Compounds of symmetric informationally complete measurements and their application in quantum key distribution

Abstract

Symmetric informationally complete measurements (SICs) are elegant, celebrated, and broadly useful discrete structures in Hilbert space. We introduce a more sophisticated discrete structure compounded by several SICs. A SIC compound is defined to be a collection of d3 vectors in d-dimensional Hilbert space that can be partitioned in two different ways: into d SICs and into d2 orthonormal bases. While a priori their existence may appear unlikely when d>2, we surprisingly find an explicit construction for d=4. Remarkably this SIC compound admits a close relation to mutually unbiased bases, as is revealed through quantum state discrimination. Going beyond fundamental considerations, we leverage these exotic properties to construct a protocol for quantum key distribution and analyze its security under general eavesdropping attacks. We show that SIC compounds enable secure key generation in the presence of errors that are large enough to prevent the success of the generalization of the six-state protocol.