A continuous variable quantum key distribution system, comprising: a transmitter configured to modulate and to transmit a plurality of quantum signals, including a first quantum signal and a second quantum signal transmitted on the same wavelength and on the same fiber, wherein the first quantum signal has a first polarization and is associated with first quadrature components X H and P H and the second quantum signal has a second polarization and is associated with second quadrature components X v and P v; and a receiver configured to receive the plurality of quantum signals transmitted by the transmitter via a quantum communication channel, and a classical communication channel for exchanging information between the transmitter and the receiver, wherein the receiver is further configured to estimate a channel matrix C representing the polarization rotation of the first and second polarization caused by the quantum communication channel; wherein the receiver is configured to modify the received first quantum signal and the received second quantum signal on the basis of the polarization rotation and to use the modified received first quantum signal and the modified received second quantum signal for generating a secret key, and/or wherein the transmitter is configured to modify the transmitted first quantum signal and the transmitted second quantum signal on the basis of the polarization rotation and to use the modified transmitted first quantum signal and the modified transmitted second quantum signal for generating a secret key, characterized in that the transmitter is further configured to transmit the first quantum signal and the second quantum signal simultaneously wherein the first polarization is orthogonal to the second polarization, and to receive, over the classical communication channel, the channel matrix or a unitary part of a singular value decomposition of the channel matrix from the receiving device, wherein the plurality of quantum signals comprise at least one training signal and wherein the receiver is configured to: estimate the polarization rotation of the first and second polarization caused by the quantum communication channel on the basis of the at least one training signal, and transmit, over the classical communication channel, the channel matrix, or the unitary part of the singular value decomposition of the channel matrix, to the transmitter. a transmitter configured to modulate and to transmit a plurality of quantum signals, including a first quantum signal and a second quantum signal transmitted on the same wavelength and on the same fiber, wherein the first quantum signal has a first polarization and is associated with first quadrature components X H and P H and the second quantum signal has a second polarization and is associated with second quadrature components X v and P v; and a receiver configured to receive the plurality of quantum signals transmitted by the transmitter via a quantum communication channel, and a classical communication channel for exchanging information between the transmitter and the receiver, wherein the receiver is further configured to estimate a channel matrix C representing the polarization rotation of the first and second polarization caused by the quantum communication channel; wherein the receiver is configured to modify the received first quantum signal and the received second quantum signal on the basis of the polarization rotation and to use the modified received first quantum signal and the modified received second quantum signal for generating a secret key, and/or wherein the transmitter is configured to modify the transmitted first quantum signal and the transmitted second quantum signal on the basis of the polarization rotation and to use the modified transmitted first quantum signal and the modified transmitted second quantum signal for generating a secret key, characterized in that the transmitter is further configured to transmit the first quantum signal and the second quantum signal simultaneously wherein the first polarization is orthogonal to the second polarization, and to receive, over the classical communication channel, the channel matrix or a unitary part of a singular value decomposition of the channel matrix from the receiving device, wherein the plurality of quantum signals comprise at least one training signal and wherein the receiver is configured to: estimate the polarization rotation of the first and second polarization caused by the quantum communication channel on the basis of the at least one training signal, and transmit, over the classical communication channel, the channel matrix, or the unitary part of the singular value decomposition of the channel matrix, to the transmitter.