JP4264735B2 - Polarized entangled photon pair generator - Google Patents

Description

  The present invention relates to a polarization entangled photon pair generation device, and more particularly to a polarization entangled photon pair generation device composed of a waveguide type PPLN and an optical fiber Sagnac interferometer.

  Research on quantum information processing has been energetically promoted by the development of quantum cryptography protocols by Charles Bennet (IBM) in 1984 and the announcement of quantum computation algorithms by Peter Shore (AT & T) in 1994. Among them, quantum cryptography has already reached the level of practical use, and experiments using existing optical fibers in the communication wavelength band (wavelength 1550 nm) are being conducted. In these experiments, laser light attenuated to the level of a single photon is used. In order to put advanced quantum information communication (quantum teleportation, etc.) using nonlocality of photons into practical use, entanglement A photon pair in the entangled state is required. Photon pairs in this entangled state can be generated by natural parametric down conversion, which is a nonlinear optical process.

  In the field of quantum information communication research, proof-of-principle experiments such as quantum cryptography and quantum teleportation using the Bell state have been carried out mainly using photon pairs in the polarization entangled state with a wavelength of 700 to 800 nm. The reason for this is that single-photon detectors with high quantum efficiency and low dark count are commercially available in this wavelength band, and crystals such as BBO that allow highly efficient down-conversion and argon lasers as high-power pump lasers ( The wavelength 351 nm) and the second harmonic of a titanium sapphire laser (wavelength around 400 nm) can be used.

  Two studies on the generation of entangled photon pairs with a wavelength of 1550 nm have been reported so far. They are those that use periodically poled quasi-phase-matched silica fibers (University of Southampton) and those that use four-wave mixing with dispersion-shifted fibers (Northwestern University). The photon pair generation rates per second for a pump power of 1 mW are 1.7 and 500, respectively. An experiment in which a photon pair in a polarization entangled state with a wavelength of 1550 nm is generated by a femtosecond pulse laser has not been reported yet.

  FIG. 3 shows an optical system used to generate a polarized entangled state with a wavelength of 800 nm by a femtosecond laser. In this system, it is necessary to adjust and stabilize the arm length of the Mach-Zehnder interferometer in order to coherently superimpose photon pairs generated in two BBO crystals. Moreover, since alignment is difficult, the expected generation rate of photon pairs and high-purity entanglement cannot be obtained.

  The “entangled photon pair generating device” disclosed in Patent Document 1 is an entangled photon pair generating device capable of generating an entangled photon pair in a beam shape with high efficiency. As shown in FIG. 4, two polarization-determined photon pair generators using a BBO or the like that generate a photon pair in the form of a beam in a state of “not entangled” with the vertical polarization and the other fixed to the horizontal polarization in series. To place. A half-wave plate that rotates the polarization by 90 degrees is inserted between these two devices. By making the pump light incident, the horizontally polarized light and vertically polarized photon beams respectively generated by the two polarization-determined photon pair generators are superposed with different polarizations. An entangled photon pair beam can be generated with higher efficiency than before.

The photon pair generating device disclosed in Non-Patent Document 1 is a device that generates a polarization correlated photon pair having a wavelength of 1550 nm using two PPLN waveguides as shown in FIG. In the two-photon interference experiment, the simultaneous detection rate is 0.53 kHz. The photon pair generating device disclosed in Non-Patent Document 2 is a device that generates a pulse of a polarized entangled photon pair using a Sagnac interferometer, as shown in FIG.
JP2003-228091 A. Yoshizawa, R. Kaji and H. Tsuchida, "Generation of polarization-entangled photon pairs at 1550nm using PPLN waveguides", Electron. Lett., Vol.39, pp.621-622. (3 April 2003). BS Shi, A, Tomita, "Generation of Pulsed Polarization Entangled Photon Pair Using a Sagnac Interferometer", Phys. Rev. A (2004).

  However, the conventional photon pair generating device has a problem that the optical loss of this wavelength band in an optical fiber is as large as 2 dB / km, and long-distance transmission is impossible.

  An object of the present invention is to solve the above-described conventional problems and efficiently generate photon pairs in a polarization entangled state with a wavelength of 1550 nm that minimizes optical loss in an optical fiber for practical use of quantum information communication. It is.

In order to solve the above-described problems, in the present invention, a polarization entangled photon pair generating device includes a polarization beam splitter that branches 45-degree polarized light from a femtosecond pulse laser, and a first waveguide connected to the polarization beam splitter. comprising a Sagnac interferometer made of a polarization maintaining optical fiber that incorporates a mold PPLN and second PPLN waveguide, the first PPLN waveguide and the second waveguide PPLN, first By pumping with light from a femtosecond pulse laser , a beam turning in the direction pumps the first waveguide type PPLN and a beam turning in the second direction pumps the second waveguide type PPLN. The photon pair in a polarization entangled state with a wavelength of 1550 nm is generated.

  In the present invention, the configuration as described above can efficiently generate photon pairs in a highly polarized entangled state with a wavelength of 1550 nm.

  Hereinafter, the best mode for carrying out the present invention will be described in detail with reference to FIGS. 1 and 2. FIG.

In the first embodiment of the present invention, 45-degree polarized light from a femtosecond pulse laser is branched by a polarization beam splitter, and is incorporated into a Sagnac interferometer composed of a polarization-maintaining optical fiber connected to the polarization beam splitter. A beam traveling in the first direction pumps the first waveguide type PPLN through the waveguide type PPLN and the second waveguide type PPLN, and a beam traveling in the second direction passes through the second waveguide type PPLN. as the pump, by pumping with light from the femtosecond pulsed laser, a polarization-entangled photon pair generating device for generating a photon pair in the polarization entangled state of the wavelength 1550 nm.

  The configuration of the polarized entangled photon pair generation apparatus according to Embodiment 1 of the present invention will be described. FIG. 1 is a conceptual diagram of a polarization entangled photon pair generation apparatus according to Embodiment 1 of the present invention. In FIG. 1, a femtosecond pulse 1 is a laser light pulse having a pulse width of about femtoseconds. A polarization beam splitter (PBS) 2 is an optical element that separates a light beam into two by a birefringent crystal. The first waveguide type PPLN 3 and the second waveguide type PPLN 4 are waveguide type PPLN (Periodically Poled Lithium Niobate) developed as a wavelength conversion element in optical communication. The polarization-maintaining fiber 5 is a single mode optical fiber that propagates light while maintaining the vibration plane (polarization plane) of light in a fixed direction. The interference filter 6 is a filter that partially controls the spectral component of the transmitted energy by the interference effect. The birefringent crystal 7 is a crystal that causes birefringence in which an extraordinarily polarized light whose refractive index varies depending on the propagation direction of the light beam and an ordinary polarized light that does not vary depending on the propagation direction.

  The 50/50 coupler 8 is an optical element in which optical fibers are joined to split incident light into 50%. The half-wave plate 9 is an optical element that gives a phase difference of π to incident light and rotates the vibration direction by 2θ ° when the vibration direction of the electric field of the light is incident at an azimuth angle of θ ° with respect to the optical axis of the wave plate. . A maximum rotation angle of 90 ° is obtained when incident at an azimuth angle of 45 °. When circularly polarized light is incident, the vibration direction of the light is changed to the opposite direction. The Glan-Thompson analyzer 10 is an optical element that emits an extraordinary ray (s-polarized light) and absorbs an ordinary ray (p-polarized light) among incident non-polarized light. The single photon detector 11 is a photodetector having a sensitivity capable of detecting a single photon. The simultaneous measuring instrument 12 is a measuring instrument that responds when two single photon detectors simultaneously detect photons.

The operation of the polarized entangled photon pair generating apparatus according to the first embodiment of the present invention configured as described above will be described. First, an outline of the operation will be described with reference to FIG. Waveguide-type PPLN developed as a wavelength conversion element in optical communication is incorporated into a Sagnac interferometer composed of polarization- maintaining fiber 5, and the clockwise beam pumps PPLN4 and the counterclockwise beam is By pumping with light from a femtosecond laser so as to pump PPLN3, photon pairs in a highly polarized entangled state are generated with high efficiency.

  A method for generating correlated photon pairs by femtosecond pulses will be described. In order to perform advanced quantum information communication using a photon pair in a polarization entangled state, the photon pair must be generated at a specific time. For this reason, it is necessary to pump the wavelength conversion element with an ultrashort pulse laser. Therefore, a photon pair is generated by pumping the waveguide type PPLN with a femtosecond titanium sapphire laser. The waveguide type PPLN has a length of 50 mm in order to obtain the largest possible photon pair generation rate. In order to efficiently couple the pump light to the waveguide and collect the generated photon pairs, a waveguide with a polarization maintaining fiber attached thereto is used.

  A method for generating a photon pair in a polarization entangled state by a Sagnac interferometer will be described. In the natural parametric down-conversion by the waveguide type PPLN, a photon pair having the same polarization as the pump light is generated. Therefore, in order to generate an entangled state related to two orthogonal polarizations, it is necessary to pump two waveguide type PPLNs with pump lights having orthogonal polarizations. In order to generate a highly pure entangled state, it is necessary to superimpose photon pairs generated in each waveguide type PPLN in a coherent manner.

  As shown in Fig. 1, a Sagnac interferometer composed of optical fibers is used to automatically coherently superimpose photon pairs generated by two waveguide-type PPLNs (with phase-matching directions orthogonal to each other). Do. By passing the generated photon pair through the same path, the problem of adjustment and stabilization of the arm length of the interferometer can be avoided. Furthermore, by configuring the Sagnac interferometer with an optical fiber, it is possible to avoid alignment problems caused by chromatic aberration when collecting pump light and generated photon pairs by a lens in free space.

  A method for evaluating the purity of the polarized entangled state will be described. The photon pair in the polarization entangled state generated by the above method is divided into two by the 50/50 coupler 8, and the polarization correlation is measured using the Glan-Thompson analyzer 10, and the purity of the entanglement is evaluated. Furthermore, it can be confirmed that four bell states can be generated by adjusting the phase difference and polarization state of a photon pair having two different polarizations with the birefringent crystal 7 and the half-wave plate 9. Here, all optical components use fiber optics.

  In this manner, a photon pair in a highly polarized entangled state with a wavelength of 1550 nm is efficiently generated by a femtosecond pulse laser using a waveguide type wavelength conversion element. Since the generated photon pair is output by an optical fiber, it is optimal for quantum information communication using a long-distance fiber as a communication path. The generation rate obtained by the photon pair generation experiment with waveguide type PPLN is 13,600. The photon pair generation rate by the waveguide type PPLN is much more efficient than other methods.

  Since the waveguide type PPLN has a very high conversion efficiency, there is a high possibility that two to three photon pairs are generated per pulse even when pumped with a low-power semiconductor pulse laser. When a large number of photon pairs are generated per pulse, the purity of polarization entanglement is reduced. Therefore, it is necessary to adjust the laser output so that one pair of photons is generated per pulse.

As described above, in the first embodiment of the present invention, the polarization entangled photon pair generating device uses a polarization-maintaining optical fiber in which 45-degree polarized light from a femtosecond pulse laser is branched by a polarization beam splitter and connected to the polarization beam splitter. The first waveguide-type PPLN and the second waveguide-type PPLN incorporated in the Sagnac interferometer configured as shown in FIG. 1 and the beam rotating in the first direction pumps the first waveguide-type PPLN, and the second waveguide-type PPLN Since the beam turning in the direction is pumped by the light from the femtosecond pulse laser so as to pump the second waveguide type PPLN , the photon pair in the polarization entangled state with the wavelength of 1550 nm is generated. The photon pair in a highly polarized entangled state can be generated with high efficiency.

  In the second embodiment of the present invention, 45-degree polarized light from a femtosecond pulse laser is transmitted through a dichroic mirror, the transmitted light is branched by a non-polarizing beam splitter, and incorporated in a Sagnac interferometer configured with a polarization-maintaining optical fiber. The first waveguide type PPLN and the second waveguide type PPLN are pumped from both sides with the light from the femtosecond pulse laser to generate a photon pair in the polarization entangled state with a wavelength of 1550 nm and reflected by the dichroic mirror This is a polarized entangled photon pair generator.

  The configuration of the polarized entangled photon pair generating apparatus according to the second embodiment of the present invention will be described. FIG. 2 is a conceptual diagram of a polarized entangled photon pair generating apparatus according to Embodiment 2 of the present invention. In FIG. 2, a dichroic mirror 13 is a mirror that separates incident light by reflecting light in a specific region of incident light and transmitting other light. The non-polarization beam splitter (NPBS) 14 is a beam splitter that branches light regardless of the polarization state. Other configurations are the same as those in the first embodiment.

  The operation of the polarized entangled photon pair generating apparatus according to Embodiment 2 of the present invention configured as described above will be described. This is another method for generating photon pairs in a polarization entangled state by a Sagnac interferometer. The waveguide type PPLN can generate correlated photon pairs regardless of which side of the waveguide is pumped. Using this feature, the Sagnac interferometer is configured as shown in FIG. Here, the horizontal component of the pump light propagating counterclockwise through the loop of the Sagnac interferometer generates a photon pair in the first waveguide type PPLN3, and the vertical component is a photon pair in the second waveguide type PPLN4. Is generated. The same applies to the clockwise pump light. This interferometer is characterized in that two entangled photon pairs interfere with each other with a non-polarizing beam splitter (NPBS) 14. By measuring the polarization correlation of the two outputs of the Sagnac interferometer with the Glan-Thompson analyzer 10, the nature and purity of the generated entangled state can be examined in detail. Four bell states can be generated.

  As described above, in the embodiment 2 of the present invention, the polarization entangled photon pair generating device transmits the waveguide type PPLN, the 45-degree polarized light from the femtosecond pulse laser, the dichroic mirror, and transmits no light. The first waveguide type PPLN and the second waveguide type PPLN that are branched by a polarization beam splitter and built in a Sagnac interferometer composed of polarization-maintaining optical fibers are pumped from both sides with light from a femtosecond pulse laser. Since the photon pair in the polarization entangled state with a wavelength of 1550 nm is generated and reflected by the dichroic mirror, the photon pair in the highly polarized entangled state can be generated with high efficiency. .

  The polarized entangled photon pair generating apparatus of the present invention is optimal as an apparatus for generating photon pairs for quantum information communication using a long-distance fiber as a communication channel.

The conceptual diagram of the polarization entangled photon pair generator in Example 1 of this invention, The conceptual diagram of the polarization entangled photon pair generator in Example 2 of the present invention, The conceptual diagram of the polarization entangled photon pair generator of the prior art example 1, The conceptual diagram of the polarization entangled photon pair generator of the prior art example 2, Conceptual diagram of a polarization entangled photon pair generation device of Conventional Example 3, It is a conceptual diagram of the polarization entangled photon pair generation device of Conventional Example 4.

Explanation of symbols

1 Femtosecond pulse 2 Polarizing beam splitter (PBS)
3 First waveguide type PPLN
4 Second waveguide type PPLN
5 Polarization-maintaining fiber 6 Interference filter 7 Birefringent crystal 8 50/50 coupler 9 Half-wave plate
10 Glan Thompson Analyzer
11 Single photon detector
12 Simultaneous measuring instruments
13 Dichroic mirror
14 Non-polarizing beam splitter (NPBS)

Claims (2)

A polarization beam splitter that branches 45-degree polarized light from a femtosecond pulse laser, and a polarization maintaining optical fiber that is connected to the polarization beam splitter and incorporates a first waveguide type PPLN and a second waveguide type PPLN. A Sagnac interferometer, and the beam that travels in the first direction through the first waveguide type PPLN and the second waveguide type PPLN pumps the first waveguide type PPLN, The beam rotating in the direction of the light is generated by generating a photon pair in a polarization entangled state with a wavelength of 1550 nm by pumping with light from a femtosecond pulse laser so as to pump the second waveguide type PPLN. A polarization entangled photon pair generator. A dichroic mirror that transmits 45-degree polarized light from the femtosecond pulse laser and reflects a photon pair in a polarization entangled state with a wavelength λ / 2; a non-polarizing beam splitter that splits the light transmitted through the dichroic mirror; A Sagnac interferometer that is connected to the non-polarizing beam splitter and is composed of a polarization-maintaining optical fiber incorporating a first waveguide type PPLN and a second waveguide type PPLN; in turn each of the first polarization component of the beam pumping the first waveguide PPLN, so that the polarization component of the second to pump said second waveguide PPLN, the first The waveguide-type PPLN and the second waveguide-type PPLN are in a polarization entangled state with a wavelength of 1550 nm by pumping light from a femtosecond pulse laser from both sides. Polarization-entangled photon pair generating apparatus characterized by generating child pairs.

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