Multiuser Quadrature Allocation for Continuous-Variable Quantum Key Distribution

We propose the adaptive multicarrier quadrature division–multiuser quadrature allocation (AMQD-MQA) multiple access technique for continuous-variable quantum key distribution (CVQKD). The MQA scheme is based on the AMQD modulation, which granulates the inputs of the users into Gaussian subcarrier continuous-variables (CVs). The subcarrier coherent states formulate Gaussian sub-channels from the physical link with diverse transmittance coefficients. In an AMQD-MQA multiple access scenario, the simultaneous reliable transmission of the users is handled by the dynamic allocation of the Gaussian subcarrier CVs. We propose two different settings of AMQD-MQA for multiple input– multiple output communication. We introduce a rate-selection strategy that tunes the modulation variances and allocates adaptively the quadratures of the users over the subchannels. We also prove the rate formulas if only partial channel side information is available for the users of the sub-channel conditions. In an experimental CVQKD scenario, an ideal Gaussian input modulation can only be approximated, which affects the quadrature adaption and the efficiency of the transmission. We show a technique for the compensation of a nonideal Gaussian input modulation, which allows the users to overwhelm the modulation imperfections to reach optimal capacity-achieving communication over the Gaussian sub-channels. We investigate the diversity amplification of the sub-channel transmittance coefficients and reveal that a strong diversity can be exploited by opportunistic Gaussian modulation.