# US 12,010,225 B2 · Discrete variable quantum key distribution using conjugate homodyne detection > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 12,010,225 B2 - Document ID: US12010225B2 - Status: Granted - Kind: B2 - Application number: 17/699,904 - Filed: 2022-03-21 - Published: 2022-09-22 - Granted: 2024-06-11 - Earliest claimed: 2021-03-22 - Assignees: UT-Battelle, LLC - Applicants: UT-Battelle, LLC - Inventors: Bing Qi - Web page: https://patentgrep.com/patent/US12010225B2 ## 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} - H04L: TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION (arrangements common to telegraphic and telephonic communication H04M) ## Abstract A system and a receiver for generating quantum key(s) using conjugated homodyne detection is provided. The receiver may communicate with a transmitter via an insecure quantum channel and a classical channel to generate the quantum key(s). A decoder, in the receiver, may determine, based at least in part on quadratures X, P measured by conjugated homodyne detectors, a raw-key signal corresponding to a key signal generated by the transmitter, and a distribution of photon numbers corresponding to a quantum signal received via the insecure quantum channel. Information about the key signal is exchanged between the receiver and the transmitter via the classical channel and used to determine a quantum bit error rate of the determined raw-key signal. A gain is also obtained. A secure-key rate is calculated based at least in part on the gain, the quantum bit error rate, and the photon number distribution. ## Claims ### Claim 1 A system comprising: a transmitter (TX) device comprising: a photon source configured to emit a sequence of photons having a source rate, a TX controller configured to generate a key signal having a bit rate equal to the source rate, an encoder configured to: encode, based on a discrete-variable encoding protocol, the key signal on the sequence of photons to form a quantum signal, and launch the quantum signal on an insecure quantum channel with a transmission rate equal to the source rate; and a receiver (RX) device comprising: a modulator communicatively coupled with the encoder through the insecure quantum channel and configured to: receive the quantum signal, and randomly modulate the received quantum signal to obtain a modulated quantum signal having the transmission rate, a first polarization-beam splitter (PBS) configured to project the modulated quantum signal in either a first polarization or a second polarization orthogonal to the first polarization, a first conjugate homodyne detector configured to: receive the modulated quantum signal projected in the first polarization, and simultaneously measure conjugate quadratures X, P of the modulated quantum signal projected in the first polarization, a second conjugate homodyne detector configured to: receive the modulated quantum signal projected in the second polarization, and simultaneously measure conjugate quadratures X, P of the modulated quantum signal projected in the second polarization, a decoder configured to determine, based at least in part on the measured conjugate quadratures X, P, a raw-key signal corresponding to the key signal, and a distribution of photon numbers corresponding to the received quantum signal, and an RX controller configured to: exchange, with the TX controller over a classical communication channel, information about the key signal, obtain a gain Q based on the determined raw-key signal and obtain a quantum bit error rate E based on the determined raw-key signal and the exchanged information, and calculate a secure-key rate R based at least in part on the obtained gain Q and quantum bit error rate E, and the determined photon number distribution. ### Claim 2 (depends on 1) The system of claim 1, wherein the RX device comprises a laser configured to emit light as a local oscillator for the first conjugate homodyne detector and the second conjugate homodyne detector, a second PBS configured to redirect a first portion of the local oscillator in the first polarization to the first conjugate homodyne detector, and a second portion of the local oscillator in the second polarization to the second conjugate homodyne detector. ### Claim 3 (depends on 2) The system of claim 2, wherein the laser has a pulse repetition rate that matches the source rate. ### Claim 4 (depends on 1) The system of claim 1, wherein the decoder is configured to determine the raw-key signal using a detection mode selected from a plurality of detection modes. ### Claim 5 (depends on 4) The signal of claim 4, wherein the selected detection mode is an independent detection mode and the decoder is configured to determine the raw-key signal using outputs of the first conjugate homodyne detector and the second conjugate homodyne detector independently by comparing the outputs with a predetermined detection threshold τ associated with the first and second conjugate homodyne detectors, and the RX controller is configured to calculate the secure-key rate R based further on the predetermined detection threshold τ. ### Claim 6 (depends on 5) The system of claim 5, wherein the predetermined detection threshold τ is in a range of 1 to 10. ### Claim 7 (depends on 5) The system of claim 5, wherein a contribution Ed to the quantum bit error rate E, which is due to polarization misalignment, is in a range of 0 to 0.05. ### Claim 8 (depends on 4) The system of claim 4, wherein the selected detection mode is a differential detection mode, and the decoder is configured to determine the raw-key signal using outputs of the first conjugate homodyne detector and the second conjugate homodyne detector jointly by comparing the outputs with each other, and the RX controller is configured to calculate the secure-key rate R based on the obtained gain Q and quantum bit error rate E, and the determined photon number distribution without a determined detection threshold τ. ### Claim 9 (depends on 8) The system of claim 8, wherein a contribution Ed to the quantum bit error rate E, which is due to polarization misalignment, is in a range of 0 to 0.01. ### Claim 10 (depends on 1) The system of claim 1, wherein the insecure quantum channel comprises an optical fiber having a length less than about 10 km. ### Claim 11 (depends on 1) The system of claim 1, wherein the insecure quantum channel comprises a free-space channel. ### Claim 12 (depends on 1) The system of claim 1, wherein each of the first and second conjugate homodyne detectors comprise shot-noise limited balanced photodiodes with a bandwidth of 5 GHz. ### Claim 13 (depends on 12) The system of claim 12, wherein the shot-noise limited balanced photodiodes are configured to be operated at room temperature. ### Claim 14 (depends on 1) The system of claim 1, wherein the transmission rate is in a range of 1 MHz to 10 GHz. ### Claim 15 (depends on 1) The system of claim 1, wherein to a secure key K is produced by exchanging information between the TX controller and the RX controller. ### Claim 16 (depends on 15) The system of claim 15, wherein the RX controller and the TX controller are configured to produce a plurality of secure keys from respective key signals and raw key signals for a communication session over the classical communication channel. ### Claim 17 (depends on 1) The system of claim 1, wherein the discrete-variable encoding protocol is a polarization encoding protocol. ### Claim 18 (depends on 1) The system of claim 1, wherein the discrete-variable encoding protocol uses BB84. ### Claim 19 A receiver comprising: a modulator communicatively coupled with a transmitter (TX) through an insecure quantum channel and configured to receive a quantum signal formed by the TX as a sequence of photons encoded, based on a discrete-variable encoding protocol, with a key signal, and randomly modulate the received signal to obtain a modulated quantum signal; a first polarization-beam splitter (PBS) configured to project the modulated quantum signal in either a first polarization or a second polarization orthogonal to the first polarization; a first conjugate homodyne detector configured to receive the modulated quantum signal projected in the first polarization, and simultaneously measure conjugate quadratures X, P of the modulated quantum signal projected in the first polarization; a second conjugate homodyne detector configured to receive the modulated quantum signal projected in the second polarization, and simultaneously measure conjugate quadratures X, P of the modulated quantum signal projected in the second polarization; a decoder configured to determine, based at least in part on the measured conjugate quadratures X, P, a raw-key signal corresponding to the key signal, and a distribution of photon numbers corresponding to the received quantum signal; and an RX controller configured to exchange, with the TX over a classical communication channel, information about the key signal, obtain a gain Q based on the determined raw-key signal and obtain a quantum bit error rate E based on the determined raw-key signal and the exchanged information, and calculate a secure-key rate R based at least in part on the obtained gain Q and quantum bit error rate E, and the determined photon number distribution. ### Claim 20 (depends on 19) The receiver of claim 19, further comprising: a laser configured to emit light as a local oscillator, a second PBS configured to redirect a first portion of the local oscillator in the first polarization to the first conjugate homodyne detector, and a second portion of the local oscillator in the second polarization to the second conjugate homodyne detector. ### Claim 21 (depends on 20) The receiver of claim 20, wherein the laser has a pulse repetition rate that matches the source rate. ### Claim 22 (depends on 19) The receiver of claim 19, wherein the decoder is configured to determine the raw-key signal using a detection mode selected from a plurality of detection modes. ### Claim 23 (depends on 22) The receiver of claim 22, wherein the selected detection mode is an independent detection mode and the decoder is configured to determine the raw-key signal using outputs of the first conjugate homodyne detector and the second conjugate homodyne detector independently by comparing the outputs with a predetermined detection threshold τ associated with the first and second conjugate homodyne detectors, and the RX controller is configured to calculate the secure-key rate R based further on the predetermined detection threshold τ. ### Claim 24 (depends on 22) The receiver of claim 22, wherein the selected detection mode is an differential detection mode and the decoder is configured to determine the raw-key signal using outputs of the first conjugate homodyne detector and the second conjugate homodyne detector jointly by comparing the outputs with each other, and the RX controller is configured to calculate the secure-key rate R based on the obtained gain Q and quantum bit error rate E, and the determined photon number distribution without a predetermined detection threshold τ. ### Claim 25 (depends on 19) The receiver of claim 19, wherein each of the first and second conjugate homodyne detectors comprise shot-noise limited balanced photodiodes with a bandwidth of 5 GHz. ### Claim 26 (depends on 25) The receiver of claim 25, wherein the shot-noise limited balanced photodiodes are configured to operate at room temperature.