Capability of anti-degradable quantum channel with additional entanglement

Abstract: Quantum communication theory focuses on the study of quantum channels for transmitting quantum information, where the transmission rate is measured by quantum channel capacity. This quantity exhibits several intriguing properties, such as non-additivity, superactivation and so on. In this work, we show that a type of quantum channel known as the anti-degradable one-mode Gaussian channel -- whose capacity is believed to be zero -- can be ``activated" to transmit quantum information through the introduction of quantum entanglement. Although the channel's output alone cannot be used to retrieve the input signal, combining it with extra entanglement makes this possible. Beyond its theoretical implications, this activation can also be realized in practical systems. For example, in electro-optic systems used for quantum transduction in the two-mode squeezing interaction regime, the transduction channel is anti-degradable. We demonstrate that this system can transmit microwave-optical quantum information with the assistance of entanglement with an ancillary mode. This results in a new type of quantum transducer that exhibits positive quantum capacity over a wide parameter space.