  A quantum transducer converts an input signal to an output at a different
frequency, while maintaining the quantum information with high fidelity. When
operating between the microwave and optical frequencies, it is crucial for
quantum networking between quantum computers via low-loss optical links, and
thereby enabling distributed quantum computing. However, the state-of-the-art
quantum transducers suffer from low transduction efficiency due to weak
nonlinear coupling, wherein increasing pump power to enhance efficiency leads
to inevitable thermal noise from heating. Moreover, we reveal that the
efficiency-bandwidth product in such systems is fundamentally limited by pump
power and nonlinear coupling coefficient, irrespective of cavity engineering
efforts. To resolve the conundrum, we propose to boost the transduction
efficiency by consuming entanglement within the same frequency band (e.g.,
microwave-microwave or optical-optical entanglement). Via a
squeezer-coupler-antisqueezer sandwich structure, the protocol enhances the
transduction efficiency to unity in the ideal lossless case, given an
arbitrarily weak nonlinear coupling, which establishes a high-fidelity quantum
communication link without any signal encoding. In practical cavity systems,
our entanglement-assisted protocol surpasses the non-assisted fundamental limit
of the efficiency-bandwidth product and reduces the threshold cooperativity for
positive quantum capacity by a factor proportional to two-mode squeezing gain.
Given a fixed cooperativity, our approach increases the broadband quantum
capacity by orders of magnitude.
