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arXiv:2112.00090 · Open Systems & Information Dynamics

A Rigidity Property of Complete Systems of Mutually Unbiased Bases

Máté Matolcsi, Mihály Weiner

Published Sep 1, 2021• 1 citation
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Abstract

Suppose that for some unit vectors [Formula: see text] in [Formula: see text] we have that for any [Formula: see text] [Formula: see text] is either orthogonal to [Formula: see text] or [Formula: see text] (i.e., [Formula: see text] and [Formula: see text] are unbiased). We prove that if [Formula: see text], then these vectors necessarily form a complete system of mutually unbiased bases, that is, they can be arranged into [Formula: see text] orthonormal bases, all being mutually unbiased with respect to each other.

DOI
10.1142/s1230161221500128

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Suppose that for some unit vectors [Formula: see text] in [Formula: see text] we have that for any [Formula: see text] [Formula: see text] is either orthogonal to [Formula: see text] or [Formula: see text] (i.e., [Formula: see text] and [Formula: see text] are unbiased). We prove that if [Formula: see text], then these vectors necessarily form a complete system of mutually unbiased bases, that is, they can be arranged into [Formula: see text] orthonormal bases, all being mutually unbiased with respect to each other.
Two orthonormal bases [Formula: see text] and [Formula: see text] of a [Formula: see text]-dimensional complex inner-product space [Formula: see text] are called mutually unbiased bases (MUBs) if and only if [Formula: see text] holds for all [Formula: see text] and [Formula: see text]. The size of any set containing pairwise MUBs of [Formula: see text] cannot exceed [Formula: see text]. If [Formula: see text] is a power of a prime, then extremal sets containing [Formula: see text] MUBs are known to exist, which are called the complete MUBs of [Formula: see text]. We have not known whether there exist complete MUBs when [Formula: see text] is not a power of a prime so far. Therefore, many researchers focus their attention on approximately mutually unbiased bases (AMUB). In this paper, two new constructions of AMUB of [Formula: see text] are provided based on the mixed character sum of two special kinds of functions over finite fields.
Analogous to the notion of mutually unbiased bases for Hilbert spaces, we consider mutually unbiased unitary bases (MUUBs) for the space of operators, [Formula: see text], acting on such Hilbert spaces. The notion of MUUB reflects the equiprobable guesses of unitary operators in one basis of [Formula: see text] when estimating a unitary operator in another. Though, for prime dimension [Formula: see text], the maximal number of MUUBs is known to be [Formula: see text], there is no known recipe for constructing them, assuming they exist. However, one can always construct a minimum of three MUUBs, and the maximal number is approached for very large values of [Formula: see text]. MUUBs can also exist for some [Formula: see text]-dimensional subspace of [Formula: see text] with the maximal number being [Formula: see text].
Two orthonormal bases in the complex space of dimension d, are said to be mutually unbiased if the square of the magnitude of the inner product of any vector from one basis with any vector in other basis is equal to the reciprocal of the dimension d. Mutually unbiased bases are used for optimal state determination of mixed quantum states. It is known that in any dimension d, the number of mutually unbiased bases is at most d+1. Ivanovic found a complete set of mutually unbiased bases for prime dimensions. His construction was generalized by Wootters and Fields for prime power dimensions. There is a strong connection between maximally commuting bases of orthogonal unitary matrices and mutually unbiased bases. Based on this connection, there exits a constructive proof of the existence of a complete set of mutually unbiased bases for prime power dimensions. This thesis is an exploration on construction of mutually unbiased bases.
In a quantum system having a finite number N of orthogonal states, two orthonormal bases {ai} and {bj} are called mutually unbiased if all inner products ⟨ai∣bj⟩ have the same modulus 1∕N. This concept appears in several quantum information problems. The number of pairwise mutually unbiased bases is at most N+1 and various constructions of such N+1 bases have been found when N is a power of a prime number. We study families of formulas that generalize these constructions to arbitrary dimensions using finite rings. We then prove that there exists a set of N+1 mutually unbiased bases described by such formulas, if and only if N is a power of a prime number.
Two orthonormal bases A and B are said to be mutually unbiased when, for every pair of vectors |αi〉 ∈ A, |βj〉 ∈ B, |〈αi|βj〉|2 = 1/d, where d is the dimension of the system. We have tried different methods to construct MUB’s in six dimensions. These include numerical searches for orthonormal bases, extension of Wootters and Fields’ method using the concept of field, and trying to partition the tensor product of qubits and qutrits operators to generate MUB’s as their eigenbases as suggested by Lawrence et al. Our studies have yielded no more than three MUB’s, and we are led to the conclusion that either only these three bases exist or the method for constructing such bases is radically different from those used so far. We shall present the difficulties encountered in each of these methods while searching for MUB’s in composite dimensions.