Note: This article assumes the existence of the multiverse — the many-worlds interpretation (MWI) of quantum mechanics. Nothing beats MWI for its sheer ability to explain quantum phenomena. If you believe that science is primarily about explaining things and not just predicting them, then MWI is for you.
Quantum computers will be in the spotlight soon with the upcoming announcement of their supremacy. It’s common knowledge that a quantum computer is implemented with qubits, and qubits take on multiple values at the same time. But how exactly do they do that? And how exactly does the computation take place? Enter the multiverse.
First some background: modeling quantum phenomena is a computational task that quickly becomes intractable using normal, classical computers. In the early 80s, Richard Feynman realized that the quantum behavior *itself* was a form of computation. For example, the pattern made by interfering photons on the wall of the double-slit experiment is a form of computation. Feynman figured out how to harness the computational ability of simple quantum phenomena to model more complicated ones. Soon after, David Deutsch extended this work to prove its universality — the universal quantum computer.
David Deutsch is also the foremost proponent of the multiverse — the explanation of quantum mechanics that all possibilities do actually occur in parallel universes. The reality of these parallel universes can be shown by the computation they perform inside of a quantum computer.
A qubit is built with an isolated piece of quantum behavior, such as an electron’s spin. The electron is put into superposition and differentiates into parallel versions of itself containing each possible qubit value — the direction of its spin in this case. Entangle multiple qubits together and the number of universes skyrockets exponentially. When an operation is performed on the qubits — such as rotating the electron spin — each parallel universe then runs a part of the computation. This parallelization is what gives the quantum computer its incredible power.
The key to quantum computing, and what makes it so difficult, is that these qubits must be absolutely isolated from the surrounding environment. This keeps the multiverse from differentiating outside of the qubits (otherwise known as decoherence). At the end of the computation, all of the universes interact and come to the same result. The universes inside the computer are now exactly the same, and the result becomes available to the outside environment.
If a qubit does prematurely interact with the surrounding system, the surroundings will differentiate into parallel universes as well, in what’s known as a “wave of differentiation”. Each universe’s computation is now lost to the others. If you were the operator of the computer, there would be many copies of you, each seeing a small part of the failed computation that your universe ran. Essentially, it’s now impossible to make the universes identical and put them back together — aka decoherence.
This, by the way, is our normal experience of the macroscopic world — the parallel universes, the parallel versions of us, cannot communicate with each other. A quantum computer creates a special environment that allows communication between parallel universes.
Explaining a quantum computer with anything other than the multiverse is imprecise and confusing. As Deutsch puts it in his book, The Fabric of Reality:
To those who still cling to a single-universe world-view, I issue this challenge: explain how Shor’s algorithm (how quantum computers will break encryption) works. I do not merely mean predict that it will work, which is merely a matter of solving a few uncontroversial equations. I mean provide an explanation. When Shor’s algorithm has factorized a number, using 10^500 or so times the computational resources than can be seen to be present, where was the number factorized? There are only about 10^80 atoms in the entire visible universe, an utterly minuscule number compared with 10^500. So if the visible universe were the extent of physical reality, physical reality would not even remotely contain the resources required to factorize such a large number. Who did factorize it, then? How, and where, was the computation performed?
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Thanks @DavidDeutschOxf for the sanity check on this article.
