  We introduce the multidimensional manifold extraction for multicarrier
continuous-variable (CV) quantum key distribution (QKD). The manifold
extraction utilizes the resources that are injected into the transmission by
the additional degrees of freedom of the multicarrier modulation. We
demonstrate the results through the AMQD (adaptive multicarrier quadrature
division) scheme, which granulates the information into Gaussian subcarrier CVs
and divides the physical link into several Gaussian sub-channels for the
transmission. We prove that the exploitable extra degree of freedom in a
multicarrier CVQKD scenario significantly extends the possibilities of
single-carrier CVQKD. The manifold extraction allows for the parties to reach
decreased error probabilities by utilizing those extra resources of a
multicarrier transmission that are not available in a single-carrier CVQKD
setting. We define the multidimensional manifold space of multicarrier CVQKD
and the optimal tradeoff between the available degrees of freedom of the
multicarrier transmission. We also extend the manifold extraction for the
multiple-access AMQD-MQA (multiuser quadrature allocation) multicarrier
protocol. The additional resources of multicarrier CVQKD allow the achievement
of significant performance improvements that are particularly crucial in an
experimental scenario.
