JP4764210B2 - Single photon generator - Google Patents

Description

  The present invention relates to a single photon generator for generating an optical pulse composed of a single photon.

  In recent years, single photon generators have become increasingly important in the fields of quantum communication and quantum computers that perform information processing using quantum mechanical superposition states, and spintronics that controls the spin of matter.

  Secure and reliable encryption communication is indispensable for the realization of next-generation information society such as e-government and e-commerce. At present, it is considered that a public encryption key widely used in the Internet or the like can be deciphered if a quantum computer capable of executing parallel computation at high speed is realized. In that case, it is impossible to completely prevent data duplication, wiretapping, and alteration by a third party. As a means for improving such a security problem, quantum cryptography, which is a principle safe encryption scheme, has attracted attention.

  The quantum cryptography is C.I. H. Bennett and G.M. It has been actively studied with a specific protocol proposed by Brassard et al. In 1984 (so-called BB84) (IEEE. Int. Conf. On Computers, Systems, and Signal Processing, Bangalore, India (I984). ) Pp.175-pp.179). Quantum cryptography, which has been proposed as a cryptographic method whose security is unconditionally proved by information theory, shows a method for safely delivering cryptographic keys. In quantum cryptography, quantum mechanical physics ensures the security of cryptography, so it is possible to guarantee ultimate security that does not depend on the limits of computer capabilities. This quantum cryptography is based on security that an eavesdropper cannot fully know the state of one photon. That is, when 1 bit of information is given to one photon in the form of polarization information, the information held by each photon follows the uncertainty principle and the no-cloning theorem, so without destroying the state of the photon. Polarization information cannot be extracted. For example, if an eavesdropper observes or copies in the middle of transmission, the polarization information changes, and different polarization information reaches the legitimate receiver, so that it is easy to know that there was an eavesdropping. it can. However, when two photons carry certain 1-bit information, even if an eavesdropper extracts one of them, a legitimate information receiver cannot detect it. Therefore, in quantum cryptography, it is necessary to ensure the security to use only one photon to transmit 1-bit information.

  Conventionally, single photon generators that generate single photons by controlling excitation pulse intensity and time width have been proposed (see Non-Patent Documents 1 and 2).

  In addition, a single photon is generated by injecting one electron into the semiconductor active layer using a so-called Coulomb blockade phenomenon in which electron tunneling is not possible due to a change in electric field due to one electron in the semiconductor thin film. A one-photon generating element has been proposed (see Patent Document 1). In addition, a single photon generator that generates single photons by injecting one electron into a semiconductor active layer by a method called turn style similar to the technique disclosed in Patent Document 1 has been reported (Non-Non-Patent Document) (See Patent Document 3).

  On the other hand, by controlling the size of semiconductor quantum dots, a single photon generator using the generation of only one electron and one hole by photoexcitation has been proposed (see Patent Document 2). In fact, single photon production has been reported. (Refer nonpatent literature 4.).

In addition, single photon generators using Raman scattering (see Patent Document 3) and single photon generators combining quantum dots, optical resonators, and refractive index control have also been proposed (see Patent Document 4). ).
JP-A-4-61176 JP 2004-253657 A JP 2003-298153 A Japanese Patent Laid-Open No. 2001-230445 Phys. Rev. Let. 76 (1996) pp. 900-pp. 903 J. et al. Modern Optics, 44 (1997) pp. 2067-pp. 2074) Nature vol. 397 (1999) p. 500-pp. 503 Jpn. J. et al. Appl. Phys. Vol. 43 (2004) L993-L995

  However, in the technique disclosed in Non-Patent Document 1 or 2, since the timing at which a single photon is generated follows a Poisson and sub-Boisson distribution, there is a probability that no photon is generated or a plurality of photons are generated simultaneously. There is a problem that it exists at a finite rate. In this case, single photons usable for quantum cryptography cannot be efficiently extracted, and the security of quantum cryptography is not guaranteed.

  Further, in the techniques disclosed in Patent Document 1 and Non-Patent Document 3, since the effect of the Coulomb blockade is used, the operating temperature is very low (for example, 50 mK), and extra photons are generated due to the background current. There is a problem that it ends up. For these reasons, the security of the quantum cryptography is not guaranteed and the practicality of the quantum cryptography device is low.

  In the technique disclosed in Non-Patent Document 4 or Patent Document 2, single photon generation is realized by current injection operation, and the operating temperature is higher than that reported in Non-Patent Document 3, but it is still at an extremely low temperature of 10K. There is a problem that it is operation and is not practical.

  In the technique disclosed in Patent Document 3, a precise optical resonator must be configured in a vacuum vessel, which lacks practicality, and a single atom such as cesium is always captured by a cooling laser beam. There is a problem that once two or more atoms are captured, a plurality of photons are generated simultaneously.

  In the technique disclosed in Patent Document 4, the single photon generation device stores the single photons generated by the quantum dots in a resonator composed of microspheres, and then waits for more time before refraction of the connecting member. It has a structure in which the rate is changed and taken out again. Due to this complicated structure, optical loss is large, and it is considered that it is extremely difficult to extract photons generated by quantum dots without loss, and there is a problem that practicality is low.

  As described above, the problems of the single photon generator are (i) that a single photon can be generated at a desired timing (arbitrary time), and (ii) that a plurality of photons cannot be generated at the same time.

  Accordingly, an object of the present invention is to provide a novel and useful single photon generator capable of solving the above two problems. Furthermore, a specific object of the present invention is to provide a single photon generator having a simple structure and high practicality.

  According to one aspect of the present invention, a single photon generator having a phosphor composed of a phosphor matrix material and one metal ion that emits light by inner-shell transition in the phosphor matrix material; There is provided a single-photon generator characterized by comprising an excitation light pulse to be excited or an excitation unit that gives electrons conducted in a phosphor in a time shorter than the emission lifetime of metal ions.

  According to the present invention, an excitation light pulse for exciting a metal ion or an electron conducted in the phosphor is given to the metal ion causing the inner shell transition in a time shorter than the emission lifetime of the metal ion. Since only one metal ion is placed in the region where the excitation light pulse is irradiated or in the region where the electron passes, the application of an electric field that generates the excitation light pulse irradiation or the electrons conducted in the phosphor Thus, a single photon can be reliably generated at a desired timing. Furthermore, the single photon generator of the present invention utilizes light emission by the inner shell transition of metal ions, and the inner shell transition of metal ions can generate photons in a wide temperature range from a very low temperature of several mK to about room temperature. Therefore, the practicality of the single photon generator is high. In addition, since the single photon generator of the present invention has a simple configuration and light emitting action, and has high optical reliability with little optical loss, it is highly practical in this respect as well. The light emission lifetime is defined as “a time during which a light emission probability of a metal ion exists” in the claims and specification of the present application.

  In addition, since many metal ions exist in the conventional phosphor, when the phosphor is irradiated with an excitation light pulse for exciting the metal ions, a large number of photons are generated. In the present invention, since only one metal ion is disposed in a region to which excitation light or an electric field is applied, photons are generated only from the metal ion. As a result, the present invention has an effect that single photons can be reliably extracted.

  According to the present invention, the metal ion that causes the inner shell transition is given a time in which the excitation light pulse for exciting it or the electron passing through the phosphor passes in a time shorter than the emission lifetime of the metal ion, Since only one metal ion is arranged in the region where the excitation light pulse in the phosphor is irradiated, or in the region where the electrons conducting in the phosphor pass, the excitation light pulse irradiation or electron flow occurs. By applying the electric field to be generated, a single photon can be reliably generated at a desired timing. Furthermore, according to the present invention, it is possible to provide a highly practical single photon generator capable of emitting light in a simple configuration and capable of emitting light at room temperature or a low temperature environment close to room temperature.

  Embodiments will be described below with reference to the drawings.

(First embodiment)
FIG. 1 is a configuration diagram of a single photon generator according to a first embodiment of the present invention.

  Referring to FIG. 1, a single photon generator 10 includes a crystal substrate 11, a single photon generator 14 formed of quantum dots 12 formed on the crystal substrate 11, and an excitation light pulse 15 </ b> A applied to the quantum dots 12. It comprises a laser irradiation device 15 for irradiation, a control unit 16 for controlling the irradiation timing, pulse width, and the like.

  Here, the quantum dot 12 is a crystal having a diameter of several nanometers to several tens of nanometers in which several hundred to several thousand atoms are gathered. The quantum dot 12 has an operating mechanism different from that of the quantum dot disclosed in Patent Document 2 in the background art column.

  The crystal substrate 11 is selected from materials capable of epitaxial growth of the quantum dots 12 according to the material of the quantum dots 12. Examples of the crystal substrate 11 include GaAs, GaN, and sapphire.

The quantum dot 12 is composed of a phosphor base material 12a and one metal ion 13 that forms a light emission center. A known material can be used as the phosphor base material 12a, but is selected from thiogallate-based materials (X = Sr, Ca, and Ba) represented by XGa ₂ S ₄ in terms of improving the luminance of the metal ions 13. Thioaluminate-based material represented by XAl ₂ S ₄ (X is the same as above), Zn ₂ SiO ₄ , and BaMg ₂ Al ₁₆ O ₂₇ are suitable. Also, from the same point as described above, as the phosphor base material 12a, ZnS, CaS, SrS, CdS, GaN, ZnO, ZnSe, BaS, and AlN are also suitable.

  Only one metal ion 13 is included in the quantum dot 12. For this reason, by irradiating the quantum dot with the excitation light pulse 15A, a photon is generated from only one metal ion 13, so that a single photon is reliably generated according to the irradiation condition of the excitation light pulse 15A described later. it can.

  The metal ion 13 is selected from ions that generate one photon by the inner-shell transition. In the inner-shell transition, electrons in the inner shell of an atom are excited by photon absorption or collision with high-energy electrons, and the electrons are in a high energy state (excited state). In the present invention, at this time, the metal ion 13 that is excited by only one electron is selected. Then, a single photon is generated when one excited electron returns from the excited state to the ground state. Examples of such metal ions 13 include Ag, Au, As, Bi, Cd, Ce, Cr, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ge, Ho, Hg, In, Mn, Nd, and Pb. , Pr, Sb, Sm, Sn, Tb, Tm, Ti, Tl, V, W, Yb, and Zn ions. These metal ions 13 can emit light in a wide temperature range from a very low temperature of several mK to room temperature by combination with a phosphor base material.

In addition, the combination of the phosphor base material 12a and the metal ions 13 can arbitrarily combine the respective materials. In particular, a thiogallate-based material represented by XGa ₂ S ₄ (X = Sr, Ca, and 1 type selected from Ba.), 1 type of fluorescence selected from aluminate materials represented by XAl ₂ S ₄ (X is the same as above), Zn ₂ SiO ₄ , and BaMg ₂ Al ₁₆ O ₂₇ It is preferable to combine the base material 12a and one metal ion 13 selected from Eu ²⁺ , Ce ³⁺ , and Mn ²⁺ .

In addition, a phosphor base material 12a selected from ZnS, CaS, SrS, CdS, GaN, ZnO, ZnSeBaS, and AlN, and Eu ²⁺ , Ce ³⁺ , and Mn ²⁺ are selected. One kind of metal ion 13 may be combined.

The light emission mechanism of Eu ²⁺ as an example of the metal ion 13 will be described. The electron configuration in the ground state and excited state of Eu ²⁺ is as follows.

Ground state Eu ²⁺ : [Kr] (4d) ¹⁰ (4f) ⁷ (5s) ² (5p) ⁶
Excited state Eu ²⁺ : [Kr] (4d) ¹⁰ (4f) ⁶ (5s) ² (5p) ⁶ (5d) ¹
In addition, said [Kr] has shown having the same electronic arrangement as the element Kr.

Eu ²⁺ excitation occurs when one electron makes a transition from the 4f level to the 5d level due to photon absorption or the like, and its light emission occurs at the time of transition from the 5d level to the 4f level. This emission generates one photon. In the present embodiment, since only one Eu ²⁺ exists in the quantum dot 12, only one photon is generated by one excitation-emission process of Eu ²⁺ .

FIG. 2 is a diagram showing an excitation spectrum and an emission spectrum of Eu ²⁺ . FIG. 2 shows a spectrum in the case where one Eu ²⁺ is contained in the SrGa ₂ S ₄ crystal.

Although not shown, the band gap of the SrGa ₂ S ₄ crystal is known to be 4.4 eV from the absorption spectrum. On the other hand, the strong and broad excitation band of the excitation spectrum at 350 nm to 500 nm shown in FIG. 2 is in the range of 3.5 eV to _2.5 eV, so that it is about 1 eV more than the band gap of the SrGa ₂ S ₄ crystal. small. From this, the excitation of Eu ^{2+ in} the SrGa ₂ S ₄ crystal indicates that the host material of SrGa ₂ S ₄ is not excited, but Eu ²⁺ is directly excited by irradiation of excitation light. Show. Therefore, even if a defect occurs in the SrGa ₂ S ₄ crystal, Eu ²⁺ can generate photons without being affected by the defect. That is, even if a defect occurs in the quantum dot 12 shown in FIG. 1, the metal ion 13 can generate a photon without being affected by the defect. Moreover, the phenomenon in which the emission intensity decreases as the temperature rises is called temperature quenching. Eu ²⁺ in the SrGa ₂ S ₄ crystal is small in temperature quenching, so that photons are generated not only at extremely low temperatures of several mK but also at room temperature. Is possible.

Further, the wavelength of the excitation band exists on the shorter wavelength side than the wavelength at which the emission intensity of the emission spectrum increases. In the excitation band, it can be seen that the absorptance increases in the ultraviolet wavelength region, and Eu ²⁺ can be excited by the excitation light pulse 15A having a lower intensity. For this reason, the laser irradiation device 15 preferably uses a pulse laser whose excitation light pulse 15A has a wavelength shorter than the emission wavelength of the metal ions 13, for example, a pulse laser that emits light with ultraviolet rays.

  Furthermore, the time width of the excitation light pulse 15 </ b> A is set by the control unit 16 at a time shorter than the light emission lifetime of the metal ions 13. This is because if the time width of the excitation light pulse 15A is set longer than the light emission lifetime of the metal ions 13, for example, two photons are continuously generated. The light emission lifetime is a time when the light emission probability of the metal ion 13 exists, and is a time obtained from, for example, a light emission waveform obtained by integrating the light emission of a large number of single photons.

  FIG. 3 is a schematic diagram for explaining a single photon generation operation. Note that the horizontal axis of FIG. 3 represents time linearly, and the vertical axis of FIG. 3B represents the light emission probability logarithmically.

Referring to FIG. 3 together with FIG. 1, when the quantum dot 12 is irradiated with the excitation light pulse 15A having the pulse width Ta shown in (A), the metal ions 13 become excited after a short time ΔT ₁ and start to relax.

  When the pulse width Ta of the excitation light pulse 15A is shorter than the emission lifetime Tb of the metal ion 13, the metal ion 13 enters an excited state during the pulse width Ta of the excitation light pulse 15A and starts relaxation. Since the light emission lifetime Tb is longer than the pulse width Ta, there is no transition to the ground state as a stochastic event while the excitation light pulse is irradiated (Ta). Then, after the excitation light pulse 15A is turned off, the metal ion 13 generates an output light pulse SP composed of a single photon while returning to the ground state. That is, as shown in FIG. 3B, the light emission probability is high immediately after the excited state is reached. The light emission probability decreases exponentially with time. As shown in FIG. 3C, the metal ion 13 generates an output light pulse SP composed of a single photon during the light emission lifetime Tb.

  If the pulse width Ta of the excitation light pulse 15A is longer than the emission lifetime Tb of the metal ion 13, a photon pulse is generated while returning to the ground state when the emission lifetime Tb elapses after excitation, and then excited again. The possibility that a second photon is generated when the excitation light pulse 15A is turned off is a stochastic event. Therefore, in order to generate a single photon, it is necessary to set the pulse width Ta of the excitation light pulse 15A to be shorter than the light emission lifetime Tb of the metal ion 13.

For example, since the emission lifetime of Eu ²⁺ is about 1 μsec, the laser irradiation device 15 can be supplied with a pulse laser capable of emitting a pulse having a duration shorter than the emission lifetime, for example, several picoseconds to several tens of femto (f ) A pulse laser capable of emitting a pulse width of about 1 second (1f seconds = 10 ⁻¹⁵ seconds) may be used. The laser irradiation device 15 includes, for example, a pulse laser (for example, product name: UV Power, manufactured by Tokyo Instruments Co., Ltd.) capable of emitting pulse light having a third harmonic (wavelength 355 nm) of YAG laser light and a pulse width of several picoseconds. 355 (pulse width of 10 psec or less) can be used. In this way, a single photon can be reliably generated by using a pulse laser having a pulse width of about one millionth or shorter than the emission lifetime of Eu ²⁺ .

  In the present invention, since a pulse laser having a relatively long pulse width of about several picoseconds can be used, the apparatus cost can be reduced as compared with the above-described prior art, for example, the single photon light emitting device of Patent Document 2. Moreover, the intensity | strength of the excitation light pulse 15A will not be specifically limited if it is the intensity | strength which can fully excite the metal ion 13. FIG.

As another example of the metal ion 13, the light emission mechanism will be described for Ce ³⁺ . The electron configuration of the ground state and the excited state of Ce ³⁺ is as follows.

Ground state Ce ³⁺ : [Kr] (4d) ¹⁰ (4f) ¹ (5s) ² (5p) ⁶
Excited state Ce ³⁺ : [Kr] (4d) ¹⁰ (5s) ² (5p) ⁶ (5d) ¹
The excitation of Ce ³⁺ occurs when one electron makes a transition from the 4f level to the 5d level due to photon absorption or the like, and the emission thereof occurs when the transition from the 5d level to the 4f level occurs. This emission generates one photon. In the present embodiment, since only one Ce ³⁺ exists in the quantum dot 12, one photon is generated by one excitation-emission process of Ce ³⁺ .

Ce ^{3+ has} an emission lifetime of 100 nsec. As described above, a single photon can be generated reliably by using a pulse laser having a pulse width of several picoseconds to several tens of femtoseconds.

Examples of such inner-shell transition include (4f) ⁿ → (4f) ^n-1 (5d) ¹ transition of rare earth elements such as Eu ²⁺ described above, and (3d) ⁵ → Mn ²⁺ described later. Examples include (3d) ⁵ transition and Cu ²⁺ (3d) ⁹ (4s) ¹ → (3d) ¹⁰ transition. In the present invention, any of these metal ions 13 that cause the inner shell transition can be used.

  Returning to FIG. 1, the excitation light pulse 15 </ b> A is applied to the entire quantum dot 12 or a range where the metal ions 13 in the quantum dot 12 exist. In FIG. 1, the condenser lens is not shown, but it is preferable that the excitation light pulse 15 </ b> A is condensed by the condenser lens and irradiated. Alternatively, a large number of quantum dots 12 including one metal ion 13 may be arranged on the crystal substrate 11 and the plurality of quantum dots 12 may be irradiated with the excitation light pulse 15A simultaneously. Single photons SP can be taken out in parallel in time by taking out a plurality of single photons separately.

  The angle at which the excitation light pulse 15A is applied to the quantum dots 12 is not particularly limited, but it is preferable to irradiate the quantum dots 12 from above or obliquely in view of easy irradiation.

  Moreover, it is preferable that the crystal substrate 11 and the quantum dot 12 which are the single photon generation | occurrence | production part 14 are mounted in a vacuum vessel (not shown). Thereby, the cleanliness of the surface of the quantum dots 12 can be maintained, and the reliability of single photon generation is increased. The temperature of the single photon generator 14 can be set in a wide temperature range from a very low temperature of several mK to room temperature. For this reason, since it operates even without a special cooling device, the configuration of the device becomes simple, and this point is also highly practical. Further, the excitation light pulse 15A may be applied to the quantum dots 12 from the outside of the vacuum vessel through an optical window provided in the vacuum vessel. Further, the single photon can be taken out via an optical filter that removes the excitation light.

Next, a method for forming the single photon generator 14 will be described with reference to FIGS. 4 (A) and 4 (B). 4A and 4B are cross-sectional views showing a process for forming a single photon generator. Here, an example will be described in which GaAs is used as the crystal substrate 11 shown in FIG. 1, SrGa ₂ S ₄ is used as the phosphor matrix material 12 a of the quantum dots 12, and Eu ²⁺ is used as the metal ion 13.

First, in the step of FIG. 4A, a quantum dot base layer 12a-1 is epitaxially grown on a GaAs substrate 11. Specifically, the GaAs substrate 11 is mounted on a molecular beam epitaxial (MBE) apparatus, and the substrate temperature is set in a range of 530 ° C. to 605 ° C. in a vacuum atmosphere (for example, a degree of vacuum of 10 ⁻⁷ Pa). Furthermore, an Sr cell (for example, heating temperature 510 ° C.) and a Ga ₂ S ₃ cell (for example, heating temperature 990 ° C.) are used as the molecular beam source. Quantum dots (base layer 12a-) composed of SrGa ₂ S _{4 of} about 2 to 5 molecular layers are formed by irradiating a molecular beam to the GaAs substrate 11 by controlling opening and closing of the shutter, and epitaxially growing SrGa ₂ S ₄ on the GaAs substrate 11. 1) is formed. At this time, since the SrGa ₂ S ₄ that has reached the surface of the GaAs substrate 11 is thin, the SrGa ₂ S ₄ aggregates, and a quantum dot base layer 12a-1 having a diameter of about 20 nm to 30 nm is formed.

In the step of FIG. 4A, after cooling, one Eu ²⁺ is adsorbed on the base layer 12a-1 in a vacuum atmosphere using a scanning tunneling microscope (STM) without being exposed to the outside air. Specifically, the metal ions 13 of Eu ²⁺ is one adsorbed from Eu ²⁺ source at the tip of the probe of the STM. Further, a probe is positioned on the surface of the base layer 12a-1, and Eu ²⁺ is adsorbed on the surface of the base layer 12a-1.

Next, in the step of FIG. 4B, the GaAs substrate 11 on which the structure of FIG. 4A is formed is returned to the MBE apparatus again without being exposed to the outside air, and is similar to the above-described formation conditions of the base layer 12a-1. in conditions, to form the base layer 12a-1 SrGa ₂ S ₄ 1 molecular layer to 5 molecular layers about the cap layer 12a-2 formed of the above. Thus, one Eu ²⁺ is covered with the cap layer 12a-2. As described above, the single photon generator 14 composed of the quantum dots 12 including one Eu ²⁺ metal ion 13 is formed on the GaAs substrate 11.

Note that when a material other than the above-described GaAs substrate, SrGa ₂ S ₄ , and Eu ²⁺ is used, it can be formed in the same manner as the above formation method. However, temperature conditions etc. are suitably selected according to those materials.

  Further, the formation of the single photon generator 14 described above uses an integrated vacuum manufacturing apparatus in which an MBE apparatus and an STM apparatus are connected by a transfer chamber capable of maintaining a vacuum atmosphere. In place of the MBE apparatus, another thin film manufacturing apparatus such as an organic metal (MO) MBE apparatus or a metal organic chemical vapor deposition (MOCVD) apparatus can be used.

  According to the first embodiment, a single photon generator 14 including only one metal ion 13 that causes an inner shell transition in one quantum dot 12 made of the phosphor base material 12a is provided. By irradiating the quantum dot 12 with the excitation light pulse 15A having a pulse width shorter than the light emission lifetime of the metal ion 13, a single photon can be reliably generated. Further, the inner-shell transition of the metal ion 13 is practical because it can generate photons in a wide temperature range from a very low temperature of several mK to about room temperature. In addition, since the configuration and the light emitting action of the single photon generator 10 are simple, the optical loss is small and the reliability is high, so that this point is also practical.

(Second Embodiment)
FIG. 5 is a block diagram of a single photon generator according to the second embodiment of the present invention. In the figure, portions corresponding to the portions described above are denoted by the same reference numerals, and description thereof is omitted.

  Referring to FIG. 5, a single photon generator 20 generates a single photon comprising a crystal substrate 11 and a phosphor base material 22 a formed on the crystal substrate 11 and a phosphor layer 22 made of metal ions 13. Unit 24, a laser irradiation device 15 that irradiates a region of phosphor layer 22 where one metal ion 13 is present with excitation light pulse 15A, a control unit 16 that controls the irradiation timing, pulse width, and the like. The The single photon generator 20 has the same configuration as the single photon generator 10 according to the first embodiment shown in FIG. 1 except that the phosphor layer 22 is formed instead of the quantum dots.

  The phosphor layer 22 is epitaxially grown on the crystal substrate 11 and formed, for example, in the form of a thin film having a thickness of 100 nm to 500 nm. The phosphor host material 22a is selected from the phosphor host materials described in the first embodiment.

  Only one metal ion 13 exists in the phosphor layer 22 in a predetermined range. Here, the predetermined range is a range irradiated with one excitation light pulse 15A. The metal ion 13 is selected from the metal ions described in the first embodiment.

  The excitation light pulse 15A may be incident on the surface of the phosphor layer 22 at any angle, but is preferably irradiated from the upper side or the side surface of the phosphor layer 22. Further, it is preferable that the excitation light pulse 15 </ b> A irradiated into the phosphor layer 22 is incident at an angle causing total reflection at the interface of the crystal substrate 11. This increases the probability that the metal ions 13 are excited by the excitation light pulse 15A. For the total reflection of the excitation light pulse 15 </ b> A, a material for which the refractive index of the crystal substrate 11 is smaller than the refractive index of the phosphor layer 22 is selected. On the other hand, since the refractive index of the phosphor layer 22 is higher than the refractive index of a vacuum or an inert gas on the surface of the phosphor layer 22, total reflection occurs.

Next, a method for forming the single photon generator 24 will be described with reference to FIGS. 6 (A) and 6 (B). 6 (A) and 6 (B) are cross-sectional views showing a process for forming a single photon generator. Here, an example will be described in which a GaAs substrate is used as the crystal substrate 11, SrGa ₂ S ₄ is used as the phosphor base material 22a, and Eu ²⁺ is used as the metal ion 13. Such a forming method is substantially the same as the forming method described with reference to FIG. 4, and it goes without saying that the forming method can be used.

First, in the process of FIG. 6A, the base layer 22a-1 made of SrGa ₂ S ₄ is epitaxially grown on the GaAs substrate 11 by the MBE apparatus. Specifically, the method is the same as that described in FIG. 4A of the first embodiment. However, the thickness of the base layer 22a-1 is set to 50 nm to 200 nm.

In the step of FIG. 6A, after cooling, one Eu ²⁺ is adsorbed on the base layer 22a-1 in a vacuum atmosphere using a scanning tunneling microscope (STM) without being exposed to the outside air.

6B, a cap layer 22a-2 having a thickness of 50 nm to 200 nm made of SrGa ₂ S ₄ covering Eu ²⁺ is formed on the base layer 22a-1 by the MBE apparatus. In this way, the single photon generator 24 composed of the phosphor layer 22 containing one Eu ²⁺ metal ion 13 is formed on the GaAs substrate 11.

  According to the second embodiment, the same effect as that of the single photon generator according to the first embodiment is obtained. That is, the phosphor layer 22 includes a single photon generation unit 24 that includes only one metal ion 13 that causes an inner shell transition, and the phosphor layer 22 that includes only one metal ion 13 includes a metal ion. By irradiating the excitation light pulse 15A having a pulse width shorter than 13 light emission lifetimes, the output light pulse SP of a single photon can be reliably generated. Furthermore, in the second embodiment, since the phosphor layer 22 is layered, the probability that the metal ions 13 are excited can be increased by making the excitation light pulse 15A incident so as to be totally reflected in the phosphor layer 22. Can be high.

(Third embodiment)
FIG. 7 is a configuration diagram of a single photon generator according to the third embodiment of the present invention. In the figure, portions corresponding to the portions described above are denoted by the same reference numerals, and description thereof is omitted.

  Referring to FIG. 7, a single photon generator 30 includes a substrate 31, a lower electrode 32 on the substrate 31, a first insulating layer 33, a phosphor base material 34a, and one metal ion 13 contained therein. The phosphor layer 34, the second insulating layer 35, and the upper electrode 36 are electrically connected between the lower electrode 32 and the upper electrode 36, and the single photon generator 38 in which the upper electrode 36 is deposited in this order. And a pulse power supply unit 39 for applying a shaped electric field. The single photon generator 30 causes an inner shell transition in the metal ion 13 by causing electrons to collide with one metal ion 13 in the phosphor layer 34 to generate a single photon.

  The material of the substrate 31 is not particularly limited, but it is preferable to use a transparent substrate including a case where a single photon is taken out from the substrate 31 side. As the transparent substrate, a transparent insulating substrate such as glass or quartz, a film such as PET or PEN, a resin substrate such as PVA, or the like can be used. In the case where single photons are taken out from above the plane of FIG. 7 (upper electrode 36 side), a semiconductor substrate such as Si can be used for the substrate 31.

  For the lower electrode 32 and the upper electrode 36, for example, a metal or alloy having a thickness of 0.5 μm, a conductive oxide, or the like can be used. The lower electrode 32 and the upper electrode 36 are preferably transparent electrodes such as ITO (Indium Tin Oxide) when taking out a single photon.

The first insulating layer 33 and the second insulating layer 35 are made of, for example, a nitride insulating material or an oxide insulating material having a thickness of 0.5 μm. Examples of the nitride insulating material include Si ₃ N ₄ and AlN. Further, as the oxide insulating _{material, SiO 2, Ta 2 O 5} , and TiO ₂ and the like. The first insulating layer 33 and the second insulating layer 35 may be a laminated thin film including a plurality of layers.

  The phosphor layer 34 is made of, for example, a phosphor base material 34a having a thickness of 1 μm, and is selected from the same materials as the phosphor base material described in the first embodiment.

  The metal ion 13 is selected from the same metal ions as those described in the first embodiment. Only one metal ion 13 exists in the phosphor layer 34 within a predetermined range. The predetermined range is a range of the phosphor layer 34 sandwiched between the lower electrode 32 and the upper electrode 36 via the first insulating layer 33 and the second insulating layer 35.

The phosphor layer 34 can be arbitrarily combined with various phosphor matrix materials 34a and metal ions 13, and in particular, a thiogallate-based material represented by XGa ₂ S ₄ (X = Sr, Ca, And a phosphor base material 34a selected from the group consisting of a thioaluminate-based material represented by XAl ₂ S ₄ (X is the same as above) and Zn ₂ SiO ₄ , Eu ²⁺ , Ce ³⁺ , and Mn ²⁺ are preferably combined with one type of metal ion 13.

In addition, one phosphor base material 34a selected from ZnS, CaS, SrS, CdS, GaN, ZnO, ZnSe, BaS, and AlN, and Eu ²⁺ , Ce ³⁺ , and Mn ²⁺ You may combine with the 1 type of metal ion 13 selected from these.

Here, as an example of the metal ion 13, the light emission mechanism will be described in the case of Mn ^{2+ causing} an inner shell transition. The electron configuration of the ground state and the excited state of Mn ²⁺ is the same as follows.

Mn ²⁺ : (1s) ² (2s) ² (2p) ⁶ (3s) ² (3p) ⁶ (3d) ⁵
Excitation and emission of Mn ²⁺ are caused by electronic transitions in the multi-electron level in the (3d) ⁵ incomplete shell. That is, excitation is caused by a transition of one electron from the ground state ⁶ A ₁ to the excited state ⁴ T ₁ . On the other hand, light emission is caused by the transition of one electron from the excited state ⁴ T ₁ to the ground state ⁶ A ₁ .

FIG. 8 is a diagram showing an emission spectrum of Mn ²⁺ . 8, later in single photon generating unit 38 that includes one Mn ²⁺ in the form of substituting the Zn ²⁺ in ZnS film (thickness 1 [mu] m) by forming method described, a frequency 1 kHz, the peak value Is an emission spectrum obtained by applying a pulsed input voltage signal of 120 V (electric field intensity of 1.2 × 10 ⁸ V / m).

Referring to FIG. 8, Mn ²⁺ exhibits a broad emission spectrum having a peak wavelength of about 580 nm due to the transition from the excited state ⁴ T ₁ to the ground state ⁶ A ₁ . In this way, Mn ²⁺ generates a single photon by the transition from the excited state of one electron to the ground state. The transition between the ground state ⁶ A ₁ and the excited state ⁴ T ₁ is a spin-forbidden transition of electrons, and the spin of one electron transitioning between the ground state ⁶ A ₁ and the excited state ⁴ T ₁ is Invert. Therefore, the light emission lifetime is much longer than that of the other metal ions 13 and is about 1 to 10 milliseconds. Therefore, as will be described below, when Mn ²⁺ is used as the metal ion, the single photon generator 30 can generate single photons more reliably. Further, since the temperature quenching of Mn ²⁺ is small, it can emit light at room temperature.

  Next, the operation of the single photon generator 30 will be described.

  FIG. 9 is a diagram for explaining the operation of the single photon generation device according to the third embodiment. Note that the horizontal axis of FIG. 9 represents time linearly, and the vertical axis of FIG. 9C represents the light emission probability logarithmically.

Referring to FIG. 9 together with FIG. 7, the single photon generator 30 has a pulsed input between the lower electrode 32 and the upper electrode 36 from the pulse power supply unit 39 and has an electric field intensity of about 10 ⁸ V / m, for example. A voltage signal (FIG. 9A) is applied. For example, when a positive voltage is applied to the upper electrode 36 with respect to the lower electrode 32 by this input voltage signal, a large number of electrons are emitted from the interface between the first insulating layer 33 and the phosphor layer 34, and the phosphor layer 34. Electrons are conducted inside. The electrons are accelerated by a high electric field of about 10 ⁸ V / m, and high-speed electrons (hereinafter referred to as “hot electrons”) move toward the upper electrode 36. As shown in FIG. 9B, hot electrons are conducted in the phosphor layer 34 during the pulse width Tc. Here, Tc represents the pulse width of hot electrons. Tc indicates the pulse width used to excite a single metal ion, similarly to the pulse width of the excitation light indicated by Ta in FIG. In this way, hot electrons and the excitation light shown in FIG. 3A have the same function in principle, but the definition of time is different. Therefore, here, the pulse width of hot electrons is Tc, and Ta is different from Ta. Separately written.

After a short time ΔT ₂ from the start of conduction of hot electrons, one hot electron collides with the metal ion 13. Then, one electron of the metal ion 13 immediately enters an excited state and starts relaxation. However, since the emission lifetime Tb shown in FIG. 9C is longer than the pulse width Tc, There is no transition to the ground state as a random event.

  After the hot electron conduction is turned off, the metal ion 13 returns to the ground state and generates an output light pulse SP consisting of a single photon. At that time, even if a plurality of hot electrons collide with the metal ions 13 during the time Tc during which the input voltage signal is applied, the metal ions 13 have already started to relax from the excited state. Is never excited again. After all the hot electrons have passed through the position of the metal ion 13, the metal ion 13 transitions to the ground state and generates an output light pulse of only one photon with the light emission probability shown in FIG. 9 (D)).

Here, the moving time of hot electrons is set to be shorter than the light emission lifetime Tb of the metal ions 13. The movement time of hot electrons is the time from when the input voltage signal is turned on until all hot electrons pass through the positions of the metal ions 13, and is several microseconds or less. On the other hand, the emission lifetime Tb of the metal ion 13 is about 1 to 10 milliseconds in the case of Mn ²⁺ , for example. Therefore, all the hot electrons can pass through the position of the metal ion 13 within the time of the emission lifetime Tb of the metal ion 13, and the single photon SP can be generated. That is, in the case of Mn ²⁺ , all the hot electrons can sufficiently pass through the position of the metal ion 13 within the light emission lifetime, and the single photon SP can be reliably generated.

  The pulse width of the input voltage signal is not particularly limited. This is because even if the pulse width of the input voltage signal is set long, if all hot electrons pass through the position of the metal ion 13 under a single polarity of the electric field, the metal ion 13 is not changed thereafter. This is because there is no influence.

  Further, by continuously applying an input voltage signal having a polarity opposite to that of the previous input voltage signal from the pulse power supply unit 39 between the lower electrode 32 and the upper electrode 36, the reverse of the case of the previous input voltage signal. Hot electron movement occurs in the direction. For example, when a negative voltage is applied to the upper electrode 36 with respect to the lower electrode 32, a large number of electrons accumulated at the interface between the second insulating layer 35 and the phosphor layer 34 move toward the lower electrode 32 (FIG. 9). (B)). Thereby, a single photon is generated by the same action as before. As described above, the single photon generator 30 can continuously generate single photons at a desired timing by supplying an input voltage signal with alternating voltage polarities at a desired timing. A single photon SP can be reliably generated by a pulse of an input voltage signal.

  Next, a method for forming the single photon generator 38 will be described with reference to FIG. FIGS. 10A and 10B are cross-sectional views showing a process for forming a single photon generator.

First, in the process of FIG. 10A, an ITO film (for example, a thickness of 0.5 μm) is formed as a lower electrode 32 on a glass substrate 31 (for example, product name 1737 glass manufactured by Corning) by sputtering or vapor deposition. Further, a Ta ₂ O ₅ film (for example, a thickness of 0.5 μm) is formed as the first insulating layer 33.

In the step of FIG. 10A, the glass substrate 31 formed up to the Ta ₂ O ₅ film is further placed on the MBE apparatus, and the substrate temperature is 120 ° C. to 160 ° C. in a vacuum atmosphere (for example, a vacuum degree of 10 ⁻⁷ Pa). Set to the range. Furthermore, using a ZnS cell (e.g. heating temperature 900 ° C.) as the molecular beam source, irradiating the the Ta ₂ O ₅ film by opening and closing control of the shutter molecular beam, a thickness of 0.5μm on the Ta ₂ O ₅ film ZnS A phosphor layer base layer 34a-1 made of a film is grown.

In the step of FIG. 10A, after cooling, a single metal ion 13, such as Mn ^2+, is adsorbed on the base layer 34a-1 in a vacuum atmosphere using STM without being exposed to the outside air.

  Next, in the step of FIG. 10B, the glass substrate 31 on which the structure of FIG. 10A is formed is returned to the MBE apparatus again without being exposed to the outside air, and is the same as the formation of the base layer 34a-1 described above. Under the conditions, a phosphor layer cap layer 34a-2 made of a 0.5 μm thick ZnS film is grown on the base layer 34a-1.

  In the step of FIG. 10B, the laminated body is further subjected to heat treatment to activate the ZnS film and the metal ions 13. The heat treatment condition is set to, for example, 1 hour at a temperature of 400 ° C.

In the step of FIG. 10B, after cooling, a Ta ₂ O ₅ film (for example, 0.5 μm thick) is formed on the phosphor layer 34 by sputtering or vapor deposition without exposing to the outside air. Further, an Al film (for example, a thickness of 0.5 μm) is formed as the upper electrode 36. As described above, the single photon generator 38 including the phosphor layer 34 containing one metal ion 13 is formed at a position sandwiched between the lower electrode 32 and the upper electrode 36.

  In the formation of the single photon generator 38, the steps of forming the base layer 34a-1 of the phosphor layer 34, the adsorption treatment of the metal ions 13 and the formation of the cap layer 34a-2 of the phosphor layer are as follows. A manufacturing apparatus similar to the method for forming the single photon generation unit of the first embodiment is used.

  According to the third embodiment, one metal ion 13 is placed in the range of the phosphor layer 34 sandwiched between the lower electrode 32 and the upper electrode 36 via the first insulating layer 33 and the second insulating layer 35. By arranging and applying a voltage pulse between the lower electrode 32 and the upper electrode 36, electrons collide with the metal ions 13, an inner shell transition occurs, and a single photon can be reliably generated. Furthermore, the inner shell transition of the metal ion 13 is practical because it can emit light not only at an extremely low temperature of several mK but also at about room temperature. In addition, since the structure and the light emitting action of the single photon generator are simple, the optical loss is small and the reliability is high.

  In the third embodiment, a single photon can be extracted by adopting a light emission mechanism based on the inner shell transition of the metal ion 13 in the phosphor layer 34. Single light photons cannot be extracted by other light emission mechanisms, for example, light emitted by the phosphor layer 34 (for example, light emitted by excitons).

  Moreover, the single photon generation part 38 of the third embodiment may omit either the first insulating layer 33 or the second insulating layer 35. Even in this case, a single photon can be generated by the same operation as described above.

(Fourth embodiment)
The single photon generator according to the fourth embodiment of the present invention has a single photon generator that generates single photons arranged two-dimensionally, and the single photon generator according to the third embodiment. It is an application example of a one-photon generator.

  FIG. 11 is a schematic plan view of a single photon generator according to the fourth embodiment of the present invention, and FIG. 12 is a configuration diagram of a single photon generator according to the fourth embodiment. In the figure, portions corresponding to the portions described above are denoted by the same reference numerals, and description thereof is omitted.

  Referring to FIGS. 11 and 12, a single photon generator 40 includes a substrate 31, a plurality of lower electrodes 32 extending in one direction on the substrate 31 and arranged in parallel to each other, and a first insulating layer 33. The phosphor layer 34 in which the metal ions 13 are arranged at predetermined positions, the second insulating layer 35, and the lower electrode 32 extend in a direction orthogonal to each other, and a plurality of upper electrodes 36 arranged in parallel to each other include this A single photon generator 41, a lower electrode drive circuit 42, and an upper electrode drive circuit 43 are sequentially deposited. The layer configuration of the single photon generator 41 is the same as that of the single photon generator 38 of the third embodiment shown in FIG. 7 except that a plurality of lower electrodes 32 and upper electrodes 36 are provided.

  Only one metal ion 13 is disposed in the phosphor layer 34 at a position where the lower electrode 32 and the upper electrode 36 intersect when viewed in plan. Therefore, the metal ion 13 at the position where the lower electrode 32 and the upper electrode 36 to which the input voltage signal is applied is excited, and a single photon is generated when the metal ion 13 transitions to the ground state. Therefore, a single photon can be generated in parallel by the single photon generator 41 by inputting a desired serial signal (for example, a signal carrying information) to the lower electrode drive circuit 42 and the upper electrode drive circuit 43. Therefore, for example, by combining a single photon generator with a large number of optical waveguides, a single photon can be simultaneously transmitted to each of the multiple optical waveguides, so that it can be used as a large-capacity optical transmission device.

  Moreover, it can use also as a wavelength multiplexing single photon generator by making the kind of metal ion 13 of the single photon generator 41 different from each other.

  According to the fourth embodiment, the same effect as that of the third embodiment is achieved, and one metal ion that causes an inner shell transition at each of the positions where the lower electrode 32 and the upper electrode 36 cross each other. By arranging 13, a single photon generator capable of generating single photons in parallel can be realized.

  The preferred embodiments of the present invention have been described in detail above, but the present invention is not limited to the specific embodiments, and various modifications and changes can be made within the scope of the present invention described in the claims. It can be changed.

It is a block diagram of the single photon generator which concerns on the 1st Embodiment of this invention. Is a diagram showing the excitation and emission spectra of Eu ²⁺ in SrGa ₂ S ₄ crystal. It is the schematic for demonstrating operation | movement of the single photon generator which concerns on 1st Embodiment. (A) And (B) is sectional drawing which shows the formation process of the single photon generating part of 1st Embodiment. It is a block diagram of the single photon generator which concerns on the 2nd Embodiment of this invention. (A) And (B) is sectional drawing which shows the formation process of the single photon generating part of 2nd Embodiment. It is a block diagram of the single photon generator which concerns on the 3rd Embodiment of this invention. It is a figure which shows the emission spectrum of Mn ^<2+ > in a ZnS film | membrane. It is a figure for demonstrating operation | movement of the single photon generator which concerns on 3rd Embodiment. (A) And (B) is sectional drawing which shows the formation process of a single photon generating part. It is a schematic plan view of the single photon generator which concerns on the 4th Embodiment of this invention. It is a block diagram of the single photon generation part which concerns on 4th Embodiment.

Explanation of symbols

10, 20, 30, 40 Single photon generator 11 Crystal substrate (GaAs substrate)
12 Quantum dots 12a, 22a, 34a Phosphor matrix material 12a-1, 22a-1, 34a-1 Base layer 12a-2, 22a-2, 34a-2 Cap layer 13 Metal ions 14, 24, 38, 41 Single Photon generator 15 Laser irradiation device 15A Excitation light pulse 16 Control unit 22, 34 Phosphor layer 31 Substrate (glass substrate)
32 Lower electrode 33 1st insulating layer 35 2nd insulating layer 36 Upper electrode 39 Pulse power supply part 42 Lower electrode drive circuit 43 Upper electrode drive circuit

Claims (8)

A single-photon generator having a phosphor matrix material, and a phosphor composed of one metal ion that emits light by inner-shell transition in the phosphor matrix material;
A single photon generator comprising: an excitation light pulse that excites the metal ions or an excitation unit that gives electrons conducted in the phosphor in a time shorter than the light emission lifetime of the metal ions.   The single-photon generator according to claim 1, wherein the excitation light pulse has a wavelength shorter than the emission wavelength of the metal ion. A single-photon generator including a phosphor matrix material, a phosphor having one metal ion that emits light by inner shell transition at each predetermined position in the phosphor matrix material, and at least one of the predetermined areas A single photon generator comprising an excitation unit that gives electrons that excite the metal ions by colliding with metal ions at a position,
The single photon generator includes a substrate, a plurality of lower electrodes extending in one direction on the substrate and arranged in parallel to each other, the phosphor formed on the lower electrode, and the phosphor a plurality of upper electrodes arranged parallel to one another extending in the direction perpendicular to the lower electrode is formed in a top, and between the phosphor and the upper electrode of the fluorescent body and the lower electrode An insulating layer formed on at least one of the
Said predetermined position, said when single photon generating unit viewed from a single photon generating apparatus, wherein a respective said lower electrode and said upper electrode is a position intersecting. The single-photon generator according to any one of claims 1 to 3, wherein the phosphor includes quantum dots, and the metal ions are contained in one quantum dot. The single-photon generator according to any one of claims 1 to 3, wherein the phosphor is a layered phosphor layer, and the metal ion is contained in the phosphor layer. The single photon generator includes a lower electrode, a layered phosphor formed on the lower electrode, an upper electrode formed on the phosphor, a space between the lower electrode and the phosphor, and fluorescence. An insulating layer formed between at least one of the body and the upper electrode,
4. The excitation unit according to any one of claims 1 to 3, wherein an electron conducted through the phosphor collides with a metal ion by a pulse voltage applied between a lower electrode and an upper electrode . The single photon generator according to one item . The metal ions are Ag, Au, As, Bi, Cd, Ce, Cr, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ge, Ho, Hg, In, Mn, Nd, Pb, Pr, Sb. , Sm, Sn, Tb, Tm , Ti, Tl, V, W, Yb, of the group consisting of Zn, one of claims 1-6, characterized in that the ions of any one element is selected The single photon generator according to any one of the above. The phosphor base material is a thiogallate-based material represented by a general formula of XGa ₂ S ₄ , a thioaluminate-based material represented by a general formula of XAl ₂ S ₄ , Zn ₂ SiO ₄ , BaMg ₂ Al ₁₆ O _27. , ZnS, CaS, SrS, CdS , GaN, ZnO, ZnSe, BaS, and one of claims 1-7, characterized in that of the group consisting of AlN is any one single of any one claim One-photon generator (where X is one selected from Sr, Ca, and Ba).

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