JP2006060143A - Optical-fiber type single photon generating element and single photon generating device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an optical fiber type single photon generating element capable of making temporal fluctuations of photon emission as small as possible and efficiently taking a photon out. <P>SOLUTION: An optical fiber 2 has at least one quantum dot 9 which emits a single photon, a core 6 has a lateral mode as a single mode at the wavelength of the single photon, and a hollow pore 8 which is provided at the center of the core 6 and filled with the quantum dot 9. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an element and apparatus for generating a single photon used in quantum cryptography communication. Here, “single” means that there is only one.

  A semiconductor quantum dot (hereinafter simply referred to as a quantum dot) is a semiconductor fine particle having a diameter of about several nanometers to a hundred nanometers, and exciton levels are discretized by electron-hole pairs being spatially confined. Provides an ideal two-level system. When one electron is excited in the two-level system, the second electron cannot be excited to the same energy level as the first electron according to the Pauli exclusion rate.

  The time interval at which the excited electrons combine with the holes to emit photons is about the radiation lifetime of the excitons. For this reason, the time interval between photons emitted from the quantum dots is always longer than the radiation lifetime. Due to this property, the statistical properties of the photons emitted from the quantum dots change, and light called anti-bunching light is generated (see Non-Patent Document 1).

  From the above property, it is possible to generate a single photon at a desired time by controlling the timing of exciting electrons with pulsed light. Thereby, the single photon light source in quantum information communication is realizable.

  By the way, the radiation lifetime of the quantum dot is about 100 ps to several ns, although it depends on the type of semiconductor. This means that the timing at which a single photon is emitted fluctuates within this radiation lifetime after the electrons in the quantum dot are excited. That is, there is a time fluctuation of photon emission in the emission of a single photon from a quantum dot. In a quantum cryptography communication system, a single photon time pulse train is transmitted from the transmitting side, and a single photon time pulse train is received on the receiving side, thereby sharing the encryption key between the sender and the receiver. Realize. When the time fluctuation of the photon emission is larger than the processing time required in the quantum cryptography communication system, for example, the gate time width of the single photon detector on the receiver side, the time pulse sequence of the single photon is calculated on the receiver side. In addition, the number of photons is counted down, and the probability of photon counting is lowered.

  Increasing the probability of spontaneous emission at which a quantum dot emits a single photon (which is given by the reciprocal of the radiation lifetime) can reduce the time fluctuation of photon emission. The spontaneous emission probability of photons per unit time is 1) the square of the amplitude of the electric field in the spatial mode in which the quantum dots exist, 2) the frequency mode density in the space in which the quantum dots exist, and 3) the dipole moment of the excited electrons. Is proportional to the product of three items such as the square of. Among these items, item 3) is a quantity specific to quantum dots made of semiconductor. The remaining items 1) and 2) depend on the environment in which the quantum dots are arranged.

  In a quantum cryptography communication system, it is necessary to take out photons emitted from quantum dots through an optical system. Photons are emitted from quantum dots into all solid angles, but when photons are extracted via an optical system, the number of photons that can be extracted is limited by the solid angle of the optical system. .

  From the above, in order to use photons emitted from a single quantum dot for quantum cryptography communication, 1) To increase the spontaneous emission probability and to minimize the time fluctuation of photon emission (first) To solve the two problems of limiting the space where the photons are emitted (second technical problem) so that the photons emitted in all solid angles can be efficiently extracted. However, it is necessary to improve the encryption key generation rate.

  As a technique for solving the first and second technical problems, a method of efficiently coupling a single quantum dot with an electric field using a microresonator is conceivable. In a minute resonator having a resonator length of about a wavelength, the number of resonator modes allowed inside is reduced, and in an allowed resonator mode, electric field confinement occurs strongly. By matching the resonator mode with the resonance energy of the quantum dot, the quantum dot can be strongly coupled to the electric field, and the probability of spontaneous emission can be increased. Non-Patent Documents 2 and 3 report examples in which coupling between an electric field in a microresonator and a quantum dot is realized. These microresonators are known to exhibit high Q values, and the coupling efficiency between quantum dots and electric fields is high.

  As a method for achieving efficient coupling between the quantum dots and the electric field, use of an optical fiber (single mode fiber) is conceivable in addition to the method using the microresonator. Since only light of a specific spatial mode is propagated in the optical fiber, it is possible to efficiently extract photons by coupling the photons emitted from the quantum dots to the specific spatial mode. Patent Document 1 describes an optical fiber core layer that uses confinement of an electric field in the radial direction of an optical fiber. In the thing described in this gazette, the optical fiber is comprised by the tubular glass clad, and the whole quantum dot filling part is made into the core. An optical fiber filled with quantum dots is advantageous in that it can be easily combined with other optical fiber elements and is easy to handle.

Recently, a photonic crystal fiber having a large number of pores in the central axis direction in parallel with the core of the optical fiber has been devised (see Non-Patent Document 4). In this photonic crystal fiber, the refractive index difference between the core and the clad can be increased by the pores periodically formed in the clad region, and the mode field diameter of the transverse mode in the fiber is reduced to the normal single mode. It can be made smaller than the fiber. Patent Document 2 describes an example in which an electric field concentrated on the core portion of a photonic crystal fiber is used by utilizing this feature. Further, Patent Document 2 proposes that a core part of a photonic crystal fiber is doped with a functional material that enhances an optical amplification function or a nonlinear optical effect.
Special table flat 10-506502 JP 2002-55239 A Michler et al., Nature 406, 968, 2000) Solomon et al., Physical Review Letters, 86, 3903, 2001 Michler et al., Science, 290, 2282, 2000 Knight et al., Optics Letters, Vol. 21, p. 1547, 1996.

  However, when the single photon generator is realized by applying the conventional method described above, there are the following problems.

  It is known that a microresonator has a high Q value and a limited allowable frequency mode, so that the probability of spontaneous emission in which a quantum dot emits a single photon is significantly increased. For this reason, it can be said that it is a very effective method from the viewpoint of increasing the spontaneous emission probability. However, since the Q value is large, it takes a long time for the photons emitted from the quantum dots to stay in the resonator for a long time and to exit the resonator. This is disadvantageous in terms of ease of control (controllability) of the generation time of a single photon. Controllability of the generation time of this single photon is also important in realizing a single photon generator.

  In the optical fiber described in Patent Document 1 in which the quantum dot is filled in the entire core, photons emitted from the quantum dot can be coupled to a specific spatial mode in the optical fiber and propagate in the optical fiber. This is very effective in solving the second technical problem. Further, since the photon is not confined in the resonator as seen in the microresonator, it takes no time to take out the photon. However, this case has the following problems.

  In the case of a single mode fiber, the electric field density formed in the radial direction of the core is highest at the center of the core, and gradually decreases as the distance from the center increases. Since the optical fiber described in Patent Document 1 has a structure in which a liquid in which quantum dots are suspended in a solvent is filled in a tubular glass clad, the filled quantum dots flow through a hollow portion (core portion). . Since this quantum dot flow also occurs in the radial direction, the quantum dot cannot stably exist at a position where the transverse mode electric field in the optical fiber is constant. For this reason, a certain spontaneous emission probability cannot be obtained. In addition, the electric field density when the quantum dots are located in the vicinity of the core wall is lower than that of the core center, and in this case, the spontaneous emission probability is low. For this reason, when the optical fiber described in Patent Document 1 is applied, it is insufficient in terms of solving the first technical problem.

  Even when the optical fiber described in Patent Document 2 is applied, the entire core is filled with quantum dots. For this reason, when a quantum dot is located near the wall of a core, an electric field density becomes a low value compared with a core center, and spontaneous emission probability becomes low. Therefore, in this case, it is insufficient in terms of solving the first technical problem.

  SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, to reduce the time fluctuation of photon emission as much as possible, and to use an optical fiber type single photon generator capable of efficiently extracting photons and the same. It is to provide a single photon generator.

  In order to achieve the above object, the present invention provides a core in which a hollow pore filled with at least one quantum dot emitting a single photon has a transverse mode of a single mode at the wavelength of the single photon. Is provided at the center. According to this configuration, since the electric field density in the single transverse mode formed in the radial direction of the core becomes higher toward the center of the core, the hollow pores are provided in a portion where the electric field density is higher. In the hollow pores formed in this way, a sufficiently high electric field density can be obtained even at the position farthest from the center in the radial direction (near the wall), and the difference between the electric field density and the electric field density at the center is also sufficient. It will be small. Therefore, even if the quantum dot flows in the radial direction in the hollow pore, it is possible to always couple the quantum dot with a high electric field. It is also possible to couple the quantum dot with a high electric field when the quantum dot is located near the wall in the hollow pore. As described above, since the quantum dot is stably coupled with a high electric field at any position in the hollow pore, the spontaneous emission probability is compared with the conventional one in which the entire core is filled with the quantum dot. Higher and more constant.

  According to the present invention, since the spontaneous emission probability can be made higher and constant, the time fluctuation of the photon emission can be made as small as possible, thereby reducing the counting off of photons. There is an effect that the probability of photon counting is improved.

  In addition, there is an effect that the controllability of the generation time of a single photon is improved by reducing the time fluctuation of the photon emission.

  Next, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1)
FIG. 1 is a diagram showing a configuration of a single photon generation apparatus according to the first embodiment of the present invention. This single photon generator comprises an excitation light source 1 and an optical fiber 2 filled with semiconductor quantum dots (hereinafter simply referred to as quantum dots).

  The excitation light source 1 is driven by an external signal and generates pulsed excitation light. The pulsed excitation light generated by the excitation light source 1 is supplied into the optical fiber 2 and excites the quantum dots contained in the optical fiber 2. This excitation light has energy that resonates in an excited state with higher energy than the photons emitted from the excited quantum dots. The excitation light source 1 has a function of adjusting the intensity of the excitation light so that the intensity of the excitation light is such that only one electron of the quantum dot is excited.

  The optical fiber 2 is a part constituting the most characteristic optical fiber type single photon generating element, and a single quantum dot that emits a photon having a wavelength to be generated is converted into an electric field density in a transverse mode in the optical fiber. Is characterized by being held at a higher position. Hereinafter, the configuration of the optical fiber 2 will be specifically described.

FIG. 2 shows the configuration of the optical fiber 2. Referring to FIG. 2, the optical fiber 2 includes a core 6 having a hollow pore 8 at the center and a clad 7 provided so as to cover the outer periphery of the core 6. The hollow pore 8 is filled with at least one quantum dot 9. The quantum dot 9 emits a single photon at a wavelength to be generated, and is synthesized organically such as PbSe or CdSe. The quantum dot 9 may be formed of a direct gap semiconductor of III-V group, II-VI group, or I-VII group that can generate a single photon, in addition to PbSe or CdSe. Examples of the direct gap semiconductor include ZnS, ZnO, TiO ₂ , AgI, AgBr, ZnTe, CdTe, and GaAs.

  The core 6 takes into account the medium that fills the inside of the hollow pore 8, the effective refractive index of the core 6 and the hollow pore 8, and the like so that the transverse mode in the core becomes a single mode in a desired wavelength range. It is configured. Here, the desired wavelength range includes a wavelength corresponding to the resonance energy of the quantum dots 9.

  The hollow pore 8 is provided in the central portion (near the central axis) of the core 6 so as to be located in a portion where the electric field density in the core 6 is the highest. FIG. 3 shows the radial direction dependence of the electric field density in a cross section obtained by cutting a part of the core 6 in a direction perpendicular to the central axis. In FIG. 3, the cross section of the core 6 is shown on the upper side, and the electric field density 10 in the cross section is shown on the lower side. As can be seen from FIG. 3, when the transverse mode in the core 6 is a single mode, the electric field density 10 is highest at the center of the core 6 and gradually decreases as the distance from the center increases. At the outermost position of the mode distribution, the electric field density 10 is almost zero.

  As described above, the spontaneous emission probability of photons per unit time is 1) the square of the electric field amplitude in the spatial mode in which the quantum dots exist, 2) the frequency mode density in the space in which the quantum dots exist, and 3) the excitation. Since it is proportional to the product of the three items of the square of the electron dipole moment, when the quantum dot 9 is at a position where the electric field density 10 is high, the spontaneous emission probability that the quantum dot 9 emits a photon increases. According to the structure of FIG. 2, the hollow pore 8 is formed in the portion with the highest electric field density 10 in the core 6 (the portion corresponding to the top of the distribution in FIG. 3). The filled quantum dots 9 are stably present at a position where the electric field density 10 is high at any position. Therefore, the spontaneous emission probability that the quantum dot 9 emits a single photon can be made higher and more stable than a structure in which the entire core is filled with quantum dots.

  Since the magnitude of the electric field density in the radial direction follows the distribution shown in FIG. 3, the quantum dots 9 can be arranged at higher electric field densities as the diameter of the hollow pores 8 is smaller. If the diameter of the hollow pore 8 is made too large, the quantum dots 9 may be arranged at a position where the electric field density is low. In that case, the spontaneous emission probability that the quantum dots 9 emit photons decreases, and photon emission is caused. The fluctuation of time increases.

The minimum electric field strength in the radial direction of the hollow pore 8, that is, the electric field strength at the outermost peripheral portion (boundary portion with the core portion 6) of the hollow pore 8 is the maximum electric field strength E ₀ at the center of the core 6. , XE ₀ (0 ≦ x ≦ 1). The closer x is to 1, the higher the spontaneous emission probability that a single photon is emitted from the quantum dot 9. By reducing the diameter of the hollow pore 8, x can be made close to 1.

  The lower limit of the diameter of the hollow pore 8 may be any size as long as it can be filled with quantum dots, but in practice, it is determined by the current manufacturing limit. On the other hand, the upper limit of the diameter of the hollow pore 8 is the processing time required for the quantum cryptography communication system (for example, the photon detector) in which the spontaneous emission probability determined by the electric field strength at the outermost peripheral part (boundary part with the core part 6) It is determined so as to satisfy the condition defined by (time resolution). When the processing time required in the quantum cryptography communication system is T and the radiation lifetime when the quantum dot is located at the center of the core 6 is τ, the outermost peripheral part of the hollow pore 8 (the boundary part with the core part 6) ) Is characterized by the following Equation 1.

  Furthermore, based on x determined in this way, the radius of the hollow pore 8 can be obtained by solving for the following equation r.

  Here, a single mode in an optical fiber is approximated by a Gaussian function. W is the radius of the mode field in the radial direction of the core, and 2W is the mode field diameter. The radius W of the mode field is a radius at which the electric field at the center of the core becomes 1 / e, and is uniquely determined by the refractive index profile of the core 6 and the cladding and the wavelength propagating in the optical fiber. Solving Equation 1 and Equation 2, the radius r of the hollow pore 8 is expressed by the following equation.

  By determining the radius of the hollow pore 8 so as to satisfy the condition of the above expression 3, the time fluctuation of the photon emission can be reduced, and the condition defined by the processing time required in the quantum cryptography communication system is Can be satisfied. As a result, counting down of photons on the receiving side can be reduced, and the probability of photon counting can be improved.

  Hereinafter, the processing time required in the quantum cryptography communication system is assumed to be the time resolution of a single photon detector using an avalanche photodiode in the 1.55 μm band, and the radius of the hollow pore 8 is calculated by the above equation 3 I will give you.

  The time resolution of a single photon detector using an avalanche photodiode is about 1 ns, which is mainly determined by the time width of the gate pulse of the photon detector. In order to detect a single photon, it is necessary for a single photon generated by the single photon generator shown in FIG. 1 to reach the photon detector on the receiving side within this gate time. Therefore, it is necessary that the time fluctuation of the photon emission in the single photon generator does not exceed the gate pulse time width. When the time fluctuation of the photon emission exceeds the gate pulse time width, the photon is counted down and the probability of photon counting is lowered.

By setting the radius of the hollow pore 8 so as to satisfy the condition of the above expression 3, the time fluctuation of the photon emission can be prevented from exceeding the gate pulse time width, and the counting off of the photons can be reduced. it can. Specifically, the electric field strength in the outermost peripheral part (boundary part with the core part 6) of the hollow pore 8 is obtained from the above equation 1, where T is the gate pulse time width of 1 ns of the photon detector. Assuming that the radiation lifetime at which the quantum dot located at the center of the core 6 emits a single photon is 500 ps, x is 0.7. In this case, the electric field strength at the outermost peripheral portion (boundary portion with the core portion 6) of the hollow pore 8 has a value of “(electric field strength E _{0 at} the center of the core 6) × 0.7”. The radius of the hollow pore 8 is set so as to allow this value. Specifically, it is assumed that the mode field diameter of the single mode realized in the core 6 including the hollow pore 8 is 2W = 9 μm. At this time, when the mode field is approximated by a Gaussian function, the radius r of the hollow pore corresponding to x = 0.7 is about 2.75 μm.

In the hollow pore 8 having the radius set as described above, the electric field density (minimum electric field density) at the position farthest from the center in the radial direction (near the boundary between the core 6 and the hollow pore 8) is “(core 6 The electric field intensity at the center of E ₀ ) × 0.7 ”. In this case, in any position in the hollow pore 8, the quantum dot 9 is always coupled with an electric field equal to or greater than the value of “(electric field intensity E _{0 at} the center of the core 6) × 0.7”. Become. Since the quantum dots and the high electric field can be coupled in this way, a high probability of spontaneous emission can be obtained, and the problem that the time fluctuation of photon emission exceeds the gate pulse time width does not occur. As a result, photon counting is reduced and the probability of photon counting is improved.

  Next, the operation of the single photon generator of this embodiment will be described with reference to FIGS.

  The excitation light source 1 is driven by an external signal to generate pulsed light. Pulsed excitation light from the excitation light source 1 is input into the optical fiber 2, propagates through the core 6, and excites the quantum dots 9 filled in the hollow pores 8. This pulsed excitation light excites only one electron of the quantum dot 9.

  The quantum dots 9 generate photons when the excited electrons recombine. That is, the quantum dot 9 emits a single photon of energy that resonates with its excitation level. A single photon emitted from the quantum dot 9 propagates efficiently in the core 6 and is taken out from the emission side end face of the optical fiber 2.

  It should be noted that the excitation light used to excite the quantum dots is usually much stronger than the single photon level, and all of the excitation light is not absorbed by the quantum dots. For this reason, it is necessary to separately add a wavelength selection element (residual light removing means) for removing the remaining light of the excitation light used for excitation to the photon extraction end of the optical fiber element. Since the wavelength of the excitation light source is shorter than the desired wavelength from which a single photon is extracted, a shortcut filter (blocking light having a shorter wavelength from the desired wavelength) can be used as the wavelength selection element. The shortcut filter may be realized by a periodic structure of refractive index in an optical fiber such as a fiber Bragg grating, or may be realized by a periodic structure of a dielectric multilayer film.

  In the single photon generator of the embodiment described above, the hollow pore 8 can be filled with one or a plurality of quantum dots that emit a single photon. When a plurality of quantum dots are filled, there are a plurality of single photons having different wavelengths in the core 6, but a single photon having a desired wavelength is selectively extracted from the plurality of single photons. By doing so, a single photon generator can be realized.

  Several methods are conceivable as a method of filling the quantum dots into the hollow pores. In general, the size of the quantum dot is about several to hundreds of nanometers in diameter, and is smaller than the wavelength of the interacting light. In addition, many quantum dots are manufactured at a time due to its manufacturing method, but the size of the manufactured quantum dots is not uniform. For this reason, when a single quantum dot 9 is filled in the hollow pore 8, how to select a single quantum dot that emits photons of a desired wavelength from a large number of quantum dots is selected. It becomes a problem. To solve this problem, a large number of quantum dots are dispersed at a low density on a suitable substrate, and a single quantum dot that emits photons of a desired wavelength is spatially separated using microspectroscopy. Several methods have been reported. In addition, there is also a technique of spatially separating single quantum dots using a near-field microscope that obtains a spatial resolution exceeding the diffraction limit of light using the near-field effect. By applying these techniques, single quantum dots 9 that emit photons of a desired wavelength can be selected and filled into the hollow pores 8.

(Embodiment 2)
Next, the structure of the single photon generator which is the 2nd Embodiment of this invention is demonstrated. This single photon generator has the same configuration as that of the first embodiment described above except that the configuration of the optical fiber including the quantum dots is different.

  FIG. 4 is a schematic diagram showing a configuration of an optical fiber including quantum dots used in the single photon generation apparatus according to the second embodiment of the present invention. This optical fiber has an outer periphery of a core portion 14 provided with a hollow pore 16 filled with a quantum dot 17 that emits photons of a desired wavelength at the central portion (central axis), and is periodically parallel to the central axis and within a cross section. It is constituted so as to be surrounded by a large number of pores 15 arranged in a regular manner. Such an optical fiber provided with a large number of pores around the core is known as a photonic crystal fiber.

  The core part 14, the numerous pores 15, and the hollow pores 16 correspond to the core 6, the clad 7, and the hollow pores 8 shown in FIG. The quantum dots 17 are the same as the quantum dots 9 described in the first embodiment. For filling the quantum dots 17 into the hollow pores 16, as in the case of the optical fiber used in the first embodiment, a single quantum dot is spatially separated using a microspectroscopic method or a near-field microscope. The method is applied.

  The diameter of the core portion 14 and the diameter and distribution of each pore 15 are determined by taking into consideration the effective refractive index of each pore 15 and hollow pore 16 (determined depending on the medium filled therein), The transverse mode in the core portion 14 is set to be a single mode at a wavelength corresponding to the resonance energy.

  Also in the present embodiment, the radius of the hollow pore 16 is set so as to satisfy the condition of Expression 3 described above. In the case of a photonic crystal fiber in which a clad is formed by a large number of pores 15, the effective refractive index of the clad can be made smaller than the refractive index of the glass member itself constituting the optical fiber. Compared with the optical fiber used in the first embodiment, the refractive index difference between the core and the clad can be increased, and the mode field diameter of the transverse mode (the distribution width shown in FIG. 3) can be reduced. Can do.

Electric field E ₀ of the core center from the inversely proportional to the mode field diameter, in the case of photonic crystal fiber used in the present embodiment, as compared with the optical fiber used in the first embodiment, and more at the core center A strong electric field is obtained. Therefore, according to the present embodiment, by limiting the diameter of the hollow pore 16, the electric field density at the position where the quantum dot exists is made constant, and the spontaneous emission probability is made constant. Compared with the optical fiber used in this embodiment, the spontaneous emission probability can be further increased.

  For example, in the optical fiber used in the first embodiment, it is assumed that the mode field diameter is 9 μm and the radiation life of the quantum dot located at the core center is 500 ps. On the other hand, in the case of the photonic crystal fiber used in this embodiment, the mode field diameter can be set to 3 μm. In this case, the radiation lifetime of the quantum dot located at the center of the core is shorter than the optical fiber used in the first embodiment by the ratio of the square of the mode field diameter, and the value is about 55 ps. Since the spontaneous emission probability is given by the reciprocal of the radiation lifetime, the spontaneous emission probability is higher in the photonic crystal fiber used in this embodiment than in the optical fiber used in the first embodiment. I understand that. Therefore, the temporal fluctuation of the photon emission can be further reduced, the photon counting is further reduced, and the probability of photon counting is improved.

  Also in the single photon generator of this embodiment, it is desirable to provide a wavelength selection element (residual light removing means) that removes the remaining light of the excitation light used for excitation, as in the case of the first embodiment. .

(Other embodiments)
In the first and second embodiments, a hollow pore provided in the center of the core is applied by applying a technique of spatially separating a single quantum dot using a microspectroscopic method or a near-field microscope. Only one quantum dot that emits a photon of a desired wavelength is filled therein. However, as described above, the size of the quantum dots is not uniform due to the manufacturing method. For this reason, the method of selecting a single quantum dot that emits photons of a desired wavelength using microspectroscopy or a near-field microscope by dispersing a large number of quantum dots in a spatially low density is desired. Quantum dots that emit photons of this type are very inefficient because they can only be selected probabilistically. As a method for efficiently filling a single quantum dot that emits photons of a desired wavelength into the hollow pore, a method of dispersing the quantum dots in the hollow pore at a low density can be considered. Here, an example in which a filling method using low density dispersion is applied will be described in detail.

  First, a filling method using low density dispersion will be specifically described by taking the optical fiber shown in FIG. 2 as an example.

  FIG. 5 shows the relationship between the spectral broadening 11 due to size nonuniformity in a large number of quantum dots and the spectral broadening 12 of a single quantum dot on the wavelength spectrum. As shown in FIG. 5, when the number of quantum dots exceeds a certain limit, the spectral spread of each quantum dot overlaps, resulting in a spectral spread 11 with a non-uniform width σ. At this time, even if a wavelength dispersion element is used, a single photon having a specific wavelength emitted from a single quantum dot cannot be taken out.

  When the number of quantum dots is sufficiently small, as shown in FIG. 6, the spectrum overlap of the quantum dots at the same wavelength is statistically reduced, and as a result, the peaks 13 of individual single quantum dots are separated from each other. It becomes a state. At this time, if the spectral spread δ of each single quantum dot is equal to or less than the wavelength resolution Δ of the wavelength dispersion element and the difference in wavelength between adjacent peaks 13 is equal to or greater than the wavelength resolution Δ, the wavelength dispersion element is used. Thus, a single photon at the wavelength of the photon to be generated can be extracted.

Suppose that the spectral spread due to the size non-uniformity of the quantum dots follows a standard normal distribution characterized by a center wavelength λ ₀ and chromatic dispersion σ (non-uniform width). The standard normal distribution in this case is given by the following equation.

  In addition, when the spectral spread shown by a single quantum dot is δ, the wavelength resolution of the wavelength dispersion element is Δ, and Δ> δ is satisfied, when one quantum dot is selected from a large number of quantum dots, The probability p that a quantum dot emits a photon of wavelength λ ′ to be generated within the range of wavelength resolution Δ of the wavelength dispersion element is given by the following equation.

  When N quantum dots are filled in the hollow pore 8, the probability that there is only one quantum dot that emits a photon of the wavelength λ ′ to be generated within the range of the resolution Δ of the wavelength selection element is Given in. This probability gives the upper limit of the yield rate when manufacturing the optical fiber element.

Here, the yield rate indicates how many of the optical fiber elements include only one quantum dot in a desired wavelength range when a large number of optical fiber elements are manufactured. Therefore, when 1000 optical fiber elements are produced, the number of optical fiber elements of “1000 × Equation 6” includes only one quantum dot in a desired range. N _{max at} which the upper limit of the yield rate is maximized is given by the following equation.

  The descriptions related to the above equations 5 to 7 can be applied not only to the normal distribution function but also to an arbitrary probability distribution function f (λ).

Here, the example which calculated the number of quantum dots using the said Formulas 4-7 is given. The spectral spread σ due to the size non-uniformity of the quantum dots is 40 nm, the spectral spread δ inherent to a single quantum dot is 0.1 nm, and the wavelength resolution Δ of the wavelength dispersion element is 1 nm. Further, the center wavelength λ ₀ and the desired wavelength λ ′ of the spectrum spread σ are both 1550 nm. In this case, the probability p obtained by the above equation 5 is 0.00997, and the number of quantum dots given by the equation 7 is about 100. At this time, the probability that the optical fiber element contains only one quantum dot in the desired wavelength range is 0.3697, which is the maximum yield in manufacturing an optical fiber element that emits photons of the desired wavelength. Gives a probability.

The quantum dot density in the solution is adjusted so that the number N of quantum dots determined by the above method is sucked into the hollow pores 8 of the optical fiber having a finite length d. The cross-sectional area of the hollow pore 8 is given by πc ² , and the density of quantum dots in the solution is given by the following equation.

A calculation example in which the density is given using Equation 8 will be described with respect to the calculation example in which N = 100 is obtained. When the diameter of the hollow pore 8 is 2c = 1 μm and the length of the optical fiber is d = 100 mm, the density of quantum dots in the solution obtained by Equation 8 is about 1.27 × 10 ⁹ pieces / ml. The suspension having the density thus obtained is filled into the hollow pores 8 by using, for example, capillary action.

When the suspension is filled in the hollow pores 8 and the solvent in which the quantum dots are suspended is volatile, the solvent is volatilized so that only the quantum dots remain on the walls of the hollow pores 8. Also good. Alternatively, both ends of the optical fiber 2 may be sealed in a state where the solvent in which the quantum dots are suspended is filled in the hollow pores 8. In this case, since the quantum dots are protected by the solvent and the quantum dots are not exposed to the air, the deterioration (for example, oxidation) of the quantum dots can be prevented. As the organic solvent, toluene, hexane, or the like can be used. In addition, when the solvent is volatilized, for example, the end face of the fiber element is sealed in a state filled with an inert gas such as N ₂ gas, He gas, or Ne gas, and the quantum dots are deteriorated by exposure to air (for example, oxidation) Etc.) may be prevented.

  The filling method by low density dispersion described above can also be applied to the photonic crystal fiber used in the second embodiment.

  When quantum dots are dispersed at a low density in the hollow pores, a plurality of photons of other wavelengths are emitted in the hollow pores in addition to a single quantum dot that emits photons of a desired wavelength. There are quantum dots. Therefore, in the configuration of the single photon generator shown in FIG. 1, a plurality of photons having different wavelengths emitted from the quantum dots in the hollow pores are emitted from the emission end face of the optical fiber 2. become. In such a case, wavelength selection means (such as a wavelength dispersion element) for selecting a photon having a desired wavelength from the photons emitted from the optical fiber 2 is required. Several configurations are conceivable as an apparatus including a wavelength selection unit. In the following, apparatus examples 1 to 5 are given as the configuration examples.

(Device Example 1)
FIG. 7 is a diagram illustrating a first configuration example of a single photon generation apparatus according to another embodiment of the present invention. This single photon generator has an optical circulator 3, a reflective fiber Bragg grating 4, and an output optical fiber 5 in addition to the excitation light source 1 and the optical fiber 2 shown in FIG.

  The optical fiber 2 has the structure shown in FIG. 2 or FIG. 4, and the hollow pores are filled with quantum dots by a filling method using low density dispersion. The optical circulator 3 is a directional waveguide that couples photons incident from the optical fiber 2 to the fiber Bragg grating 4 and couples photons incident from the fiber Bragg grating 4 to the output optical fiber 5. The fiber Bragg grating 4 is a wavelength selector having a grating period capable of selecting the wavelength of a photon to be generated, and reflects a photon having a wavelength corresponding to the grating period among the photons incident from the optical circulator 3. The wavelength of the reflected photon can be arbitrarily set by arbitrarily setting the period of the grating.

  In this example apparatus, the excitation light source 1 is driven by an external signal to generate pulsed light. Pulsed excitation light from the excitation light source 1 is input into the optical fiber 2 and propagates through the core to excite each quantum dot filled in the hollow pore. This pulsed excitation light excites only one electron of each quantum dot.

  Each quantum dot generates a photon when the excited electrons recombine. That is, each quantum dot emits a single photon of energy that resonates with its respective excitation level. As a result, the number of photons in the core of the optical fiber 2 is equal to or less than the number of quantum dots filled in the hollow pores. However, the wavelength of each photon is different, and the interval between adjacent wavelengths is larger than the resolution of the fiber Bragg grating 5.

  Each photon emitted from the quantum dot propagates through the core and is coupled to the fiber Bragg grating 5 via the optical circulator 3. In the fiber Bragg grating 5, only a single photon at a desired wavelength is reflected and returned to the optical circulator 3. In the optical circulator 3, the returned single photon is coupled to the output fiber 5. Thus, a single photon having a desired wavelength is extracted from the output fiber 5 to the outside.

(Device example 2)
FIG. 8 is a diagram illustrating a second configuration example of a single photon generation apparatus according to another embodiment of the present invention. This single photon generator is composed of an excitation light source 18, an optical fiber 19 including quantum dots, and a transmission type fiber Bragg grating 20. The excitation light source 18 is the same as the excitation light source 1 shown in FIG. The optical fiber 19 has the structure shown in FIG. 2 or FIG. 4, and the hollow pores are filled with quantum dots by a filling method using low density dispersion.

  In the present apparatus example, the excitation light source 18 is driven by an external signal to generate pulsed light. Each quantum dot in the optical fiber 19 is excited by the pulsed excitation light from the excitation light source 18. This pulsed excitation light excites only one electron of each quantum dot. Each excited quantum dot emits a single photon of energy resonating with the respective excitation level. Each photon has a different wavelength, and the interval between adjacent wavelengths is larger than the resolution of the fiber Bragg grating 20.

  Each photon emitted from each quantum dot propagates through the core and is coupled to the fiber Bragg grating 20. In the fiber Bragg grating 20, only a single photon at the desired wavelength is transmitted. Thus, a single photon is extracted from the fiber Bragg grating 20 to the outside.

  According to this apparatus example, the optical circulator 3 and the output optical fiber 5 can be omitted as compared with the configuration shown in FIG. 7, and the cost can be reduced accordingly.

(Device Example 3)
FIG. 9 is a diagram illustrating a third configuration example of a single photon generation apparatus according to another embodiment of the present invention. This single photon generator comprises an excitation light source 21 and an optical fiber 22 including a transmission fiber Bragg grating and quantum dots. The excitation light source 21 is the same as the excitation light source 1 shown in FIG.

  FIG. 10 shows the configuration of the optical fiber 22. The optical fiber 22 has a transmission fiber Bragg grating 25 provided in a part of the core of the optical fiber shown in FIG. 2, and the hollow pore 8 provided at the center of the core 6 has a low density dispersion. The quantum dots 9 are filled by a filling method.

  Excitation light from the excitation light source 21 is supplied from one end face 23a, and a single photon having a desired wavelength is extracted from the other end face 23b. The fiber Bragg grating 25 is located on the other end face 23b side, and the quantum dots 9 are dispersed on the end face 23a side from this position.

  The excitation light source 21 is driven by an external signal to generate pulsed light. Each quantum dot in the optical fiber 22 is excited with pulsed excitation light from the excitation light source 21. This pulsed excitation light excites only one electron of each quantum dot.

  Each excited quantum dot emits a single photon of energy resonating with the respective excitation level. The wavelength of each photon is different, and the interval between adjacent wavelengths is larger than the resolution of the fiber Bragg grating 25.

  Each photon emitted from each quantum dot propagates through the core 6, and only a single photon having a desired wavelength passes through the fiber Bragg grating 25. Thus, a single photon that has passed through the fiber Bragg grating 25 is taken out from the other end face 23b.

  Also in this apparatus example, the optical circulator 3 and the output optical fiber 5 can be omitted as compared with the configuration shown in FIG. 7, and the cost can be reduced accordingly.

(Example 4)
Next, the 4th structural example of the single photon generator which is other embodiment of this invention is demonstrated. The single photon generator of the fourth configuration example has the same configuration as that of the third configuration example described above except that the configuration of the optical fiber is different.

  FIG. 11 shows the configuration of an optical fiber used in the single photon generator of the fourth configuration example. In this optical fiber, a transmission type fiber Bragg grating 28 is provided in a part of the core portion of the optical fiber shown in FIG. 4, and a low density dispersion is provided in the hollow pore 16 provided in the center of the core 14. The quantum dots 17 are filled by the filling method according to.

  Excitation light from an excitation light source is supplied from one end face 26, and a single photon having a desired wavelength is extracted from the other end face. The fiber Bragg grating 28 is located on the other end face side, and the quantum dots 17 are dispersed on the end face 26 side from this position.

  Each quantum dot 17 in the optical fiber is excited by the pulsed excitation light from the excitation light source 21. Each excited quantum dot emits a single photon of energy resonating with the respective excitation level. Each photon has a different wavelength, and the interval between adjacent wavelengths is larger than the resolution of the fiber Bragg grating 28.

  Each photon emitted from each quantum dot propagates through the core portion 14, and only a single photon having a desired wavelength passes through the fiber Bragg grating 28. Thus, a single photon that has passed through the fiber Bragg grating 28 is taken out from the other end face.

  Also in this apparatus example, the optical circulator 3 and the output optical fiber 5 can be omitted as compared with the configuration shown in FIG. 7, and the cost can be reduced accordingly.

(Device Example 5)
Next, the 5th structural example of the single photon generator which is other embodiment of this invention is demonstrated.

  As a method of exciting electrons of a quantum dot using light, (1) a method of irradiating light having energy higher than the band gap energy of a semiconductor constituting the quantum dot (first excitation method), (2) quantum There are two methods (second excitation method) of irradiating light having energy resonating with a higher-order excited state of dots.

  In the first to fourth device examples, the first excitation method is used, and electrons are excited in all the quantum dots included in the optical fiber. From each quantum dot, a single photon having a wavelength corresponding to the size of the respective quantum dot is emitted. Of the photons emitted from each quantum dot, a single photon having a desired wavelength is extracted by the wavelength selection element.

  In this apparatus example, by using the second excitation method, it is possible to perform similar wavelength selection without using a wavelength selection element such as a fiber Bragg grating. The principle will be described below.

  In a quantum dot, an electron-hole pair is confined in a very small region spatially, so that exciton levels are discretized. When a quantum dot is used as a single photon light source, fluorescence emitted from the lowest energy level among the discretized exciton levels is used. This state in which excitation to the lowest energy level is called the first excited state. In the quantum dot, in addition to the first excited state, higher-order excited states such as a second excited state and a third excited state in which excitation to a higher energy level is also present. Since the energy of such higher-order excited states is also converted depending on the size of the quantum dot, when handling a large number of single quantum dots at the same time, the energy position in the higher-order excited state Variations caused by the size distribution of the quantum dots occur.

  When the number of quantum dots is sufficiently reduced, the overlap of energy levels of quantum dots at the same wavelength is statistically reduced even in a higher-order excited state. That is, in an optical fiber filled with a small number of single quantum dots, the energy levels of the single quantum dots are separated from each other, and as a result, the respective fluorescence spectra are separated. In this case, excitation light having a spectral width equal to or less than the spectral spread δ inherent to the quantum dots is selectively excited to excite higher-order excitation states of the quantum dots in which the resonance wavelength of the first excitation state is at a desired wavelength. Then, a single photon having a desired wavelength is emitted from the quantum dot. For example, when an electron is excited to the second excited state of a specific single quantum dot, the excited electron relaxes from the second excited state to the first excited state, and from the first excited state to its resonance. A single photon with a wavelength corresponding to the wavelength is emitted.

  FIG. 12 shows a fluorescence spectrum emitted from each single quantum dot when interband excitation is performed in a small number of single quantum dots filled in an optical fiber. Since the energy levels of the single quantum dots are separated from each other, the fluorescence spectra in the respective quantum dots are also separated from each other. In FIG. 12, the fluorescence spectrum of each quantum dot in the first excited state is shown on the right side of the drawing, and the fluorescence spectrum of each quantum dot in the second excited state is shown on the left side. The fluorescence peak 102 is a fluorescence peak in a first excited state of a certain quantum dot A. In the second excited state of the quantum dot A, a fluorescence peak 101 corresponding to the fluorescence peak 102 exists. When a small number of single quantum dots filled in an optical fiber are irradiated with excitation light having the same wavelength as the fluorescence peak 101, light absorption occurs as a reverse process of fluorescence emission, and the quantum dots filled in the optical fiber are absorbed. Of these, only the quantum dot A of the fluorescence peak 101 is selectively excited. Here, the excited electrons undergo energy relaxation from the second excited state of the fluorescent peak 101 to the first excited state, and as a result, photons having the wavelength of the fluorescent peak 102 are emitted from the quantum dots A.

  In this apparatus example, by using the above principle, a single photon having a desired wavelength is extracted without using a wavelength selection element. A specific device configuration includes an excitation light source and an optical fiber filled with a small number of single quantum dots. The pulse width and wavelength of the excitation light from the excitation light source are determined based on the above principle. By supplying excitation light from this excitation light source from one end face of the optical fiber, a single photon can be extracted from the other end face of the optical fiber.

  Also in the single photon generator of the other embodiment described above, similarly to the first embodiment, a wavelength selection element (residual light removing means) for removing the remaining light of the excitation light used for excitation is provided. It is desirable to provide it.

  Each embodiment described above is an example of the present invention, and the configuration thereof can be appropriately changed without departing from the spirit of the invention. For example, in a configuration using a fiber Bragg grating as a wavelength selection element for extracting photons of a desired wavelength, an interference filter or a diffraction grating type spectrometer may be used instead of the fiber Bragg grating. The interference filter can be formed in a connector portion that connects the optical fiber type single photon generating element and an external device.

  Moreover, in each embodiment, the optical fiber which is a fiber type single photon generating element can be fundamentally produced by the existing manufacturing method. In the first and second embodiments, a liquid containing quantum dots may be filled in the hollow pores. In this case, when the liquid containing the quantum dots is volatile, the liquid may be volatilized to leave only the quantum dots on the walls in the hollow pores. Moreover, you may make it seal the both ends of an optical fiber in the state with which the liquid containing a quantum dot was filled in the hollow pore.

  In addition, an existing method using a pump or a capillary phenomenon may be applied to the filling of the quantum dots into the hollow pores.

It is a figure which shows the structure of the single photon generator which is the 1st Embodiment of this invention. It is a schematic diagram which shows an example of a structure of the optical fiber shown in FIG. It is a schematic diagram correlating the radial direction dependence of the electric field density in the core and the cross section of the single mode fiber with hollow pores. It is a schematic diagram which shows the structure of the optical fiber containing the quantum dot used for the single photon generator which is the 2nd Embodiment of this invention. It is a figure for demonstrating the relationship between the spectrum spread which many quantum dots show, and the spectrum spread which a single quantum dot shows. It is a figure for demonstrating the relationship between the spectrum spread which a few single quantum dot shows, and the wavelength resolution of a wavelength dispersion element. It is a figure which shows the 1st structural example of the single photon generator which is other embodiment of this invention. It is a figure which shows the 2nd structural example of the single photon generator which is other embodiment of this invention. It is a figure which shows the 3rd structural example of the single photon generator which is other embodiment of this invention. It is a schematic diagram which shows the structure of the optical fiber shown in FIG. It is a schematic diagram which shows the structure of the optical fiber used in the 4th structural example of the single photon generator which is other embodiment of this invention. It is a figure for demonstrating the principle of the 5th structural example of the single photon generator which is other embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1, 18, 21 Excitation light source 2, 19, 22 Optical fiber 3 Optical circulator 4 Reflective fiber Bragg grating 5 Optical fiber for output 6 Core 7 Cladding 8, 16 Hollow pore 9, 17 Quantum dot 10 Electric field density 11, 12 Spectrum Spread 13 Spectrum peak 14 Core portion 15 Pore 20, 25, 28 Transmission type fiber Bragg grating 23a, 23b, 26 End face 101, 102 Fluorescence peak

Claims (13)

At least one quantum dot emitting a single photon;
A core whose transverse mode is single mode at the wavelength of the single photon;
An optical fiber type single photon generating element having a hollow pore provided at the center of the core and filled with the quantum dots.   The optical fiber type single photon generating element according to claim 1, wherein the core is surrounded by a plurality of pores parallel to the central axis of the core. The processing time determined by the gate time width of an external photon detector that detects a single photon emitted from the quantum dot is T, and when the quantum dot is located at the center of the core, the quantum dot When the radiation life to be emitted is τ and the radius of the mode field in the radial direction of the core is W, the radius r of the hollow pore is

The optical fiber type single photon generating element according to claim 1 or 2, wherein the optical fiber type single photon generating element is defined so as to satisfy the following condition.   The optical fiber type single photon generating element according to any one of claims 1 to 3, further comprising wavelength selection means for selecting a single photon emitted from the quantum dot.   The optical fiber type single photon generating element according to claim 4, wherein the wavelength selecting means is a fiber Bragg grating formed in a part of the core.   6. The apparatus according to claim 1, further comprising: a residual light removing unit that removes light remaining in the core out of light for exciting the quantum dots supplied from the outside into the core. 7. 2. An optical fiber type single photon generating element described in 1. A hollow pore filled with at least one quantum dot that emits a single photon, an optical fiber provided in the center of the core in which the transverse mode is a single mode at the wavelength of the single photon;
A single-photon generator comprising: an excitation light source that generates pulsed light for exciting the quantum dots.   The single-photon generator according to claim 7, wherein the optical fiber is a photonic crystal fiber in which the core is surrounded by a plurality of pores parallel to a central axis of the core. The processing time determined by the gate time width of an external photon detector that detects a single photon emitted from the quantum dot is T, and when the quantum dot is located at the center of the core, the quantum dot When the radiation life to be emitted is τ and the radius of the mode field in the radial direction of the core is W, the radius r of the hollow pore is

The single photon generator according to claim 7 or 8, wherein the single photon generator is defined so as to satisfy the following condition.   The single-photon generator according to any one of claims 7 to 9, further comprising wavelength selection means for selecting a single photon emitted from the quantum dot.   The single-photon generator according to claim 10, wherein the wavelength selection means is a fiber Bragg grating formed in a part of the core of the optical fiber.   The single photon generator according to any one of claims 7 to 11, wherein the excitation light source is configured to selectively excite the quantum dots to a higher-order excitation state.   The single-photon generator according to any one of claims 7 to 12, further comprising residual light removing means for removing light remaining in the core of the optical fiber from light from the excitation light source.

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