Fundamental Limit on the Power of Entanglement Assistance in Quantum Communication

The optimal rate of reliable communication over a quantum channel can be enhanced by preshared entanglement. Whereas the enhancement may be unbounded in infinite-dimensional settings even when the input power is constrained, a long-standing conjecture asserts that the ratio between the entanglement-assisted and unassisted classical capacities is bounded in finite-dimensional settings [Bennett et al., IEEE Trans. Inf. Theory 48, 2637 (2002)IETTAW0018-944810.1109/TIT.2002.802612]. In this Letter, we prove this conjecture by showing that their ratio is upper bounded by o(d2), where d is the input dimension of the channel. An application to quantum communication with noisy encoders and decoders is given. © 2025 authors. Published by the American Physical Society.