JP5765783B2 - Quantum entangled photon pair generation method - Google Patents

Description

The present invention relates to entangled photon pairs onset UBUKATA method for generating a photon pair of entangled state.

  2. Description of the Related Art Conventionally, techniques for generating entangled photon pairs have been developed in order to realize quantum information processing with extremely high processing speed and use it for highly secure quantum information communication. Quantum entanglement indicates an entangled state with a strong correlation that cannot be distinguished from each other. As a method of generating such a entangled photon pair, there is a method of down-converting laser light (see Non-Patent Document 1). However, this method has the following problems. First, the timing of generating photon pairs cannot be controlled. In addition, only one photon pair cannot be generated at a time, and a plurality of photon pairs are always generated with a certain probability. Also, the speed of generating photon pairs is slow.

P. G. Kwiat et al., "New high-intensity source of polarization-entangled photon pairs", Physical Review Letters, Vol.75, No.24, pp.4337-4342, 1995. A. J. Hudson et al., "Coherence of an Entangled Exciton-Photon State", Physical Review Letters, Vol.99, No.26, 266802, 2007.

  In contrast to the technique described above, a technique for generating one photon pair using a semiconductor quantum dot has been proposed (see Non-Patent Document 2). However, in this technique, it is not possible to control the state in which two photons are generated simultaneously, and the two generated photons are generated with a time difference from each other and have different photon energies. For this reason, the technique of Non-Patent Document 2 has a problem that the two generated photon pairs are not necessarily entangled photon pairs, and the entangled photon pairs cannot be generated intentionally (controlled). there were.

  The present invention has been made to solve the above problems, and an object of the present invention is to enable one quantum entangled photon pair to be generated in a controlled state.

Quantum entangled photon pair generating device includes a first semiconductor layer made of a semiconductor of n-type formed on the superconductor layer made of a superconductor, i-type formed on the first semiconductor layer At least a second semiconductor layer made of the semiconductor, a third semiconductor layer made of a p-type semiconductor formed on the second semiconductor layer, and one quantum dot formed in the second semiconductor layer. .

  In addition, the entangled photon pair generation method according to the present invention includes a first semiconductor layer made of an n-type semiconductor formed on a superconductor layer made of a superconductor, and a first semiconductor layer. The formed second semiconductor layer made of i-type semiconductor, the third semiconductor layer made of p-type semiconductor formed on the second semiconductor layer, and one quantum formed in the second semiconductor layer An element including at least a dot and having a depletion layer formed in the second semiconductor layer by a built-in potential of a pin structure including the first semiconductor layer, the second semiconductor layer, and the third semiconductor layer is produced to form a superconductor layer The first step of allowing the electron cooper pair to penetrate into the first semiconductor layer and the first light continuous to the element from the third semiconductor layer side are irradiated to reduce the built-in potential of the pin structure, thereby depleting the depletion layer. Second step to reduce and first light irradiation In this state, the depletion layer is further reduced by irradiating the device with the second light from the third semiconductor layer side, one electron cooper pair is disposed in the conduction band of the quantum dot, and generated by the first light irradiation. At least a third step of generating a pair of quantum entangled photons by arranging one hole pair in the valence band of the quantum dot, and the irradiation with the second light includes a new pair of quantum entangled photons Stop before it occurs.

  As described above, according to the present invention, it is possible to obtain an excellent effect that one quantum entangled photon pair can be generated in a controlled state.

FIG. 1 is a configuration diagram illustrating a configuration of a entangled photon pair generating element according to an embodiment of the present invention. FIG. 2 is a flowchart for explaining a method of generating entangled photon pairs in the embodiment of the present invention. FIG. 3 is a configuration diagram illustrating a configuration example of the entangled photon pair generation device according to the embodiment of the present invention. FIG. 4 is a characteristic diagram showing the relationship between the element diameter of the columnar element portion of the entangled photon pair generating element and the photon extraction efficiency. FIG. 5 is a characteristic diagram showing changes in the photoelectromotive force and the photocurrent with respect to the power of the excitation light input to the entangled photon pair generating element in the embodiment of the present invention.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram illustrating a configuration of a entangled photon pair generating element according to an embodiment of the present invention. FIG. 1 schematically shows a cross section of the entangled photon pair generating element.

  The quantum entangled photon pair generating element is formed on a first semiconductor layer 102 made of an n-type semiconductor formed on a superconductor layer 101 made of a superconductor, and on the first semiconductor layer 102. A second semiconductor layer 103 made of i-type semiconductor, and a third semiconductor layer 104 made of p-type semiconductor formed on the second semiconductor layer 103. In addition, one quantum dot 105 formed in the second semiconductor layer 103 is provided. The superconductor layer 101 is formed on the support substrate 111.

  The superconductor layer 101 may be made of, for example, Nb and may be formed with a layer thickness of about 100 to 200 nm. The first semiconductor layer 102 may be made of, for example, n-type InGaAlAs and may have a layer thickness of about 50 to 300 nm. The third semiconductor layer 104 may be made of, for example, p-type InGaAlAs and may have a layer thickness of about 50 to 300 nm.

  The second semiconductor layer 103 may be made of non-doped (i-type) InGaAlAs, and the quantum dots 105 may be made of InAs. For example, the second semiconductor layer 103 includes a first barrier layer 131 made of i-type InGaAlAs and a second barrier layer 132 made of i-type InGaAlAs, and the first barrier layer 131 and the second barrier layer 132 are Quantum dots 105 are formed between them. The first barrier layer 131 and the second barrier layer 132 may be formed to have a thickness of about 10 to 100 nm.

  Here, a method for manufacturing the element in the present embodiment will be briefly described. For example, using an InP substrate, p-type InGaAlAs (third semiconductor layer 104) and i-type InGaAlAs (second barrier layer 132) are epitaxially grown on the InP substrate. Subsequently, InAs is grown on the i-type InGaAlAs in an island shape under a predetermined condition such as a condition that does not result in a continuous film shape.

  Subsequently, i-type InGaAlAs (first barrier layer 131) and n-type InGaAlAs (first semiconductor layer 102) are crystal-grown. These may be performed by a well-known metal organic chemical vapor deposition method, molecular beam epitaxy method or the like. Next, Nb is deposited on the uppermost n-type InGaAlAs (first semiconductor layer 102), for example, by vapor deposition. Through the above steps, a semiconductor superconductor laminated structure of a p-InGaAlAs layer, an i-InGaAlAs layer, an InAs island-like layer, an i-InGaAlAs layer, an n-InGaAlAs layer, and an Nb layer is obtained.

  As described above, by using InGaAlAs, each semiconductor layer can be crystal-grown in a lattice-matched state with the InP substrate. Also, by changing the ratio of each composition of InGaAlAs, the band gap energy can be varied in a wide range from the band gap energy of InGaAs of 0.8 eV to the band gap energy of InAlAs of 1.53 eV, and the emission wavelength of InAs quantum dots Is easy to change.

  Next, the above-described semiconductor superconductor laminated structure is attached on the support substrate 111. The Nb layer is attached to the support substrate 111. As the support substrate 111, for example, a glass substrate or a semiconductor substrate may be used. After pasting in this way, the InP substrate is removed. For example, the InP substrate is polished and thinned to a thickness of 50 μm, and the thinned InP substrate is removed by wet etching.

  Next, the semiconductor multilayer structure is patterned by a known lithography technique and reactive ion etching to form a columnar element portion including the first semiconductor layer 102, the second semiconductor layer 103, and the third semiconductor layer 104. The columnar element portion may be formed in a cylindrical shape having a diameter of 300 to 600 nm, for example. As described above, the quantum entangled photon pair generating element in the present embodiment can be obtained.

  Here, in the InAs island-shaped layer described above, a plurality of InAs island portions are formed on the i-InGaAlAs layer. Therefore, in the semiconductor multilayer structure, a plurality of quantum dots are provided. On the other hand, as described above, by patterning in a columnar shape with a predetermined diameter, it is possible to make one quantum dot exist in the columnar element portion. In other words, the diameter of the columnar element portion may be appropriately set so that one quantum dot exists in the columnar element portion.

  Next, a method for generating entangled photon pairs by the above-described entangled photon pair generating element will be described with reference to the flowchart of FIG. In addition, the quantum entangled photon pair generation method described below is performed in a state where the element is cooled to a temperature (9.2 K) at which the superconductor (Nb) constituting the superconductor layer 101 exhibits superconductivity. Is.

  First, in step S201, an electron cooper pair is made to penetrate into the first semiconductor layer 102 from the superconductor layer 101 of the entangled photon pair generating element. As is well known as the superconducting proximity effect, when a superconductor such as Nb is brought into contact with a semiconductor having a low Schottky barrier height such as InGaAs, the electron cooper pair in the superconductor is Fermi energy in the semiconductor. Along the semiconductor.

  However, in the above-described quantum entangled photon pair generating element, a depletion layer is formed in the second semiconductor layer 103 by the built-in potential of the pin structure including the first semiconductor layer 102, the second semiconductor layer 103, and the third semiconductor layer 104. Yes. For this reason, the above-described electron Cooper pair cannot penetrate into the quantum dots 105 in the second semiconductor layer 103.

  Next, in step S202, the device (quantum entangled photon pair generating device) is irradiated with continuous first light from the third semiconductor layer 104 side to reduce the built-in potential of the pin structure and reduce the depletion layer. Let For example, a semiconductor laser is used as the light source, and irradiation with the first light having a wavelength of 980 nm is started. By this light irradiation, electrons and holes are generated by photoexcitation in the pin structure, and electrons are separated into the first semiconductor layer 102 and holes are separated into the third semiconductor layer 104 by the built-in potential.

  In this way, when the electrons and holes that have been generated by light and separated in space accumulate in the first semiconductor layer 102 and the third semiconductor layer 104, respectively, the built-in potential is canceled and reduced by the generated photovoltaic force. Go. In this state, the electron Cooper pair that has entered the first semiconductor layer 102 enters a state near the quantum dots 105 due to the reduction of the depletion layer accompanying the decrease in the built-in potential described above.

  Here, in the irradiation with the first light, it is important that the electron Cooper pair that has entered from the superconductor layer 101 enters a state in which it enters close to the quantum dots 105. At this time, the electron Cooper pair is not in a state of entering the quantum dot 105 or a state in which the electron Cooper pair is too far from the quantum dot 105. In other words, by controlling the irradiation state of the first light, the electron Cooper pair is controlled to enter into the vicinity of the quantum dot 105 (DC bias).

  Next, in step S203, irradiation of the second light is started from the third semiconductor layer 104 side to the element while the first light is irradiated as described above. When this second light irradiation is performed for a preset time (y in step S204), the second light irradiation is stopped (step S205). In addition, by repeating Step S203 to Step S205 at a predetermined time interval, so-called pulsed light is emitted. The depletion layer is further reduced by the irradiation of the second light described above. As a result, the electron Cooper pair that has entered close to the quantum dot 105 enters a state.

  As a result, an electron Cooper pair is arranged in the conduction band of the quantum dot 105, and one hole pair generated by the irradiation of the second light is arranged in the valence band of the quantum dot 105. Quantum entangled photon pairs are generated by causing luminescence recombination of pairs. Thus, according to the present embodiment, since one electron cooper pair and one hole pair are recombined to emit light, two photons are generated simultaneously, and a entangled photon pair is generated. It becomes like this. Further, the above-described state is controlled by the on / off (pulse) of the second light irradiation, and the entangled photon pair can be generated in an intended state.

  Here, it is important to stop the irradiation of the second light before entering a state where a new entangled photon pair is generated (can be generated). For example, a semiconductor laser may be used as the light source, and the second light having a wavelength of 1300 nm and a pulse width of 50 ps may be irradiated. The pulse width may be, for example, an interval for stopping before the light emission recombination described above occurs. In addition, since the light emission lifetime in the quantum dot 105 is about 1 ns, the second light may be a pulse shorter than this, and may be, for example, a pulse light with a width of 100 ps. In this way, a pair of entangled photon pairs can be generated for each pulse of the second light. Also, entangled photon pairs can be generated at high speed by high-speed pulse excitation. The excitation wavelength can be excited at any wavelength as long as it is shorter than the emission wavelength of the quantum dots 105. The repetition frequency of the pulse can be about 1 GHz which is the reciprocal of the light emission lifetime at the maximum.

  Next, an apparatus for realizing the above-described quantum entangled photon pair generation method will be described. FIG. 3 is a configuration diagram illustrating a configuration example of the entangled photon pair generation device. In this apparatus, the first light emitted from the first light source 301 is guided by the single mode optical fiber 302 and the optical multiplexer 303, reflected by the semi-reflective mirror 304, guided by the single mode optical fiber 305, and collected. The light is collected by the optical lens 306 and enters the quantum entangled photon pair generating element 311. The first light source 301 is, for example, a semiconductor laser that emits continuous first light having a wavelength of 980 nm.

  As described above, the second light emitted from the second light source 307 is guided by the single mode optical fiber 308 while the first light is incident on the quantum entangled photon pair generating element 311 and is optically coupled. The light is combined with the first light by the waver 303, reflected by the semi-reflective mirror 304, guided by the single mode optical fiber 305, collected by the condenser lens 306, and incident on the quantum entangled photon pair generating element 311. The second light source 307 is, for example, a semiconductor laser that emits second light that is pulsed light having a wavelength of 1300 nm and a pulse width of 50 ps.

  As described above, when the second light is incident in addition to the first light, the quantum entangled photon pair generated by the quantum entangled photon pair generating element 311 passes through the condenser lens 306 and is simply transmitted. The light is guided by the one-mode optical fiber 305, transmitted through the semi-reflective mirror 304, guided by the single-mode optical fiber 309, and detected by the superconducting single-photon detector 310.

  Here, as shown in the calculation result of FIG. 4, by configuring the condenser lens 306 from a lens having a numerical aperture (NA) of 0.8, high photon pair extraction efficiency can be obtained. For example, as can be seen from FIG. 4, when the element diameter of the columnar element portion of the entangled photon pair generating element is 400 nm, the photon pair extraction efficiency can be set to 80% and maximized. The calculation for obtaining the result shown in FIG. 4 uses an FDTD (finite difference time domain) method. This calculation method is known to obtain a strictly accurate calculation result because Maxwell's basic differential equation related to the electromagnetic field is calculated as it is.

  Next, FIG. 5 shows the results of measurement of photoelectromotive force and photocurrent by photoexcitation in the method of generating entangled photon pairs using the entangled photon pair generating element in this embodiment. FIG. 5 is a characteristic diagram showing changes in the photovoltaic power and the photocurrent with respect to the power of the input excitation light. From FIG. 5, it can be seen that the built-in potential is canceled by the photovoltaic power generated by photoexcitation of about 0.3 mW, and the photocurrent begins to flow.

  As described above, in the present invention, one quantum dot is provided in the i layer (second semiconductor layer) in the pin structure, and the superconductor layer is provided in contact with the n layer (first semiconductor layer). A state in which holes are accumulated in the p-layer (third semiconductor layer) by light irradiation and an electron Cooper pair can enter near the quantum dot, and one hole pair is generated in the quantum dot by pulse light irradiation. I tried to make it. As a result, according to the present invention, one entangled photon pair can be generated in a controlled state.

  Thus, according to the present invention, the quantum level of a semiconductor quantum dot, also called an artificial atom, is used, and an electron Cooper pair is distributed in the conduction band of the quantum dot and one hole pair is distributed in the quantum level of the valence band. Thus, a semiconductor light emitting device that generates only one pair of entangled photon pairs at a time can be realized.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious. For example, the columnar element portion is not limited to a cylindrical shape, and may be formed in a rectangular column state.

  DESCRIPTION OF SYMBOLS 101 ... Superconductor layer, 102 ... 1st semiconductor layer, 103 ... 2nd semiconductor layer, 104 ... 3rd semiconductor layer, 105 ... Quantum dot, 111 ... Support substrate, 131 ... 1st barrier layer, 132 ... 2nd barrier layer.

Claims (1)

A first semiconductor layer made of an n-type semiconductor formed on a superconductor layer made of a superconductor, and a second semiconductor layer made of an i-type semiconductor formed on the first semiconductor layer And a third semiconductor layer made of a p-type semiconductor formed on the second semiconductor layer, and one quantum dot formed in the second semiconductor layer, the first semiconductor layer , An element in which a depletion layer is formed in the second semiconductor layer by a built-in potential of a pin structure including the second semiconductor layer and the third semiconductor layer, and the first semiconductor layer is formed from the superconductor layer. A first step of intruding an electronic Cooper pair against
A second step of reducing the depletion layer by irradiating the device with first continuous light from the side of the third semiconductor layer to reduce the internal potential of the pin structure;
In the state of irradiating the first light, the depletion layer is further reduced by irradiating the element with the second light from the third semiconductor layer side, and one electron in the conduction band of the quantum dot And at least a third step of generating a entangled photon pair by arranging a Cooper pair and arranging one hole pair generated by the irradiation of the first light in the valence band of the quantum dot. ,
The method of generating a entangled photon pair, wherein the irradiation of the second light is stopped before a new quantum entangled photon pair is generated.

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