**Resolvable Block Designs in Construction of Approximate Real MUBs that are Sparse**

*Ajeet Kumar and Subhamoy Maitra*

**Abstract: **Several constructions of Mutually Unbiased Bases (MUBs) borrow tools from combinatorial objects. In this paper we focus
how one can construct Approximate Real MUBs (ARMUBs) with improved parameters using results from the domain of Resolvable Block Designs (RBDs). We first explain the generic idea of our strategy in relating the RBDs with MUBs/ARMUBs, which are sparse (the basis vectors have small number of non-zero co-ordinates). Then specific parameters are presented, for which we can obtain new classes and improve the existing results. To be specific, we present an infinite family of $\lceil\sqrt{d}\rceil$ many ARMUBs for dimension $d = q(q+1)$, where $q \equiv 3 \bmod 4$ and it is a prime power, such that for any two vectors $v_1, v_2$ belonging to different bases, $|\braket{v_1|v_2}| < \frac{2}{\sqrt{d}}$. We also demonstrate certain cases, such as $d = sq^2$, where $q$ is a prime power and $sq \equiv 0 \bmod 4$. These findings subsume and improve our earlier results in [Cryptogr. Commun. 13, 321-329, January 2021]. This present construction idea
provides several infinite families of such objects, not known in the literature, which can find efficient applications in quantum information processing for the sparsity, apart from suggesting that parallel classes of RBDs are intimately linked with MUBs/ARMUBs.

**Category / Keywords: **foundations / (Approximate Real) Mutually Unbiased Bases, Combinatorial Design, Cryptology, Hadamard Matrices, Quantum Information Theory, Resolvable Block Design.

**Date: **received 1 Jul 2021

**Contact author: **subho at isical ac in

**Available format(s): **PDF | BibTeX Citation

**Version: **20210705:184751 (All versions of this report)

**Short URL: **ia.cr/2021/901

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