Rewindable Quantum Computation and Its Equivalence to Cloning and Adaptive Postselection

Abstract

We define rewinding operators that invert quantum measurements. Then, we define complexity classes RwBQP, CBQP, and AdPostBQP as sets of decision problems solvable by polynomial-size quantum circuits with a polynomial number of rewinding operators, cloning operators, and adaptive postselections, respectively. Our main result is that BPPPP⊆RwBQP=CBQP=AdPostBQP⊆PSPACE. As a byproduct of this result, we show that any problem in PostBQP can be solved with only postselections of events that occur with probabilities polynomially close to one. Under the strongly believed assumption that BQP⊉SZK, or the shortest independent vectors problem cannot be efficiently solved with quantum computers, we also show that a single rewinding operator is sufficient to achieve tasks that are intractable for quantum computation. Finally, we show that rewindable Clifford circuits remain classically simulatable, but rewindable instantaneous quantum polynomial time circuits can solve any problem in PP.