Mutually Unbiased Measurements, Hadamard Matrices, and Superdense Coding

TL;DR

This paper explores mutually unbiased measurements (MUMs), extending the concept of MUBs, and demonstrates their applications in quantum communication, including superdense coding, revealing new protocols and disproving existing conjectures.

Contribution

It characterizes MUMs as direct sums of MUBs, constructs new non-decomposable MUMs using quaternionic Hadamard matrices, and applies these to advance superdense coding protocols.

Findings

MUMs are more general than MUBs and can be unbounded in number.

Constructed new MUMs not decomposable into MUBs using quaternionic Hadamard matrices.

Disproved a conjecture on superdense coding rigidity using non-decomposable MUMs.

Abstract

Mutually unbiased bases (MUBs) are highly symmetric bases on complex Hilbert spaces, and the corresponding rank-1 projective measurements are ubiquitous in quantum information theory. In this work, we study a recently introduced generalization of MUBs called mutually unbiased measurements (MUMs). These measurements inherit the essential property of complementarity from MUBs, but the Hilbert space dimension is no longer required to match the number of outcomes. This operational complementarity property renders MUMs highly useful for device-independent quantum information processing. It has been shown that MUMs are strictly more general than MUBs. In this work we provide a complete proof of the characterization of MUMs that are direct sums of MUBs. We then proceed to construct new examples of MUMs that are not direct sums of MUBs. A crucial technical tool for these construction is a correspondence with quaternionic Hadamard matrices, which allows us to map known examples of such matrices to MUMs that are not direct sums of MUBs. Furthermore, we show that -- in stark contrast with MUBs -- the number of MUMs for a fixed outcome number is unbounded. Next, we focus on the use of MUMs in quantum communication. We demonstrate how any pair of MUMs with d outcomes defines a d-dimensional superdense coding protocol. Using MUMs that are not direct sums of MUBs, we disprove a recent conjecture due to Nayak and Yuen on the rigidity of superdense coding for infinitely many dimensions. The superdense coding protocols arising in the refutation reveal how shared entanglement may be used in a manner heretofore unknown.