Even if quantum computers do not exist, a theoretical model of them does exist, so we can discuss them the same way we discuss a turring machine. (Although we might need to clarify which model we are talking about).
However, that is irrelevant because quantum computers do exist. For example, in 2012, researcher's factored the number 21. [1].
However, even the existence of quantum computers is irrelevant, because programming languages designed for them still exist. [2] Not only that, but you can still run programs written in such languages on a classical computer.
Of course, you could also do the type of Bayesian programming in a language designed for that, instead of one designed for Quantom computation.
However, that is irrelevant because quantum computers do exist. For example, in 2012, researcher's factored the number 21. [1].
However, even the existence of quantum computers is irrelevant, because programming languages designed for them still exist. [2] Not only that, but you can still run programs written in such languages on a classical computer.
Of course, you could also do the type of Bayesian programming in a language designed for that, instead of one designed for Quantom computation.
[1] http://arxiv.org/pdf/1111.4147v2.pdf [2] http://tph.tuwien.ac.at/~oemer/qcl.html