JP6399508B2 - Photon generator and photon generation method - Google Patents

Description

  The present invention relates to a photon generator and a photon generation method for generating a single photon or entangled photon pair.

  In recent years, there is an increasing expectation for quantum information technology that manipulates the quantum state of light and materials and uses it for information processing. The quantum state of light can be manipulated by finely controlling the interaction between light and matter. Typical applications of quantum information technology are quantum cryptography (quantum key distribution) and quantum computation. By realizing these, it is possible to realize highly secure information communication and high-speed computation with a specific algorithm. The foundation of quantum information technology is the creation of controlled quantum states. In particular, it is extremely important to generate single-photon and entangled photon pairs with good controllability because the possibility of application to quantum cryptography communication is expanded.

  Many single-photon and entangled-photon pair generators using semiconductor quantum dots have been studied from the viewpoints of solid materials, stable operation, and emission wavelength control. To date, many single photon generators have been realized from the near-ultraviolet wavelength region to the wavelength band of optical fiber communications. In advanced quantum cryptography communication, in addition to suppressing noise and improving light extraction efficiency, high coherence is desired.

As a method for preventing noise (S / N degradation) due to scattering of excitation light,
(1) A method of increasing the S / N by controlling the polarization of the excitation laser having a narrow line width and separating the polarization of the excitation light and emission (for example, see Non-Patent Document 1),
(2) A method in which excitation light is propagated through a waveguide and a generated single photon is taken out as leakage light from the waveguide (for example, see Non-Patent Document 2).
Etc. have been proposed.

However, even if the method (1) is used,
(a) S / N at the saturation point of single photon generation is poor due to scattered light,
(b) A CW laser is used to narrow the excitation beam width, and on-demand single photon generation cannot be realized.
(c) The degree of freedom in designing the optical system is reduced to suppress scattered light with polarized light.
Issues such as remain.

Even if the method (2) is used,
(a) There is emission of excitation light leaking from the waveguide, and the S / N of single photon detection cannot be sufficiently suppressed.
Issues such as remain.

  The semiconductor material is irradiated with two parent photons to generate exciton molecules with zero angular momentum by two-photon resonance excitation, and the generated exciton molecules are simultaneously split into two photons. There is known a method of generating a entangled photon pair having the same wavelength as that of the parent photon (see, for example, Patent Document 1). In this method, entangled photon pairs are generated using nonlinearity of bulk semiconductor crystals and bulk exciton molecules, but an incident light pulse with a certain intensity or more is required to cause nonlinear effects. Also, the generated entangled photon pair does not necessarily become a single entangled photon pair for one pulse event, and therefore does not satisfy the on-demand property.

Japanese Patent No. 3649408

Phys. Rev. Lett. 108, 093602 (2012), Appl. Phys. Lett. 99, 261904 (2011) Phys. Rev. B 78, 073301 (2008)

  In order to realize generation of single photons having high coherence, it is considered effective to suppress energy dissipation during recombination emission of excitons and exciton molecules. However, when photoexcitation is performed using an optical pulse that completely resonates with the emission energy of the quantum dots, the energy of the excitation light and the energy of the generated single photons become the same. Therefore, S / N deteriorates due to scattering of excitation light when taking out a single photon.

  It is an object of the present invention to provide a photon generator and a photon generation method that can reduce noise due to energy dissipation of excitation light while realizing high coherence by resonance light excitation.

In order to solve the above problems, the photon generator is
A single quantum dot formed on a semiconductor substrate;
An excitation light source for two-photon resonance excitation of the exciton molecular state of the quantum dots;
An optical filter for separating at least one of exciton recombination luminescence and exciton molecule recombination luminescence from the quantum dots from excitation light from the excitation light source;
With
Recombination energy E _X of excitons of the quantum _dots, the recombination energy E _BX exciton molecules of the quantum _dots, the energy of the excitation light and E _P, the excitation light source,
E _P = (E _X + E _BX ) / 2
The quantum dots are irradiated with excitation light that satisfies the above conditions, and the exciton molecule state is resonantly excited.

  By suppressing energy dissipation during recombination emission of a single photon or entangled photon pair, photon generation with high coherence can be realized.

As principles of the embodiments, the excitation light energy (E _P), exciton recombination energy (E _X), is a diagram showing the relationship between recombination energy (E _BX) exciton molecules. It is a schematic block diagram of the photon generator of 1st Embodiment. It is a figure which shows the number of excitons when exciter light time integration intensity is changed, and the number of excitons of an exciton molecular state. It is a schematic block diagram of the photon generator of 2nd Embodiment. It is a figure which shows the population of an exciton molecule when an excitation pulse time width is 20 ps using a direct modulation laser diode and exciton molecule binding energy ΔE _BX is changed from 0.5 meV to 3.0 meV. It is a schematic block diagram of the photon generator of 3rd Embodiment.

  Hereinafter, embodiments of the invention will be described with reference to the drawings. In the embodiment, one quantum dot is irradiated with resonance excitation light, and the exciton molecule state is resonance excited by two-photon excitation. In the process of relaxation from the resonantly excited exciton molecule state to the ground state, recombination luminescence of the exciton molecule and recombination luminescence of the exciton occur continuously. The recombination luminescence of the exciton molecule and the recombination luminescence of the exciton are separated from the excitation light (scattered light) separately or together. By this method, single photons or entangled photon pairs can be generated with good controllability.

FIG. 1 is a diagram illustrating a relationship among excitation light energy (E _P ), exciton recombination energy (E _X ), and exciton molecule recombination energy (E _BX ) as the principle of the embodiment.

  Quantum dots have discrete energy levels due to their quantum mechanical confinement effect. A pair of electron holes with opposite spins forms an “exciton” state (| X>), and the exciton molecular state (| BX>) is excited when two excitons are excited into the quantum dot. Form.

The excitons excited in the quantum dot are affected by the Coulomb interaction, and the recombination energy changes between the exciton state and the exciton molecular state. In FIG. 1, the recombination energy of the exciton molecular state (| BX>) is lower than the energy of (| XX>) simply doubled the exciton energy. This amount of energy change is referred to as exciton molecule binding energy ΔE _BX .

The exciton molecule binding energy ΔE _BX is represented by ΔE _BX = E _X −E _BX . Here, E _X is the recombination energy of the s-shell exciton, and E _BX is the recombination energy of the s-shell exciton molecule. In this specification and claims, the term “exciton molecule” refers to an exciton having a smaller recombination energy of ΔE _BX among two excitons constituting an exciton molecule state. Shall be pointed to.

By simultaneously absorbing two photons of the resonant light pulse applied to the quantum dot, two excitons are generated in the quantum dot and form an exciton molecular state (| BX>). At this time, the energy E _P of the resonance light pulse used is expressed by the equation (1) using the recombination energy E _X of the exciton of the s-shell and the recombination energy E _BX of the exciton molecule of the s-shell. It is represented by

E _P = (E _X + E _BX ) / 2 = E _X − (ΔE _BX / 2) (1)
According to this relationship, the two-photon resonance, recombination energy E _BX exciton molecule recombination energy E _X excitons, respectively have a difference of ± ΔE _{BX / 2} relative to the energy E _P of the pumping light pulse I understand that. Since E _P ≠ E _X , E _P ≠ E _BX , and E _P <E _X , the exciton photoexcitation due to the one-photon absorption of the excitation light pulse is suppressed.

ΔE _BX is a parameter determined by physical properties. In a typical InAs / InP quantum dot, ΔE _BX / 2 is about 1 meV, and ΔE _BX / 2 is about 0.5 meV. By selecting the wavelength of the recombination luminescence of the exciton and the recombination luminescence of the exciton molecule using an optical filter, it is possible to extract either or both of the exciton recombination luminescence and the exciton molecule recombination luminescence. it can. Thereby, scattering of excitation light can be suppressed and single photon or entangled photon pairs can be efficiently generated.

(First embodiment)
FIG. 2 is a schematic configuration diagram of the photon generator 10 of the first embodiment. The photon generator 10 includes quantum dots 13 formed on a semiconductor substrate 12, an excitation pulse generator (excitation light source) 11, and an optical filter 15. The excitation pulse generator 11 irradiates the quantum dot 13 and resonantly excites the s-shell exciton molecular state of the quantum dot 13. In the process of relaxation from the exciton molecule state to the ground state, the recombination emission of the exciton molecule and the recombination emission of the exciton are continuously generated from the quantum dot 13. The optical filter 15 separates each or both of the exciton molecule recombination emission (energy E _BX ) and the exciton recombination emission (energy E _X ) from the scattered light of the excitation light.

Here, as described with reference to FIG. 1, the energy E _P of the pumping light pulse emitted from the excitation pulse generator 11, and s- shell exciton recombination energy E _X of the quantum dots 13, the quantum dots The relationship of 13 s-shell exciton molecule recombination energy E _BX is
E _P = (E _X + E _BX ) / 2, and E _P <E _X
The two-photon absorption is performed efficiently.

  The quantum dots 13 are InAs quantum dots 13 formed on, for example, the InP substrate 13. Although not shown, a buffer layer is formed on the InP substrate 12, InAs quantum dots are formed by molecular beam epitaxial growth, and a cap layer (not shown) is formed on the quantum dots 13.

  The excitation pulse generator 11 is a pulse light source such as a semiconductor laser diode. As will be described later, the excitation pulse generator 11 has variable excitation energy (wavelength) and pulse area (time integrated intensity of excitation light: the intensity at which one exciton is generated by photon absorption is called a π pulse). Excitation light source. In the example of FIG. 2, the angle between the optical path P1 of the excitation pulse generator 11 and the photon detection path P2 of recombination light emission is set to θ, but is not limited to this example. The range of θ is not limited as long as the pulsed light incident on the semiconductor substrate 12 is not totally reflected, and the excitation pulse path P1 and the photon detection path P2 may have a coaxial relationship such as θ = 0.

When the excitation light pulse is irradiated, the carriers confined in the quantum dots 13 simultaneously absorb two photons of energy E _P to be in an exciton molecule state, and then recombination energy E _BX of the exciton molecule, releasing recombination energy E _X exciton continuously. The photon detection path P2 includes reflected and scattered light of excitation light pulses in addition to the emission of exciton molecules and the emission of excitons.

Optical filter 15 disposed in the photon detection path P2 is either or both of the recombination energy E _X of the recombination energy E _BX and excitons biexcitons, separated from the energy E _P of the pumping light pulse.

For InAs quantum dots 13 formed on the InP substrate 12, since the exciton molecules binding energy Delta] E _BX of about 1 _meV, each _{E BX} and _{E X,} a difference of about 0.5meV from the energy _{E P} of the pumping light pulse Have. Therefore, it cuts out only one of E _BX or E _X using a band-pass filter as the optical filter 15. In this case, the photon generator 10 functions as a single photon generator.

It may be configured to block E _P using a notch filter as the optical filter 15. In this case, since both the E _BX and E _X is transmitted through the optical filter 15 can be used as a photon pair generating device entangled photons generator 10.

FIG. 3 is a diagram showing the number of excitons and the number of excitons in the exciton molecular state when the excitation light time integrated intensity is changed. In resonance excitation, an arbitrary exciton molecular population can be created by controlling the time integrated intensity of excitation light. Note that the two-photon absorption in the exciton molecular state is a non-linear process that does not involve real levels, and the probability of two-photon absorption is higher when the time width of the excitation pulse is shorter and ΔE _BX is smaller. In FIG. 3, ΔE _BX / 2 is a general value of 0.5 meV, and the number of excitons and excitons in the exciton molecular state when the time width of the excitation light pulse is changed are calculated. Here, the pulse area is a time integral value of the optical pulse intensity, and the point where the number of excitons becomes 1 by one-photon absorption is defined as π. When an excitation pulse having a time width of 20 ps that can be realized by a general semiconductor laser diode or the like is used, a pulse area of 4π is required to cause two-photon absorption. When converted into the power of light at which excitons are saturated, an excitation light pulse 16 times in intensity is required, but considering the wavelength selectivity of the optical filter 15, it can be sufficiently suppressed.

  When the recombination emission of the exciton molecule is used, the emission recombination lifetime is about half that of the exciton, so that the probability of emitting light as a single photon while maintaining coherence is increased. In addition, when the quantum dots 13 having no fine structure splitting are used, an entangled photon pair with high coherence can be realized by extracting both exciton molecular emission and exciton emission with the optical filter 15.

(Second Embodiment)
FIG. 4 is a schematic configuration diagram of the photon generator 20 of the second embodiment. In the second embodiment, _ΔBX is controlled by applying an electric field. The basic configuration of the photon generator 20 is the same as that of the photon generator 10 of the first embodiment. The same components as those of the photon generator 10 of the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

The semiconductor substrate 22 has a mesa structure in a portion where the quantum dots 23 are present. By arranging the electrodes 23 on both sides of the mesa structure and applying an electric field between the electrodes 23, the exciton molecule binding energy ΔE _BX is modulated, and the light intensity of the necessary excitation light pulse is controlled.

  The mesa structure can be formed, for example, by growing a layer including the quantum dots 23 on the semiconductor substrate 22 and then etching the periphery of the quantum dots 23. The width d between the electrodes 23 is, for example, 200 to 1000 nm.

The exciton molecular binding energy ΔE _BX of the quantum dot 23 is mainly determined by the Coulomb integral energy between two electrons and two holes in the quantum dot 23, but when an electric field is applied from the outside, The overlap of wave functions changes. Therefore, the exciton molecule binding energy ΔE _BX changes.

In general self-forming quantum dots, the size in the horizontal direction is larger than the size in the height direction. When an electric field in the lateral direction (direction parallel to the substrate) is applied to the quantum dots 23, a large difference occurs in the displacement of the center of gravity of the wave function of electrons and holes due to the difference in effective mass between electrons and holes. In the second embodiment, ΔE _BX can be changed by about 1 meV by introducing a lateral electric field of about 30 kV / cm (applied voltage conversion: about 0.6 to 3.0 V). By controlling the excitation energy of the excitation pulse generator 11 according to the change in ΔE _BX, the probability of occurrence of two-photon absorption can be controlled.

FIG. 5 is a diagram showing the population of exciton molecules when exciton molecule binding energy ΔE _BX is changed from 0.5 to 3.0 meV with a predetermined excitation pulse time width. In the example of FIG. 5, the excitation pulse time width is set to 20 ps, which is a typical time width when a directly modulated laser diode is used.

When ΔE _BX is changed from 1.0 meV to 2.0 meV, a pulse area of 4π is required to generate exciton molecules by two-photon absorption, but a pulse area of 5.5π is required. come. This means a change from 16 times the exciton saturation intensity to 30 times the intensity. However, since the binding energy of the exciton molecule has expanded to 2.0 meV, the energy difference between the excitation light pulses E _P and E _X or the energy difference between E _P and E _BX becomes larger, and discrimination by the optical filter 15 is performed. Becomes easier. Therefore, single photon emission with less noise can be extracted. By controlling ΔE _BX by this electric field, two-photon absorption with low noise matched to the wavelength width of the excitation light pulse and the transmission wavelength characteristic of the filter can be realized. Further, when the optical filter 15 is used as a notch filter to block the scattered light of the excitation light pulse, the entangled photon pair maintaining coherence can be taken out.

(Third embodiment)
FIG. 6 is a schematic configuration diagram of the photon generator 30 of the third embodiment. In the third embodiment, a photon generator 30 is provided in which ΔE _BX control by applying an electric field and light extraction efficiency are enhanced. The configuration of the basic part of the photon generator 30 is the same as that of the photon generator 10 of the first embodiment. The same components as those of the photon generator 10 of the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

  In the third embodiment, photoexcitation is performed from the back surface 32 a of the semiconductor substrate 32, and light emitted from the quantum dots 33 is extracted outside the semiconductor substrate 32. After a layer including quantum dots 33 is grown on the semiconductor substrate 32 via a buffer layer (not shown), the semiconductor substrate 32 is processed into a conical shape, and the electrodes 34 are arranged on the side surfaces. Photoexcitation and light detection are performed from the back surface 32a of the semiconductor substrate 32. The quantum dot 33 exists at a position away from the apex of the cone by about 50 to 300 nm, for example. Light emitted from the quantum dots 33 is reflected by the conical side wall of the semiconductor substrate 32 and most of the light returns in the direction of the back surface 32a of the semiconductor substrate 32, so that light emitted from the quantum dots 33 can be efficiently extracted to the outside.

  The substrate back surface 32a may be mirror-finished to improve light transmission. Alternatively, an antireflection film 36 may be formed to suppress reflection loss.

The distance d between the electrodes 34 at the bottom of the cone is about 1 μm. By applying a voltage between the electrodes 34, an electric field is applied to the quantum dots 33 in the lateral direction (the direction parallel to the substrate surface of the semiconductor substrate 32), and the overlap of the wave functions of electrons and holes is changed to set ΔE _BX Control. Since ΔE _BX is increased to accurately separate the two photons from the scattered light of the excitation light and the required increase in excitation power is a trade-off, the range of ΔE _BX is 1.0 to 2.0 meV. It is desirable to control so that

  Exciton molecule recombination emission and exciton recombination emission appearing in the photon detection path P2 from the back surface 32a of the semiconductor substrate 32 are output from the photon generator 30 as a single photon or entangled photon pair by the optical filter 15. .

As described above, the energy E _P of the resonance excitation light can be separated from the recombination energy E _BX of the exciton molecule and the recombination energy E _X of the exciton by the configuration and method of the embodiment. Further, high coherence can be realized by suppressing energy dissipation in the recombination emission process of s-shell excitons and s-shell exciton molecules. This realizes an on-demand high-coherence photon generator that suppresses the influence of excitation light scattering regardless of the optical structure.

  In the case where the recombination emission of the exciton molecule is taken out as a single photon, the emission recombination time is half that of the exciton emission, so that it is possible to generate a single photon that can generate photons at a higher repetition rate. By taking out both exciton light emission and exciton molecular light emission using a quantum dot having a high structure symmetry, it becomes possible to generate an entangled photon pair having high coherence.

The photon generators 10, 20, and 30 of the embodiments are useful for generating single photons and entangled photons. In the embodiment, InAs / InP quantum dots are used. However, the present invention is not limited to this, and any quantum dots such as InAs / GaAs quantum dots, GaN / AlN quantum dots, and InP / GaP quantum dots can be used. In any case, when the recombination energy E _{X of} the s-shell exciton of the quantum dot and the recombination energy E _BX of the s-shell exciton molecule, the pulse energy E _P of the excitation light source is expressed as E _P = ( E _X + E _BX ) / 2 is selected and set. The excitation pulse generator is any excitation light source that can vary the excitation light energy or the time integrated intensity of the excitation light.

The following notes are presented for the above explanation.
(Appendix 1)
A single quantum dot formed on a semiconductor substrate (12, 22, 32);
An excitation light source for two-photon resonance excitation of the exciton molecular state of the quantum dots;
An optical filter for separating at least one of exciton recombination luminescence and exciton molecule recombination luminescence from the quantum dots from excitation light from the excitation light source;
With
Recombination energy E _X of excitons of the quantum _dots, the recombination energy E _BX exciton molecules of the quantum _dots, the energy of the excitation light and E _P, the excitation light source,
E _P = (E _X + E _BX ) / 2
A photon generator characterized by irradiating the quantum dots with excitation light that satisfies the above conditions to excite the exciton molecules in two-photon resonance.
(Appendix 2)
The photon generator according to appendix 1, wherein the excitation light source is a light source having a variable excitation light time integrated intensity.
(Appendix 3)
A pair of electrodes disposed on the sides of the quantum dots and applying an electric field in a direction parallel to the semiconductor substrate to the quantum dots;
Or an exciton molecule binding energy represented by a difference between the recombination energy of the exciton and the recombination energy of the exciton molecule can be controlled by applying the electric field. 2. The photon generator according to 2.
(Appendix 4)
The photon generator according to appendix 3, wherein the excitation light source changes the time integration intensity according to a change in the exciton molecule binding energy due to the application of the electric field.
(Appendix 5)
5. The photon generator according to any one of appendices 1 to 4, wherein the optical filter transmits one of recombination light emission of the exciton and recombination light emission of the exciton molecule.
(Appendix 6)
5. The photon generation according to any one of appendices 1 to 4, wherein the optical filter blocks the excitation light and transmits both the recombination light emission of the exciton and the recombination light emission of the exciton molecule. apparatus.
(Appendix 7)
Any one of Supplementary notes 1 to 6, wherein the back surface of the semiconductor substrate is irradiated with the excitation light, and the recombination light emission of the exciton and the recombination light emission of the exciton molecule are extracted from the back surface of the semiconductor substrate. The photon generator as described.
(Appendix 8)
A single quantum dot is irradiated with excitation light that excites two-photon resonance of the exciton molecular state of the quantum dot,
At least one of exciton recombination luminescence and exciton molecule recombination luminescence from the quantum dots is optically separated from excitation light from the excitation light source;
The recombination energy E _X of excitons of the quantum _dots, the recombination energy E _BX exciton molecules of the quantum _dots, when a and E _P energy of the excitation light, energy of the excitation light,
E _P = (E _X + E _BX ) / 2
A photon generation method characterized by satisfying:
(Appendix 9)
9. The photon generation method according to appendix 8, wherein the population of excitons in an exciton molecular state is controlled by controlling the time integration intensity of the excitation light.
(Appendix 10)
Applying an electric field perpendicular to the growth direction of the quantum dots to control the exciton molecule binding energy represented by the difference between the recombination energy of the exciton and the recombination energy of the exciton molecule, The photon generating method according to appendix 8 or 9,
(Appendix 11)
The photon generation method according to appendix 10, wherein the time-integrated intensity of the excitation light is adjusted according to a change in the exciton molecule binding energy due to the application of the electric field.

10, 20, 30 Photon generator 11 Excitation pulse generator (excitation light source)
DESCRIPTION OF SYMBOLS 15 Optical filter 12, 22, 32 Semiconductor substrate 13, 23, 33 Quantum dot 24, 34 Electrode P1 Optical path P2 of excitation light source P2 Photon detection path

Claims (4)

A single quantum dot formed on a semiconductor substrate;
An excitation light source for two-photon resonance excitation of the exciton molecular state of the quantum dots;
An optical filter for separating at least one of exciton recombination luminescence and exciton molecule recombination luminescence from the quantum dots from excitation light from the excitation light source;
With
Recombination energy E _X of excitons of the quantum _dots, the recombination energy E _BX exciton molecules of the quantum _dots, the energy of the excitation light and E _P, the excitation light source,
E _P = (E _X + E _BX ) / 2
The Biexciton state resonant excitation the excitation light satisfying by irradiating the quantum dot,
A pair of electrodes disposed on the sides of the quantum dots and applying an electric field in a direction parallel to the semiconductor substrate to the quantum dots;
The exciton molecular binding energy represented by the difference between the recombination energy of the exciton and the recombination energy of the exciton molecule can be controlled by applying the electric field. Photon generator.   The photon generator according to claim 1, wherein the excitation light source is a light source having a variable excitation light time integrated intensity. The photon generator according to claim 2 , wherein the excitation light source changes the excitation light time integrated intensity in accordance with a change in the exciton molecule binding energy due to the application of the electric field. A single quantum dot is irradiated with excitation light that excites two-photon resonance of the exciton molecular state of the quantum dot,
Wherein at least one of the recombination radiation of the recombination radiation and biexcitons of excitons from the quantum dots, and optically separated from the front Ki励 Okoshiko,
The recombination energy E _X of excitons of the quantum _dots, the recombination energy E _BX exciton molecules of the quantum _dots, when a and E _P energy of the excitation light, energy of the excitation light,
E _P = (E _X + E _BX ) / 2
Meet the,
A pair of electrodes are arranged on the sides of the quantum dots, and an electric field in a direction horizontal to the semiconductor substrate on which the quantum dots are formed is applied to the quantum dots,
A photon generation method characterized in that, by applying the electric field, exciton molecule binding energy expressed by a difference between recombination energy of the exciton and recombination energy of the exciton molecule can be controlled .

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