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Communication system and communication method using the same

外国特許コード F110003745
整理番号 E06203WO
掲載日 2011年7月4日
出願国 アメリカ合衆国
出願番号 58103704
公報番号 20070110242
公報番号 7974540
出願日 平成16年11月29日(2004.11.29)
公報発行日 平成19年5月17日(2007.5.17)
公報発行日 平成23年7月5日(2011.7.5)
国際出願番号 JP2004017681
国際公開番号 WO2005053219
国際出願日 平成16年11月29日(2004.11.29)
国際公開日 平成17年6月9日(2005.6.9)
優先権データ
  • 特願2003-398491 (2003.11.28) JP
  • 特願2004-335228 (2004.11.19) JP
  • 2004WO-JP17681 (2004.11.29) WO
発明の名称 (英語) Communication system and communication method using the same
発明の概要(英語) (US7974540)
A communication system capable of employing polarization-dependent phase modulators with a reversing configuration that preserves security against disturbance of a polarization state at a transmission path but without using Faraday mirrors and a communication method using the same are provided.
A quantum cryptography system of the present invention includes a first station 1, a transmission path 2, and a second station 3.
The first station 1 has means for emitting time-divided optical pulses into the transmission path 2 and measuring a phase difference between the optical pulses returning from the transmission path 2.
The transmission path 2 is a medium of light.
The second station 3 has means for reversing traveling directions of the optical pulses, means for producing a phase difference, corresponding to a random number bit value to be transmitted, between the time-divided optical pulses, means for splitting the entering optical pulse into orthogonally polarized components and producing a 180-degree phase difference therebetween, means for rotating each polarization direction by 90 degrees, means for eliminating a component resulting from a deviation from the polarization rotation angle of 90 degrees, and means for attenuating optical pulse intensity to include no more than 1 photon per bit.
特許請求の範囲(英語) [claim1]
1. A communication system comprising: a transmission path for serving as a transmission medium of light;
a first station which emits time-divided optical pulses into the transmission path, returns optical pulses modulated at a second station into the transmission path, and measures a phase difference between the optical pulses returning from the transmission path; and
the second station including, a polarization beam splitter splitting the time-divided optical pulses into first orthogonally polarized components and second orthogonally polarized components,
a first phase modulator receiving the first of the split optical pulses and producing the phase difference corresponding to a value of a random number bit between the time-divided optical pulses, and
a second phase modulator receiving the second of the split optical pulses after polarization direction of the second of the split optical pulses is rotated by 90 degrees, producing a same phase difference as the first phase modulator between the first of the optical pulses, and modulating the orthogonally polarized components of each optical pulse to have a phase difference of 180 degrees therebetween,
wherein an output of the first phase modulator is combined with an output of the second phase modulator after the polarization direction is rotated by 90 degrees, and then the combined output returns into the transmission path.
[claim2]
2. The communication system according to claim 1, wherein a single phase modulator is configured as the first phase modulator and configured as the second phase modulator.
[claim3]
3. The communication system according to claim 2, wherein after each optical pulse entering the second station is split into the orthogonally polarized components, distances along which the split polarized components propagate before entering the phase modulator are set to be different for each polarized component, and by temporally varying driving voltage, the phase difference corresponding to the value of the random number bit and the 180-degree phase difference between the orthogonally polarized components are produced at the same time.
[claim4]
4. The communication system according to claim 2, wherein after each optical pulse entering the second station is split into the orthogonally polarized components, optical paths along which the split polarized components propagate before entering the different terminals of the phase modulator are composed of a polarization-maintaining optical fiber.
[claim5]
5. The communication system according to claim 4, wherein by orienting a polarizing axis of the polarization-maintaining optical fiber along electric-field vectors of the orthogonally polarized components of the entering optical pulse, the split polarized components are combined with their polarization directions rotated by 90 degrees.
[claim6]
6. The communication system according to claim 3 or 4, wherein a Faraday rotator modulates the orthogonally polarized components of each optical pulse to have a phase difference of 180 degrees therebetween and rotates the polarization direction by 90 degrees.
[claim7]
7. The communication system according to claim 1, wherein antireflection termination is provided at a port, from which a polarized component resulting from a deviation from the polarization rotation angle of 90 degrees is output, of the polarization beam splitter.
[claim8]
8. The communication system according to claim 1, wherein the second station has means for attenuating intensity of each optical pulse to include no more than 1 photon per bit when reemitting the optical pulses into the transmission path after combining the output of the first phase modulator with the output of the second phase modulator, so that a quantum cryptographic key is distributed.
[claim9]
9. A communication method comprising: emitting, at a first station, time-divided optical pulses into a transmission path, returning optical pulses modulated at a second station into the transmission path, and measuring a phase difference between the optical pulses returning from the transmission path;
splitting, by a polarization beam splitter at the second station, the time-divided optical pulses into first orthogonally polarized components and second orthogonally polarized components;
receiving, by a first phase modulator at the second station, the first of the split optical pulses, and producing the phase difference corresponding to a value of a random number bit between the time-divided optical pulses;
receiving, by a second phase modulator at the second station, the second of the split optical pulses after polarization direction of the second of the split optical pulses is rotated by 90 degrees, producing a same phase difference as the first phase modulator between the first of the optical pulses, and modulating the orthogonally polarized components of each optical pulse to have a phase difference of 180 degrees therebetween; and
combining, at the second station, an output of the first phase modulator with an output of the second phase modulator after the polarization direction is rotated by 90 degrees, and then returning the combined output into the transmission path.
[claim10]
10. The communication method according to claim 9, wherein a single phase modulator is configured as the first phase modulator and configured as the second phase modulator.
[claim11]
11. The communication method according to claim 10, wherein after each optical pulse entering the second station is split into the orthogonally polarized components, distances along which the split polarized components propagate before entering the phase modulator are set to be different for each polarized component, and by temporally varying driving voltage, the phase difference corresponding to the value of the random number bit and the 180-degree phase difference between the orthogonally polarized components are produced at the same time.
[claim12]
12. The communication method according to claim 10, wherein after each optical pulse entering the second station is split into the orthogonally polarized components, optical paths along which the split polarized components propagate before entering from the different terminals of the phase modulator to the polarization beam splitter is composed of a polarization-maintaining optical fiber.
[claim13]
13. The communication method according to claim 12, wherein by orienting a polarizing axis of the polarization-maintaining optical fiber along electric-field vectors of the orthogonally polarized components of the entering optical pulse, the split polarized components are combined with their polarization directions rotated by 90 degrees.
[claim14]
14. The communication method according to claim 11 or 12, wherein a Faraday rotator modulates the orthogonally polarized components of each optical pulse to have a phase difference of 180 degrees therebetween rotates the polarization direction by 90 degrees.
[claim15]
15. The communication method according to claim 9, wherein antireflection termination is provided at a port, from which a polarized component resulting from a deviation from a polarization rotation angle of 90 degrees is output, of the polarization beam splitter.
[claim16]
16. The communication method according to claim 9, wherein the second station has means for attenuating intensity of each optical pulse to include no more than 1 photon per bit when reemitting the optical pulses into the transmission path after the combining of the output of the first phase modulator with the output of the second phase modulator, so that a quantum cryptographic key is distributed.
  • 発明者/出願人(英語)
  • TOMITA AKIHISA
  • NAKAMURA KAZUO
  • TAJIMA AKIO
  • TANAKA AKIHIRO
  • NANBU YOSHIHIRO
  • SUZUKI SHUUJI
  • TAKEUCHI TAKESHI
  • MAEDA WAKAKO
  • TAKAHASHI SEIGO
  • AKIHISA TOMITA
  • JAPAN SCIENCE AND TECHNOLOGY AGENCY
  • NEC
国際特許分類(IPC)
参考情報 (研究プロジェクト等) ERATO IMAI Quantum Computation and Information AREA
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