Quantum key distribution information leakage due to backflashes in single photon avalanche photodiodes

1. A quantum cryptography apparatus, comprising:

a photon emitter configured to emit a photon at a wavelength;

a photon receiver coupled to the photon emitter by at least one quantum channel and comprising a first polarization optic configured to output the emitted photon in a polarization state;

a first photodetector configured to detect the emitted photon from the output of the first polarization optic;

a second photodetector configured to detect a backflash from the first photodetector; and

a second polarization optic between the first photodetector and the second photodetector, wherein the second polarization optic is configured to detect a polarization dependence of the backflash from the first photodetector,

wherein the second photodetector is a four channel single photon counting avalanche photodiode (APD) array configured to simultaneously detect the polarization dependence of the backflash from the first photodetector, and

wherein the second photodetector and the second polarization optic are configured to internally calibrate the photon receiver.

2. The apparatus of claim 1 , wherein:

the photon receiver comprises two arms, each arm associated with a BB84 basis; and

a non-polarizing 50:50 beamsplitter that couples the two arms.

3. The apparatus of claim 1 , wherein:

the at least one quantum channel is a free space channel; and

the wavelength of the emitted photon is 400 nm to 1100 nm.

4. The apparatus of claim 1 , wherein:

the at least one quantum channel is a fiber optic channel; and

the wavelength of the emitted photon is 1100 nm to 1600 nm.

5. The apparatus of claim 1 , wherein the second polarization optic comprises an adjustable linear polarizer.

6. The apparatus of claim 1 , wherein the emitted photon from the photon emitter is circularly polarized.

7. A quantum key distribution system for characterizing backflashes, comprising:

a photon emitter configured to emit a photon at a wavelength;

a photon receiver coupled to the photon emitter by at least one quantum channel and comprising a first polarization optic configured to output the emitted photon in a polarization state;

a first photodetector configured to detect the photon emitted from the output of the first polarization optic;

a second photodetector configured to detect a backflash from the first photodetector;

a second polarization optic between the first photodetector and the second photodetector, wherein the second polarization optic is configured to detect a polarization dependence of the backflash from the first photodetector; and

a data acquisition subsystem coupled to the second photodetector, wherein the data acquisition subsystem characterizes the polarization dependence of the backflash and deduces the polarization state of the emitted photon,

wherein the photon receiver comprises a backflash receiver comprising an optical circulator, the second photodetector, and the second polarization optic, and

wherein the backflash receiver and the data acquisition subsystem are configured to internally calibrate the photon receiver.

8. The system of claim 7 , wherein:

the photon receiver comprises two arms, each arm associated with a BB84 basis; and

a non-polarizing 50:50 beamsplitter that couples the two arms.

9. The system of claim 7 , wherein:

the at least one quantum channel is a free space channel; and

the wavelength of the emitted photon is 400 nm to 1100 nm.

10. The system of claim 7 , wherein:

the at least one quantum channel is a fiber optic channel; and

the wavelength of the emitted photon is 1100 nm to 1600 nm.

11. The system of claim 7 , wherein:

the second photodetector is a four channel single photon counting avalanche photodiode (APD) array, the second photodetector being configured to simultaneously detect the polarization dependence of the backflash from the first photodetector.

12. The system of claim 7 , wherein the data acquisition subsystem comprises an alarm subsystem based on the polarization dependence of the backflash.

13. The system of claim 7 , wherein the data acquisition subsystem is configured to decrypt a quantum key distribution between the photon emitter and the photon receiver.

14. A method for characterizing backflashes in a quantum key distribution system, comprising:

emitting a photon from a photon emitter at a wavelength;

transferring the emitted photon by at least one quantum channel to a photon receiver;

receiving the emitted photon through a first polarization optic with a first photodetector;

detecting a backflash from the first photodetector with a second photodetector;

characterizing a polarization dependence of the backflash through a second polarization optic; and

calibrating the quantum key distribution system based on the polarization dependence of the backflash.

15. The method of claim 14 , further comprising triggering an alarm subsystem based on the polarization dependence of the backflash.

16. The method of claim 14 , further comprising deterring subsequent measurement of the backflash based on the polarization dependence of the backflash.

17. The method of claim 16 , wherein deterring subsequent measurement of the backflash comprises implementing an optical isolator, an optical circulator, an optical modulator, an optical filter, or some combination thereof.

18. The method of claim 14 , wherein:

the at least one quantum channel is a free space channel; and

the wavelength of the emitted photon is 400 nm to 1100 nm.