Matrix product state

A matrix product state (MPS) is a structured representation of a quantum many-body wave function for systems with a one-dimensional lattice geometry, where the state is parameterized as a chain of local tensors contracted along virtual bonds, with the physical indices corresponding to the local Hilbert space at each site.^[1] This form allows for an efficient description of states exhibiting limited entanglement, scaling polynomially with system size rather than exponentially, making it particularly suitable for simulating strongly correlated quantum systems.^[2] Mathematically, for a chain of N sites with local dimension d, an MPS takes the form |\psi\rangle = \sum_{\{s_i\}} \mathrm{Tr} \left( \prod_{i=1}^N A^{}_{s_i} \right) |s_1, \dots, s_N\rangle, where each A^{}_{s_i} is a D × D matrix (with D the bond dimension controlling entanglement) and the trace enforces periodicity in canonical forms.^[3] The concept of MPS originated in the early 1990s as a mathematical framework for translationally invariant states on quantum spin chains, introduced by Fannes, Nachtergaele, and Werner to characterize finitely correlated states with exponentially decaying correlations.^[4] Concurrently, White developed the density matrix renormalization group (DMRG) algorithm in 1992, which implicitly relies on MPS-like truncations to compute ground states of one-dimensional Hamiltonians with high accuracy, revolutionizing numerical simulations of quantum lattice models. Subsequent advances in the 2000s, driven by insights from quantum information theory, formalized MPS explicitly and extended their use to variational methods like time-evolving block decimation (TEBD) for real-time dynamics.^[1] MPS are defined by their adherence to an area law for entanglement entropy in gapped one-dimensional systems, where the entanglement across any cut scales logarithmically with the bond dimension D, enabling approximation of low-energy states with modest computational resources.^[2] Canonical forms, such as left- or right-normalized MPS, facilitate efficient algorithms for optimization, contraction, and manipulation, with the bond dimension serving as a tunable parameter for balancing accuracy and efficiency.^[3] Symmetries, including U(1) charge conservation or SU(2) spin, can be incorporated into the tensors to reduce parameters and enhance simulations of realistic models like the Heisenberg chain.^[1] In applications, MPS underpin DMRG and its variants for finding ground states of interacting spin or fermionic systems, achieving near-exact results for gapful Hamiltonians and providing benchmarks for higher-dimensional extensions like projected entangled pair states (PEPS). They also enable studies of quantum phases, topological order, and critical phenomena, where the entanglement structure reveals universal properties.^[2] Beyond condensed matter, MPS find use in quantum information for state tomography, circuit compilation, and classifying entanglement in multipartite systems.^[3] Ongoing developments integrate MPS with machine learning for pattern recognition in quantum data and with quantum hardware for variational quantum eigensolvers.^[1]

Fundamentals

Definition and Motivation

In quantum many-body physics, simulating the behavior of systems composed of many interacting particles presents a formidable challenge due to the exponential growth of the Hilbert space dimension. For a one-dimensional chain of L sites, each with a local Hilbert space of dimension d (such as d=2 for qubits or higher for qudits), the total number of basis states scales as d^L, making exact representations and computations intractable for large L.^[5] Matrix product states (MPS) emerge as a powerful solution to this complexity, offering a compact parametrization of quantum wave functions that exploits the typically low entanglement structure of physically relevant states in one-dimensional systems. By representing states with polynomial storage and computational cost—scaling as O(L D^3 d) where D is a controllable bond dimension—MPS enable efficient numerical studies of ground states, dynamics, and correlations without needing to store the full exponential Hilbert space. This approach is particularly effective for states obeying an area law of entanglement, where the entanglement entropy across any cut remains bounded independent of system size, allowing truncation to low-rank approximations with negligible error.^[5]^[6] At its core, an MPS factorizes the many-body wave function into a chain of local tensors, one per site, where each tensor encodes the contribution from the local degree of freedom while connecting to neighboring sites via auxiliary indices. The full wave function coefficients in the product basis arise from successive contractions along these bonds, effectively chaining matrix multiplications that capture inter-site correlations. This tensor network structure draws from the mathematical framework of finitely correlated states, initially formalized for translation-invariant quantum spin chains.^[5] A prerequisite for understanding MPS is the tensor product construction of quantum states: the total Hilbert space \mathcal{H} = \bigotimes_{i=1}^L \mathcal{H}_i forms a multi-dimensional array (tensor) whose elements are the wave function amplitudes, with each local space \mathcal{H}_i spanned by basis states like |0\rangle or |1\rangle for qubits. MPS are exact for unentangled product states, requiring only bond dimension D=1, and serve as high-fidelity approximations for entangled states with bounded correlation length, such as those in gapped local Hamiltonians, where increasing D systematically improves accuracy. Canonical forms of MPS provide normalized representations that facilitate computations like overlaps and optimizations.^[5]

Historical Development

The origins of matrix product states (MPS) trace back to the early 1990s through parallel developments in numerical methods and mathematical theory. Concurrently, Steven R. White introduced the density matrix renormalization group (DMRG) algorithm in 1992 as a method to approximate ground states of strongly correlated quantum systems using a variational wavefunction that implicitly relied on a matrix product structure, and Fannes, Nachtergaele, and Werner characterized finitely correlated states on quantum spin chains, providing a mathematical framework for translationally invariant MPS with exponentially decaying correlations.^[4] This approach was further refined in White's 1993 work, establishing DMRG as a powerful tool for one-dimensional systems where the ansatz effectively captured low-entanglement states through iterative renormalization. Although not explicitly termed MPS at the time, this variational framework laid the groundwork for representing quantum states with bounded entanglement, with the bond dimension emerging as a key parameter to control approximation accuracy. An explicit formulation of MPS appeared in 1995 through the work of Stellan Östlund and Stefan Rommer, who introduced "matrix product ground states" to represent quantum states in the thermodynamic limit of infinite periodic systems, enabling efficient computation of correlation functions and ground-state properties. This formulation connected directly to DMRG by showing how infinite-system limits could be handled variationally without finite-size effects dominating. Earlier influences on MPS concepts stemmed from studies of valence bond solids in the 1980s, particularly the Affleck-Kennedy-Lieb-Tasaki (AKLT) model proposed in 1987, which demonstrated exactly solvable spin-1 chains with gapped excitations and short-range entangled ground states constructed from singlet projectors, inspiring later MPS representations of such ordered phases. In the 2000s, Frank Verstraete and J. Ignacio Cirac extended MPS to both finite and infinite chain configurations, developing algorithms like infinite time-evolving block decimation (iTEBD) for dynamics and ground-state optimization, which formalized MPS as versatile tools for simulating open-boundary and translationally invariant systems. These extensions built on earlier infinite MPS ideas to handle realistic boundary conditions and real-time evolution efficiently. The transition to a broader tensor network framework occurred in the mid-2000s, with Cirac, Verstraete, and collaborators recognizing MPS as the one-dimensional case of projected entangled pair states (PEPS) for higher dimensions and matrix product operators (MPO) for Hamiltonians, unifying variational methods across lattice geometries. This perspective, detailed in their 2004 work, enabled scalable approximations for two- and higher-dimensional systems by generalizing the contraction and optimization procedures from MPS. MPS gained widespread prominence after 2004 through this unified tensor network language, which facilitated applications in quantum information science, such as entanglement renormalization and state tomography, transforming DMRG from a specialized condensed matter tool into a cornerstone of many-body quantum simulation.

Mathematical Representation

Tensor Network Structure

A matrix product state (MPS) provides a structured representation of a many-body quantum state as a one-dimensional chain in a tensor network, where local tensors are connected via auxiliary indices to encode correlations efficiently. This formulation assumes that the state exhibits limited entanglement across bipartitions, allowing compression of the full wave function tensor.^[7] For a pure state |\psi\rangle of N sites, each with local physical dimension d, the MPS under open boundary conditions is expressed as

|\psi\rangle = \sum_{s_1 \dots s_N=1}^d \psi_{s_1 \dots s_N} |s_1 \dots s_N\rangle,

where the wave function coefficients are obtained via contractions over virtual bond indices:

\psi_{s_1 \dots s_N} = \sum_{\alpha_0 \alpha_1 \dots \alpha_N} A^{s_1}_{\alpha_0 \alpha_1} A^{s_2}_{\alpha_1 \alpha_2} \cdots A^{s_N}_{\alpha_{N-1} \alpha_N},

with \alpha_0 = \alpha_N = 1 (scalars at the boundaries) and each tensor A^{s_i} having dimensions \chi_{i-1} \times \chi_i for the virtual indices, where \chi_i denotes the bond dimension at position i. Often, a uniform bond dimension \chi is used throughout for simplicity.^[7]^[1] In tensor diagram notation, the MPS is visualized as a linear chain of N nodes, each representing a tensor A^{s_i} with an upward physical leg of dimension d (index s_i) and horizontal virtual legs of dimensions \chi_{i-1} and \chi_i, contracted sequentially along the chain to form the scalar coefficient for each basis state.^[1] For periodic boundary conditions, the representation modifies to a closed loop, with each A^{s_i} a \chi \times \chi matrix and the coefficients given by

\psi_{s_1 \dots s_N} = \operatorname{Tr} \left( A^{s_1} A^{s_2} \cdots A^{s_N} \right),

enabling descriptions of translationally invariant systems in the thermodynamic limit. This tensor network structure contrasts with the full wave function tensor of dimension d^N, achieving exponential compression by restricting virtual dimensions to low-rank approximations that capture essential entanglement structure without storing all elements explicitly.^[1] As a brief extension to mixed states, MPS generalizes to matrix product operators (MPO), which represent operators or density matrices as similar tensor chains acting on the physical indices.^[1]

Bond Dimension and Approximation Error

In matrix product states (MPS), the bond dimension \chi refers to the size of the virtual indices connecting adjacent tensors, which governs both the storage requirements and the representational power of the state. For a chain of N sites with local physical dimension D, the total number of parameters scales as O(N D \chi^2), making \chi a key factor in computational efficiency. Larger values of \chi allow the MPS to capture more intricate correlations and entanglement structures, enhancing its expressiveness, while smaller \chi promotes efficiency at the cost of potential loss in accuracy. Any pure state in a one-dimensional Hilbert space of N qubits can be exactly represented as an MPS with bond dimension \chi = 2^{N/2}, though this grows exponentially with system size and is impractical for large N. However, for states obeying an area law of entanglement—common in many-body systems—the required \chi remains polynomially bounded or even constant, enabling efficient approximations without full exponential resources. In practice, truncation during MPS construction or optimization compresses the bond dimension by retaining only the dominant components, typically via singular value decomposition (SVD) applied across bipartitions. The resulting approximation error, measured in the Hilbert-Schmidt (Frobenius) norm, is bounded by the sum of the squares of the discarded singular values \sum_{i > \chi} \sigma_i^2, providing a quantifiable measure of the fidelity loss. A pivotal feature for one-dimensional gapped systems is the saturation of entanglement entropy S at a constant value, which scales approximately as S \sim \log \chi when the Schmidt rank is fully utilized. This relation implies that modest \chi values suffice to achieve high-fidelity approximations, as the bounded S limits the necessary virtual dimension. Furthermore, the exponential decay of spatial correlations, characterized by a finite correlation length \xi, ensures that entanglement does not grow unbounded, thereby keeping \chi effectively constant even for large N and allowing precise simulations with controlled error.

Canonical Forms

Left-Canonical Form

The left-canonical form of a matrix product state (MPS) is a normalization convention in which the tensors are sequentially orthonormalized from the left end of the chain. For an MPS represented as |\psi\rangle = \sum_{\{s_i\}} \prod_{i=1}^N A_i^{s_i} |s_1 \dots s_N\rangle, where A_i^{s_i} are matrices with physical index s_i and bond dimensions \chi_{i-1} \times \chi_i, the left-canonical form requires that the tensors A_i^{s_i} for sites i = 1 to k satisfy the isometry condition \sum_{s_i} (A_i^{s_i})^\dagger A_i^{s_i} = I_{\chi_i}, ensuring that the partial contractions over the physical indices from the left form an orthonormal basis.^[5] This condition implies that each A_i acts as a partial isometry, mapping the left bond space to the combined left-physical space without distortion.^[8] To construct the left-canonical form, one performs a sequential singular value decomposition (SVD) starting from the left. Begin with the full state coefficient tensor, reshape the first two sites into a matrix over the leftmost bond and the remaining indices, and apply SVD to obtain a left-orthonormal matrix for the first site and absorb the singular values and right singular vectors into the second site; iterate this process across all sites until the chain is fully left-normalized.^[5] If no truncation of the bond dimension \chi occurs during the SVD steps, this procedure exactly preserves the original state norm, which is then equal to 1 up to a global scalar factor.^[9] In this form, the center of orthogonality lies at the right end of the chain, as the entire left block up to the last site is orthonormalized.^[8] A key property of the left-canonical form is the isometry of the transfer matrix E = \sum_s A^s \otimes \overline{A^s}, where the overline denotes complex conjugation; this operator satisfies E^\dagger E = I when acting on the left bond space, reflecting the left-orthonormality and enabling efficient contractions.^[5] This isometric nature simplifies the computation of the state norm and overlaps, as the left partial trace yields the identity.^[9] The left-canonical form offers advantages in numerical algorithms, particularly by simplifying expectation values of operators acting on left-local sites, where contractions reduce to tracing over orthonormal bases without additional normalizations.^[5] It is especially useful in the initial forward sweeps of the density matrix renormalization group (DMRG) method, where left-orthonormality facilitates variational optimization and truncation by maintaining a compact representation of the left block's entanglement structure.^[5]

Right-Canonical Form

The right-canonical form of a matrix product state (MPS) represents a quantum many-body wave function where the tensors from site k to N are normalized such that they satisfy the right-isometry condition \sum_{s_i} A^{s_i} (A^{s_i})^\dagger = I, ensuring orthonormality of the basis states to the right of any bipartition. This normalization preserves the overall state under gauge transformations and facilitates efficient computations by making the right fixed point of the associated transfer operator the identity matrix. In this form, the MPS tensors A^{s_i} act as isometric maps from the right virtual bond to the physical and left virtual bonds, which is particularly useful for systems where correlations decay from the right end. To construct the right-canonical form, one performs successive singular value decompositions (SVDs) in sweeps starting from the rightmost site and moving leftward, enforcing the right-isometry at each step by absorbing the unitary factors appropriately. During these sweeps, the MPS is iteratively decomposed and renormalized, with optional truncation of the bond dimension to control approximation error. The transfer matrix E_{\alpha \alpha', \beta \beta'} = \sum_{s_i} A^{s_i}_{\alpha \beta} \overline{A^{s_i}_{\alpha' \beta'}} then satisfies E E^\dagger = I when evaluated from the right, reflecting the isometry and simplifying the evaluation of norms and overlaps. This form is especially efficient for applying right-local operators, as the orthonormality allows expectation values to be computed with a scaling of O(D^2 d^2), where D is the bond dimension and d the physical dimension. It complements the left-canonical form in alternating optimization sweeps, enabling bidirectional refinement in variational algorithms for ground state searches and time evolution. In infinite MPS representations of translationally invariant systems, the left- and right-canonical forms coincide, yielding a consistent periodic structure for analyzing long-range correlations.

Mixed-Canonical Form

The mixed-canonical form of a matrix product state (MPS) is a hybrid representation that achieves left-canonical normalization for sites 1 through k-1, right-canonical normalization for sites k+1 through N, and leaves the central tensor B^{} at site k unnormalized. This structure localizes the non-orthonormality at the chosen center, leveraging the orthonormality properties established in pure left- and right-canonical forms. In tensor notation, the state is expressed as |\psi\rangle = \sum_{i_1,\dots,i_N} \left( \prod_{j=1}^{k-1} A^{}_{i_j \alpha_{j-1} \alpha_j} \right) B^{}_{i_k \alpha_{k-1} \alpha_k} \left( \prod_{j=k+1}^N B^{}_{i_j \alpha_{j-1} \alpha_j} \right) |i_1 \dots i_N\rangle, where the A^{} tensors satisfy left-orthonormality \sum_{i_j} (A^{}_{i_j})^\dagger A^{}_{i_j} = I and the B^{} tensors (for j > k) satisfy right-orthonormality \sum_{i_j} B^{}_{i_j} (B^{}_{i_j})^\dagger = I. To construct the mixed-canonical form with center at site k, one performs a sweep across the chain, starting from an initial MPS representation and successively applying QR decompositions or singular value decompositions (SVDs) to enforce canonicality on the left and right segments while absorbing the resulting diagonal singular value matrices into the central tensor B^{}. This process ensures the left block up to k-1 is isometric and the right block from k+1 to N is co-isometric, with the computational cost scaling as O(N D^3 d) for bond dimension D and local Hilbert space dimension d. The center position can then be shifted to an adjacent site by a single SVD or QR step, updating the neighboring tensors and redistributing the singular values accordingly. A key property of this form is that the squared norm of the state simplifies to the Frobenius norm of the central tensor:

\langle \psi | \psi \rangle = \| B^{} \|_F^2 = \sum_{i_k, \alpha_{k-1}, \alpha_k} |B^{}_{i_k \alpha_{k-1} \alpha_k}|^2,

which follows directly from the orthonormality of the surrounding tensors and enables straightforward rescaling of the state by normalizing B^{}. This localization of the norm facilitates efficient computation of overlaps and normalization without full chain contraction. The mixed-canonical form is particularly advantageous for operations centered at site k, such as applying local operators or performing updates, as these require only contractions involving the central and immediate neighboring tensors, with complexity O(D^3 d^2) rather than a full sweep. It was introduced by Vidal in the context of time-evolving block decimation to enable efficient local gate applications and state updates in one-dimensional quantum simulations.

Properties

Entanglement and Schmidt Rank

In matrix product states (MPS), the Schmidt decomposition across a bipartition defined by a bond index k reveals the entanglement structure, where the singular value decomposition of the MPS tensors yields the entanglement spectrum consisting of non-negative singular values \lambda_i^{(\alpha_k)} with i = 1, \dots, r_{\alpha_k}, and the Schmidt rank r_{\alpha_k} \leq \chi_k, the bond dimension at that site. This decomposition expresses the wave function as |\psi\rangle = \sum_{i=1}^{r_{\alpha_k}} \lambda_i^{(\alpha_k)} |L_i\rangle \otimes |R_i\rangle, where |L_i\rangle and |R_i\rangle are orthonormal states in the left and right subsystems, respectively, directly quantifying the bipartite entanglement across the cut. The von Neumann entanglement entropy S for the reduced density matrix \rho_L = \mathrm{Tr}_R(|\psi\rangle\langle\psi|) = \sum_i (\lambda_i^{(\alpha_k)})^2 |L_i\rangle\langle L_i| of the left subsystem is given by S = -\mathrm{Tr}(\rho_L \log \rho_L) = -\sum_i (\lambda_i^{(\alpha_k)})^2 \log (\lambda_i^{(\alpha_k)})^2, which serves as a measure of entanglement and is upper-bounded by \log \chi_k since the eigenvalues sum to 1 and there are at most \chi_k non-zero terms. Similarly, Rényi entropies S_\alpha = \frac{1}{1-\alpha} \log \mathrm{Tr}(\rho_L^\alpha) for \alpha > 0, \alpha \neq 1 are computed as S_\alpha = \frac{1}{1-\alpha} \log \sum_i (\lambda_i^{(\alpha_k)})^{2\alpha}, providing a family of entanglement measures that capture different aspects of the spectrum, with S_\alpha \leq \log \chi_k as well. In one-dimensional quantum systems, ground states of local Hamiltonians obey an area law for entanglement entropy, where S scales at most constantly with subsystem size for gapped systems, allowing efficient representation by MPS with polynomially bounded bond dimension \chi. For gapless critical systems described by conformal field theories, S instead scales logarithmically as S \sim \frac{c}{3} \log L + \mathrm{const}, where c is the central charge and L is the subsystem length, implying a bond dimension \chi \sim L^{c/6} that grows slowly but requires larger \chi for high accuracy. Matrix product ground states (MPSGS), such as those of frustration-free Hamiltonians like the AKLT model, are exactly represented by MPS with finite \chi, strictly obeying the area law; conversely, states violating the area law, like those with volume-law entanglement S \sim L, demand exponentially large \chi \sim \exp(L) for exact representation.

Symmetries and Invariants

Matrix product states (MPS) can incorporate physical symmetries of the underlying Hamiltonian to enforce conserved quantities and reduce computational costs. For systems with global symmetries, the MPS tensors are adapted such that virtual indices carry quantum numbers corresponding to irreducible representations of the symmetry group, ensuring the state transforms correctly under symmetry operations. In the case of Abelian symmetries, such as the U(1) symmetry associated with particle number conservation, the virtual indices are labeled by integer quantum numbers q that add under tensor contractions. The physical basis states s_i also carry a charge k, leading to a block-diagonal structure in the MPS tensors where blocks correspond to different charge sectors. Specifically, the tensor components satisfy A^{s_i}_{\alpha \beta} = 0 unless \mathrm{charge}(\alpha) + \mathrm{charge}(s_i) = \mathrm{charge}(\beta), enforcing charge conservation across sites. This structure was introduced by McCulloch in 2007 for U(1) symmetries, allowing simulations in targeted charge sectors and reducing the effective bond dimension \chi by restricting to relevant blocks. For non-Abelian symmetries, such as SU(2) spin symmetry, the adaptation uses a multiplet structure where virtual bonds are decomposed into direct sums of irreducible representations (irreps), with fusion rules governed by Clebsch-Gordan coefficients to form fusion trees. This groups equivalent irreps, significantly reducing the effective bond dimension; for example, an SU(2)-invariant tensor with 225 components can be compressed to 11 by exploiting the symmetry. The extension to SU(2) was developed by Singh et al. in 2012, enabling efficient handling of spin-conserving Hamiltonians through block-sparse operations on multiplets. Symmetry-adapted MPS enforce invariants such as total particle number or total spin, which are encoded in the quantum numbers of the boundary virtual indices (e.g., the leftmost index carries the total charge for the entire state). These conserved quantities can be extracted by tracing the MPS over physical indices while preserving the boundary quantum numbers, yielding expectation values in specific symmetry sectors. Such adaptations accelerate algorithms like the density matrix renormalization group (DMRG) by factors of 10–100 through reduced effective \chi and sparse computations, with U(1) implementations from McCulloch achieving up to 10× gains and SU(2) extensions from Singh et al. enabling even larger effective dimensions at comparable cost (e.g., \chi \approx 1300 vs. 220 without symmetry).

Numerical Methods

Density Matrix Renormalization Group

The density-matrix renormalization group (DMRG) serves as the primary variational method for optimizing matrix product states (MPS) to approximate the ground states of one-dimensional quantum Hamiltonians, particularly those exhibiting strong correlations. Introduced by Steven R. White in 1992, this technique revolutionized the numerical simulation of quantum many-body systems by efficiently truncating the Hilbert space while preserving essential low-energy physics.^[10] DMRG operates by iteratively refining an MPS representation through local optimizations, leveraging the low entanglement structure of ground states in one dimension to achieve high accuracy for systems with up to thousands of sites.^[5] The algorithm exists in two main variants: finite-system DMRG for open-boundary chains of fixed length and infinite-system DMRG (IS-DMRG) for approximating periodic or translationally invariant infinite chains. In finite DMRG, the process begins with an initial guess for the MPS, such as a random product state or a simple trial wavefunction. Local tensors are then optimized sequentially: for a given site, an effective Hamiltonian H_{\mathrm{eff}} is constructed for the surrounding superblock (comprising the left block, the current site(s), and the right block), typically via contractions of the MPS tensors. This H_{\mathrm{eff}} is solved as a small eigenvalue problem to find the lowest-energy eigenvector, which updates the local tensor (e.g., A) while keeping the blocks fixed. Singular value decomposition (SVD) is applied to canonicalize the MPS and truncate the bond dimension to a fixed \chi, retaining only the \chi largest singular values to minimize the discarded weight from the reduced density matrix. The optimization proceeds in sweeps, alternating left-to-right and right-to-left across the chain, with multiple sweeps (often 6–10) until convergence in energy or truncation error is achieved.^[5] IS-DMRG, in contrast, iteratively grows the system by adding unit cells around a central seed, optimizing two-site unit cells under translational invariance assumptions, and is particularly suited for extracting thermodynamic properties without finite-size effects dominating.^[5] A core step in both variants involves solving the effective Hamiltonian for a block plus one or two sites, given by

H_{\mathrm{eff}} = \sum_{b_{l-1}, b_l} L_{a_{l-1}, a'_{l-1}, b_{l-1}} \, W_{\sigma_l, \sigma'_l, b_{l-1}, b_l} \, R_{a_l, a'_l, b_l},

where L and R represent the contracted left and right blocks, W encodes the local Hamiltonian terms, and indices run over physical and virtual (bond) dimensions; the ground state of H_{\mathrm{eff}} yields the updated tensor via the lowest eigenvector.^[5] Truncation occurs post-optimization by SVD, ensuring the MPS remains in a canonical form during sweeps. Accuracy improves systematically with increasing bond dimension \chi, as the variational energy converges to the true ground state; for gapped systems, the truncation error scales exponentially with \chi, while finite-size energy errors decay as \exp(-L / \xi), with L the system length and \xi the correlation length.^[5] This enables precise simulations of chains up to 1000 sites or more, often reaching relative errors below $10^{-10} for local observables.^[11]

Time Evolution and Real-Time Dynamics

The time evolution of matrix product states (MPS) under unitary dynamics is crucial for simulating real-time quantum many-body processes, such as quench dynamics or transport phenomena in one-dimensional systems. Two primary algorithms dominate this domain: the time-evolving block decimation (TEBD) method, which relies on Trotter decomposition for approximate evolution, and the time-dependent variational principle (TDVP), which projects the exact time evolution onto the tangent space of the MPS manifold. These approaches enable efficient simulations by exploiting the low entanglement structure of MPS, allowing access to dynamics inaccessible to exact diagonalization for system sizes beyond a few dozen sites.^[12]^[13] TEBD approximates the time-evolution operator for a time step \Delta t using a Trotter-Suzuki decomposition, particularly suited for Hamiltonians with nearest-neighbor interactions H = \sum_j H_{j,j+1}. The unitary evolution U(\Delta t) = e^{-i H \Delta t} is factorized as

U(\Delta t) \approx \prod_j e^{-i H_{j,j+1} \Delta t},

with higher-order variants reducing the error. This decomposition is applied sequentially to pairs of sites by contracting the corresponding two-site gates onto the MPS tensors in a mixed-canonical form, followed by singular value decomposition (SVD) to truncate the bond dimension \chi and restore the canonical form. The process sweeps across the chain, ensuring locality and scalability, with computational cost scaling as O(\chi^3 d^3) per time step, where d is the local Hilbert space dimension. Introduced by Vidal in 2004, TEBD marked a breakthrough by enabling real-time simulations of entangled quantum states in one dimension, previously limited by exponential resource demands.^[12]^[12] In contrast, TDVP provides a variational approximation to the Schrödinger equation i \partial_t |\psi\rangle = H |\psi\rangle by restricting the evolution to the manifold of MPS with fixed or adaptive bond dimension. The method projects the Hamiltonian onto the tangent space of the MPS, yielding equations of motion for the tensor parameters that minimize the deviation from exact dynamics. For a site-centered MPS, this involves solving local eigenvalue problems to update each tensor, preserving normalization and offering second-order accuracy in \Delta t without explicit Trotterization. Haegeman et al. formalized this approach in 2011, demonstrating its efficacy for both real- and imaginary-time evolution, with extensions to infinite systems via uniform MPS. TDVP excels in capturing smooth dynamics but requires solving small linear systems, scaling similarly to TEBD yet with better long-time accuracy for non-local Hamiltonians when combined with matrix product operator representations.^[13]^[13] Both methods face challenges from entanglement growth, particularly in critical or chaotic systems where the Schmidt rank across cuts increases linearly or faster with time, demanding adaptive increases in \chi to control truncation errors. For TEBD, the Trotter error accumulates as O((\Delta t)^2), necessitating small steps for precision, while in gapped systems, entanglement saturates, allowing fixed \chi. Extensions like time-dependent DMRG (t-DMRG) refine TEBD for finite \Delta t by incorporating effective Hamiltonians, improving accuracy for short-time dynamics in lattice models. These techniques have been pivotal in studying phenomena like thermalization and operator spreading, with error bounds tied to the discarded singular values during truncation.^[12]^[13]

Examples

Greenberger–Horne–Zeilinger State

The Greenberger–Horne–Zeilinger (GHZ) state for N qubits is a canonical example of a highly entangled multipartite quantum state, defined as

|\text{GHZ}\rangle = \frac{1}{\sqrt{2}} \left( |0\rangle^{\otimes N} + |1\rangle^{\otimes N} \right).

This state exhibits perfect correlations across all sites, such that measuring any qubit in the X-basis collapses the entire system to an eigenstate of the global parity operator. The GHZ state can be exactly represented as a matrix product state (MPS) with bond dimension \chi = 2. In this representation, the physical tensors for each site are simple diagonal 2×2 matrices:

A^0 = \begin{pmatrix} 1 & 0 \\ 0 & 0 \end{pmatrix}, \quad A^1 = \begin{pmatrix} 0 & 0 \\ 0 & 1 \end{pmatrix},

combined with appropriate boundary vectors: the left boundary is the row vector \frac{1}{\sqrt{2}} (1, 1) and the right boundary is the column vector \begin{pmatrix} 1 \\ 1 \end{pmatrix}. The contraction of this MPS yields non-zero amplitudes only for the all-zero and all-one computational basis states, each equal to $1/\sqrt{2}, reproducing the GHZ state exactly. This form arises because the two basis vectors in the superposition propagate independently along the two virtual bonds without mixing, reflecting the state's rigid global structure.^[1] Despite its maximal multipartite entanglement—where the state cannot be written as a product across any non-trivial partition—the GHZ state requires only \chi = 2 due to its special form, which leads to zero local correlations beyond the global phase coherence between the two terms. Across any bipartition, the Schmidt rank is exactly 2, resulting in a constant entanglement entropy S = \log 2, independent of N. This area-law scaling enables the efficient exact MPS representation, in contrast to generic volume-law entangled states that generally demand exponentially large bond dimensions. The GHZ MPS is commonly employed to benchmark numerical algorithms, such as tensor contractions for overlap computations, owing to its analytic simplicity and exact solvability.^[14]

W State

The W state for N qubits is defined as |W_N\rangle = \frac{1}{\sqrt{N}} \sum_{i=1}^N |00\dots 1_i \dots 00\rangle, where the sum runs over all basis states with exactly one excitation delocalized across the chain. This state was introduced in the context of quantum information to exemplify a distinct class of multipartite entanglement inequivalent to the GHZ state under stochastic local operations and classical communication (SLOCC). In the matrix product state (MPS) formalism, the W state can be exactly represented with bond dimension \chi = 2, by tracking whether the excitation has been placed or remains to be placed along the chain. The tensors encode the propagation of the no-excitation subspace on one virtual bond and the placement of the excitation on the other, with the physical index distinguishing the local qubit state (s=0 or s=1). Specifically, the tensors have an upper triangular form for s=0 propagating the "not placed" state and off-diagonal for placing the excitation at s=1, terminating further propagation; the final site closes the chain. In canonical form, the singular values across bonds reflect the uniform amplitudes $1/\sqrt{N} for each position.^[15] This constant bond dimension is similar to the GHZ state, as the MPS structure captures the W state's delocalized single excitation efficiently.^[15] The entanglement structure of the W state, as revealed by its MPS, features bipartite entanglement entropy S = -p \log_2 p - (1-p) \log_2 (1-p) across a cut separating l sites from the remaining N-l, with p = l/N, reaching a maximum of 1 bit for balanced bipartitions. However, its multipartite entanglement differs fundamentally from the GHZ state: the W state remains bipartite entangled upon tracing out any single qubit (with residual entanglement entropy approaching 1 bit for large N), making it more robust to local losses, whereas the GHZ state reduces to a product state. The Schmidt rank is 2 across any cut, underscoring the structure captured by the low bond-dimension MPS.^[15]

AKLT Model

The Affleck–Kennedy–Lieb–Tasaki (AKLT) model is a paradigmatic one-dimensional quantum spin-1 antiferromagnetic chain introduced in 1987, whose exact ground state exemplifies the application of matrix product states (MPS) to strongly correlated systems.^[16] The model features a valence bond solid (VBS) phase with short-range spin correlations yet hidden long-range order, serving as the first exactly solvable example of an MPS ground state and motivating subsequent numerical methods like the density matrix renormalization group (DMRG).^[17] The Hamiltonian of the AKLT model is given by

H = \sum_i \left( \mathbf{S}_i \cdot \mathbf{S}_{i+1} + \frac{1}{3} P_{i,i+1}^{S=2} \right),

where \mathbf{S}_i are the spin-1 operators at site i, and P_{i,i+1}^{S=2} is the projector onto the total spin-2 subspace of neighboring sites (equivalent to \frac{1}{3} (\mathbf{S}_i \cdot \mathbf{S}_{i+1})^2 + \frac{1}{3} \mathbf{S}_i \cdot \mathbf{S}_{i+1}).^[18] This nearest-neighbor interaction frustrates the system, yielding a unique gapped ground state under periodic boundary conditions, with the lowest-energy excitations separated by the Haldane gap of approximately 0.35J (in units where the exchange J=1).^[19] The ground state is constructed as a VBS by representing each physical spin-1 as a symmetric projection of two virtual spin-1/2 degrees of freedom, forming singlets between neighboring virtual spins across sites.^[18] This VBS ground state admits an exact MPS representation with bond dimension \chi = 2, reflecting the underlying virtual spin-1/2 structure.^[19] In the standard basis where the physical index corresponds to m = +1, 0, -1, the MPS tensors A^m (each of size $2 \times 3 \times 2) are

A^{+1} = -\frac{\sigma^{+}}{\sqrt{2}}, \quad A^{-1} = \frac{\sigma^{-}}{\sqrt{2}}, \quad A^{0} = \frac{\sigma^{z}}{\sqrt{2}},

where \sigma^{\pm} = (\sigma^{x} \pm i \sigma^{y})/2.^[19] These tensors encode the local projection from virtual to physical space, ensuring the overall state is SU(2) invariant through the spin-1/2 representation in the virtual bonds.^[18] The full ground state is then |\psi\rangle = \sum_{\{m\}} \mathrm{Tr} \left( \prod_i A^{m_i} \right) |m_1, \dots, m_L \rangle, normalized appropriately. Key properties of the AKLT ground state include exponential decay of two-point spin correlations, \langle S^z_i S^z_j \rangle \propto (-1)^{j-i} e^{-|j-i| / \xi} with correlation length \xi = 1 / \ln 3 \approx 0.91, alongside a non-zero string order parameter \langle S^z_i \exp(i \pi \sum_{k=i+1}^{j-1} S^z_k) S^z_j \rangle \to -4/9 as |j-i| \to \infty, signaling topological order protected by the underlying symmetry.^[19] Contracting the MPS representation exactly reproduces the ground-state energy per site of -4/3 and facilitates computation of these observables via transfer matrix techniques, with complexity scaling as O(\chi^3) per site.^[19] The exact solvability via MPS highlighted the efficiency of low bond-dimension representations for gapped phases, influencing the development of variational MPS methods for more general Hamiltonians.^[17]

Majumdar–Ghosh Model

The Majumdar–Ghosh model is a one-dimensional antiferromagnetic spin-1/2 chain with nearest-neighbor and next-nearest-neighbor interactions, introduced in 1969 to study frustration effects in quantum spin systems. The Hamiltonian takes the form

H = J \sum_i \left( \mathbf{S}_i \cdot \mathbf{S}_{i+1} + \frac{1}{2} \mathbf{S}_i \cdot \mathbf{S}_{i+2} \right)

for J > 0, which is exactly solvable and exhibits a quantum phase transition at this specific coupling ratio. This form incorporates frustration through the competing antiferromagnetic tendencies that favor dimerization over uniform Néel order. The ground state of the model is twofold degenerate, corresponding to two distinct perfect dimer coverings of the lattice: one pairing spins on sites i and i+1 for even i, and the other for odd i. Each individual dimer covering is an exact eigenstate and can be represented precisely as a matrix product state with bond dimension \chi = 2, the minimal value required for exactness in this case. The translationally invariant ground state under periodic boundary conditions is the equal superposition of these two dimer states, preserving the total [S^z](/page/S/Z) = 0 sector and ensuring zero total spin. For the dimer pattern with singlets on even-odd bonds, the MPS is constructed using site-dependent 2\times2 tensors that enforce local singlet formation. On odd sites (start of a dimer), the tensors are

A^{\uparrow} = \begin{pmatrix} 0 & 1 \\ 0 & 0 \end{pmatrix}, \quad A^{\downarrow} = \begin{pmatrix} 0 & 0 \\ 1 & 0 \end{pmatrix},

while on even sites (end of a dimer), they are transposed to complete the singlet: A^{\uparrow} = \begin{pmatrix} 0 & 0 \\ -1 & 0 \end{pmatrix}, A^{\downarrow} = \begin{pmatrix} 1 & 0 \\ 0 & 0 \end{pmatrix} (up to normalization and gauge choice). This alternating structure yields the product-of-singlets wavefunction when contracted, with the bond dimension \chi=2 capturing the exact entanglement across dimer boundaries. The model possesses an exact ground-state energy of -(3N/4) J, where N is the number of sites, reflecting the saturation of all nearest-neighbor bonds to singlet values of -3/4. Excitations above the ground state include soft modes at momentum k = \pi, arising from the broken translational symmetry and dimer order, with a gap opening away from the exact point. The MPS form proves particularly useful for analyzing excitations, as small perturbations around the exact point can be incorporated via low-bond-dimension MPS updates to compute dispersion relations and correlation functions efficiently. This 1969 model underscores the power of MPS in exactly representing ground states of frustrated systems with low entanglement, serving as a benchmark for validating numerical techniques like the density matrix renormalization group due to its analytic solvability and simple dimer structure.