22.04.2005

In this paper we show that one-qubit polynomial time computations are as powerful as NC¹ circuits. More generally, we define syntactic models for quantum and stochastic branching programs of bounded width and prove upper and lower bounds on their power. We show that any NC¹ language can be accepted exactly by a width-2 quantum branching program of polynomial length, in contrast to the classical case where width 5 is necessary unless NC¹ = ACC. This separates width-2 quantum programs from width-2 doubly stochastic programs as we show the latter cannot compute the middle bit of multiplication. Finally, we show that bounded-width quantum and stochastic programs can be simulated by classical programs of larger but bounded width, and thus are in NC¹.
For read-once quantum branching programs (QBPs), we give a symmetric Boolean function which is computable by a read-once QBP with O(log n) width, but not by a deterministic read-once BP with o(n) width, or by a classical randomized read-once BP with o(n) width which is "stable" in the sense that its transitions depend on the value of the queried variable but do not vary from step to step. Finally, we present a general lower bound on the width of read-once QBPs, showing that our O(log n) upper bound for this symmetric function is almost tight.