# US 12,015,700 B2 · Quantum key distribution information leakage due to backflashes in single photon avalanche photodiodes > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 12,015,700 B2 - Document ID: US12015700B2 - Status: Granted - Kind: B2 - Application number: 16/952,499 - Filed: 2020-11-19 - Published: 2021-03-11 - Granted: 2024-06-18 - Earliest claimed: 2017-09-12 - Assignees: The MITRE Corporation - Applicants: The MITRE Corporation - Inventors: Daniel T. Stack; Stephen P. Pappas; Brandon V. Rodenburg; Colin P. Lualdi - Web page: https://patentgrep.com/patent/US12015700B2 ## CPC - H04L 9/0852: {Quantum cryptography (transmission systems employing electromagnetic waves other than radio waves, e.g. light, infrared H04B10/00; wavelength-division multiplex systems H04J14/02; WDM arrangements H04J14/03)} - H04L 9/0858: {Details about key distillation or coding, e.g. reconciliation, error correction, privacy amplification, polarisation coding or phase coding} - G01J 1/0429: {using polarisation elements} - H04L 63/1475: {Passive attacks, e.g. eavesdropping or listening without modification of the traffic monitored} - G01J 2001/442: {Single-photon detection or photon counting} - H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M) - G01J: MEASUREMENT OF INTENSITY, VELOCITY, SPECTRAL CONTENT, POLARISATION, PHASE OR PULSE CHARACTERISTICS OF INFRARED, VISIBLE OR ULTRAVIOLET LIGHT; COLORIMETRY; RADIATION PYROMETRY (light sources F21, H01J, H01K, H05B; investigating properties of materials by optical means G01N) ## 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. ## Claims ### Claim 1 A method for calibrating a quantum cryptography apparatus, the method comprising: emitting a photon at a wavelength from a photon emitter; receiving the emitted photon with a photon receiver; detecting a backflash of the emitted photon; characterizing a polarization dependence of the backflash; and calibrating the quantum cryptography apparatus based on the polarization dependence of the backflash, wherein the calibrating comprises detecting correlations between a polarization state of the emitted photon and a polarization state of the backflash. ### Claim 2 (depends on 1) The method of claim 1, wherein the calibrating comprises calibrating the photon receiver based on an information leakage percentage of the quantum cryptography apparatus. ### Claim 3 (depends on 1) The method of claim 1, wherein the calibrating comprises calibrating the photon receiver based on a quantum bit error rate of the quantum cryptography apparatus. ### Claim 4 (depends on 1) The method of claim 1, further comprising triggering an alarm based on the polarization dependence of the backflash. ### Claim 5 (depends on 1) The method of claim 1, further comprising deterring subsequent measurement of the backflash based on the polarization dependence of the backflash. ### Claim 6 (depends on 1) The method of claim 1, further comprising decrypting a quantum key distribution between the photon emitter and the photon receiver. ### Claim 7 (depends on 1) The method of claim 1, further comprising transferring the emitted photon by at least one quantum channel to the photon receiver. ### Claim 8 (depends on 7) The method 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. ### Claim 9 (depends on 7) The method 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. ### Claim 10 (depends on 1) The method of claim 1, wherein the receiving comprises receiving the emitted photon through a first polarization optic configured to output the emitted photon in the polarization state. ### Claim 11 (depends on 1) The method of claim 1, wherein the receiving comprises receiving the emitted photon with a first photodetector. ### Claim 12 (depends on 1) The method of claim 1, wherein the detecting comprises detecting the backflash from a first photodetector with a second photodetector. ### Claim 13 (depends on 1) The method of claim 1, wherein the characterizing comprises characterizing the polarization dependence of the backflash through a second polarization optic. ### Claim 14 A method for calibrating a quantum key distribution system, the method comprising: emitting a photon at a wavelength from a photon emitter; receiving the emitted photon with a photon receiver, wherein the photon receiver comprises: a polarization optic configured to output the emitted photon in a polarization state; and a photodetector configured to detect the emitted photon from the output of the polarization optic; detecting a backflash of the emitted photon from the photodetector; characterizing a polarization dependence of the backflash; and calibrating the quantum key distribution system based on the polarization dependence of the backflash, wherein the calibrating comprises detecting correlations between the polarization state of the emitted photon and a polarization state of the backflash. ### Claim 15 (depends on 14) The method of claim 14, further comprising triggering an alarm based on the polarization dependence of the backflash. ### Claim 16 (depends on 15) The method of claim 15, wherein the triggering the alarm is based on an information leakage percentage, a quantum bit error rate, or both of the quantum key distribution system. ### Claim 17 (depends on 15) The method of claim 15, further comprising deterring subsequent measurement of the backflash based on the alarm. ### Claim 18 (depends on 17) The method of claim 17, wherein the deterring subsequent measurement of the backflash comprises implementing an optical isolator, an optical circulator, an optical modulator, an optical filter, an optical switch, an optical router, or a combination thereof. ### Claim 19 (depends on 18) The method of claim 18, wherein the implementing comprises activing the optical circulator to siphon off the backflash.