Quantum key distribution system and method

A transmitting device for use in a continuous variable quantum key distribution system, the transmitting device being configured to:

• modulate and transmit over a quantum communication channel to a receiving device a plurality of quantum signals, including a first quantum signal and a second quantum signal transmitted on a same wavelength and on a same fiber, wherein the first quantum signal has a first polarization and is associated with first quadrature components XH and PH and the second quantum signal has a second polarization and is associated with second quadrature components Xv and Pv;
• and
• receive over a classical communication channel C a channel matrix or a unitary part of a singular value decomposition of the channel matrix from the receiving device, the channel matrix representing a polarization rotation of the first and second polarization caused by the quantum communication channel;
• wherein the transmitting device is further 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 and to transmit the first quantum signal and the second quantum signal simultaneously, wherein the first polarization is orthogonal to the second polarization.

A receiving device for use in a continuous variable quantum key distribution system, the receiving device being configured to:

• receive a plurality of quantum signals transmitted by a transmitter via a quantum communication channel, wherein the first quantum signal has a first polarization and is associated with first quadrature components XH and PH and the second quantum signal has a second polarization and is associated with second quadrature components Xv and Pv, and wherein the first quantum signal and the second quantum signal are received on a same wavelength and on a same fiber;
• and
• estimate a channel matrix C representing a polarization rotation of the first and second polarization caused by the quantum communication channel and determine a singular value decomposition of the channel matrix;
• 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 by applying a part of the singular value decomposition of the channel matrix and to use the modified received first quantum signal and the modified received second quantum signal for generating a secret key and to receive the first quantum signal and the second quantum signal simultaneously, wherein the first polarization is orthogonal to the second polarization.

A method of operating a continuous variable quantum key distribution system, comprising:

• modulating and transmitting, by a transmitting device, a plurality of quantum signals, including a first quantum signal and a second quantum signal transmitted on a same wavelength and on a same fiber, wherein the first quantum signal has a first polarization and is associated with first quadrature components XH and PH and the second quantum signal has a second polarization and is associated with second quadrature components Xv and Pv;
• receiving, by a receiving device, the plurality of quantum signals via a quantum communication channel and estimating a channel matrix C representing a polarization rotation of the first and second polarization caused by the quantum communication channel;
• and
• implementing at least one of:
• modifying the received first quantum signal and the received second quantum signal on the basis of the polarization rotation and using the modified received first quantum signal and the modified received second quantum signal for generating a secret key, and/or
• modifying the transmitted first quantum signal and the transmitted second quantum signal on the basis of the polarization rotation and using the modified transmitted first quantum signal and the modified transmitted second quantum signal for generating a secret key and to transmit the first quantum signal and the second quantum signal simultaneously, wherein the first polarization is orthogonal to the second polarization.