A Comparative Study of Encoding Strategies for Quantum Convolutional Neural Networks

TL;DR

This paper compares three encoding strategies for quantum convolutional neural networks (QCNNs), analyzing their performance and robustness under noise for different input resolutions, providing practical guidance for encoder selection.

Contribution

It offers an implementation-level comparison of Angle, Amplitude, and Hybrid encodings for QCNNs, including a differentiable training pipeline and empirical evaluation under noise.

Findings

Angle encoding performs best on 4x4 inputs with noise robustness.

Hybrid encoding can outperform Angle at 8x8 under moderate noise.

Amplitude encoding excels in lightweight and full-resolution settings.

Abstract

Quantum convolutional neural networks (QCNNs) offer a promising architecture for near-term quantum machine learning by combining hierarchical feature extraction with modest parameter growth. However, any QCNN operating on classical data must rely on an encoding scheme to embed inputs into quantum states, and this choice can dominate both performance and resource requirements. This work presents an implementation-level comparison of three representative encodings -- Angle, Amplitude, and a Hybrid phase/angle scheme -- for QCNNs under depolarizing noise. We develop a fully differentiable PyTorch--Qiskit pipeline with a custom autograd bridge, batched parameter-shift gradients, and shot scheduling, and use it to train QCNNs on downsampled binary variants of MNIST and Fashion-MNIST at $4\times 4$ and $8\times 8$ resolutions. Our experiments reveal regime-dependent trade-offs. On aggressively downsampled $4\times 4$ inputs, Angle encoding attains higher accuracy and remains comparatively robust as noise increases, while the Hybrid encoder trails and exhibits non-monotonic trends. At $8\times 8$, the Hybrid scheme can overtake Angle under moderate noise, suggesting that mixed phase/angle encoders benefit from additional feature bandwidth. Amplitude-encoded QCNNs are sparsely represented in the downsampled grids but achieve strong performance in lightweight and full-resolution configurations, where training dynamics closely resemble classical convergence. Taken together, these results provide practical guidance for choosing QCNN encoders under joint constraints of resolution, noise strength, and simulation budget.