Quantum Encoding of Structured Data with Matrix Product States

Abstract: Quantum computing faces a fundamental challenge: the amplitude encoding of an arbitrary n-qubit state vector generally requires {\Omega}(2n) gate operations. We can, however, form dimensionality-reduced representations of quantum states using matrix product states (MPS), providing a promising pathway to the efficient amplitude encoding of states with limited entanglement entropy. In this paper, we explore the capabilities of MPS representations to encode a wide range of functions and images using O(n)-depth circuits without any ancilla qubits, computed with the so-called Matrix Product Disentangler algorithm with tensor network optimisation. We find that MPS-based state preparation enables the efficient encoding of functions up to low-degree piecewise polynomials with accuracy exceeding 99.99% accuracy. We also showcase a novel approach to encoding structured image data based on MPS approximations of the discrete wavelet transform (DWT) representation, which is shown to prepare a 128x128 ChestMNIST image on 14 qubits with fidelity exceeding 99.1% on a circuit with a total depth of just 425 single-qubit rotation and two-qubit CNOT gates.