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International Journal of Modern Physics B

MULTIPARTITE QUANTUM SYSTEMS: PHASES DO MATTER AFTER ALL

Luis Lorenzo Sánchez-Soto, A. B. Klimov, Hubert de Guise

Published May 20, 2006• 3 citations
Publisher

Abstract

A comprehensive theory of phase for finite-dimensional quantum systems is developed. The only physical requirement imposed is that phase is complementary to amplitude. This complementarity is implemented by resorting to the notion of mutually unbiased bases. For a d-dimensional system, where d is a power of a prime, we explicitly construct d + 1 classes of maximally commuting operators, each one consisting of d - 1 operators. One of this class consists of diagonal operators that represent amplitudes and, by the finite Fourier transform, operators in this class are mapped to off-diagonal operators that can be appropriately interpreted as phases. The relevant example of a system of qubits is examined in detail.

DOI
10.1142/s0217979206034376

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A comprehensive theory of phase for finite-dimensional quantum systems is developed. The only physical requirement imposed is that phase is complementary to amplitude. This complementarity is implemented by resorting to the notion of mutually unbiased bases. For a d-dimensional system, where d is a power of a prime, we explicitly construct d + 1 classes of maximally commuting operators, each one consisting of d - 1 operators. One of this class consists of diagonal operators that represent amplitudes and, by the finite Fourier transform, operators in this class are mapped to off-diagonal operators that can be appropriately interpreted as phases. The relevant example of a system of qubits is examined in detail.
We develop a comprehensive theory of phase for finite-dimensional quantum systems. The only physical requirement we impose is that phase is complementary to amplitude. To implement this complementarity we use the notion of mutually unbiased bases, which exist for dimensions that are powers of a prime. For a d-dimensional system (qudit) we explicitly construct d+1 classes of maximally commuting operators, each one consisting of d-1 operators. One of this class consists of diagonal operators that represent amplitudes (or inversions). By the finite Fourier transform, it is mapped onto ladder operators that can be appropriately interpreted as phase variables. We discuss the examples of qubits and qutrits, and show how these results generalize previous approaches.
We develop a comprehensive theory of phase for finite-dimensional quantum systems. The only physical requirement we impose is that phase is complementary to amplitude. To implement this complementarity we use the notion of mutually unbiased bases, which exist for dimensions that are powers of a prime. For a d-dimensional system (qudit) we explicitly construct d+1 classes of maximally commuting operators, each one consisting of d-1 operators. One of this class consists of diagonal operators that represent amplitudes (or inversions). By the finite Fourier transform, it is mapped onto ladder operators that can be appropriately interpreted as phase variables. We discuss the examples of qubits and qutrits, and show how these results generalize previous approaches.
A complete set of d + 1 mutually unbiased bases exists in a Hilbert space of dimension d whenever d is power of a prime. We discuss a simple construction of d + 1 disjoint classes (each one having d − 1 commuting operators) such that the corresponding eigenstates form sets of unbiased bases. One of these classes is diagonal and can be mapped to “ladder” operators by means of the finite Fourier transform. Using this idea, we naturally introduce the notion of quantum phase as complementary to the inversion. Relevant examples involving qubits and qutrits are discussed.
A complete set of d + 1 mutually unbiased bases exists in a Hilbert space of dimension d , whenever d is a power of a prime. We discuss a simple construction of d + 1 disjoint classes (each one having d − 1 commuting operators) such that the corresponding eigenstates form sets of unbiased bases. Such a construction works properly for prime dimension. We investigate an alternative construction in which the real numbers that label the classes are replaced by a finite field having d elements. One of these classes is diagonal, and can be mapped to cyclic operators by means of the finite Fourier transform, which allows one to understand complementarity in a similar way as for the position–momentum pair in standard quantum mechanics. The relevant examples of two and three qubits and two qutrits are discussed in detail.
We present a detailed computational and algebraic study of Mutually Unbiased Bases (MUBs) in finite-dimensional Hilbert spaces, with a particular focus on dimensions 2, 3, 4, and the challenging case of 6. Starting from the Hadamard-phase parametrization, we derive explicit analytical conditions for mutual unbiasedness in dimension 4, providing a tractable system of trigonometric constraints on the phase parameters. We then explore a tensor-product construction via Pauli operators, highlighting the algebraic and group-theoretical origin of MUBs in two-qubit systems, and demonstrating how these constructions yield a complete set of 5 MUBs in dimension 4. Extending our approach, we investigate the Fourier-family method in dimension 6, where the absence of a prime-power structure imposes strong rigidity constraints and limits the known constructions to sets of 3 MUBs. We provide a systematic computational framework for testing candidate phase vectors, bridging the gap between analytical insight and numerical exploration. Finally, we generalize the discussion to arbitrary prime-power dimensions, emphasizing the role of finite-field structures, Heisenberg-Weyl operators, and discrete symmetries in generating complete sets of MUBs. Our work offers a transparent, line-by-line verification methodology, highlighting both the geometric and algebraic richness of MUBs, and clarifying why certain dimensions resist full analytical constructions. This study serves as a comprehensive resource for researchers seeking both theoretical understanding and practical construction of MUBs in quantum information science.