



02008nas a2200193   4500008004100000245007500041210006900116260001400185490000700199520143800206100001801644700002101662700001801683700002401701700001701725700002101742700001401763856003701777       2013                        eng d00aUniqueness of Quantum States Compatible with Given Measurement Results0 aUniqueness of Quantum States Compatible with Given Measurement R  c2013/7/110 v883 a  We discuss the uniqueness of quantum states compatible with given results for
measuring a set of observables. For a given pure state, we consider two
different types of uniqueness: (1) no other pure state is compatible with the
same measurement results and (2) no other state, pure or mixed, is compatible
with the same measurement results. For case (1), it is known that for a
d-dimensional Hilbert space, there exists a set of 4d-5 observables that
uniquely determines any pure state. We show that for case (2), 5d-7 observables
suffice to uniquely determine any pure state. Thus there is a gap between the
results for (1) and (2), and we give some examples to illustrate this. The case
of observables corresponding to reduced density matrices (RDMs) of a
multipartite system is also discussed, where we improve known bounds on local
dimensions for case (2) in which almost all pure states are uniquely determined
by their RDMs. We further discuss circumstances where (1) can imply (2). We use
convexity of the numerical range of operators to show that when only two
observables are measured, (1) always implies (2). More generally, if there is a
compact group of symmetries of the state space which has the span of the
observables measured as the set of fixed points, then (1) implies (2). We
analyze the possible dimensions for the span of such observables. Our results
extend naturally to the case of low rank quantum states.
1 aChen, Jianxin1 aDawkins, Hillary1 aJi, Zhengfeng1 aJohnston, Nathaniel1 aKribs, David1 aShultz, Frederic1 aZeng, Bei  uhttp://arxiv.org/abs/1212.3503v2