# US 11,451,307 B1 · Optical system and method > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 11,451,307 B1 - Document ID: US11451307B1 - Status: Granted - Kind: B1 - Application number: 17/403,783 - Filed: 2021-08-16 - Published: 2022-10-06 - Granted: 2022-09-20 - Earliest claimed: 2021-03-30 - Assignees: Kabushiki Kaisha Toshiba - Applicants: Kabushiki Kaisha Toshiba - Inventors: Mirko Pittaluga; Zhiliang Yuan; Andrew James Shields - Web page: https://patentgrep.com/patent/US11451307B1 ## CPC - H04B 10/70: Photonic quantum communication - H04B 10/2507: for the reduction or elimination of distortion or dispersion - H04B 10/29: Repeaters - H04B 10/50: Transmitters - H04B 10/61: Coherent receivers - H04L 9/0855: {involving additional nodes, e.g. quantum relays, repeaters, intermediate nodes or remote nodes} - H04B: TRANSMISSION - H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M) ## Abstract An optical system, comprising: an emitter configured to output a first optical signal along a first optical path to an interference unit and to output a second optical signal along a second optical path to the interference unit, the interference unit being configured to interfere the first and second optical signals, wherein the coherence length of the optical signal is longer than the path difference between first and second optical paths, and there is a path difference between the first and second paths of at least 1 km. ## Claims ### Claim 1 An optical system, comprising: an emitter configured to output a first optical signal along a first optical path to a detector node and to output a second optical signal along a second optical path to the detector node, the detector node being configured to interfere the first and second optical signals; a first node with a first encoder located in the first node, the first node forming at least part of the first optical path, the first encoder being configured to receive said first optical signal and encode said first optical signal in phase such that the first optical signal output from said first encoder is encoded in phase; and a second node with a second encoder located in the second node, the second node forming at least part of the second optical path, the second encoder being configured to receive said second optical signal and encode said second optical signal in phase such that the second optical signal output from said second encoder is encoded in phase, wherein said detector node is configured to cause interference of the encoded first optical signal and the encoded second optical signal, and coherence length of the optical signal is longer than a path difference between the first and second optical paths, and the path difference between the first and second optical paths is at least 1 Kilometer (km). ### Claim 2 (depends on 1) The optical system according to claim 1, wherein the emitter is located in the first node such that the second optical path extends from the emitter in the first node, through the second node and to the detector node. ### Claim 3 (depends on 1) The optical system according to claim 1, wherein the first optical signal and the second optical signal are continuous wave “CW” signals and the first encoder is configured to produce a pulsed signal from said first optical signal and the second encoder is configured to produce a pulsed signal from said second optical signal. ### Claim 4 (depends on 3) The optical system according to claim 3, wherein each pulse is configured to contain on average one photon or less. ### Claim 5 (depends on 1) The optical system according to claim 1, wherein said emitter is configured to output signals at n different wavelengths, where n is an integer of at least 2, such that there are n different wavelengths in the first optical signal and n different wavelengths in the second optical signal. ### Claim 6 (depends on 1) The optical signal according to claim 1, the system being configured as a quantum communication system, said detector node comprising processing circuitry, said processing circuitry being configured to determine information about the encoder phase of the first and second optical signals by measuring the interference at the detector node. ### Claim 7 (depends on 6) The optical signal according to claim 6, wherein the detector node is configured to perform first order interference of said first and second optical signals. ### Claim 8 (depends on 7) The optical system according to claim 7, wherein the first node, the second node and the detector node are configured to implement quantum communication using twin field quantum key distribution. ### Claim 9 (depends on 6) The optical system according to claim 6, wherein the emitter is not located in the detector node. ### Claim 10 (depends on 1) The optical system according to claim 1, wherein the emitter comprises a laser locked to a high finesse optical cavity. ### Claim 11 (depends on 6) The optical system according to claim 6, wherein the emitter is configured to output a reference signal. ### Claim 12 (depends on 11) The optical system according to claim 11, wherein the reference signal is a continuous wave signal. ### Claim 13 (depends on 11) The optical system according to claim 11, wherein the reference signal has a different wavelength to the encoded first optical signal and the encoded second optical signal. ### Claim 14 (depends on 11) The optical system according to claim 11, wherein the detector node is configured to interfere the reference signal and obtain tuning parameters for said optical system. ### Claim 15 (depends on 14) The optical system according to claim 14, wherein the detector node uses said tuning parameters as an input to tune said optical system to remove phase drift. ### Claim 16 (depends on 15) The optical system according to claim 15, wherein the detector node is configured to tune said optical system using at least one of active stabilisation and passive phase stabilisation. ### Claim 17 (depends on 1) The optical system according to claim 1, wherein the first node and the second node are linked by a frequency stabilising fibre. ### Claim 18 A method of optical communication, the method comprising: outputting a first optical signal from an emitter along a first optical path to a detector node and outputting a second optical signal from the same emitter along a second optical path to the detector node, wherein: at least part of the first optical path being formed by a first node with a first encoder located in the first node, the first encoder receiving said first optical signal and encoding said first optical signal in phase such that the first optical signal output from said first encoder is encoded in phase; and at least part of the second optical path being formed by a second node with a second encoder located in the second node, the second encoder receiving said second optical signal and encoding said second optical signal in phase such that the second optical signal output from said second encoder is encoded in phase, wherein said detector node is configured to cause interference of the encoded first optical signal and the encoded second optical signal; the method further comprising: causing interference of the first and second optical signals at the detector node, wherein coherence length of the optical signal is longer than a path difference between the first and second optical paths, and the path difference between the first and second paths is at least 1 kilometer (km).