Mutually Unbiased Measurements, Hadamard Matrices, and Superdense Coding
arXiv:2204.11886 · doi:10.1109/TIT.2023.3269524
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.
arXiv commentv2: Added some references and related discussion. v1: 20 pages. Comments welcome!
References in corpus (11)
- On mutually unbiased bases
- All Teleportation and Dense Coding Schemes
- Mutually unbiased measurements in finite dimensions
- Mutually unbiased bases and symmetric informationally complete measurements in Bell experiments
- Maximal nonlocality from maximal entanglement and mutually unbiased bases, and self-testing of two-qutrit quantum systems
- Mutual Unbiasedness in Coarse-grained Continuous Variables
- Mutually Unbiased Coarse-Grained Measurements of Two or More Phase-Space Variables
- Complex Hadamard matrices with noncommutative entries
- New Infinite Families of Perfect Quaternion Sequences and Williamson Sequences
- Entropic uncertainty relations for mutually unbiased periodic coarse-grained observables resemble their discrete counterparts
- Periodic discretized continuous observables are neither continuous nor discrete