Patent Number: 
Section: claims

1. A quantum entangled photon pair generating device comprising:a polarization maintaining loop path;a loop input unit for receiving excitation light from a first optical path, separating the excitation light into a first excitation light component and a second excitation light component, the first excitation light component and the second excitation light component having mutually orthogonal polarization, feeding the first excitation light component into the polarization maintaining loop path in a clockwise direction, and feeding the second excitation light component into the polarization maintaining loop path in a counterclockwise direction;an optical conversion generation unit including a first second-order nonlinear optical medium disposed in the polarization maintaining loop path for generating first up-converted light from the first excitation light component by second harmonic generation, a second second-order nonlinear optical medium disposed in the polarization maintaining loop path for generating second up-converted light from the second excitation light component by second harmonic generation, and a first half-wave plate disposed in the polarization maintaining loop path between the first and second nonlinear optical media for rotating planes of polarization of the first and second excitation light components by ninety degrees, the second second-order nonlinear optical medium also generating first down-converted light from the first up-converted light by spontaneous parametric down-conversion, the first second-order nonlinear optical medium also generating second down-converted light from the second up-converted light by spontaneous parametric down-conversion, the first up-converted light and the first down-converted light propagating on the polarization maintaining loop path in the clockwise direction, the second up-converted light and the second down-converted light propagating on the polarization maintaining loop path in the counterclockwise direction;a loop output unit for receiving the first down-converted light and the second down-converted light from the polarization maintaining loop path, combining the first down-converted light and the second down-converted light to obtain combined light, and outputting the combined light on a second optical path; anda polarization manipulation unit for manipulating a polarization direction of at least one of the first excitation light component, the second excitation light component, the first down-converted light, and the second down-converted light, thereby causing the loop output unit to output the combined light as polarization entangled light. 2. The quantum entangled photon pair generating device of claim 1, wherein the first and second up-converted light and the first and second down-converted light are generated entirely by the optical conversion generation unit. 3. The quantum entangled photon pair generating device of claim 1, wherein a single polarization splitting-combining module is used as both the loop input unit and the loop output unit. 4. The quantum entangled photon pair generating device of claim 3, wherein the polarization manipulation unit is a second half-wave plate disposed in the polarization maintaining loop path to rotate a plane of polarization of the first down-converted light by ninety degrees. 5. The quantum entangled photon pair generating device of claim 4, wherein the first optical path and the second optical path share a common section, further comprising an optical circulator disposed at one end of the common section. 6. The quantum entangled photon pair generating device of claim 4, wherein the first optical path and the second optical path share a common section, further comprising a first wavelength selective filter disposed at an end of the common section to reflect one of the excitation light and the combined light and transmit another one of the excitation light and the combined light. 7. The quantum entangled photon pair generating device of claim 3, wherein the polarization manipulation unit further comprises:a first nonreciprocal polarization converter preceding the optical conversion generation unit in the counterclockwise direction on the polarization maintaining loop path, the first nonreciprocal polarization converter including a first Faraday rotator and a third half-wave plate, the first Faraday rotator preceding the third half-wave plate in the counterclockwise direction on the polarization maintaining loop path; anda second nonreciprocal polarization converter following the optical conversion generation unit in the counterclockwise direction on the polarization maintaining loop path, the second nonreciprocal polarization converter including a second Faraday rotator and a fourth half-wave plate, the second Faraday rotator preceding the fourth half-wave plate in the counterclockwise direction on the polarization maintaining loop path. 8. The quantum entangled photon pair generating device of claim 3, wherein the polarization manipulation unit further comprises:a second half-wave plate following the optical conversion generation unit in the clockwise direction on the polarization maintaining loop path, for rotating a plane of polarization of the first down-converted light by ninety degrees; anda polarization converter following the optical conversion generation unit in the counterclockwise direction on the polarization maintaining loop path to rotate a plane of polarization of the second up-converted light by ninety degrees, the polarization converter including a fifth half-wave plate, a quarter-wave plate following the fifth half-wave plate in the clockwise direction on the polarization maintaining loop path, and a sixth half-wave plate following the quarter-wave plate in the clockwise direction on the polarization maintaining loop path. 9. The quantum entangled photon pair generating device of claim 8, wherein the first optical path and the second optical path share a common section, further comprising an optical circulator disposed at one end of the common section. 10. The quantum entangled photon pair generating device of claim 8, wherein the first optical path and the second optical path share a common section, further comprising a first wavelength selective filter disposed at an end of the common section to reflect one of the excitation light and the combined light and transmit another one of the excitation light and the combined light. 11. The quantum entangled photon pair generating device of claim 3, wherein the polarization manipulation unit further comprises:a first nonreciprocal polarization converter preceding the optical conversion generation unit in the counterclockwise direction on the polarization maintaining loop path, the first nonreciprocal polarization converter including a first Faraday rotator and a third half-wave plate, the first Faraday rotator preceding the third half-wave plate in the counterclockwise direction on the polarization maintaining loop path;a second nonreciprocal polarization converter following the optical conversion generation unit in the counterclockwise direction on the polarization maintaining loop path, the second nonreciprocal polarization converter including a second Faraday rotator and a fourth half-wave plate, the second Faraday rotator preceding the fourth half-wave plate in the counterclockwise direction on the polarization maintaining loop path; anda polarization converter disposed between the second nonreciprocal polarization converter and the optical conversion generation unit on the polarization maintaining loop path, the polarization converter including a fifth half-wave plate, a quarter-wave plate following the fifth half-wave plate in the clockwise direction on the polarization maintaining loop path, and a sixth half-wave plate following the quarter-wave plate in the clockwise direction on the polarization maintaining loop path. 12. The quantum entangled photon pair generating device of claim 1, further comprising a second wavelength selective filter having two output ports, disposed on the second optical path to reject the excitation light, output a signal wavelength component of the combined light from one of the two output ports, and output an idler wavelength component of the combined light from another one of the two output ports. 13. The quantum entangled photon pair generating device of claim 12, wherein the second wavelength selective filter comprises an arrayed waveguide grating. 14. The quantum entangled photon pair generating device of claim 1, further comprising an optical low-pass filter disposed on the second optical path to reject at least one of the first up-converted light and the second up-converted light.