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arXiv:2301.12137 · Quantum Information Processing

Unitarily inequivalent local and global Fourier transforms in multipartite quantum systems

Chao Lei, Apostol Vourdas

Published Jan 19, 2023• 2 citations
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Abstract

Abstract A multipartite system comprised of n subsystems, each of which is described with ‘local variables’ in $${{\mathbb {Z}}}(d)$$ Z ( d ) and with a d-dimensional Hilbert space H(d), is considered. Local Fourier transforms in each subsystem are defined and related phase space methods are discussed (displacement operators, Wigner and Weyl functions, etc). A holistic view of the same system might be more appropriate in the case of strong interactions, which uses ‘global variables’ in $${{\mathbb {Z}}}(d^n)$$ Z ( d n ) and a $$d^n$$ d n -dimensional Hilbert space $$H(d^n)$$ H ( d n ) . A global Fourier transform is then defined and related phase space methods are discussed. The local formalism is compared and contrasted with the global formalism. Depending on the values of d, n the local Fourier transform is unitarily inequivalent or unitarily equivalent to the global Fourier transform. Time evolution of the system in terms of both local and global variables, is discussed. The formalism can be useful in the general area of Fast Fourier transforms.

DOI
10.1007/s11128-022-03820-2

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Abstract A multipartite system comprised of n subsystems, each of which is described with ‘local variables’ in Z(d){{\mathbb {Z}}}(d)Z(d) Z ( d ) and with a d-dimensional Hilbert space H(d), is considered. Local Fourier transforms in each subsystem are defined and related phase space methods are discussed (displacement operators, Wigner and Weyl functions, etc). A holistic view of the same system might be more appropriate in the case of strong interactions, which uses ‘global variables’ in Z(dn){{\mathbb {Z}}}(d^n)Z(dn) Z ( d n ) and a dnd^ndn d n -dimensional Hilbert space H(dn)H(d^n)H(dn) H ( d n ) . A global Fourier transform is then defined and related phase space methods are discussed. The local formalism is compared and contrasted with the global formalism. Depending on the values of d, n the local Fourier transform is unitarily inequivalent or unitarily equivalent to the global Fourier transform. Time evolution of the system in terms of both local and global variables, is discussed. The formalism can be useful in the general area of Fast Fourier transforms.
Abstract Two methods for fast Fourier transforms are used in a quantum context. The first method is for systems with dimension of the Hilbert space D=dnD=d^nD=dn D = d n with d an odd integer, and is inspired by the Cooley-Tukey formalism. The ‘large Fourier transform’ is expressed as a sequence of n ‘small Fourier transforms’ (together with some other transforms) in quantum systems with d-dimensional Hilbert space. Limitations of the method are discussed. In some special cases, the n Fourier transforms can be performed in parallel. The second method is for systems with dimension of the Hilbert space D=d0...dn−1D=d_0...d_{n-1}D=d0​...dn−1​ D = d 0 . . . d n - 1 with d0,...,dn−1d_0,...,d_{n-1}d0​,...,dn−1​ d 0 , . . . , d n - 1 odd integers coprime to each other. It is inspired by the Good formalism, which in turn is based on the Chinese reminder theorem. In this case also the ‘large Fourier transform’ is expressed as a sequence of n ‘small Fourier transforms’ (that involve some constants related to the number theory that describes the formalism). The ‘small Fourier transforms’ can be performed in a classical computer or in a quantum computer (in which case we have the additional well known advantages of quantum Fourier transform circuits). In the case that the small Fourier transforms are performed with a classical computer, complexity arguments for both methods show the reduction in computational time from O(D2)\mathcal{O}(D^2)O(D2) O ( D 2 ) to O(Dlog⁡D)\mathcal{O}(D\log D)O(DlogD) O ( D log D ) . The second method is also used for the fast calculation of Wigner and Weyl functions, in quantum systems with large finite dimension of the Hilbert space.
The Heisenberg-Weyl group HW(d)HW(d)HW(d) related to a ddd-dimensional Hilbert space H(d)H(d)H(d), is enlarged into the Heisenberg-Weyl-parity group HWP(d)HWP(d)HWP(d) that incorporates parity transformations. It consists of 2d32d^32d3 elements, of which d3d^3d3 elements belong to the HW(d)HW(d)HW(d) subgroup, and extra d3d^3d3 elements which are related through a Fourier transform with the former ones. It is shown that HWP(d)HWP(d)HWP(d) is a generalised version of the dihedral group. The properties of operators that combine displacements and parity, are discussed. HWP(d)HWP(d)HWP(d) is shown to be a solvable group, and commutators of its elements perform displacement and parity transformations of quantum states, along loops in the discrete phase space.2d22d^22d2 coherent states related to the HWP(d)HWP(d)HWP(d) group are introduced, which consist of d2d^2d2 coherent states related to the HW(d)HW(d)HW(d) subgroup, and extra d2d^2d2 coherent states which are related through a Fourier transform with the former ones. In noisy cases, expansion of an arbitrary state in terms of the 2d22d^22d2 coherent states with Bargmann coefficients, is advantageous in comparison to expansion in terms of the d2d^2d2 coherent states related to HW(d)HW(d)HW(d). One of the consequences of the HWP(d)HWP(d)HWP(d) group, is a natural unification of the Wigner and Weyl functions. The properties of the unified Wigner-Weyl function are discussed.
The Heisenberg-Weyl group H W ( d ) related to a d -dimensional Hilbert space H ( d ) , is enlarged into the Heisenberg-Weyl-parity group H W P ( d ) that incorporates parity transformations. It consists of 2 d 3 elements, of which d 3 elements belong to the H W ( d ) subgroup, and extra d 3 elements which are related through a Fourier transform with the former ones. It is shown that H W P ( d ) is a generalised version of the dihedral group. The properties of operators that combine displacements and parity, are discussed. H W P ( d ) is shown to be a solvable group, and commutators of its elements perform displacement and parity transformations of quantum states, along loops in the discrete phase space. 2 d 2 coherent states related to the H W P ( d ) group are introduced, which consist of d 2 coherent states related to the H W ( d ) subgroup, and extra d 2 coherent states which are related through a Fourier transform with the former ones. In noisy cases, expansion of an arbitrary state in terms of the 2 d 2 coherent states with Bargmann coefficients, is advantageous in comparison to expansion in terms of the d 2 coherent states related to H W ( d ) . One of the consequences of the H W P ( d ) group, is a natural unification of the Wigner and Weyl functions. The properties of the unified Wigner–Weyl function are discussed.
Quantum systems with finite Hilbert space are considered, and phase-space methods like the Heisenberg–Weyl group, symplectic transformations and Wigner and Weyl functions are discussed. A factorization of such systems in terms of smaller subsystems, based on the Chinese remainder theorem, is studied. The general formalism is applied to the case of angular momentum. In this context, SU (2) coherent states are used for analytic representations. Links between the theory of finite quantum systems and other fields of research are discussed.
A discussion of discrete Wigner functions in phase space related to mutually unbiased bases is presented. This approach requires mathematical assumptions, which limits it to systems with density matrices defined on complex Hilbert spaces of dimension p n where p is a prime number. With this limitation, it is possible to define a phase space and Wigner functions in close analogy to the continuous case. That is, we use a phase space that is a direct sum of n two-dimensional vector spaces each containing p 2 points. This is in contrast to the more usual choice of a two-dimensional phase space containing p 2 n points. A useful aspect of this approach is that we can relate complete separability of density matrices and their Wigner functions in a natural way. We discuss this in detail for bipartite systems and present the generalization to arbitrary numbers of subsystems when p is odd. Special attention is required for two qubits ( p = 2) and our technique fails to establish the separability property for more than two qubits. Finally, we give a brief discussion of Hamiltonian dynamics in the language developed in the paper.