Mutually Unbiased Unitaries Bases

TL;DR

This paper explores special sets of unitary bases with uniform inner product properties across various dimensions, constructing maximal sets in some cases, proving nonexistence in others, and analyzing their applications in quantum information and cryptography.

Contribution

It constructs maximal mutually unbiased unitary bases for certain dimensions, proves their nonexistence in others, and analyzes their properties and applications in quantum systems.

Findings

Maximal sets of such bases are constructed for 4D and 2D subspaces.

Nonexistence of such bases is proven for 3D subspaces.

Average fidelity for estimating these unitaries matches that of estimating a completely unknown unitary.

Abstract

We consider the notion of unitary transformations forming bases for subspaces of $M(d,\mathbb{C})$ such that the square of Hilbert-Schmidt inner product of matrices from the differing bases is a constant. Moving from the qubit case, weconstruct the maximal number of such bases for the 4 and 2 dimensional subspaces while proving the nonexistence of such a construction for the 3 dimensional case. Extending this to higher dimensions, we commit to such a construct for the case of qutrits and provide evidence for the existence of such unitaries for prime dimensional quantum systems. Focusing on the qubit case, we show that the average fidelity for estimating any of such a transformation is equal to the case for estimating a completely unknown unitary from $\text{SU}(2)$. This is then followed by a quick application for such unitaries in a quantum cryptographic setup.