Revised: January 27, 2015

Published: March 26, 2015

**Keywords:**quantum entanglement, quantum complexity theory, quantum interactive proofs, quantum statistical zero knowledge, BQP, QMA, QSZK, QIP, separability testing

**Categories:**quantum computing, entanglement, complexity theory, interactive proofs, quantum interactive proofs, zero knowledge, BQP, QMA, QIP, separability

**ACM Classification:**F.1.3

**AMS Classification:**68Q10, 68Q15, 68Q17, 81P68

**Abstract:**
[Plain Text Version]

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 $n$.