Local quantum interactions generate dynamics in an exponentially large Hilbert space, yet locality and entanglement restrict the information that can spread and accumulate. Bounds on propagation and entanglement growth, together with tensor networks, exploit these restrictions to discard information unnecessary for describing the evolution. This leads to a sharper question: how much information must any low-rank representation retain? Here we determine these limits, up to logarithmic factors, for short-range interactions. For time-independent evolution, the optimal rank obeys $\log D=\widetilde O(t+\sqrt{\log(1/ε)})$, with matching lower bounds fixing the accuracy exponent $1/2$; for arbitrary driving, matching fixed-time bounds instead give $2/3$. Correspondingly, the corresponding dynamical entanglement spectra exhibit distinct small-$α$ Rényi laws, $α^{-1}$ and $α^{-2}$. In one dimension, the static limit is constructively attained by an explicit MPO algorithm, with an analogous extension to Liouvillian dynamics. Together, these results determine the irreducible information required to represent local quantum evolution and uncover distinct entanglement structures in static and driven dynamics.
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How accurately can local quantum evolution be represented with low operator Schmidt rank, beyond what light-cone bounds imply? This paper derives precision-dependent compression bounds for finite-range and exponentially decaying interactions, relates them to entanglement-spectrum tails, and gives constructive one-dimensional MPO algorithms.
For bounded boundary size, time-independent evolution admits an operator-norm-$\epsilon$ approximation with $\log D=\widetilde O(t+\sqrt{\log(1/\epsilon)})$. Arbitrary driving instead gives $\log D=\widetilde O(t+\sqrt{t\log(1/\epsilon)}+\log^{2/3}(1/\epsilon))$. Fixed-time lower-bound families match the accuracy powers $1/2$ and $2/3$, respectively, up to logarithmic factors. Corresponding small-$\alpha$ Rényi-entanglement upper bounds scale as $\widetilde O(\alpha^{-1})$ and $\widetilde O(\alpha^{-2})$, with matching power-law lower bounds. Suitably controlled piecewise-polynomial or analytic driving recovers the static compression scale.
The proof uses differential expansions and discrete Fourier filters to assign terms to buffer cuts with few explicit interaction insertions. “Buffered Schmidt rank” retains these different cuts until conversion to a prescribed cut; optimizing buffer width yields the stated bounds. For one-dimensional finite-range static dynamics, an explicit MPO construction achieves bond dimension $\exp[\widetilde O(t+\sqrt{\log(n/\epsilon)})]$ and construction time $n\exp[\widetilde O(t+\sqrt{\log(n/\epsilon)})]$. Liouvillian analogues use diamond-norm error.
The lower bounds allow system size to grow with accuracy; the static construction uses exponentially decaying interactions, and the driven construction permits arbitrarily fast switching. Algorithms use exact arithmetic, require computable temporal input for structured drives, and do not establish finite-precision stability. Liouville-MPO approximants need not be CPTP.
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Generative-AI tools were used extensively in the preparation of this manuscript and, in several parts, in the technical development of the proofs.
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14 pages, 4 figures; 134 pages of Supplementary Material
Local quantum interactions generate dynamics in an exponentially large Hilbert space, yet locality and entanglement restrict the information that can spread and accumulate. Bounds on propagation and entanglement growth, together with tensor networks, exploit these restrictions to discard information unnecessary for describing the evolution. This leads to a sharper question: how much information must any low-rank representation retain? Here we determine these limits, up to logarithmic factors, for short-range interactions. For time-independent evolution, the optimal rank obeys logD=O(t+log(1/ε)), with matching lower bounds fixing the accuracy exponent 1/2; for arbitrary driving, matching fixed-time bounds instead give 2/3. Correspondingly, the corresponding dynamical entanglement spectra exhibit distinct small-α Rényi laws, α−1 and α−2. In one dimension, the static limit is constructively attained by an explicit MPO algorithm, with an analogous extension to Liouvillian dynamics. Together, these results determine the irreducible information required to represent local quantum evolution and uncover distinct entanglement structures in static and driven dynamics.
How accurately can local quantum evolution be represented with low operator Schmidt rank, beyond what light-cone bounds imply? This paper derives precision-dependent compression bounds for finite-range and exponentially decaying interactions, relates them to entanglement-spectrum tails, and gives constructive one-dimensional MPO algorithms.
For bounded boundary size, time-independent evolution admits an operator-norm-ϵ approximation with logD=O(t+log(1/ϵ)). Arbitrary driving instead gives logD=O(t+tlog(1/ϵ)+log2/3(1/ϵ)). Fixed-time lower-bound families match the accuracy powers 1/2 and 2/3, respectively, up to logarithmic factors. Corresponding small-α Rényi-entanglement upper bounds scale as O(α−1) and O(α−2), with matching power-law lower bounds. Suitably controlled piecewise-polynomial or analytic driving recovers the static compression scale.
The proof uses differential expansions and discrete Fourier filters to assign terms to buffer cuts with few explicit interaction insertions. “Buffered Schmidt rank” retains these different cuts until conversion to a prescribed cut; optimizing buffer width yields the stated bounds. For one-dimensional finite-range static dynamics, an explicit MPO construction achieves bond dimension exp[O(t+log(n/ϵ))] and construction time nexp[O(t+log(n/ϵ))]. Liouvillian analogues use diamond-norm error.
The lower bounds allow system size to grow with accuracy; the static construction uses exponentially decaying interactions, and the driven construction permits arbitrarily fast switching. Algorithms use exact arithmetic, require computable temporal input for structured drives, and do not establish finite-precision stability. Liouville-MPO approximants need not be CPTP.
Near-Optimal Bounds on the Density of Low-Energy States of k-Local Hamiltonians and Faster Quantum Algorithms
Low-energy estimation and state preparation for general k-local Hamiltonians are fundamental challenges in quantum complexity theory. Buhrman et al.~ [BGLGST, PRL 2025] recently broke the natural Grover bound O∗(2n/2) for both problems, with the improvement depending on the relative accuracy ε and the locality k. In this work, we present faster exponential quantum algorithms for these problems, where the binary entropy function governs the runtime exponent. For sufficiently small ε/k, our algorithms improve the exponent by a factor of log(k/ε) over [BGLGST, PRL 2025]. Our main technical result is an entropy-governed lower bound on the dimension of the Hamiltonian's low-energy subspace, obtained by depolarizing its ground state. For fixed k, this bound is optimal up to constant factors in the exponent. The same framework yields tighter bounds for Heisenberg, XY, and Ising models on arbitrary interaction graphs.
We study entanglement reachability in quantum many-body systems evolving under physically admissible local dynamics. While Lieb--Robinson bounds constrain the propagation of information, and complexity-based approaches characterize the resources required to represent or prepare quantum states, these frameworks do not by themselves determine which entanglement structures are dynamically attainable. The central question addressed in this work is therefore: which entanglement structures can arise under constrained quantum evolution, and which remain inaccessible despite being kinematically allowed? We introduce a unified axiomatic framework for entanglement reachability based on the notion of \emph{structural confinement}. The framework incorporates finite-range locality, finite-dimensional memory degrees of freedom, and abstract representability constraints expressed through operator subsystems. Rather than characterizing accessibility directly in Hilbert space, we formulate the problem on an induced structural space of entanglement configurations and quantum information patterns. We establish a structural confinement theorem showing that admissible dynamics generate only a restricted reachable region in structural space. The theorem provides a general criterion for identifying forbidden structures whenever their defining information cannot be preserved within the admissible representational framework. Thus, ocality, memory, and representability constraints do not merely limit the rate of information propagation; they impose additional dynamical restrictions on the set of quantum structures that can actually be realized. The generality of the framework is demonstrated through three explicit realizations involving multipartite coherence, tensor-network representability, and protected logical nformation in quantum error-correcting codes. Although these realizations involve distinct physical structures, they are governed by the same underlying confinement mechanism: dynamical accessibility is determined by the compatibility between physical evolution and the structural information retained by the chosen representation. These results establish structural confinement as a general organizing principle for entanglement reachability under constrained quantum dynamics. The framework provides a unified language for deriving dynamical accessibility limits beyond propagation bounds alone and for classifying which quantum structures are reachable, forbidden, or conditionally accessible in locally constrained many-body systems.
Entanglement dynamics depend not only on how a quantum system is partitioned, but critically on how interactions across that partition are structured. For a spatial bipartition of a locally interacting system, entanglement is generated near the boundary and then propagates into the bulk. By contrast, when two extended quantum fields are coupled locally along their entire length, the interaction crosses the field-space partition everywhere, and this generates correlations throughout the system. Here, we study the entanglement dynamics between two gapless one-dimensional quantum many-body systems described by Luttinger liquid theory. The systems are initially decoupled and prepared at zero or finite temperature, after which a time-dependent tunneling interaction is activated uniformly along their length. Within a Gaussian approximation, we derive general analytical expressions for the logarithmic negativity, mutual information, and Rényi entropies under arbitrary coupling protocols. At zero temperature, entanglement displays an early-time power-law growth whose exponent is fixed solely by the first non-null derivative of the tunneling protocol. Once the coupling saturates, we obtain exact long-time averages of the information-theoretic quantities and characterise how the correlations scale with temperature and the final coupling strength. We also analyse how mutual information and logarithmic negativity approach the adiabatic limit for a very slow protocol with respect to intrinsic system timescale. This work extends the study of entanglement dynamics in nonequilibrium field theory to field-space partitions and mixed initial states.
One of the major ways in which quantum mechanics differs from classical mechanics is the existence of special quantum correlations - entanglement. Typical quantum states are highly entangled, making them complex and inefficient to represent. Physically interesting states are unusual, they are only weakly entangled. By restricting ourselves to weak entanglement, efficient representations of quantum states can be found. A tensor network is constructed by taking objects called tensors that encode spatially local information and gluing them together to create a large network that describes a complex quantum state. The manner in which the tensors are connected defines the entanglement structure of the quantum state. Tensors networks are therefore a natural framework for describing physical behaviour of complex quantum systems. In this thesis we utilize tensor networks to solve a number of interesting problems. Firstly, we study a Feynman path integral written over tensor network states. As a sum over classical trajectories, a Feynman path integral can struggle to capture entanglement. Combining the path integral with tensor networks overcomes this.We consider the effect of quadratic fluctuations on the tensor network path integral and calculate corrections to observables numerically and analytically. We also study the time evolution of complex quantum systems. By projecting quantum dynamics onto a classical phase space defined using tensor networks, we relate thermal behaviour of quantum systems to classical chaos. In doing so we demonstrate a relationship between entanglement growth and chaos. By studying the dynamics of coupled quantum chains we also gain insight into how quantum correlations spread over time. As noted, tensor networks are remarkably efficient. In the final section of this thesis we use tensor networks to create compressed machine learning algorithms. Their efficiency means that tensor networks can use 50 times fewer parameters with no significant decrease in performance.
Understanding the dynamics of interacting quantum systems is a central goal of quantum simulation. For geometrically local Hamiltonians, locality suggests that digital simulation should retain much of the parallelism of the underlying dynamics. For a target accuracy ε, we construct a nearest-neighbor quantum circuit that simulates the dynamics of an n-qubit, time-independent, finite-range Hamiltonian with normalized local interaction strength on a fixed-dimensional lattice for time T≥1, with circuit depth O(Tlog(nT/ε)) and gate count O(nTlog(nT/ε)). This reduces the polylogarithmic overhead of Haah, Hastings, Kothari, and Low to a single logarithm. On logarithmic-scale blocks from the Lieb--Robinson decomposition, a shallow but highly accurate implementation is essential for reducing the circuit depth. Our key observation is that spatial localization also controls the cost of high precision. In each block, the error of a constant-order product formula is compensated for by a correction, yielding exponential accuracy. The correction can be implemented with depth proportional to the block diameter, introducing no additional asymptotic depth scale beyond that required by spatial localization.
Furthermore, we prove an unconditional circuit-depth lower bound for uniform XXZ Hamiltonians: at fixed evolution time T in the stated regime and sufficiently small fixed error ε, any nearest-neighbor quantum circuit simulating these dynamics requires depth Ω(logn/loglogn). This matches our O(logn) upper bound up to a doubly logarithmic factor, establishing near-optimal circuit depth for lattice Hamiltonian simulation in this regime. Together, these results clarify the role of locality and precision in determining the circuit complexity of quantum dynamics and suggest broader principles for spatially constrained quantum computation.
We aim to rigorously address how local noise affects classical simulability of quantum dynamics benchmarked by tensor-network methods. Using operator entanglement entropy (OEE), we prove the following: (1) For single-qubit depolarizing noise on arbitrary circuits, tensor networks with poly(n) bond dimension suffice for fixed absolute Hilbert-Schmidt error after $\order{1}$ depth, while relative error demands $\order{\log n}$ depth; and this bound is optimal. (2) For single-qubit depolarizing noise on 1D local circuits, the existence of whole-trajectory error-bounded matrix product operator (MPO) of poly(n) bond dimension at all depths. (3) For general single-qubit noise in 1D brickwall circuits, random two-design gates with contraction coefficient c<1/3 yield an $\order{1}$ OEE plateau with probability 1−Te−Ω(n), while arbitrary gates with c<1/48 give $\order{\log n}$ OEE in the worst case. (4) In higher dimensions, these bounds yield uniform-in-depth poly(n) average boundary-bond dimensions for projected entangled pair operators~(PEPO) across every cut -- under depolarizing noise at either absolute or relative accuracy, and under general noise with strong contraction at absolute accuracy. Our results establish a rigorous connection between certain noise models, circuit types, and their classical simulability.