C-QOBRA: A Study of Quantum Conditioning in Specification-Driven Molecular Generation

C-QOBRA is a conditional variational autoencoder (cVAE) for molecular design, whose encoder and decoder are parameterized quantum circuits of eight to ten qubits with at most $204$ trainable parameters. Read directly from the decoder circuit, with no classical network, it is already an effective unconditional generator: on QM9, it yields more unique-and-valid and more novel-and-valid molecules than %the fully quantum QVAE-Mole while using fewer quantum parameters, fully quantum existing VAEs while using fewer quantum parameters, and most of its novel molecules stay within QM9's size limit. Conditional control is weaker under the same direct readout: requested logP values come out in the right order but are compressed to about a third of their range, and conditioning on QED barely moves the output. Because the circuit generates well, this points to decoding rather than conditioning as the bottleneck. Adding a classical post-processing head restores controllability and reveals how little classical capacity is needed: a head roughly $1/200$ the size of the largest tested already recovers most of the attainable logP control, with nearly five times fewer classical parameters than the pre-existing equivalent, which generates only unconditionally. Overall, the quantum circuits store the latent representation, and a small classical head suffices to decode it into conditioned property values.