Polarization tracking system for free-space optical communication, including quantum communication

A quantum receiver, comprising:

• a receiver optical system configured to receive a combined optical signal from a transmitter via free space that includes quantum communication (QC) optical radiation at a QC optical wavelength and beacon optical radiation at a beacon optical wavelength, wherein the QC optical radiation and the beacon optical radiation have been transmitted by a transmitter using a dual wavelength, single mode, photonic crystal fiber that provides concentric cores for both the QC optical radiation and the beacon optical radiation, such that light at both the QC optical wavelength and the beacon optical wavelength are both able to propagate along the fiber in only one mode;
• a dichroic beam splitter configured to separate the QC optical radiation from the beacon optical radiation;
• a QC receiver configured to decode data from the received QC optical radiation based on a polarization basis;
• a tracking optical retarder situated so as to provide a variable retardation to the received QC optical radiation for delivery to the QC receiver;
• a polarization control system configured to continuously estimate, based on the beacon optical radiation, a polarization error caused by rotation of the transmitter relative to the receiver, and control, based on the estimated polarization error, the variable retardation of the tracking optical retarder to compensate for the polarization error;
• and
• a processing circuit configured to generate a secret sequence of bits that is shared with the transmitter based on the QC optical radiation for one or more cryptographic operations.

A method, comprising:

• combining, at a quantum transmitter, quantum communication (QC) optical radiation at a communication optical wavelength with beacon optical radiation at a beacon optical wavelength different from the communication optical wavelength using a dichroic beam splitter, wherein the beacon optical radiation has a state of polarization corresponding to a basis polarization of the quantum communication optical radiation;
• transmitting the combined QC and beacon optical radiation through free space, so as to enable a receiver to correct for polarization errors caused by rotation of the quantum transmitter relative to the receiver, using a dual wavelength, single mode, photonic crystal fiber that provides concentric cores for both the communication optical radiation and the beacon optical radiation, such that light at both the communication optical wavelength and the beacon optical wavelength are both able to propagate along the fiber in only one mode;
• and
• generating a secret sequence of bits that is shared with a quantum receiver based on the QC optical radiation for one or more cryptographic operations.

A method, comprising:

• receiving at a receiver a combined optical signal comprising quantum communication (QC) optical radiation at a QC optical wavelength and beacon optical radiation at a beacon optical wavelength from a transmitter via free space, wherein the transmitter transmitted the QC optical radiation and the beacon optical radiation using a dual wavelength, single mode, photonic crystal fiber that provides concentric cores for both the QC optical radiation and the beacon optical radiation, such that light at both the QC optical wavelength and the beacon optical wavelength are both able to propagate along the fiber in only one mode;
• separating, using a dichroic beam splitter, the QC optical radiation from the beacon optical radiation;
• continuously estimating a polarization error caused by rotation of the transmitter relative to the receiver based on the beacon optical radiation;
• adjusting a state of polarization of the QC optical radiation to compensate for the polarization error;
• and
• generating a secret sequence of bits that is shared with the transmitter based on the QC optical radiation for one or more cryptographic operations.