Abstract
We introduce the multi-head quantum neural network (QNN), a quantum module compatible with noisy intermediate-scale quantum (NISQ) devices that can replace transformer blocks inside pre-trained large language models (LLMs) with little loss in accuracy and with far fewer parameters than the blocks it replaces. Encoding a model's $D$ hidden neurons into a qubit register is the obstacle: amplitude encoding scales poorly in circuit depth and angle encoding in qubit count, so neither is practical at production-scale width. We instead partition the hidden space into $k$ disjoint heads, angle-encode each into an $n_q$-qubit register, process the heads in parallel, and couple them through constrained classical linear layers, the mixers. Performing surgery on Qwen3-30B-A3B-Instruct-2507, we show that a single multi-head QNN with $145\,281$ trainable parameters ($D=2048$, $k=32$, $n_q=8$, $L=4$) can replace a contiguous span of eight transformer blocks, displacing $16.33\%$ of the model's parameters. Ablations show that, for a single replaced block, the quantum heads (circuits followed by a learned readout), and not the classical mixers alone, recover the displaced function, whereas for fused spans the mixers alone come within $1$-$3\%$ of the full module in perplexity. Whether this benefit is specific to trained, entangling quantum circuits is not established: controls that switch the entangler off (two seeds), or freeze the circuit at its initial or at uniformly random angles (one seed), beat the full module on held-out perplexity, and a parameter-matched classical multilayer perceptron remains a few percent ahead of the full module. We train classically with exact statevector simulation, then execute the quantum heads of our fusion checkpoints on a $156$-qubit IBM Heron processor, showing that NISQ-scale quantum modules can be installed in a production-scale LLM and run on hardware.
Abstract (translated)
URL
https://arxiv.org/abs/2611.15250
