The State Preparation of Multivariate Normal Distributions using Tree Tensor Network

Hidetaka Manabe1 and Yuichi Sano2

1Graduate School of Engineering Science, Osaka University
2Department of Nuclear Engineering, Kyoto University

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Abstract

The quantum state preparation of probability distributions is an important subroutine for many quantum algorithms. When embedding $D$-dimensional multivariate probability distributions by discretizing each dimension into $2^n$ points, we need a state preparation circuit comprising a total of $nD$ qubits, which is often difficult to compile. In this study, we propose a scalable method to generate state preparation circuits for $D$-dimensional multivariate normal distributions, utilizing tree tensor networks (TTN). We establish theoretical guarantees that multivariate normal distributions with 1D correlation structures can be efficiently represented using TTN. Based on these analyses, we propose a compilation method that uses automatic structural optimization to find the most efficient network structure and compact circuit. We apply our method to state preparation circuits for various high-dimensional random multivariate normal distributions. The numerical results suggest that our method can dramatically reduce the circuit depth and CNOT count while maintaining fidelity compared to existing approaches.

Amplitude encoding of classical data is a key subroutine in quantum algorithms. In particular, preparing multivariate normal distributions is an important step in various practical quantum algorithms, such as quantum field theory and quantum finance. In quantum state preparation, we encode a $D$-dimensional function $f(x)$, discretizing each dimension into $2^n$ points, as an $n$-qubit quantum state. However, this generally requires a circuit with $nD$ qubits and a depth of $O(2^{nD})$, which can become a critical bottleneck in the execution of algorithms.

To address this, we propose a scalable method for generating state preparation circuits for $D$-dimensional multivariate normal distributions, utilizing tree tensor networks (TTNs). First, we prove that when variables exhibit a 1D correlation structure, such as exponentially decaying correlations, a multivariate normal distribution can be efficiently represented using a TTN. Our numerical simulations confirm this result and demonstrate that low-cost state preparation circuits for such distributions can be generated.

Second, we propose a compilation method based on automatic structural optimization of TTNs, a concept recently developed in quantum many-body physics. This technique optimizes the network structure by reconnecting the legs of local tensors based on entanglement entropy. This allows us to appropriately detect an efficient structure for the given distributions, as shown in the image. We numerically show that our method can greatly reduce the circuit cost by factors of 10 to 1000 compared to existing methods.

In summary, our work presents not only a practical compilation method for quantum state preparation but also highlights the power of tensor networks in representing high-dimensional functions and the utility of automatic structural optimization in computer science.

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[2] Josh Green and Jingbo Wang, "Quantum encoding of functions and images with matrix product states", Physical Review A 113 5, 052616 (2026).

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[6] Parker Kuklinski, Benjamin Rempfer, Kevin Obenland, and Justin Elenewski, "A simpler Gaussian state-preparation", arXiv:2508.03987, (2025).

[7] Oskari Kerppo, William Steadman, Ossi Niemimäki, and Valtteri Lahtinen, "Minimizing entanglement entropy for enhanced quantum state preparation", arXiv:2507.22562, (2025).

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