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arXiv:quant-ph/0504091

On the Matrix Representation of Quantum Operations

Yoshihiro Nambu, Kazuo Torii Nakamura

Published Apr 12, 2005• 3 citations
arXivHTMLPDF

Abstract

This paper considers two frequently used matrix representations -- what we call the $χ$- and $\mathcal{S}$-matrices -- of a quantum operation and their applications. The matrices defined with respect to an arbitrary operator basis, that is, the orthonormal basis for the space of linear operators on the state space are considered for a general operation acting on a single or two \textit{d}-level quantum system (qudit). We show that the two matrices are given by the expansion coefficients of the Liouville superoperator as well as the associated bijective, positive operator on the doubled-space defined with respect to two types of induced operator basis having different tensor product structures, i.e., Kronecker products of the relevant operator basis and dyadic products of the associated bipartite state basis. The explicit conversion formulas between the two matrices are established as a computable matrix multiplication. Extention to more qudits case is trivial. Several applications of these matrices and the conversion formulas in quantum information science and technology are presented.

DOI
10.48550/arxiv.quant-ph/0504091

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This paper considers two frequently used matrix representations -- what we call the χχχ- and S\mathcal{S}S-matrices -- of a quantum operation and their applications. The matrices defined with respect to an arbitrary operator basis, that is, the orthonormal basis for the space of linear operators on the state space are considered for a general operation acting on a single or two \textit{d}-level quantum system (qudit). We show that the two matrices are given by the expansion coefficients of the Liouville superoperator as well as the associated bijective, positive operator on the doubled-space defined with respect to two types of induced operator basis having different tensor product structures, i.e., Kronecker products of the relevant operator basis and dyadic products of the associated bipartite state basis. The explicit conversion formulas between the two matrices are established as a computable matrix multiplication. Extention to more qudits case is trivial. Several applications of these matrices and the conversion formulas in quantum information science and technology are presented.
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Abstract Analysis of quantum processes, especially in the context of noise, errors, and decoherence is essential for the improvement of quantum devices. An intuitive representation of those processes modeled by quantum channels are Pauli transfer matrices. They display the action of a linear map in the n-qubit Pauli basis in a way, that is more intuitive, since Pauli strings are more tangible objects than the standard basis matrices. We set out to investigate classical algorithms that convert the various representations into Pauli transfer matrices. We propose new algorithms that make explicit use of the tensor product structure of the Pauli basis. They convert a quantum channel in a given representation (Chi or process matrix, Choi matrix, superoperator, or Kraus operators) to the corresponding Pauli transfer matrix. Moreover, the underlying principle can also be used to calculate the Pauli transfer matrix of other linear operations over n-qubit matrices such as left-, right-, and sandwich multiplication as well as forming the (anti-)commutator with a given operator. Finally, we investigate the runtime of these algorithms, derive their asymptotic scaling and demonstrate improved performance using instances with up to seven qubits.
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We introduce a bi-oriented interlaced matrix product on 2×22\times 22×2 matrices over a unital, possibly non-associative algebra, extending a matrix mnemonic for Cayley--Dickson doubling. The opposite orientations XRX_RXR​ and XLX_LXL​ coincide precisely when the underlying algebra is commutative. For the canonical position and momentum operators Q,PQ,PQ,P on L2(R)L^2(\mathbb{R})L2(R), a matrix DDD and its companion D~\widetilde{D}D satisfy, on the common Schwartz domain, D~XRD=D~XLD=2HId2\widetilde{D} X_R D =\widetilde{D} X_L D =2H Id_2DXR​D=DXL​D=2HId2​, where H=(P2+Q2)/2H=(P^2+Q^2)/2H=(P2+Q2)/2. We study the associated interlaced eigenvalue equations for operator-valued columns. The combinations Φ=A+iBΦ=A+iBΦ=A+iB and Ψ=A−iBΨ=A-iBΨ=A−iB decouple the equations into two-sided Sylvester equations. We construct explicit rank-one generalized eigenoperators from position and momentum eigenstates. Their distributional kernels belong to the doubled rigged Hilbert space S(R2)⊂L2(R2)⊂S′(R2)S(\mathbb{R}^2)\subset L^2(\mathbb{R}^2) \subset S'(\mathbb{R}^2)S(R2)⊂L2(R2)⊂S′(R2); the corresponding position and momentum operators act on different tensor factors and therefore commute. The rank-one families do not exhaust the distributional solutions: higher-order generalized solutions are also exhibited. We distinguish the algebraic identities on the Schwartz core from questions concerning closed-operator realizations.
Tensor product operators on finite dimensional Hilbert spaces are studied. The focus is on bilinear tensor product operators. A tensor product operator on a pair of Hilbert spaces is a maximally general bilinear operator into a target Hilbert space. By 'maximally general' is meant every bilinear operator from the same pair of spaces to any Hilbert space factors into the composition of the tensor product operator with a uniquely determined linear mapping on the target space. There are multiple distinct tensor product operators of the same type; there is no "the" tensor product. Distinctly different tensor product operators can be associated with different parts of a multipartite system without difficulty. Separability of states, and locality of operators and observables is tensor product operator dependent. The same state in the target state space can be inseparable with respect to one tensor product operator but separable with respect to another, and no tensor product operator is distinguished relative to the others; the unitary operator used to construct a Bell state from a pair of |0>'s being highly tensor product operator-dependent is a prime example. The relationship between two tensor product operators of the same type is given by composition with a unitary operator. There is an equivalence between change of tensor product operator and change of basis in the target space. Among the gains from change of tensor product operator is the localization of some nonlocal operators as well as separability of inseparable states. Examples are given.
The bases used in quantum mechanics, in general, are orthonormal. The relations in orthonormal basis vectors are simpler than those in nonorthogonal basis vectors. Also, the operators that represent different physical quantities are Hermitian and the eigenvectors of these operators belonging to different eigenvalues are orthonormal to each other. But, in many cases, such as sets of screened hydrogenic wavefunctions, nonorthogonal sets of basis vectors occur. It is, therefore, necessary to consider the representation of vectors and operators in nonorthogonal bases. Making use of the set of vectors reciprocal to the given set of nonorthogonal base vectors, the expression for the norms of vectors and for the matrix elements has been found. Also, adjoint and Hermitian matrices in nonorthogonal bases have been worked out using a set of reciprocal basis vectors.