a possible state of X - call it X' - by inducing a temporary superposition in X - schematically, |X> goes to |X>+|X'>, and then back to |X> - while it is coupled to some other quantum system. We find out something about X' by examining the final state of that other system, but X' itself never actually existed, just X.

So the core claim is that by having quantum control over an entity, you can find out about how it would behave, without actually making it behave that way. This applies to any entity or combination of entities, though it will be much easier for some than others.

Now first I want to point out that being a single-world theorist does not immediately make you a counterfactualist about these measurements. All a single-world theorist has to do is to explain quantum mechanics without talking about a multiverse. Suppose someone were to say of the above process that what actually existed was X, then X', and then X again, and that X' while it existed interacted a little with the auxiliary quantum system. Suddenly the counterfactualist magic is gone, and we know about X' simply because situation X' really did exist for a while, and it left a trace of its existence in something else.

So here is the real issue: The discourse of quantum mechanics is full of "superpositions". Not just E-V bomb-testing and a superposition which goes from one component to two and back to one - but superpositions in great multitudes. Quantum computers in exponentially large superpositions; atoms and molecules in persistent multi-component superpositions; complicated macro-superpositions which appear to be theoretically possible. A single-world interpretation of quantum theory has to deal with superpositions of all sorts, in a way such that multiplicity of possibility does not equate to multiplicity of actuality. That might be achieved by saying that only one component of the superposition is the reality, or even by saying that none of them is, and that what is real (the hidden variables) is something else entirely.

The Copenhagen interpretation does this, but it does it in a non-explanatory way. The wavefunction is just a predictive device; the particle is always somewhere in particular; and it always happens to be where the predictive device says it will be. You can say that, but really you need to say how that manages to be true. You need a model of microscopic dynamics which explains why the wavefunction works. So we can agree that the Copenhagen interpretation is inadequate for a final theory.

Bohm's theory has a single world, but then it uses the wavefunction to guide the events there, so the multiverse seems to be there implicitly. You can, however, rewrite Bohm's equation of motion so it makes no reference to a pilot wave. Instead, you have a nonlocal potential. I'm not aware of significant work in this direction so I don't know if it is capable of truly banishing the multiverse from its explanation of QM.

Anyway, let me return to this issue of interpreting a generic superposition in a single-world way and make some further comments. You should not assume that just because, formally and mathematically, we can write about something like |cat dead>+|cat alive>, that there simply must be some physical reality where both cat-dead information and cat-alive information can be extracted with equal ease. This is very far from having been demonstrated.

First of all, on a purely classical level, I can easily cook up a probability distribution like "50% probability the cat is dead and 50% probability the cat is alive", but I cannot deduce from that, that the multiverse must be real. That the formalism can talk about macro-superpositions doesn't yet make them real.

So in order to make things difficult for the single-world theorist, we need superpositions where the various branches seem to have an equal claim on reality, e.g. where we can probe at will for information about macroscopically distinct situations existing within the superposition. Schrodinger's cat doesn't really work because you only ever see the cat dead or alive. If you could somehow open the lid and see the cat alive, and yet also get a photo from a video camera in the box which showed the cat to be dead - now that would be evidence of a multiverse!

Counterfactual measurement and counterfactual computation certainly sound like this. Thus, in counterfactual computation, you couple your readout system to a quantum computer, and then you do the |X> to |X>+|X'> thing and back again, where X' is "quantum computer makes a computation". So the computer is back in the X state, and the computation never was, but it left its counterfactual trace in the readout system. It's as if you opened the lid on the box and the cat was alive, yet the video camera gave you a photo of the cat dead.

However, the superpositions and entanglements involved in these experiments are so micro, and so far from anything macroscopic and familiar, that to talk about them in these ways is very much a rhetorical choice. A common-sense interpretation of counterfactual measurement would be that you are simply measuring an existing property which would produce the counterfactual behavior under the right circumstances. Thus, I might look at a house of cards and claim that it would fall down in the presence of a strong wind. But I didn't arrive at this correct conclusion by mysteriously probing a parallel world where a strong wind actually blows; I arrived at the conclusion by observing the very fine balance of forces existing among the cards in this world. I learnt something true by observing something actual, not by paranormally observing something possible.

The argument that counterfactual measurement implies many-worlds is applying this common-sense principle, but it's also assuming that the actuality which provides the information cannot be anything less than a duplicate world. Because counterfactual measurement has so far only been performed on microscopic quantum systems, we do not get to apply macroscopic intuitions as we could in the situation of a counterfactual photograph of Schrodinger's cat.

To really make progress here, what we need is a thought-experiment in which a macroscopic superposition is made to yield information about more than one branch, as the counterfactualist rhetoric claims. Unfortunately, your needle-in-the-arm experiment is not there yet, because we haven't gone into the exact details of how it's supposed to work. You can't just say, 'If we did a quantum experiment where we could produce data about glucose levels in someone's bloodstream, without the needle having gone into their arm, why, that would prove that the multiverse is real!' Even just as a hypothetical, that's not enough. You need to explain how the decoherence shielding works and what the quantum readout system is - the one which remembers the interaction with the branch where the needle did go in. We need a physically complete description of the thought-experiment in order to reason about the interpretive possibilities.

Sampling the bloodstream is too complicated because of all the complexities of human metabolism. But something like reversibly sampling a salt crystal and measuring ion density - that sounds a more plausible candidate for analysis, something where the experimental setup can be described and where the analysis is tractable. I'll have to see if there's a proposed experiment or known thought-experiment which is suitable...