  One of the most important practical realizations of the fundamentals of
quantum mechanics is continuous-variable quantum key distribution (CVQKD). Here
we propose the adaptive multicarrier quadrature division-multiuser quadrature
allocation (AMQD-MQA) multiple access technique for continuous-variable quantum
key distribution. The MQA scheme is based on the AMQD modulation, which
granulates the inputs of the users into Gaussian subcarrier
continuous-variables (CVs). 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 sub-channels. We also prove
the rate formulas if only partial channel side information is available for the
users of the sub-channel conditions. 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.
