Multipartite subspaces containing no locally inaccessible information

One notion of nonlocality in quantum theory is the fact that information may be encoded in a composite system in such a way that it is not accessible through local measurements, even with the assistance of classical communication. Thus, contrary to the classical case, there exists information in quantum many-body systems which cannot be accessed locally. We show, however, that, remarkably, two-dimensional subspaces do not have this property: Any physically allowed measurement on information encoded in any two-dimensional subspace, regardless of entanglement or multipartite structure, may be performed locally. Further, this requires only local measurement and feed-forward of classical information, readily achievable in many experimental platforms. As an application to quantum secret sharing we suggest a twist on a well-known quantum information splitting protocol, which ensures that no receiving party ever has access to the full state sent, but parties must work together to perform measurements on the state. These results may have practical applications to the measurement of encoded qubits in, e.g., quantum secret sharing, quantum error correction, and reveal a fundamental property unique to two-dimensional subspaces.

Figure 1

The $x\text{−}z$ plane of the Bloch sphere showing the Breidbart basis states $|B0\rangle ,|B1\rangle$, intermediate between the $x$ and $z$ bases.