  Self-testing is a method to characterise an arbitrary quantum system based
only on its classical input-output correlations, and plays an important role in
device-independent quantum information processing as well as quantum complexity
theory. Prior works on self-testing require the assumption that the system's
state is shared among multiple parties that only perform local measurements and
cannot communicate. Here, we replace the setting of multiple non-communicating
parties, which is difficult to enforce in practice, by a single computationally
bounded party. Specifically, we construct a protocol that allows a classical
verifier to robustly certify that a single computationally bounded quantum
device must have prepared a Bell pair and performed single-qubit measurements
on it, up to a change of basis applied to both the device's state and
measurements. This means that under computational assumptions, the verifier is
able to certify the presence of entanglement, a property usually closely
associated with two separated subsystems, inside a single quantum device. To
achieve this, we build on techniques first introduced by Brakerski et al.
(2018) and Mahadev (2018) which allow a classical verifier to constrain the
actions of a quantum device assuming the device does not break post-quantum
cryptography.
