JP2016018853A - Photon generator and photon generation method, and control program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a highly reliable photon generator capable of enhancing the extraction efficiency of a single photon, by suppressing the blinking phenomenon.SOLUTION: A photon generator has a quantum dot unit 1 including a quantum dot 1a generating a single photon, electrodes 1A, 1B for applying a voltage to the quantum dot unit 1, an exciting light generation unit 2 for irradiating the quantum dot 1a with an exciting light pulse generated by adjusting the energy of the exciting light pulse so as to resonate with the excitation level in the quantum dot 1a, a first voltage source 3 for applying a reverse bias voltage pulse to the electrodes 1A, 1B, and a synchronous operation unit 4 for operating the exciting light generation unit 2 and first voltage source 3 synchronously so as to repeat irradiation of the exciting light pulse and application of a voltage pulse alternately.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a photon generation device, a photon generation method, and a control program provided with quantum dots.

  In recent years, there is an increasing expectation for quantum information technology that manipulates the quantum state of light and matter and uses it for information processing. Typical applications of quantum information technology are quantum cryptography communication and quantum computation, and by realizing these, high-speed computation can be realized in highly secure information communications and specific algorithms. The technology that forms the basis of these is to create a controlled quantum state. In particular, it is extremely important to generate a single photon with good controllability because it can be applied to quantum cryptography communication and the like.

  As a photon generator of an apparatus that generates a single photon (single photon generator), a semiconductor quantum dot that can realize a pseudo quantum two-level system is used. Quantum dots are particularly studied worldwide because they have advantages such as the possibility of stable operation due to solid materials and the possibility of controlling the emission wavelength by selecting materials and shapes. Hereinafter, the single photon generator is assumed to use quantum dots.

  There are two types of driving methods for the single photon generator, roughly divided into the photoexcitation method and the current injection method. The former is a method in which excitons photoexcited in a semiconductor material are generated by irradiating a device including quantum dots, and light emitted from the excitons in the quantum dots is extracted as a single photon. The latter is a method in which an electric current is passed instead of light irradiation, electrons and holes are injected into the quantum dots, and light emitted from the recombination is used as a single photon.

Among the optical excitation methods, there is an excitation method called quasi-resonant excitation. FIG. 10 is a diagram for explaining that single photons are generated by quasi-resonant excitation. In FIG. 10, the upper stage shows the energy band diagram and electron level of the conduction band, and the lower stage shows the energy band diagram and hole level of the valence band. The same applies to FIG. 12 described later.
In quasi-resonant excitation, an exciton in an excited state is directly generated in the quantum dot by externally irradiating an excitation light pulse having energy that resonates with the exciton level in the excited state of the quantum dot. The light that is recombined after the excited excitons are relaxed to the ground state excitons (neutral excitons: X ⁰ ) is used as a single photon. By using quasi-resonant excitation, it is possible to suppress the generation of excess carriers in the quantum dots, thereby increasing the purity and light extraction efficiency, which are important characteristic parameters of the single photon generator.

In a single photon generator, photons are generated one by one and the characteristics of these individual photons are used for information processing, so there is no point in amplifying and using the optical output. Therefore, to improve the performance of the single photon light source itself, it is extremely important to increase the light extraction efficiency as much as possible. Here, the light extraction efficiency is defined as “a generation probability of a single photon emitted to an external light extraction unit (for example, an optical fiber or the like) in one driving event of the single photon generation device”. The light extraction efficiency (hereinafter referred to as η) is mainly determined from three factors. The first is the probability (η _cap ) that excitons are injected into a quantum level that generates a single photon. The second is the light emission efficiency (η _iqe : internal quantum efficiency) when excitons in the quantum dots recombine. The third is the optical coupling efficiency (η _opt ) from the optical structure including the quantum dots to the light extraction portion when taking out the photons emitted from the quantum dots to the outside of the apparatus.

These are in the following relationship.
η = η _cap × η _iqe × η _opt
Here, η _iqe is determined by the material, shape, and crystal quality of the quantum dot, and η _opt is a parameter determined by the optical structure including the quantum dot and the light extraction unit.

JP 2004-253657 A

Phys. Rev. B 69, 205324 (2004). Applied Physics Express 3 (2010) 092802.

In order to improve the light extraction efficiency η of a single photon, it has already been reported to improve η _cap using quasi-resonant excitation (see Non-Patent Documents 1 and 2). However, even with this approach, it is difficult to suppress the blinking phenomenon to reduce the time average <eta _cap> of eta _cap. Blinking phenomenon is a state transition with a certain time constant between a state where one exciton (generally X ⁰ ) in a quantum dot can emit light (on state) and a state where it cannot emit light (off state). The name is given because the light emission appears to blink when the on state and the off state are switched in order.

  As specific states of the on state and the off state of the blinking phenomenon, so-called bright excitons and dark excitons can be considered. A bright exciton is an exciton that is an optically acceptable transition, in which the spin states of electrons and holes are sequentially combined as shown in ↑ ↓. On the other hand, a dark exciton is an exciton that is an optically forbidden transition, in which the spin states of electrons and holes are in the order of ↑↑ or ↓↓.

  Originally, excitons formed when absorbing an excitation light pulse should be bright excitons. However, the spin of electrons or holes rarely reverses in the process of phonon scattering or the like of electrons or holes, thereby forming dark excitons (see FIGS. 11 and 12). This phenomenon is called “Spin Flip”. The state of dark excitons formed by spin flip is an optically forbidden transition, so the recombination probability is extremely low, and once dark excitons are formed in quantum dots, dark excitations are prolonged It will remain as a child.

At this time, in the ON state, 0 <η _cap <1, and after the generation of a single photon, the exciton in the quantum dot is in an empty state until the next excitation light pulse is irradiated. The pulse is ready to be absorbed by resonant excitation. On the other hand, in the off state, since the carrier state immediately before the photoexcitation pulse irradiation is changed, the exciton energy in the excited state is changed by the Coulomb interaction between carriers, and as a result, resonance absorption of the excitation light pulse does not occur. η _cap = 0. At this time, as shown in FIG. 12, eta time average of _cap <η _cap> is reduced due to the contribution of off-state.

The blinking phenomenon has been reported in Non-Patent Documents 1 and 2, and has an overly long time constant (about several μs) compared to the recombination emission time (about 1 ns) of neutral exciton emission of quantum dots. The on state and the off state are interchanged. When the light extraction efficiency η in the single photon generator at this time is rewritten using <η _cap >,
η = <η _cap > × η _iqe × η _opt
It becomes. Since the on-state and the off-state transition in about several μs due to the blinking phenomenon, <ηcap> decreases as shown in FIG. As a result, there is a problem that the light extraction efficiency η also decreases.

  The present invention has been made in view of the above problems, and an object of the present invention is to propose a photon generator and a photon generation method capable of suppressing the blinking phenomenon and improving the single photon extraction efficiency. And

  One aspect of the photon generator includes a quantum dot unit including a quantum dot that generates a single photon, at least a pair of electrodes for applying a voltage to the quantum dot unit, and energy of an excitation light pulse Adjusting to resonate with the exciton level in the quantum dot, generating the excitation light pulse to irradiate the quantum dot, and generating a single photon from the quantum dot, and The first voltage source that applies a reverse-bias voltage pulse to the electrode, the excitation light generation unit, and the first voltage source are alternately and repeatedly irradiated with the excitation light pulse and applied with the voltage pulse. As described above, a synchronous operation unit that performs a synchronous operation is included.

  One aspect of the photon generation method is a photon generation method for generating a single photon from a quantum dot, wherein the energy of an excitation light pulse is adjusted to resonate with an exciton level in the quantum dot, and A first step of generating an excitation light pulse and irradiating the quantum dot to generate a single photon from the quantum dot and a second step of applying a reverse bias voltage pulse to the quantum dot are alternately performed. Repeatedly.

  One aspect of the control program is a control program for generating a single photon from a quantum dot, wherein the energy of an excitation light pulse is adjusted to resonate with an exciton level in the quantum dot, and A first step of generating an excitation light pulse and irradiating the quantum dot to generate a single photon from the quantum dot and a second step of applying a reverse bias voltage pulse to the quantum dot are alternately performed. And causing the computer to execute a procedure of repeatedly repeating the second step repeatedly.

  According to the above aspects, it is possible to suppress the blinking phenomenon and improve the single photon extraction efficiency.

It is a schematic diagram which shows schematic structure of the single photon generator by 1st Embodiment. It is sectional drawing which shows the specific structure of the quantum dot part in 1st Embodiment. It is a figure which shows the carrier extraction (initialization) by reverse bias application. It is a figure which shows the operation sequence of the photon generation method by 1st Embodiment. It is a figure which shows the timing of the excitation light pulse and voltage pulse in 1st Embodiment. It is a schematic diagram which shows schematic structure of the single photon generator by 2nd Embodiment. It is sectional drawing which shows an example of the specific structure of the quantum dot part in 2nd Embodiment. It is sectional drawing which shows the other example of the specific structure of the quantum dot part in 2nd Embodiment. It is a figure which shows the timing of the excitation light pulse and voltage pulse in 2nd Embodiment. It is a figure for demonstrating generation | occurrence | production of the single photon by quasi-resonance excitation. It is a figure for demonstrating that a dark exciton is produced | generated by spin flip. It is a figure for demonstrating that an ON state and an OFF state generate | occur | produce by a blinking phenomenon.

  Hereinafter, specific embodiments of the photon generator and the generation method will be described in detail with reference to the drawings.

(First embodiment)
First, the first embodiment will be described. FIG. 1 is a schematic diagram showing a schematic configuration of a single photon generator according to the first embodiment.
The single photon generator includes a quantum dot unit 1, an excitation light pulse generator 2, an alternating current (AC) voltage unit 3, a synchronization circuit 4, an optical filter unit 5, and an overall control unit 10.

  The quantum dot unit 1 includes a quantum dot 1a that generates a single photon, and includes a pair of electrodes 1A and 1B for applying a voltage to the quantum dot. In addition, in FIG. 1, the structure of the quantum dot part 1 is modeled and described, and as shown below, its specific structure is slightly different.

The excitation light pulse generator 2 adjusts the energy of the excitation light pulse so as to resonate with the exciton level in the quantum dot 1a, and generates the excitation light pulse E _P. The generated excitation light pulse E _P irradiates the quantum dot 1a.
The AC voltage unit 3 is a power source for applying an AC voltage to the electrodes 1A and 1B of the quantum dot unit 1 as a first voltage source. A voltage pulse from the AC voltage unit 3 is applied to the electrodes 1A and 1B.

The synchronization circuit 4 is a circuit that controls the timing of the excitation light pulse E _P irradiation by the excitation light pulse generation unit 2 and the application of the AC voltage by the AC voltage unit 3. Are operated synchronously so as to be repeated alternately.
The optical filter unit 5 receives a photon generated in the quantum dot 1a by irradiation with the excitation light pulse E _P , selects only a predetermined wavelength, removes excess background light, and transmits the photon. Photons transmitted through the optical filter unit 5 is output as a single-photon E _X.

The overall control unit 10 generates the excitation light pulse E _P by the excitation light pulse generation unit 2, the application of the AC voltage by the AC voltage unit 3, the generation of the excitation light pulse E _P by the synchronization circuit 4, and the application timing of the AC voltage. It controls and executes control. The overall control unit 10 is realized by a central processing unit (CPU) of a computer, for example.

In the present embodiment, the quantum dot unit 1 specifically adopts the configuration shown in FIG.
In the quantum dot portion 1 of the present embodiment, InAs quantum dots are arranged as a photon generating portion in an i layer portion of a pin structure formed of InP. The doping amount of the p layer and the n layer is 1 × 10 ^{18 in} order to obtain an activated carrier density even at a low temperature of about 10 K (−263 ° C.), which is essential when operating as a single photon generator. / Cm ³ or more and 5 × 10 ¹⁷ / cm ³ or more. The p layer on the upper side of the quantum dot suppresses the effect of free carrier absorption that occurs when an externally applied excitation light pulse is transmitted and when photons generated in the quantum dot are transmitted and extracted outside. It is necessary to make it thinner. On the other hand, it is necessary to have a thickness that can achieve electrical contact even at low temperatures. A typical thickness that can satisfy both of these is 100 nm or more. The thickness of the i layer including the quantum dots cannot be made too thick considering that the carriers in the quantum dots are extracted by an electric field. In order to realize carrier extraction from the quantum dots with a realistic applied voltage (about −3 V or less), it is considered that an electric field strength of 100 kV / cm or more is necessary, so that the thickness of the i layer is about 160 nm or less. good. Regarding the n layer, there are few restrictions on the thickness, and it may be about 200 nm or more.

Specifically, the n-InP buffer layer 12 (doping amount: 5 × 10 ¹⁷ / cm ³ ) having a thickness of about 200 nm is formed on the n-InP substrate 11 (about 5 × 10 ¹⁷ / cm ³ ) having a thickness of about 500 μm. Degree) to grow. Subsequently, an i-InP layer 13 having a thickness of about 100 nm, an InAs quantum dot layer 14 including InAs quantum dots 14a, an i-InP layer 15 having a thickness of about 150 nm are grown, and a p-InP layer 16 having a thickness of about 300 nm is formed on the uppermost surface. (Dope amount: about 1 × 10 ¹⁸ / cm ³ ) is grown. An n-InP substrate 11 is used as electrodes 1A and 1B, which are electrode pairs, in which a part of a pin junction including quantum dots 14a that generate single photons is processed into a mesa structure having a width W of about 500 nm. An n-side contact electrode 17 made of Ti / Pt / Au is formed thereon, and a p-side contact electrode 18 made of Au / Zn / Au is formed on the p-InP layer 16.

  In the photon generation method according to the present embodiment, as shown in FIG. 3, in order to suppress the blinking phenomenon described above, an AC voltage of reverse bias is applied to the quantum dots during the irradiation of one excitation light pulse that generates a single photon. Apply. In FIG. 3, the upper part shows the electron level, and the rear part shows the hole level. When dark excitons remain inside the quantum dot by applying a reverse bias AC voltage, electrons and holes are extracted by the reverse bias applied electric field to remove the dark excitons. On the other hand, when dark excitons are not generated inside the quantum dot, the applied electric field of reverse bias does not particularly affect the quantum dot. As described above, this embodiment employs a configuration in which a reverse bias voltage pulse is always applied between irradiations of one excitation light pulse (excitation light pulse application and reverse bias voltage pulse application are alternately repeated). . In order to confirm whether or not dark excitons are generated during irradiation of one excitation light pulse, single photons must be detected, and single photons cannot be output. In the present embodiment, by adopting the above configuration, it is possible to reliably remove dark excitons and output single photons without adding a special device or the like.

  Hereinafter, the photon generation method according to the present embodiment will be described. FIG. 4 is a diagram illustrating an operation sequence of the photon generation method according to the first embodiment. FIG. 5 is a diagram illustrating the timing of the excitation light pulse and the voltage pulse in the first embodiment.

The pumping light pulse is generated at a repetition rate (period) T as shown in FIG. First, at time t ₀ = 0, the excitation light pulse generator 2 generates an excitation light pulse and irradiates the quantum dots (step S1). A quantum dot generates a single photon upon irradiation with an excitation light pulse. At this time, the AC voltage unit 3 is set in a standby state (V = 0 (V)) (step S2). The standby state of the AC voltage unit 3 continues from time t ₋₁ (t _sta ) including time when the excitation light pulse is generated to time t ₁ . _Let this duration be t _ope . The time from time t _-1 to time t ₀ is the AC voltage stabilization time described later.

At time t ₁ , the AC voltage unit 3 generates a reverse bias voltage pulse (V _ini = V _AC ), and the voltage pulse is applied to the quantum dot (step S3). At this time, if dark excitons remain in the quantum dots, electrons and holes are extracted and the dark excitons are removed (step S4). The application of the voltage pulse continues from time t ₁ to time t ₂ . This time is _defined as application time t _ini .

At time t _2, AC voltage unit 3 cancels the generation of the voltage pulse (step S5). The AC voltage unit 3 waits for the stabilization time of the voltage pulse (step S6).
The generation of one photon is completed in the above steps S1 to S6, and steps S1 to S6 are repeatedly executed.

Time t ₁ is the timing to apply a reverse bias (t _ope -t _sta), as in Figure 5, the light emitting recombination time of time after irradiation with the excitation light neutral exciton X ⁰ in the quantum dots (approximately 1 ns) or more, and a typical value may be 9 ns. The voltage pulse application time t _ini is a tunneling time of electrons and holes (depending on the applied voltage, carrier extraction). It is sufficient that it is sufficiently large as compared with 0.1 ns when used in a typical value, and a typical value of about 0.8 ns or more is considered sufficient. The repetition rate T needs to be a value sufficiently larger than the luminescence recombination time of the neutral exciton X ⁰ , and is 10 ns as an example here.

In this embodiment, when dark excitons remain in the quantum dots, the dark excitons are removed by extracting electrons and holes by applying a voltage pulse. With this configuration, the quantum dot is always turned on at the timing of irradiating the excitation light pulse. As a result, the light extraction efficiency <η _cap > is improved. In other words, if the decay time constants of the nm love on state and off state where a blinking phenomenon occurs without applying an electric field are τ _on and τ _off , the light extraction efficiency <η _cap > is approximately the ratio of the time constants. Τ _on / (τ _on + τ _off ). When the blinking phenomenon is suppressed by the configuration of the present embodiment, the light extraction efficiency <η _cap > is improved to about 1.

  As described above, according to the present embodiment, it is possible to suppress the blinking phenomenon and improve the single photon extraction efficiency.

(Second Embodiment)
Next, a second embodiment will be described. FIG. 6 is a schematic diagram showing a schematic configuration of a single photon generator according to the second embodiment. In addition, about the same structural member as FIG. 1 of 1st Embodiment, the same code | symbol is attached | subjected and detailed description is abbreviate | omitted.

  As in the first embodiment, the single photon generator of this embodiment includes a quantum dot unit 1, an excitation light pulse generator 2, an alternating current (AC) voltage unit 3, a synchronization circuit 4, an optical filter unit 5, and a DC. A bias superimposing unit 6 and an overall control unit 10 are provided. In addition to the AC voltage unit 3 that is the first voltage source, the single photon generator includes a DC voltage unit 21 that applies a direct current (DC) voltage to the quantum dots 1a as the second voltage source. Yes.

  The DC voltage unit 21 is a power source for applying a DC voltage to the electrodes 1A and 1B of the quantum dot unit 1 when the quantum dot 1a absorbs excitation light and generates photons. The DC voltage unit 21 mainly applies a forward bias DC voltage to the electrodes 1 </ b> A and 1 </ b> B via the DC bias superimposing unit 6.

In the present embodiment, the quantum dot unit 1 specifically adopts the configuration shown in FIG. 7 or FIG.
The quantum dot part 1 of FIG. 7 has InAs quantum dots arranged as a photon generating part in i-GaAs. The i layer including the quantum dots is processed into a mesa structure having a width W of about 500 nm, and a pair of electrodes are formed so as to sandwich the mesa structure. Extraction of excess excitons by applying an electric field, which is essential in this embodiment, is realized by applying a voltage pulse between these electrode pairs. When the voltage value of the voltage pulse is 5 V, the electric field applied to the quantum dots is about 100 kV / cm, and the electric field strength is sufficient to extract the carriers in the quantum dots.

  Specifically, an i-GaAs buffer layer 23 having a thickness of about 200 nm is grown on an i-GaAs substrate 22 having a thickness of about 500 μm. Subsequently, an i-GaAs layer 24 having a thickness of about 100 nm, an InAs quantum dot layer 25 including an InAs quantum dot 25a, and an i-GaAs layer 26 having a thickness of about 100 nm are grown, and an i including a quantum dot 25a that generates a single photon. A part of the layer is processed into a mesa structure having a width W of about 500 nm. Electric field application electrodes 27 and 28 as electrodes 1A and 1B as electrode pairs are formed on both sides of the mesa structure.

The quantum dot part 1 of FIG. 7 arrange | positions an InAs quantum dot as a photon generating part in the i layer part of the ni structure formed with GaAs. An n-side contact electrode is disposed on the back surface, and an i-side Schottky electrode is disposed on the i layer. The Schottky electrode has a fine opening having a diameter of about 500 nm, and an excitation light pulse is irradiated through the opening to realize quasi-resonant excitation to the quantum dot. The structure is such that an electric field can be applied to the i layer by applying a voltage between both electrodes. The doping amount of the n layer is 5 × 10 ¹⁷ / cm ³ or more in order to obtain an activated carrier density even at a low temperature of about 10 K (−263 ° C.), which is essential when operating as a single photon generator. And The thickness of the i layer including the quantum dot portion cannot be so thick considering that the carriers in the quantum dot are extracted by an electric field. In order to realize carrier extraction from the quantum dots with a realistic applied voltage (about −3 V or less), it is considered that an electric field strength of 100 kV / cm or more is necessary. Therefore, the thickness of the i layer is about 350 nm or less. I just need it. Regarding the n layer, there are few restrictions on the thickness, and it should be about 200 nm or more. As shown in FIG. 7, it is also possible to form the electrode using the n-doped structure for the entire substrate.

Specifically, an n-GaAs buffer layer 32 (doping amount: about 5 × 10 ¹⁷ / cm ³ ) having a thickness of about 200 nm is grown on the n-GaAs substrate 31 (doping amount: about 5 × 10 ¹⁷ / cm ³ ). To do. Subsequently, an i-GaAs layer 33 having a thickness of about 100 nm, an InAs quantum dot layer 34 including InAs quantum dots 34a, and an i-GaAs layer 35 having a thickness of about 100 nm are grown. As electrodes 1A and 1B, which are electrode pairs, an n-side contact electrode 36 made of AuGeNi / Au on the back surface of the n-GaAs substrate 31, and an i-side Schottky electrode 37 made of Ti / Al / Au on the surface of the i-GaAs layer 35. Form. A minute opening 37 a having a diameter D of about 500 nm is formed in the i-side Schottky electrode 37. The excitation light pulse is applied to the InAs quantum dots 34a through the opening 37a.

  In the photon generation method according to the present embodiment, in order to suppress the above-described blinking phenomenon, a reverse bias AC voltage is applied to the quantum dots during irradiation of one excitation light pulse that generates a single photon. As a result, when dark excitons remain inside the quantum dot, electrons and holes are extracted by an applied electric field of reverse bias, and the dark excitons are removed. On the other hand, when dark excitons are not generated inside the quantum dot, the applied electric field of reverse bias does not particularly affect the quantum dot. As described above, this embodiment employs a configuration in which a reverse bias voltage pulse is always applied between irradiations of one excitation light pulse (excitation light pulse application and reverse bias voltage pulse application are alternately repeated). . In order to confirm whether or not dark excitons are generated during irradiation of one excitation light pulse, single photons must be detected, and single photons cannot be output. In the present embodiment, by adopting the above configuration, it is possible to reliably remove dark excitons and output single photons without adding a special device or the like.

Hereinafter, the photon generation method according to the present embodiment will be described. FIG. 9 is a diagram illustrating the timing of the excitation light pulse and the voltage pulse in the second embodiment.
In the present embodiment, an operation sequence similar to that in FIG. 4 of the first embodiment is performed.

In the present embodiment, as shown in FIG. 9, the DC voltage unit 21 applies a forward-biased DC voltage (V _DC ) to the quantum dots in an overlapping manner throughout the entire operation sequence.
The pumping light pulse is generated at a repetition rate (period) T as shown in FIG. First, at time t ₀ = 0, the excitation light pulse generator 2 generates an excitation light pulse and irradiates the quantum dots (step S1). A quantum dot generates a single photon upon irradiation with an excitation light pulse. At this time, the AC voltage unit 3 is set in a standby state (V = 0 (V)) (step S2). The standby state of the AC voltage unit 3 continues from time t ₋₁ (t _sta ) including time when the excitation light pulse is generated to time t ₁ . _Let this duration be t _ope . The time from time t _-1 to time t ₀ is the AC voltage stabilization time described later.

At time t ₁ , the AC voltage unit 3 generates a reverse-bias voltage pulse (V _AC ), and V _DC is superimposed and applied to the quantum dot as V _ini (= V _AC + V _DC ) (step) S3). At this time, if dark excitons remain in the quantum dots, electrons and holes are extracted and the dark excitons are removed (step S4). The application of the voltage pulse continues from time t ₁ to time t ₂ . This time is _defined as application time t _ini .

At time t _2, AC voltage unit 3 cancels the generation of the voltage pulse (step S5). The AC voltage unit 3 waits for the stabilization time of the voltage pulse (step S6).
The generation of one photon is completed in the above steps S1 to S6, and steps S1 to S6 are repeatedly executed.

In this embodiment, when dark excitons remain in the quantum dots, the dark excitons are removed by extracting electrons and holes by applying a voltage pulse. With this configuration, the quantum dot is always turned on at the timing of irradiating the excitation light pulse. As a result, the light extraction efficiency <η _cap > is improved. In other words, if the decay time constants of the nm love on state and off state where a blinking phenomenon occurs without applying an electric field are τ _on and τ _off , the light extraction efficiency <η _cap > is approximately the ratio of the time constants. Τ _on / (τ _on + τ _off ). When the blinking phenomenon is suppressed by the configuration of the present embodiment, the light extraction efficiency <η _cap > is improved to about 1.

The quantum dots (InAs / GaAs) formed in i-GaAs in the present embodiment have a relatively large height (about 5 nm to about 10 nm). For this reason, when excitons are generated inside the quantum dots, the gravity center positions of the wave functions of electrons and holes tend to shift in the height direction, and a spontaneous dipole moment tends to occur. The dipole moment varies depending on the electric field applied to the quantum dots. Therefore, by applying a low DC voltage in consideration of the built-in voltage of the pin junction (in this case, +0.6 V is taken as an example), a decrease in the light emission efficiency of X ^{0 in} the quantum dots is suppressed.

A part of the built-in voltage is canceled by the forward biased DC voltage, and the effective electric field applied to the quantum dots can be reduced. By adjusting the DC voltage, the dipole moment of excitons during photon generation is reduced, and there is also an effect that the internal quantum efficiency η _iqe is improved by increasing the overlap integral of the electron-hole wave function.

At time t ₁ (t _ope −t _sta ), which is the timing for applying the reverse bias, as shown in FIG. 9, the time after irradiation with excitation light is neutral excitons in the quantum dots as in the first embodiment. It only needs to be several times longer than the luminescence recombination time (about 1 ns) of X ⁰ , and a typical value may be 9 ns. Further, the voltage pulse application time t _ini only needs to be sufficiently larger than the tunneling time of electrons and holes (depending on the applied voltage, but about 0.1 ns when used for carrier extraction), and is a typical value. For example, about 0.8 ns or more is considered sufficient. The repetition rate T needs to be a value sufficiently larger than the luminescence recombination time of the neutral exciton X ⁰ , and is 10 ns as an example here.

  As described above, according to the present embodiment, it is possible to suppress the blinking phenomenon and improve the single photon extraction efficiency.

(Other embodiments)
The functions of the constituent elements of the single photon generation device according to the first and second embodiments described above (such as the overall control unit 10 in FIGS. 1 and 6) are operated by programs stored in the RAM, ROM, etc. of the computer. Can be realized. Similarly, each step of the single photon generation method (steps S1 to S6 in FIG. 4) can be realized by operating a control program stored in a RAM or ROM of a computer. This control program and a computer-readable storage medium storing the control program are included in this embodiment.

  Specifically, the control program is recorded on a recording medium such as a CD-ROM, or provided to the computer via various transmission media. As a recording medium for recording the control program, a flexible disk, a hard disk, a magnetic tape, a magneto-optical disk, a nonvolatile memory card, and the like can be used in addition to the CD-ROM. On the other hand, a communication medium in a computer network system for propagating and supplying program information as a carrier wave can be used as a transmission medium for the control program. Here, the computer network is a WAN such as a LAN or the Internet, a wireless communication network, or the like, and the communication medium is a wired line such as an optical fiber or a wireless line.

  Further, the control program included in the present embodiment is not limited to the one in which the functions of the present embodiment are realized by the computer executing the supplied control program. For example, even when the control program is implemented in cooperation with an OS (operating system) or other application software running on a computer, the control program is included in the present embodiment. . The control program is also included in the present embodiment when all or part of the processing of the supplied control program is performed by the function expansion board or function expansion unit of the computer to realize the functions of the present embodiment. It is.

  A normal personal user terminal device can be used as the computer. Instead of using a normal personal user terminal device, a predetermined computer specialized for a single photon generator may be used.

  Hereinafter, various aspects of the photon generator, the generation method, and the control program will be collectively described as additional notes.

(Supplementary note 1) a quantum dot portion including a quantum dot that generates a single photon;
At least a pair of electrodes for applying a voltage to the quantum dot portion;
The excitation light pulse is adjusted so as to resonate with the exciton level in the quantum dot, and the excitation light pulse is generated and irradiated to the quantum dot to generate a single photon from the quantum dot. A light generator;
A first voltage source for applying a reverse bias voltage pulse to the electrode;
A synchronous operation unit that synchronously operates the excitation light generation unit and the first voltage source so as to alternately repeat the irradiation of the excitation light pulse and the application of the voltage pulse. Photon generator.

  (Supplementary note 2) The photon generation device according to supplementary note 1, further comprising a second voltage source for applying a forward-biased DC voltage to the photon generation device.

(Supplementary Note 3) The quantum dot portion includes an i semiconductor layer in which the quantum dots are formed.
The photon generator according to appendix 1 or 2, wherein a pair of the electrodes are arranged so as to sandwich both side surfaces of the i semiconductor layer.

  (Supplementary Note 4) The quantum dot portion includes a p semiconductor layer or an n semiconductor layer, and an i semiconductor layer formed on the p semiconductor layer or the n semiconductor layer, and the i semiconductor layer includes the i semiconductor layer. The photon generator according to appendix 1 or 2, wherein quantum dots are formed.

  (Supplementary Note 5) The quantum dot portion includes a p semiconductor layer and an n semiconductor layer, and an i semiconductor layer sandwiched between the p semiconductor layer and the n semiconductor layer, and the quantum dot portion is included in the i semiconductor layer. The photon generator according to appendix 1 or 2, wherein dots are formed.

(Appendix 6) A photon generation method for generating a single photon from a quantum dot,
Adjusting the energy of the excitation light pulse to resonate with the exciton level in the quantum dot, generating the excitation light pulse, irradiating the quantum dot, and generating a single photon from the quantum dot A photon generation method characterized by alternately and repeatedly performing a first step and a second step of applying a reverse bias voltage pulse to the quantum dots.

  (Supplementary note 7) The photon generation method according to supplementary note 6, wherein a forward-biased DC voltage is superimposed and applied to the quantum dots throughout the entire sequence in which the first step and the second step are repeated.

(Appendix 8) A control program for generating a single photon from a quantum dot,
Adjusting the energy of the excitation light pulse to resonate with the exciton level in the quantum dot, generating the excitation light pulse, irradiating the quantum dot, and generating a single photon from the quantum dot Control that causes a computer to execute a procedure of alternately repeating a first step and a second step of alternately repeating a second step of applying a reverse bias voltage pulse to the quantum dots. program.

  (Supplementary note 9) The supplementary note 8 is characterized in that the computer executes a procedure of superimposing and applying a forward-biased DC voltage to the quantum dots throughout the entire sequence in which the first step and the second step are repeated. The control program described.

DESCRIPTION OF SYMBOLS 1 Quantum dot part 1a Quantum dot 1A, 1B Electrode 2 Excitation light pulse generation part 3 AC voltage part 4 Synchronous circuit 5 Optical filter part 6 DC bias superimposition part 10 General control part 11 n-InP board | substrate 12 n-InP buffer layer 13, 15, 16 i-InP layers 14, 25, 34 InAs quantum dot layers 14a, 25a, 34a InAs quantum dots 17, 36 n-side contact electrode 18 p-side contact electrode 21 DC voltage unit 22 i-GaAs substrate 23 i-GaAs buffer Layers 24 and 26 i-GaAs layers 27 and 28 Electric field applying electrode 31 n-GaAs substrate 32 n-GaAs buffer layer 33 and 35 i-GaAs layer 37 i-side Schottky electrode 37a Opening

Claims (6)

A quantum dot portion comprising a quantum dot that generates a single photon;
At least a pair of electrodes for applying a voltage to the quantum dot portion;
Adjusting the energy of the excitation light pulse to resonate with the exciton level in the quantum dot, generating the excitation light pulse, irradiating the quantum dot, and generating a single photon from the quantum dot An excitation light generator,
A first voltage source for applying a reverse bias voltage pulse to the electrode;
A synchronous operation unit that synchronously operates the excitation light generation unit and the first voltage source so as to alternately repeat the irradiation of the excitation light pulse and the application of the voltage pulse. Photon generator.   The photon generator according to claim 1, further comprising a second voltage source for applying a forward biased DC voltage to the photon generator. A photon generation method for generating a single photon from a quantum dot,
Adjusting the energy of the excitation light pulse to resonate with the exciton level in the quantum dot, generating the excitation light pulse, irradiating the quantum dot, and generating a single photon from the quantum dot A photon generation method characterized by alternately and repeatedly performing a first step and a second step of applying a reverse bias voltage pulse to the quantum dots.   4. The photon generation method according to claim 3, wherein a forward-biased DC voltage is superimposed and applied to the quantum dots throughout the entire sequence in which the first step and the second step are repeated. 5. A control program for generating a single photon from a quantum dot,
Adjusting the energy of the excitation light pulse to resonate with the exciton level in the quantum dot, generating the excitation light pulse, irradiating the quantum dot, and generating a single photon from the quantum dot Control that causes a computer to execute a procedure of alternately repeating a first step and a second step of alternately repeating a second step of applying a reverse bias voltage pulse to the quantum dots. program.   6. The control according to claim 5, further comprising causing a computer to execute a procedure of superimposing and applying a forward-biased DC voltage to the quantum dots throughout the sequence in which the first step and the second step are repeated. program.

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