No linear stretching of single-copy quantum pseudorandomness
The paper proves that single-copy quantum pseudorandom states (1PRS) cannot be stretched arbitrarily long in a black-box setting. While 1PRS exist for lengths like 1.1n, they do not exist for lengths growing as $\Omega(n^2+\epsilon)$ relative to specific oracles. This separation is achieved by constructing an oracle that limits the effective number of quantum states a generator can use.