Quantum MIMO Diversity over Discrete-Variable Crosstalk Channels

Quantum communication plays a pivotal role in enabling distributed quantum computing and sensing. Diversity strategies can be used to increase the communication reliability (in the sense of output fidelity with respect to the input quantum state) when multiple communication channels are available. The current paper proposes a diversity strategy for quantum discrete variable (DV) multiple-input-multiple-output (MIMO) channels, utilizing approximate cloning to distribute information across multiple channels at the transmitter and purification to merge the noisy and entangled joint state into a single quantum state at the receiver. The proposed method is compared with simpler strategies, such as the best-channel selection, to identify the advantage regions over a quantum channel combining both crosstalk and depolarization, where the crosstalk is modeled by a controlled-SWAP operator. Our numerical results show that the cloning-purification strategy offers an advantage, especially in cases where full channel state information (CSI) can be exploited to optimize the cloning asymmetry. More importantly, and in contrast to the classic MIMO diversity, we demonstrate that the distribution of quantum information over all available channels does not always provide an advantage due to the dilution cost of the cloning operation.