The standard proof that BQPSPACE is in PSPACE relies on a Savitch game type analysis on path integrals. However, it assumes the time running length for BQPSPACE is at most exponentially long. This is true for PSPACE, but for closed quantum systems with a fixed number of degrees of freedom, it typically takes a doubly exponentially long time before Poincare recurrence due to the exponential nature of the state vector. So, does the proof still run through, or not?
Why is BQPSPACE in PSPACE if it can have doubly exponentially long running times?
quantum computing