Abstract: A quantum cryptography apparatus and system includes a photon emitter, a photon receiver, a first photodetector, a second photodetector, a first polarization optic, and a second polarization optic. The photon emitter is configured to emit a photon at a wavelength. The photon receiver is coupled to the photon emitter by at least one quantum channel. The photon receiver includes the first polarization optic configured to output the emitted photon in a polarization state. The first photodetector is configured to detect the emitted photon from the output of the first polarization optic. The second photodetector is configured to detect a backflash from the first photodetector. The second polarization optic is between the first photodetector and the second photodetector. The second photodetector and the second polarization optic are configured to internally calibrate the photon receiver.

Abstract: A quantum cryptography apparatus and system includes a photon emitter, a photon receiver, a first photodetector, a second photodetector, a first polarization optic, and a second polarization optic. The photon emitter is configured to emit a photon at a wavelength. The photon receiver is coupled to the photon emitter by at least one quantum channel. The photon receiver includes the first polarization optic configured to output the emitted photon in a polarization state. The first photodetector is configured to detect the emitted photon from the output of the first polarization optic. The second photodetector is configured to detect a backflash from the first photodetector. The second polarization optic is between the first photodetector and the second photodetector. The second photodetector and the second polarization optic are configured to internally calibrate the photon receiver.

Abstract: A quantum cryptography apparatus and system includes a photon emitter, a photon receiver, a first photodetector, a second photodetector, a first polarization optic, and a second polarization optic. The photon emitter is configured to emit a photon at a wavelength. The photon receiver is coupled to the photon emitter by at least one quantum channel. The photon receiver includes the first polarization optic configured to output the emitted photon in a polarization state. The first photodetector is configured to detect the emitted photon from the output of the first polarization optic. The second photodetector is configured to detect a backflash from the first photodetector. The second polarization optic is between the first photodetector and the second photodetector. The second photodetector and the second polarization optic are configured to internally calibrate the photon receiver.

Abstract: A quantum cryptography apparatus and system includes a photon emitter, a photon receiver, a first photodetector, a second photodetector, a first polarization optic, and a second polarization optic. The photon emitter is configured to emit a photon at a wavelength. The photon receiver is coupled to the photon emitter by at least one quantum channel. The photon receiver includes the first polarization optic configured to output the emitted photon in a polarization state. The first photodetector is configured to detect the emitted photon from the output of the first polarization optic. The second photodetector is configured to detect a backflash from the first photodetector. The second polarization optic is between the first photodetector and the second photodetector. The second photodetector and the second polarization optic are configured to internally calibrate the photon receiver.

Abstract: A quantum cryptography apparatus and system includes a photon emitter, a photon receiver, a first photodetector, a second photodetector, a first polarization optic, and a second polarization optic. The photon emitter is configured to emit a photon at a wavelength. The photon receiver is coupled to the photon emitter by at least one quantum channel. The photon receiver includes the first polarization optic configured to output the emitted photon in a polarization state. The first photodetector is configured to detect the emitted photon from the output of the first polarization optic. The second photodetector is configured to detect a backflash from the first photodetector. The second polarization optic is between the first photodetector and the second photodetector. The second photodetector and the second polarization optic are configured to internally calibrate the photon receiver.

Abstract: A quantum cryptography apparatus and system includes a photon emitter, a photon receiver, a first photodetector, a second photodetector, a first polarization optic, and a second polarization optic. The photon emitter is configured to emit a photon at a wavelength, wherein the photon emitter is coupled to the photon receiver by at least one quantum channel. The photon receiver includes the first polarization optic configured to output a polarization state of the emitted photon. The first photodetector is configured to detect the photon emitted from the output of the first polarization optic. The second photodetector is configured to detect a backflash from the first photodetector. The second polarization optic is between the first photodetector and the second photodetector. The quantum cryptography apparatus may be a quantum key distribution system for characterizing backflashes.