JP2007114464A - Polarization entangled photon couple generating device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a device capable of generating high-efficiency and wide-band photon couple in an optical communication wavelength band and also capable of generating wide-band two-photon polarization entangled photon couple and three-photon polarization entangled photon couple by the design of a pseudo phase matching element. <P>SOLUTION: As a generating device for the polarization entangled photon couple using pseudo phase matching optical crystal based upon periodically poled lithium niobate, obtained are a generating device for a two-photon polarization entangled photon couple characterized in a structure wherein first optical crystal of phase matching ooe of a phase matching type I and second optical crystal of phase matching eee of a phase matching type 0 are cascaded; and a generating device for a three-photon polarization entangled photon couple characterized in a structure wherein third optical crystal of eee of the phase matching type 0 and fourth optical crystal of eee of the phase matching type 0 are cascaded. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a device for generating a highly efficient entangled photon pair using a quasi-phase matching element having a high conversion efficiency, which generates a entangled photon pair in a wavelength band in which an optical communication band component can be used. .

  As for the two-photon polarization entangled photon pair generating device of the prior art, a device using Type II phase matching oeo, eoo has been reported as a polarization entangled photon pair generating device in the optical communication wavelength band using pseudo phase matching. By making excitation light incident on this device as ordinary light, the output light becomes ordinary light and extraordinary light, which is a pair of polarization entangled photons. This device can realize a entangled photon pair generation device in the optical communication wavelength band by setting the polarization inversion period to 7 μm. Since a entangled photon pair can be generated with a single device, it is expected to be a simple and reliable device. The structure of a conventional polarization entangled photon pair device and incident outgoing light are shown in FIG.

  However, since the operation line width of the conventional device is about 500 GHz, and the efficiency of wavelength conversion decreases in the future by excitation with an ultrashort pulse, it is necessary to realize a device operating in a wide band. In addition, the fact that the polarization inversion period is as short as 7 μm contributes to the reduction of the manufacturing yield, and therefore it is necessary to realize a device that operates with a longer period of polarization inversion.

  The prior art of the present invention uses two BBO crystals and appropriately adjusts the respective phase matching conditions so that the polarization entangled photon pair is oriented in the directions of +3 degrees and -3 degrees with respect to the incident pump light. There is a suggestion to take out. (See Patent Document 1) As a practical example of this prior art, the pump wavelength is 351.1 nm, and the wavelength of the generated polarization-entangled photon pair is 702.2 nm. In the method of the present invention, the polarization entangled photon pair is generated at an angle with respect to the incident pump light, so that a walk-off occurs at the time of wavelength conversion, resulting in a decrease in conversion efficiency. In addition, since the wavelength of the generated entangled photon pair is 702.2 nm, there is a problem that loss in long-distance transmission using an optical fiber is large.

  As another prior art, there is a method of generating a polarization-entangled photon pair in an optical communication wavelength band using PPLN. However, this method is not a method of directly emitting a polarization entangled photon pair with a single crystal, but a method of forming a polarization entangled photon pair by arranging a plurality of crystals in an interferometer. The configuration becomes complicated.

JP2003-228091 JP 2005-258232 A

  Therefore, high-efficiency and broadband polarization-entangled photon pairs can be generated in the optical communication wavelength band, and broadband two-photon polarization-entangled photon pairs and three-photon polarization-entangled photon pairs can be generated by the design of the quasi-phase matching element. It is an object of the present invention to provide such a device.

  According to the present invention, in a polarization entangled photon pair generating device using a periodically poled lithium niobate-based quasi-phase-matching optical crystal, by making excitation light incident as extraordinary light, Type I phase matching: from ooe The output is ordinary light, Type0 phase matching: the output from eee becomes extraordinary light, and a two-photon polarization entangled photon pair is generated. As a phase matching type, TypeI phase matching: ooe first optical crystal, Type0 phase matching: A device for producing a two-photon entangled photon pair characterized by a cascade arrangement of eee second optical crystals is obtained.

  Further, according to the present invention, the first optical crystal and the second optical crystal are two-photon structures characterized by a structure in which a single lithium niobate-based quasi-phase-matching optical crystal is configured as periodic polarization inversion. A device for generating a entangled photon pair is obtained.

  In addition, according to the present invention, parametric wavelength conversion and degenerate point operation parametric wavelength conversion can be operated in cascade in a device for generating a entangled photon pair using a periodically poled lithium niobate-based quasi-phase-matched optical crystal. So, in order to generate a three-photon deflection entangled photon pair, the phase matching type Type 0: eee third optical crystal and the phase matching type Type 0: eee fourth optical crystal are arranged in cascade. A device for producing a three-photon entangled photon pair characterized by the above structure is obtained.

  Further, according to the present invention, the third optical crystal and the fourth optical crystal each have a structure of three photons characterized by being configured as a periodic polarization inversion in a single lithium niobate-based quasi phase matching optical crystal. A device for generating a entangled photon pair is obtained.

  According to the present invention, by using a quasi-phase matching element as a polarization entangled photon pair generation device, it is possible to generate a polarization entangled photon pair with high efficiency and wide bandwidth in the optical communication wavelength band. The design of the quasi-phase matching element enables generation of broadband two-photon polarization entangled photon pairs and generation of three-photon polarization entangled photon pairs.

1.2 Configuration example of a two-photon polarization entangled photon pair generation device This device can be realized by setting the polarization inversion periods of the ordinary light generation unit and the abnormal light generation unit to 25 μm and 19 μm, respectively. The operating line width of this device is as wide as 75 THz, and parametric wavelength conversion is possible over the entire spectral region even when short pulse light of the order of 100 fsec is used as the excitation light source. A two-photon polarization entangled photon pair generation device fabricated as a single device by writing two periods of TypeI phase matching ooe and Type0 phase matching eee as phase matching types on the same wafer is shown in FIG.

  We will design and develop a quasi-phase-matching parametric device that also takes into account birefringence, and generate entangled photon pairs that are more efficient than conventional ones under collinear phase-matching conditions. Focusing on the 775 nm equivalent to the second harmonic of the optical communication wavelength band as the excitation light source, we will study device design. In consideration of actual PPLN device fabrication, a phase matching type in which the polarization inversion period is about 10 μm or more is adopted. The phase matching type that can convert the wavelength from the effective nonlinear coefficient of the lithium niobate (LN) crystal is Type0 phase matching eee, ooo, TypeI phase matching ooe, TypeII phase matching oeo, eoo There are 5 types. In the notation such as eee, e indicates that the light is extra-ordinary, and o indicates that the light is ordinary. In turn, the polarization of light contributing to the parametric process is shown. For example, eee indicates that all polarized light is extraordinary light, and ooe indicates that idler light and signal light are ordinary light, and pump light is extraordinary light.

Wavelength conversion by the quasi-phase-matching parametric process is an energy conservation law (1) Equation ωp = ωs + ωi (1)
And the equation (2) that is the law of conservation of momentum,
kp = ks + ki + 2π / Λ (2)
This is realized when and are satisfied at the same time. Where ωp, ωs, and ωi are the frequencies of the pump light, signal light, and idler light, kp, ks, and ki are the wave numbers of the pump light, signal light, and idler light, respectively, and Λ is the polarization inversion period of the quasi-phase-matched crystal. The wave number is expressed by kp = 2πnp / λp, ks = 2πns / λs and ki = 2πni / λi, where np, ns and ni are the refractive index of the crystal for pump light, signal light and idler light, λp, λs and λi are the wavelengths of pump light, signal light and idler light, respectively. The refractive index of the crystal is a function of wavelength and temperature, and is expressed by the Selmeier equation. Therefore, if the operating temperature and the wavelength of the pump light are determined, the phase inversion period in which phase matching can be obtained by the equations (1) and (2) Λ and the wavelengths of the signal light and idler light are determined. Since the polarization entangled photon pair has the same wavelength of signal light and idler light, ωs = ωi = ωp / 2, and if the polarization inversion period Λ is obtained under such conditions, it functions as a polarization entangled photon pair generating device.

  FIG. 3 shows the result of calculating the polarization reversal period with respect to the excitation wavelength when the operating temperature is 100 ° C. As a device for simultaneously generating ordinary light and extraordinary light, a device using Type II phase matching and a device using Type 0 and Type I phase matching devices in cascade can be realized. Here, the device using Type II phase matching is in the conventional report, and it can be seen that the polarization inversion period is the shortest. On the other hand, focusing on phase matching between Type 0 and Type I, Type 0 eee is excited by extraordinary pump light to generate abnormal signal light and idler light, and Type I oee is excited by extraordinary pump light. It generates ordinary signal light and idler light. If these two processes are used in cascade, the extraordinary light and ordinary light are obtained from the extraordinary pump light, which becomes a entangled photon pair. The polarization inversion period for realizing each process is longer than Type II phase matching and easy to manufacture.

  If the wavelength of the polarization entangled photon pair is 1.55 μm in the optical communication wavelength band, the wavelength of the pump light is 775 nm. FIG. 4 shows the result of calculating the polarization inversion period with respect to the operating temperature when the excitation wavelength is 775 nm. A device using Type II phase matching has a polarization inversion period of 10 μm or less, while a device using Type 0 eee and Type I ooe phase matching has a polarization inversion period of 15 μm or more.

  In other words, Type 0 eee is used for the first optical crystal, Type I ooe is used for the second optical crystal, or Type I oee is used for the first optical crystal, and Type 0 eee is used for the second optical crystal. A polarization entangled photon pair device can be realized. If the operating temperature is 100 ° C., the polarization inversion period of the first optical crystal is 19 μm, the polarization inversion period of the second optical crystal is 25 μm, or the polarization inversion period of the first optical crystal is 25 μm, and the second optical crystal The polarization inversion period of the crystal may be 19 μm. Moreover, as a form of realization, it is possible to obtain the same effect even if the first optical crystal and the second optical crystal are manufactured monolithically on the same wafer, or are formed after separate crystals are arranged. .

2.3 Configuration example of a three-photon entangled photon pair generating device By operating the parametric wavelength conversion and degenerate point parametric wavelength conversion in cascade, it is possible to realize a device that generates entangled three-photons. FIG. 2 is the resulting three-photon polarization entangled photon pair generating device and wavelength conversion. By setting the number of photons in the entangled photon pair to 3 photons, the information content of the photons is improved. When using a laser beam with a wavelength of 517 nm as the excitation light source, the first and second stage polarization inversion periods are 7 μm and 19 μm, respectively, when Type 0: eee phase matching is used, which maximizes the conversion efficiency of the polarization inversion device. .

  In order to generate a three-photon entangled photon pair by a parametric process, it is not possible with a single device, so it is necessary to design the wavelength conversion process in two stages. The third optical crystal generates a signal light having a wavelength 3/2 times that of the pump light and an idler light having a wavelength three times that of the pump light from the pump light, and the signal light and idler light are generated from the fourth optical crystal. In the third crystal and the fourth optical crystal, the wavelength of the pump light is three times that of the pump light. These three photons are generated, and these three photons are entangled photon pairs. The device design for generating the entangled photon pair of three photons in the optical communication wavelength band may be designed so as to satisfy the equations (1) and (2) as in the case of two photons. Also, only Type 0 eee that maximizes the effective nonlinear optical constant is considered here. If the wavelength of the three finally generated photons is 1550 nm, the wavelength of the pump light introduced into the third optical crystal is 516.7 nm. The relationship between the three light frequencies contributing to the parametric process is known (ωs = 3/2 * ωp, ωi = 3 * ωp), and FIG. 5 shows the relationship between the operating temperature and the polarization inversion period at this time. . The wavelengths of the signal light and idler light introduced into the fourth optical crystal are 775 nm and 1550 nm, respectively, and FIG. 6 shows the relationship between the operating temperature and the polarization inversion period at this time. In either case, if the operating temperature is 100 ° C., the necessary polarization inversion periods are 6.7 μm and 18.7 μm, respectively. As a form of realization, it is possible to obtain the same effect even if the first optical crystal and the second optical crystal are manufactured monolithically on the same wafer, or are formed after separate crystals.

  8 and 9 show the actual two-photon polarization entangled photon pair generation device configuration and the three-photon entangled photon pair generation device configuration. With this configuration, research and development of quantum computers and quantum cryptography communications for the future is possible.

A two-photon polarization entangled photon pair generating device and incident outgoing light produced as a single device by writing one period as a phase matching type on the same wafer. Parametric wavelength conversion and degenerate point operation A three-photon polarization entangled photon pair generating device, incident outgoing light and wavelength conversion that operate parametric wavelength conversion in cascade. It is the result of calculating the period of polarization inversion with respect to the excitation wavelength when the operating temperature is 100 ° C. It is the result of calculating the period of polarization inversion with respect to the operating temperature when the excitation wavelength is 775 nm. This is the relationship between the operating temperature and the polarization inversion period of the three-photon polarization entangled photon pair generating device. The wavelength of the signal light and idler light is the relationship between the operating temperature and the polarization inversion period of the three-photon polarization entangled photon pair generating device of 775 nm and 1550 nm, respectively. It is the structure of a conventional polarization entangled photon pair device and incident outgoing light. A two-photon polarization entangled photon pair generator configuration. It is a three-photon entangled photon pair generator configuration.

Claims (4)

  In a polarization-entangled photon pair generation device using a periodically poled lithium niobate-based quasi-phase-matched optical crystal, the incident light as extraordinary light makes TypeI phase matching: output from ooe normal, Type0 phase Matching: Type I phase matching: oee first optical crystal and Type0 phase matching: eee second optical so that the output from eee becomes extraordinary light and a two-photon polarization entangled photon pair is generated A device for producing a pair of two-photon entangled photons characterized by a structure in which crystals are arranged in cascade.   The first optical crystal and the second optical crystal are two-photon deflection entangled photon pair generating devices characterized by a structure in which a single lithium niobate-based quasi-phase-matching optical crystal is configured as periodic polarization inversion.   Three-photon entangled photons are generated by cascading parametric wavelength conversion and degenerate point operation parametric wavelength conversion in a polarization entangled photon pair generation device using periodically poled lithium niobate-based quasi-phase-matched optical crystal Three-photon deflection characterized by a cascade arrangement of Type0: eee third optical crystal as phase matching type and Type0: eee fourth optical crystal as cascade type to form pairs Tangle photon pair generation device.   The third optical crystal and the fourth optical crystal are a three-photon deflection entangled photon pair generating device characterized in that a single lithium niobate-based quasi-phase-matching optical crystal is configured as periodic polarization inversion.

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