JP4872302B2 - Single photon generator - Google Patents

Description

  The present invention relates to a single photon generator that is suitably used for quantum cryptography communication and the like.

  In recent years, the development of information communication and computer technology has been remarkable, and as a result, in order to maintain the confidentiality of communication information, it has become an important issue to realize secure and reliable encryption communication.

  In information communication using a normal communication channel, there is a possibility that communication is always eavesdropped by a third party. Therefore, in encrypted communication, when information is communicated between two parties, the sender side encrypts the information using an encryption key. As a result, even when the communication is wiretapped by a third party, the safety of the communication content is maintained. In this case, in order for the receiver side to decrypt the information, it is necessary to have a decryption key for returning the cipher to the plaintext. Therefore, the basic problem of cryptographic communication is how to hold the encryption key and the decryption key between the two parties.

  Currently, in the public encryption key method widely used on the Internet, the recipient makes the encryption key public and the decryption key is held only by himself / herself. Then, the sender side sends the information encrypted with the encryption key, and decrypts the ciphertext using the decryption key held by itself. In this way, the receiver can safely receive information from an unspecified number of senders by properly using two types of keys, the encryption key and the decryption key.

  The security of this public encryption key method is ensured by the fact that it is difficult to estimate the decryption key from the encryption key. For example, in a commonly used RSA cipher, it is necessary to factorize a huge integer, which is an encryption key, in order to obtain a decryption key. Enormous amount of time. For this reason, it is virtually impossible to decrypt. However, if a quantum computer or the like capable of executing parallel calculation at high speed appears, the time required to execute the above calculation will be drastically reduced, and the safety based on the limit of the calculation speed is threatened. In addition, the public encryption key method cannot prevent a third party from falsifying data.

  On the other hand, there is a one-time pad method in which a secret encryption key is shared between two communicating parties, and this secret key is discarded in one communication, and this encryption method is guaranteed unconditional safety by information theory. However, this method has a problem of how to securely share a secret key. Currently, quantum cryptography communication is expected as a means for distributing this secret key. In the secret key method, the encryption key and the decryption key may be the same.

  Whereas conventional optical communication uses a collection of many photons as an information medium and expresses information depending on the presence or absence thereof, quantum cryptography communication uses a single photon as a medium and 1-bit information in its quantum state. assign. For example, in the BB84 protocol, which is one of the representative protocols of quantum cryptography communication, four quantum states of photons (states with polarization angles of 0 degrees, 90 degrees, 45 degrees, and -45 degrees) are used as information.

  When information is given to the quantum state of one photon, the information held by each photon follows the uncertainty principle and observation theory of quantum mechanics, so information cannot be extracted without affecting the state of the photon. . Therefore, when a third party eavesdrops on information on the communication channel, such an action is immediately detected by the two parties that are communicating.

  In this way, in the secret key distribution method based on quantum cryptography communication, the security of the encryption key shared between the two parties is not limited to the calculation speed but to the quantum mechanical principle as long as the information bearer is a single photon. Guaranteed on the basis. On the other hand, in order to realize such quantum cryptography communication, a highly reliable single photon generator that repeatedly generates an optical pulse composed of a single photon is required.

  Conventionally, as a pseudo single photon generation method, a method is used in which pulse light including a large number of photons from a pulse laser is passed through an ND filter and the amount of light is attenuated until a single photon pulse is obtained (Rev. Mod). Phys., Vol. 74, 145 (2002)).

  In this case, the number of photons contained in one laser pulse light varies from pulse to pulse, and the degree of light attenuation by the ND filter also varies stochastically. For this reason, even when the number of photons contained in the laser pulse light is relatively large and the dimming effect by the ND filter is relatively small, the probability that two photons are contained in the light pulse after dimming is close to zero. If it is going to suppress to, it will be forced to carry out dimming of a laser pulse light excessively. As a result, the rate at which photons disappear in the light pulse after dimming reaches 9 out of 10 pulses, and there is a problem that the generation efficiency of a single photon pulse is lowered. Even if the light is excessively attenuated in this way, it is impossible to completely eliminate the probability that a plurality of photons are included in the light pulse after light attenuation. The problem remains from the point.

  Therefore, in Patent Document 1 described later, an apparatus that enables generation of a single photon by utilizing the light emission of excitons in a quantum dot is proposed, unlike the above method. Here, the quantum dot is a solid having the following properties.

  When the length of the movement direction of an electron system (hereinafter simply referred to as an electron system) occupying a conduction band in a solid is suppressed to about the de Broglie wavelength (several nm to several tens of nm), the electronic system Is significantly different from the behavior of an electronic system in which conduction band electrons move around in a bulk-sized solid, changing the electrical or optical properties of the solid material. A special phenomenon that occurs due to the movement of conduction band electrons restricted to a small region is called the quantum size effect. Solids that exhibit quantum size effects include quantum wells in which the movement of the electron system is restricted only in one dimension, quantum wires in which the movement of the electron system is restricted in two dimensions, and the movement of the electron system in all three dimensions. There are limited quantum dots.

  A quantum dot made of a semiconductor crystal is a small solid having a size of several nanometers to several tens of nanometers, for example, from several hundred to several thousand semiconductor atoms. The electronic system of a bulk size semiconductor crystal forms a band structure composed of continuous energy levels, whereas the electronic system of quantum dots forms discrete and discontinuous energy levels. Moreover, these energy levels are affected by the size of the quantum dot, and the smaller the size, the higher the energy level, and the band gap energy for exciting electrons from the valence band to the conduction band is growing.

  When atoms in the quantum dot absorb photons with energy equal to the band gap energy, conduction electrons and holes are generated. At this time, since the conduction electrons and holes are confined in a very narrow space in the quantum dots, there is a high probability that the conduction electrons are trapped in the holes and form excitons (electron-hole pairs). In addition, since the exciton binding energy is larger than that of the bulk crystal, exciton dissociation hardly occurs and the exciton lifetime is long. In addition, since the excitonic oscillator strength is higher than that of the bulk size crystal, the probability that electrons and holes constituting the exciton are recombined to emit light and transition to the valence band is increased. As a result, in the quantum dots, the ratio of the fluorescence intensity to the excitation light intensity is very large as compared with the bulk-size crystal.

  FIG. 8 is an explanatory diagram (a) showing the configuration of the single photon generator 100 shown in Patent Document 1 and an enlarged sectional view (b) in the vicinity of the quantum dots 103.

  As shown in FIG. 8A, in the single photon generator 100, an undoped AlAs barrier layer 102 is formed on an n-type GaAs substrate 101, and an InAs-GaSb quantum dot 103 is formed thereon. Many are formed by the Stranski-Krastanow mode growth method. As shown in FIG. 8 (b), the quantum dot 103 has a composition gradient within the dot, about 2 to 3 molecular layers are formed on the inner side, and a layer rich in GaSb is formed on the outer side. About 15 molecular layers are formed. The size of the quantum dot 103 is about 20 to 50 nm in diameter and about 5 to 10 nm in height.

  On the barrier layer 102 and the quantum dots 103, an undoped AlAs barrier layer 104 having a thickness of about 20 to 30 nm is formed so as to embed the quantum dots 103, and further on the barrier layer 104, the thickness is increased. A p-type GaAs contact layer 105 of about 20 nm is formed. Furthermore, an upper electrode 106 and a lower electrode 107 are formed ohmicly on the upper surface of the contact layer 105 and the lower surface of the GaAs substrate 101, respectively. An optical window 106A is opened in the upper electrode 106 immediately above the quantum dot 103 corresponding to one of the quantum dots 103A.

  The mechanism by which a single photon is generated by the single photon generator 100 is as follows.

  First, the switch 109A of the bias power supply 109 is opened. In this state, the excitation pulse light 108 </ b> A having a wavelength corresponding to the band gap is incident on a large number of quantum dots 103 from the semiconductor laser 108 to generate conduction electrons and holes in each quantum dot 103. Since the quantum dot 103 has a composition gradient in the dot, electrons are spatially separated and held at the lower part of the quantum dot 103 rich in InAs, and holes are held at the upper part of the quantum dot 103 rich in GaAs. In this state, excitons are formed. Such excitons can realize a long recombination lifetime because there is little overlap of electron and hole wavefunctions.

Next, the switch 109A is closed by the control circuit 109B, and a reverse bias voltage from the bias power source 109 is applied between the upper electrode 106 and the lower electrode 107. As a result, an electric field acts on the inside of the quantum dots 103, the spatial separation of electrons and holes is released, the electrons and holes are recombined, and a photon is emitted from each quantum dot 103. Of these photons, only the photons emitted from the quantum dots 103A immediately below the optical window 106A are extracted from the optical window 106A and emitted as single photons. At this time, the optical gate member 110 formed of a high-speed optical switch element using an electro-optic crystal such as KH ₂ PO ₄ is interlocked as described below to ensure the extraction of a single photon.

  That is, in the excitation process using the excitation pulsed light 108A, it is inevitable that a plurality of electrons and holes are excited and, as a result, exciton molecules are formed. However, these multiple electrons and holes recombine and emit light in a short time, typically within a few hundred picoseconds after excitation, because the degree of spatial separation is small compared to the above excitons. Disappear. Therefore, if the switch 109A is closed after a time longer than the recombination lifetime of the exciton molecule, for example, 1 nanosecond, after the excitation with the excitation pulsed light 108A, the light emission and the exciton by the exciton molecule are closed. Can be separated temporally. At this time, the gate of the optical gate member 110 is interlocked with the opening and closing of the switch 109 </ b> A so that the light emission by the exciton molecule is blocked by the optical gate member 110, and only the light emission by the exciton passes through the optical gate member 110.

  Further, Patent Document 2 described later proposes another single photon generator that uses light emission from photoexcited quantum dots.

Japanese Patent Laying-Open No. 2004-253657 (page 4-7, FIG. 1-7) Japanese Patent No. 3422482 (pages 4 and 5, FIGS. 1 and 2)

  In the single photon generator proposed in Patent Document 1 shown in FIG. 7, a large number of quantum dots 103 formed densely by the Stranski-Krastanow mode growth method are excited without distinction by the excitation pulse light 108A. Photons are emitted from a large number of quantum dots 103. Therefore, in order to extract only photons from one quantum dot 103A, it is necessary to provide an optical window 106A for selection using the upper electrode 106 as a mask. The diameter of the optical window 106A cannot be increased excessively in order not to pass photons from the quantum dots 103 other than the quantum dots 103A.

  On the other hand, since the AlAs barrier layer 104 and the GaAs contact layer 105 have thicknesses of about 20 to 30 nm and about 20 nm, respectively, the distance between the quantum dot 103A and the optical window 106A can be reduced to less than this. Can not.

  As a result, the solid angle spanned by the optical window 106A with respect to the quantum dot 103A is about one-tenth to a few tenths of all directions. Since the photons from the quantum dots 103A are emitted in all directions, the photons extracted from the optical window 106A are only about a fraction to a few tenths of all the photons emitted from the quantum dots 103A. Therefore, even if one photon is emitted from the quantum dot 103A for each excitation pulse light 108A, the photon can actually be extracted from the single photon generator 100 for several to several tens of excitation pulse lights 108A. Decrease to about once.

  In addition, since the planar shape of the quantum dot 103 is almost circular and the crystal shape has no anisotropy, the polarization direction of the emitted photons cannot be controlled to be constant, and photons having the necessary polarization direction are generated. Therefore, there is a problem that the generation efficiency of a single photon pulse that can be actually used is further reduced.

  The single photon generator proposed in Patent Document 2 is too complicated in configuration and operation. In addition, information that can estimate the proportion of photons that can be extracted as a single photon pulse out of the photons emitted from the quantum dots is not shown, but the proportion is small from FIG. It is estimated that the generation efficiency of photon pulses is low.

  In view of the above circumstances, an object of the present invention is to provide a single photon generator with high efficiency for extracting photons emitted from quantum dots as single photon pulses that can actually be used.

That is, the present invention is a single photon generator that emits a single photon in response to the incidence of excitation pulsed light,
Quantum dots made of luminescent material are embedded in the optical waveguide,
The single photon is emitted from the quantum dot excited by the absorption of the excitation pulsed light, and the single photon is extracted through the optical waveguide.
It relates to a single photon generator.

  Photons used for the purpose of transmitting information are generally often transmitted through an optical waveguide such as an optical fiber. In this way, it is possible to safely and easily transmit photons safely and easily while minimizing loss between the photon generator and the information processing device that uses the photons, or between the information processing devices. it can.

The single photon generator of the present invention is a single photon generator that emits a single photon in response to the incidence of excitation pulse light.
Quantum dots made of luminescent material are embedded in the optical waveguide,
The single photons are emitted from the quantum dots excited by the absorption of the excitation pulse light, and the single photons are extracted through the optical waveguide.

  For this reason, there is nothing intervening between the quantum dot that is the source of single photons and the optical waveguide that transmits the photons, and the shortest path, and therefore the highest efficiency, from the quantum dots. The emitted photons can be sent to an information processing device or the like that uses the photons. As a result, it is possible to realize a single photon generator with high generation efficiency of single photon pulses that can be actually used. Such a single photon generator is suitably used for quantum cryptography communication, for example, and can contribute to an improvement in the data transfer rate.

  In the present invention, the light-emitting material may be a direct transition type semiconductor material. Direct transition semiconductors emit light by recombination of electrons and holes without phonon involvement, so the probability of light emission by recombination of electrons and holes is greater than indirect transition semiconductors that require phonon involvement. Is much bigger.

  The quantum dot may be made of a crystal having a size not more than four times the exciton Bohr radius. As described above, the quantum dots have the effect of increasing the transition probability that light emission is caused by recombination of electrons and holes due to the quantum size effect, and the fluorescence intensity is increased. Such a confinement effect is said to be remarkable when the size of the quantum dots is not more than four times the exciton Bohr radius.

  The quantum dots preferably have a shape in which one principal axis is longer than the other two principal axes, and emit linearly polarized photons polarized in the major axis direction.

  The excitation pulse light may be pulse light that is shorter than the lifetime of excitons generated in the quantum dots.

  In addition, it is preferable to have means for preventing the excitation pulse light and its scattered light from being mixed and extracted into the single photon.

  Next, a preferred embodiment of the present invention will be specifically described with reference to the drawings.

  FIG. 1 is a top view (a) showing a configuration of a single photon generation device 10 according to an embodiment of the present invention, and an optical waveguide joint surface 2C showing a quantum dot 1 in an enlarged manner as viewed from the light emitting side. It is a side view (b).

  As shown in FIG. 1A, in the single photon generator 10, the quantum dots 1 are embedded in the optical waveguide 2 composed of the optical waveguides 2A and 2B. The optical waveguide 2 is connected to the optical waveguide 4 through the dichroic filter 3 that transmits the single photon 8 emitted from the quantum dot 1 and reflects the excitation pulse light 7, and transmits the excitation pulse light 7 to transmit the single photon 8. Is connected to the optical waveguide 6 through a dichroic filter 5 that reflects the light.

  The quantum dot 1 is a nanosized microcrystal of a direct transition type semiconductor such as indium phosphide. Direct transition semiconductors emit light by recombination of electrons and holes without phonon involvement, so the probability of light emission by recombination is much greater than indirect transition semiconductors that require phonon involvement. .

The size of the quantum dot 1 is preferably not more than 4 times the exciton Bohr radius a _B ^* . At this time, due to the quantum size effect, the transition probability of light emission due to recombination of electrons and holes increases, and the effect of increasing the fluorescence intensity is remarkable. The exciton Bohr radius a _B ^* is given by the following equation.

ここで、κは量子ドット１を形成している発光材料の誘電率、ｈはプランク定数、μは励起子の換算質量、ｅは電子の電荷、ｍ_eは電子の有効質量、そしてｍ_hはホールの有効質量である。

Here, kappa is the dielectric constant of the luminescent material forming the quantum dots 1, h is Planck's constant, reduced mass of μ exciton, e is the electron charge, m _e is the electron effective mass, and m _h is The effective mass of the hole.

  As shown in FIG. 1B, the quantum dot 1 preferably has a shape in which one principal axis is longer than the other two principal axes, for example, an ellipsoidal shape or a cylindrical shape. In this way, linearly polarized photons polarized in the long axis direction 9 can be emitted from the quantum dots 1.

  The material of the optical waveguides 2, 4, and 6 is not particularly limited as long as the material is light transmissive, but is preferably a transparent oxide such as plastic and quartz.

  FIG. 2 is an energy diagram illustrating a mechanism in which single photons 8 are emitted from quantum dots 1 in single photon generator 10.

  As described above, the electrons in the conduction band in the quantum dot 1 take an energy level discretized by the quantum size effect (FIG. 2A). An excitation light source 7 having a wavelength corresponding to the energy difference between the highest energy level 11 in the valence band and the lowest energy level 12 in the conduction band, that is, the band gap energy, is incident on the quantum dot 1 from an excitation light source such as a pulse laser. Then, as shown in FIG. 2A, one electron absorbs the excitation pulse light 7 and is excited from the valence band highest energy level 11 to the conduction band lowest energy level 12, so that the quantum dot 1 Conduction electrons and holes are generated inside. For example, when the quantum dot 1 is made of indium phosphide and is a microcrystal having a size of 5 nm, the wavelength of the excitation pulse light 7 is 800 nm.

  As shown in FIG. 2B, while the first electron remains at the conduction band lowest energy level 12, the second electron is not excited. This is because when the second electron is excited to the conduction band lowest energy level 12, two electrons occupy the same orbit, and the energy of the conduction band lowest energy level 12 increases due to repulsion between electrons. Therefore, the wavelength of the light that can excite the second electron deviates from the wavelength of the excitation pulse light 7 to the short wavelength side, that is, the excitation pulse light 7 has insufficient photon energy to excite the second electron. Because.

  Since the movement region is limited in the quantum dot 1, as shown in FIG. 2 (c), the conduction electrons are quickly combined with holes to form excitons as compared with the bulk size crystal. In addition, since the quantum dot 1 has a greater excitonic oscillator strength than a bulk crystal, the electrons and holes constituting the exciton quickly recombine and emit light, as shown in FIG. There is a high probability that a transition to the valence band highest energy level 11 and a single photon 8 by emission of excitons is emitted from the quantum dot 1 to the optical waveguide 2. For example, when the quantum dot 1 is made of indium phosphide and is a microcrystal having a size of 5 nm, the wavelength of the single photon 8 is 850 nm.

  At this time, it is preferable that the pulse width (duration) of the excitation pulse light 7 be sufficiently shorter than the time until conduction electrons form excitons. In this way, since the second electron is not excited by the excitation pulse light 7 as shown in FIG. 2B, the quantum dot 1 has a single excitation pulse light 7 for each time. It is guaranteed that only photons 8 are emitted. Since the series of steps shown in FIGS. 2A to 2D are promptly performed, a pulse train having a large repetition frequency is made incident on the quantum dot 1 as the excitation pulse light 7, and the pulse train of a single photon 8 having a large repetition frequency is used. Can be released.

  As shown in FIG. 2C, after the conduction electrons form excitons, the conduction band minimum energy level 12 becomes empty, so that the valence band electrons are excited again by absorption of the excitation pulse light 7. Is possible. Therefore, if the pulse width of the excitation pulse light 7 is too large and the excitation pulse light 7 is incident even when the excitation pulse light 7 enters the state shown in FIG. 2C, the second electron is excited and one excitation pulse is generated. It cannot be guaranteed that a single photon 8 is emitted per light 7.

  As shown in FIG. 1A, in the single photon generator 10, the quantum dot 1 is embedded in the optical waveguide 2 composed of the optical waveguides 2A and 2B, and the optical waveguide 2 transmits the single photon 8. The dichroic filter 3 is connected to the optical waveguide 4 on the light emission side. Accordingly, the single photon 8A emitted from the quantum dot 1 to the light emission side is guided to an information processing apparatus or the like that uses the shortest path while minimizing loss. Further, the single photon 8B emitted from the quantum dot 1 to the side opposite to the light emitting side is reflected by the dichroic filter 5 that reflects the single photon 8, and thereafter, in the shortest path as in the single photon 8A. , While minimizing the loss, it is guided to an information processing device that uses the loss. As a result, the single photon generator 10 is a device that can efficiently generate a pulse train of single photons 8 that can be actually used. For example, the single photon generator 10 is preferably used for quantum cryptography communication and contributes to an improvement in the data transfer rate. be able to.

  The excitation pulse light 7 passes through the dichroic filter 5 but is reflected by the dichroic filter 3 and is therefore guided to the optical waveguide 6 side and is hardly guided to the optical waveguide 4 side. Only a pulse train of single photons 8 is guided to the side.

  FIG. 3 is a flowchart showing a process of embedding the quantum dots 1 in the optical waveguides 2A and 2B, which is a part of the manufacturing process of the single photon generator 10. 3A to 3D, a top view is shown on each right side, and a side view seen from the right side of the top view is shown on each left side.

  First, as shown in FIG. 3A, an optical waveguide 2A is prepared. Although FIG. 3A shows an example in which the cross-sectional shape of the optical waveguide 2A is circular, other shapes such as a rectangle or an ellipse may be used.

  Next, as shown in FIG. 3B, one to several quantum dots 1 are fixed to the joint surface 2C of the optical waveguide 2A with the optical waveguide 2B by a method such as coating. This specific method will be described later with reference to FIG.

  Next, as shown in FIG. 3C, the optical waveguide 2B is formed so as to embed the quantum dots 1 by covering the joint surface 2C of the optical waveguide 2A, and the optical waveguide 2 in which the quantum dots 1 are embedded. Complete.

  Next, as shown in FIG. 3D, the cylindrical optical waveguide 2 is rotated around the central axis so that the polarization direction of the single photon is directed to a predetermined direction. Specifically, there is a method of rotating the optical waveguide 2 while measuring the polarization direction of photons emitted from the quantum dots 1 by irradiating excitation light. In addition, when the quantum dot 1 is observed with a microscope in the process shown in FIG. 3B, the major axis direction is marked at that time, and the optical waveguide 2 is rotated based on the mark. Good.

  FIG. 4 is an explanatory diagram showing a method of arranging quantum dots in the manufacturing process of the single photon generator 10.

  In the method shown in FIG. 4A, a dispersion containing a relatively high concentration of quantum dots 1 is prepared, and this is applied to the bonding surface 2C of the optical waveguide 2A. It adheres to the joint surface 2C. As a result, a state in which a relatively large number of quantum dots 1 are fixed to the bonding surface 2C as shown in (1) of FIG. 4A is formed. Thereafter, unnecessary quantum dots 1 are removed using a scanning probe microscope technique, and as shown in (2) of FIG. 4 (a), one to several quantum dots 1 are located at positions sufficiently separated from each other. Try to leave them one by one.

  When a plurality of quantum dots 1 are left, the size and irradiation position of the beam 17 of the excitation pulse light 7 are adjusted as shown by the dotted line in (2) of FIG. Do not excite dot 1. Then, the quantum dot 1 having the best characteristics is selected and used.

  Here, once the excessive number of quantum dots 1 are arranged, the scanning probe microscope technique is used to remove the unnecessary quantum dots 1, but the necessary number of quantum dots 1 are bonded to the bonding surface using the scanning probe microscope technique. It may be fixed to 2C. For these two methods, a method that is mainly easy to execute may be selected.

  In the method shown in FIG. 4 (b), a dispersion liquid containing quantum dots 1 at a relatively low concentration is prepared, and this is applied to the joint surface 2C of the optical waveguide 2A, and then the solvent is evaporated to obtain one to several The quantum dots 1 are fixed to the bonding surface 2C. After that, similarly to the method of FIG. 4A, the size and irradiation position of the beam 17 of the excitation pulse light 7 are adjusted, and the two quantum dots 1 are not excited simultaneously. Then, the quantum dot 1 having the best characteristics is selected and used.

  Instead of the above method, although not shown in the figure, a dilute dispersion liquid is prepared so as to determine whether or not one quantum dot 1 is included in one application amount, and an appropriate quantum dot 1 is bonded to the bonding surface. You may use the method of repeatedly apply | coating a dispersion liquid and evaporating a solvent until it adheres to 2C. Since these methods only select and use the quantum dots 1 fixed to the bonding surface 2C by applying the dispersion liquid and evaporating the solvent, there is no need to use a scanning probe microscope technique, and there is an advantage that it is simple. is there.

  The method shown in FIG. 4C is a method that can be said to be a modification of the method shown in FIG. In the method shown in FIGS. 4A and 4B, the quantum dots 1 are arranged directly on the joint surface 2C of the optical waveguide 2A, whereas in the method shown in FIG. After applying a dispersion containing the quantum dots 1 at an appropriate concentration on the entire surface of the waveguide sheet 18, the solvent is evaporated to fix the quantum dots 1 on the optical waveguide sheet 18. Next, similarly to the method shown in FIG. 4B, a suitable quantum dot 1 is found from the characteristics of photons obtained by actually irradiating the excitation pulse light 7. Then, a part 19 of the optical waveguide sheet 18 including the quantum dots 1 is cut and bonded to the bonding surface 2C of the optical waveguide 2A.

  FIG. 5 is a top view showing a configuration of single photon generation apparatus 20 based on a modification of the embodiment of the present invention. In the single photon generator 10 shown in FIG. 1A, the incident direction of the excitation pulse light 7 and the emission direction of the single photon 8 are the same. On the other hand, the single photon generator 20 is arranged so that the incident direction of the excitation pulse light 7 is orthogonal to the emission direction of the single photon 8, and accordingly, the excitation pulse light 7 is transmitted and the single photon 8 is transmitted. The position where the dichroic filter 5 that reflects the light is disposed is changed, and the optical waveguide 6 is omitted.

  This structure has an advantage that the excitation pulse light 7 is not easily mixed into the single photon 8. Since it is the same as that of the single photon generator 10 in other points, it goes without saying that the single photon generator 20 has the same characteristics as those of the single photon generator 10 described above.

  Hereinafter, examples of the present invention will be given to describe the single photon generator according to the present invention more specifically, and the results of measuring the performance as a single photon generator will be described.

Example 1
Indium phosphide was prepared using the method described in Reference 1 (J. Handbook of Nanostructured Materials and Nanotechnology, Nalwa, HS, Ed .; Academic Press: San Diego, 2000; v. 1, p. 427). As a constituent material, a quantum dot having a crystal size of 5 nm on a side and an emission wavelength of 850 nm was prepared.

  This quantum dot was incorporated in the single photon generator 10 shown in FIG. The optical waveguides 2, 4 and 6 are made of quartz. As an excitation light source, a mode-locked titanium (Ti) -doped sapphire laser having a wavelength of 800 nm, a repetition frequency of 80 MHz, and a pulse width of 2 ns was used.

  FIG. 6 shows a single photon demonstration system using a Hanbury Brown-Twiss correlation measuring instrument used for measuring the pulse train of the single photon 8 in this embodiment. In this system, the pulse train of the single photon 8 emitted from the single photon generator 10 is divided into two directions by the beam splitter 31, and each photon that follows each path is photomultiplier. The correlation between the time when the electrons are detected by the two photomultipliers 32 and 33 and detected by the two photoelectron multipliers 32 and 33 is measured by the Hanbury Brown-Twiss correlation measuring device 34.

  If only a single photon 8 is emitted in one pulse in the pulse train of single photons 8, the photon passes through the beam splitter 31 and is detected by the photomultiplier 32, or Either the light beam is reflected by the beam splitter 31 in the orthogonal direction and detected by the photomultiplier rod 33, and the photon multiplier rods 32 and 33 do not detect photons at the same time. On the other hand, if a plurality of photons are emitted in one pulse, photons are simultaneously detected by the photomultipliers 32 and 33 with a finite probability.

  The result of correlation measurement of the pulse train of the single photon 8 emitted from the single photon generator 10 using the single photon demonstration system of FIG. 6 is the same graph as FIG. FIG. 7 is a graph showing an example in which a pulse train of single photons emitted from a single photon generator is measured using a Hanbury Brown-Twiss correlation measuring instrument. The horizontal axis of the graph indicates one of the correlation measuring instruments. Is the time when the photon is observed at the photomultiplier of the reference time (t = 0), and the vertical axis indicates the frequency at which the photon is observed at the other photomultiplier at the time before and after the reference time. Is shown. In the graph of FIG. 7, the detection frequency of photons at the background level at t = 0 indicates that no photon is observed in the other photomultiplier at the time when the photon is observed in one photomultiplier. That is, no photons are observed simultaneously by two photomultipliers, indicating that no pulse containing a plurality of photons was present in the pulse train.

Example 2
Using the method described in Reference 2 (Nano-Letters, vol.3, no.6, p.833-837, June 2003), indium phosphide is used as a constituent material, and the major axis is 20 nm and the minor axis is 3 nm. Quantum dots having an emission wavelength of 850 nm were prepared.

  This quantum dot was incorporated into the single photon generator 10 shown in FIG. At this time, it was mounted so that the major axis was oriented in a predetermined direction.

  When the measurement was performed with the same configuration as in Example 1, a single photon array polarized in the direction of the long axis of the quantum dots was observed.

  As described above, based on the embodiments and examples of the present invention, it is possible to realize a device that generates a single photon pulse train having a desired polarization angle at a high frequency. Conventionally, quantum cryptography communication has a problem that the data transfer rate cannot be increased. However, if the single-photon generator according to the present invention is used, quantum cryptography that cannot be eavesdropped in principle and has a high data transfer rate. Communication can be realized.

  As mentioned above, although this invention was demonstrated based on embodiment and an Example, this invention is not limited to these examples at all, and it cannot be overemphasized that it can change suitably in the range which does not deviate from the main point of invention. .

  The single photon generation device of the present invention is a device that can efficiently generate a pulse train of single photons and can send the pulse train to an information processing device that uses the pulse train with little loss. And can contribute to the improvement of the data transfer speed.

The top view (a) which shows the structure of the single photon generator based on embodiment of this invention, and the side view (b) which looked at the optical waveguide joining surface 2C which expanded and shows a quantum dot from the light-projection side is there. FIG. 6 is an energy diagram for explaining a mechanism in which single photons are emitted from quantum dots in the single photon generator. It is a flowchart which shows the process of embedding the quantum dot 1 in the optical waveguides 2A and 2B which is a part of manufacturing process of a single photon generator similarly. It is explanatory drawing which shows the arrangement | positioning method of the quantum dot in the manufacturing process of a single photon generator same as the above. It is a top view which shows the structure of the single photon generator based on the modification of embodiment of this invention. It is explanatory drawing which shows the structure of the single photon generation | occurrence | production verification system using the Hanbury Brown-Twiss correlation measuring device by Example 1 of this invention. It is a graph which shows the example which measured the pulse train of the single photon emitted from the single photon generator using the Hanbury Brown-Twiss correlation measuring device. It is explanatory drawing (a) which shows the structure of the single photon generator shown by patent document 1, and an expanded sectional view (b) near a quantum dot.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Quantum dot, 2, 2A, 2B ... Optical waveguide, 2C ... Joint surface of optical waveguide 2A and 2B,
3, 5 ... Dichroic filter, 4, 6 ... Optical waveguide,
7: Excitation pulse light (for example, wavelength 800 nm),
8, 8A, 8B ... single photons (eg, wavelength 850 nm),
9 ... Long axis direction (polarization direction of a single photon), 10 ... Single photon generator,
11 ... highest energy level of valence band, 12 ... lowest energy level of conduction band,
13 ... exciton energy level, 17 ... excitation pulsed light beam, 18 ... optical waveguide sheet,
19 ... Optical waveguide sheet cut and joined to the optical waveguide 2A, 20 ... Single photon generator,
31 ... Beam splitter, 32, 33 ... Photomultiplier,
34 ... Hanbury Brown-Twiss correlation instrument, 100 ... single photon generator,
101 ... GaAs substrate (n-type), 102 ... undoped AlAs barrier layer,
103 ... Quantum dots (InAs-GaSb), 103A ... Quantum dots directly under the optical window,
104 ... undoped AlAs barrier layer, 105 ... GaAs contact layer (p-type),
106 ... Upper electrode, 106A ... Optical window, 107 ... Lower electrode, 108 ... Semiconductor laser,
108A ... excitation pulse light, 109 ... bias power supply, 109A ... switch,
109B ... Control circuit, 110 ... Optical gate member

Claims (5)

A single photon generator that emits a single photon in response to the incidence of excitation pulsed light,
Quantum dots made of luminescent material are embedded in the optical waveguide,
The single photons are emitted from the quantum dots excited by the absorption of the excitation pulse light, and the single photons are extracted through the optical waveguide.
The quantum dots, one of the spindle has a longer shape than the other two principal axes, emits photons of linearly polarized light polarized in the long axis Direction,
Single photon generator.   2. The single photon generator according to claim 1, wherein the light emitting material is a direct transition type semiconductor material.   2. The single photon generator according to claim 1, wherein the quantum dot is made of a crystal having a size of four times or less of an exciton Bohr radius.   The single-photon generator according to claim 1, wherein the excitation pulse light is pulse light shorter than a lifetime of excitons generated in the quantum dots.   2. The single photon generator according to claim 1, further comprising means for preventing the excitation pulse light and the scattered light from being mixed and extracted from the single photon.

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