Beyond Bell sampling: stabilizer state learning and quantum pseudorandomness lower bounds on qudits

Bell sampling is a simple yet powerful measurement primitive that has recently attracted a lot of attention, and has proven to be a valuable tool in studying stabiliser states. Unfortunately, however, it is known that Bell sampling fails when used on qudits of dimension d>2. In this paper, we explore and quantify the limitations of Bell sampling on qudits, and propose new quantum algorithms to circumvent the use of Bell sampling in solving two important problems: learning stabiliser states and providing pseudorandomness lower bounds on qudits. More specifically, as our first result, we characterise the output distribution corresponding to Bell sampling on copies of a stabiliser state and show that the output can be uniformly random, and hence reveal no information. As our second result, for d=p prime we devise a quantum algorithm to identify an unknown stabiliser state in (ℂ^p)^⊗ n that uses O(n) copies of the input state and runs in time O(n^4). As our third result, we provide a quantum algorithm that efficiently distinguishes a Haar-random state from a state with non-negligible stabiliser fidelity. As a corollary, any Clifford circuit on qudits of dimension d using O(logn/logd) auxiliary non-Clifford single-qudit gates cannot prepare computationally pseudorandom quantum states.