Quantum computers are real. Their capabilities may be limited at present but they exist. This may not seem like an extraordinary statement until you fully consider what a quantum computer does.
It does not matter that the practical applications of even an advanced future quantum computer would likely be limited to factoring vast numbers for secure encryption purposes. In many ways conceiving of this potential 'killer app' is just a way to secure the funding to build them in the first place. No, what really matters is that the calculations involved just aren't possible within the bounds of this Universe alone.
So where are the calculations being done? They are being done in tandem with the requisite number of equivalent quantum computers, staffed by the requisite number of equivalent copies of the person/people running the quantum computers, in the requisite number of equivalent universes required to complete the calculation.
As the wonderful John Gribbin points out in his recent book 'In Search of the Multiverse', this is not equivalent to the 'phase spaces' used by mathematicians to undertake complex calculations requiring theoretical extra dimensions. 'Phase spaces' work but you cannot use them to do the kind of calculations a quantum computer can do - because the (to all intents and purposes infinite) computing power of the Multiverse is simply not available without one.
If this very real example of how the Multiverse can be used for a practical application leaves you reeling then you are not alone. There is no guarantee that even the operators of these devices are absorbing the full reality of the Multiverse when they talk about 'spin direction' and 'superposition'. If superposition works and can be used for calculation purposes then what does it matter?
At the quantum level - the level of the almost impossibly tiny - 'objects' do not behave as they do at our level. Their position is not fixed but consists of 'clouds' of positional probabilities. They can be in multiple places at the same time. This may seem impossible to us but it is a law of the nature of the quantum level. The property of 'fixedness' only arises at larger scales, including our own. This quantum property can be harnessed for computation by using quantum superpositions to 'represent' binary digits - allowing one to undertake inconceivably large numbers of calculations all at the same time.
My reading of Gribbin's explanation of the process is that the quantum computers in the various universes 'call' for the answer all at the same time. The superpositions allow the calculation to be 'split' and undertaken across the requisite part of the Multiverse. The answer is then instantly 'collapsed' back to the observer in each universe.
Why am I trying to explain this when I don't understand it and do not have the mathematical language to describe it? Because I feel a sense of wonder at this and wish to teach myself a language for describing it to myself.
I want to internalise it and I think it might be enjoyable for others to internalise it too.