 which were common to many early quantum algorithms. Note the appearance that \(f\) is evaluated for each of its possible inputs simultaneously. The idea that we should take this at face value—that quantum computers actually do compute a function for many different input values simultaneously—is what Duwell (2018) calls the Quantum Parallelism Thesis (QPT). For Deutsch, who accepts it as true, the only reasonable explanation for the QPT is that the many worlds interpretation (MWI) of quantum mechanics is also true. For Deutsch, a quantum computer in superposition, like any other quantum system, exists in some sense in many classical universes simultaneously. These provide the physical arena within which the computer effects its parallel computations. This conclusion is defended by Hewitt-Horsman (2009) and by Wallace (2012). Wallace notes, however, that the QPT—and hence the explanatory need for many worlds—may not be true of all or even most quantum algorithms.