Volume 11 (2015) Article 3 pp. 59-103
Quantum Interactive Proofs and the Complexity of Separability Testing
We identify a formal connection between physical problems related to the detection of separable (unentangled) quantum states and complexity classes in theoretical computer science. In particular, we show that to nearly every quantum interactive proof complexity class (including $\mathsf{BQP}$, $\mathsf{QMA}$, $\mathsf{QMA}(2)$, and $\mathsf{QSZK}$), there corresponds a natural separability testing problem that is complete for that class. Of particular interest is the fact that the problem of determining whether an isometry can be made to produce a separable state is either $\mathsf{QMA}$-complete or $\mathsf{QMA}(2)$-complete, depending upon whether the distance between quantum states is measured by the one-way LOCC norm or the trace norm. We obtain strong hardness results by employing prior work on entanglement purification protocols to prove that for each $n$-qubit maximally entangled state there exists a fixed one-way LOCC measurement that distinguishes it from any separable state with error probability that decays exponentially in $n$.