Apparatus and method for realizing photon pair emission and fiber output on basis of quantum dots

Abstract

An apparatus and method for realizing photon pair emission and fiber output on the basis of quantum dots. The apparatus comprises: a single quantum dot (1), which is used for generating cascaded luminescence of single excitons and biexcitons, wherein an exciton state exhibits luminescence in two polarization states, i.e. H and V, and the two polarization states have an energy splitting; a microcavity (2), which is formed by a first Bragg reflector (3) and a second Bragg reflector (4), and is used for controlling the directional light emission of the single quantum dot (1); an optical fiber coupling component (6), which is prepared by means of vertically bonding a single-mode optical fiber (5) and encapsulated by means of curing, and is used for collecting the cascaded luminescence of single excitons and biexcitons generated by the single quantum dot (1); a local electric field, which is generated by a single dopant atom (22) in an adjacent modulation-doped layer (21), and is used for tuning the single quantum dot (1), so as to reduce exciton energy splitting and tune an exciton wave function; a luminescence output module, which is used for performing fluorescence collection on the single quantum dot (1) and outputting the collected cascaded luminescence of single excitons and biexcitons in the form of a Gaussian optical field; and a photon pair correlation test unit (11), which is used for filtering the cascaded luminescence of single excitons and biexcitons, and testing polarization correlation on the basis of filtered single excitons and biexcitons.

Description

Device and method for realizing photon pair emission and optical fiber output based on quantum dots Technical Field

[0001] The present disclosure relates to the technical field of semiconductor optical devices, and in particular to a device and method for realizing photon pair emission and optical fiber output based on quantum dots. Background Art

[0002] Photodetection utilizes the intensity (or power) of optical signals. Limited by device noise, the limit of photodetection is typically at least in the picowatt range. Recently developed single-photon counters can detect single photons (1E-19 watts), thereby enhancing the sensitivity of photodetection. The creation of polarization-correlated photon pair quantum sources has important application value and research significance for achieving quantum-enhanced photodetection (sub-shot noise).

[0003] Typically, polarization-correlated photon pairs are generated in nonlinear crystals through parametric down-conversion. To improve generation efficiency, higher-power optical pulses are required for pumping. The conversion process places high demands on the focused spot size, crystal temperature control, and cavity design, making the technology more complex. Furthermore, the wavelength distribution of photon pairs generated by nonlinear parametric conversion is relatively broad, making it unfavorable for quantum Hong-Ou-Mandel interferometry. The cascaded emission of single and double exciton states in semiconductor quantum dots is stable, with discrete and sharp spectral lines (minimum <10 microelectron volts); it can be directly measured through narrow-line filtering; it can be integrated into a microcavity for directional light emission; and it is also suitable for direct coupling to single-mode optical fibers. Therefore, the disadvantages of generating polarization-correlated photon pairs based on nonlinear parametric conversion compared to generating polarization-correlated photon pairs using quantum dots may include higher technical complexity, lower conversion efficiency, and more stringent requirements for optical design.

[0004] Summary of the Invention

[0005] In response to the above technical problems, the present disclosure provides a device and method for realizing photon pair emission and optical fiber output based on quantum dots.

[0006] On the one hand, an embodiment of the present disclosure provides a device for realizing photon pair emission and optical fiber output based on quantum dots, including: a single quantum dot, used to generate single and double exciton cascade luminescence, wherein the exciton state has H and V dual polarization states to emit light, wherein the dual polarization states have energy splitting; a microcavity, formed by a first Bragg reflector and a second Bragg reflector, used to control the single quantum dot to emit light in a directional manner; an optical fiber coupling component, prepared by vertically gluing single-mode optical fibers and encapsulated by curing, used to collect the single and double exciton cascade luminescence generated by the quantum dots; a local electric field, generated by a single doping atom in a neighboring modulation doping layer, used to regulate the single quantum dot to reduce the exciton energy splitting; a luminescence output module, used to collect fluorescence from the single quantum dot and output the collected single and double exciton cascade luminescence in the form of a Gaussian light field; a photon pair correlation test unit, used to filter the single and double exciton cascade luminescence and test the polarization correlation based on the filtered single and double excitons.

[0007] Optionally, the thickness of the microcavity is an equivalent length of the emission wavelength of a single quantum dot in the current material.

[0008] Optionally, the first Bragg reflector and the second Bragg reflector are arranged in parallel in an up-down manner and are made of a material with a periodically alternating distribution of refractive indices.

[0009] Optionally, the luminescence output module includes: a first beam splitter for exciting a single quantum dot and collecting fluorescence; and an optical fiber output unit for outputting single and biexciton cascade luminescence in the form of a Gaussian light field.

[0010] Optionally, the first beam splitter is a fiber beam splitter with a splitting ratio greater than 80:20, wherein the first beam splitter is aligned with the polarization principal axis of the single quantum dot.

[0011] Optionally, the device further includes: a light source output module for generating an excitation laser to excite a single quantum dot, wherein the excitation laser is output through an optical fiber coupling component and connected to the first beam splitter, and the output power of the light source output module is continuously adjustable.

[0012] Optionally, the photon pair correlation test unit includes: a filter for eliminating laser background and filtering out the fluorescence of a single quantum dot; a second beam splitter for evenly dividing the fluorescence signal into two paths to obtain a first fluorescence signal and a second fluorescence signal; a narrow line filter group, consisting of a first narrow line filter and a second narrow line filter, for filtering the first fluorescence signal and the second fluorescence signal respectively to obtain single exciton luminescence and biexciton luminescence; a polarizer group, consisting of a first polarizer and a second polarizer, for polarization filtering the single exciton luminescence and biexciton luminescence to obtain H polarization or V polarization; a counting and measurement module, for measuring the second-order cross-correlation of the two photons to verify the polarization correlation.

[0013] Optionally, the first narrow line filter and the second narrow line filter are both filter elements with a filtering line width of less than 0.5 nanometers and a transmittance of more than 60%, and the filtering wavelength is tuned by adjusting the angle or cavity length of the first narrow line filter and / or the second narrow line filter.

[0014] Optionally, the optical fiber coupling component is encapsulated by AB resin to generate thermal stress point defects based on temperature increase and decrease, wherein the thermal stress point defects will act as a second doping to form an in-plane double electric field in the local electric field.

[0015] Another aspect of the embodiments of the present disclosure provides a method for realizing photon pair emission and optical fiber output based on quantum dots, including: generating single and double exciton cascade luminescence from a single quantum dot; regulating the exciton luminescence characteristics by modulating the doped local electric field to generate polarization-correlated photon pairs; collecting the single and double exciton cascade luminescence based on a luminescence output module; filtering the single and double exciton luminescence based on a photon pair correlation test unit and checking the polarization correlation.

[0016] The device provided by the present disclosure for realizing photon pair emission and optical fiber output based on quantum dots has at least the following beneficial effects:

[0017] The device provided by the disclosed embodiments is based on semiconductor self-organized quantum dots, which generate cascaded luminescence with sharp and discrete single and double excitons. The quantum dot exciton luminescence exhibits fine structure splitting, anisotropy, wave function degeneracy, and light-heavy hole symmetry, and can be controlled by local electric and stress fields. Specifically, fine structure splitting can be reduced to <4 microelectronvolts through local field control, and the exciton spectral linewidth can be minimized to <10 microelectronvolts. This allows for the elimination of background noise, improved detection sensitivity, enhanced concealment, and quantum interference measurement through narrow-line filters. The local electric field generated by quantum dot neighbor modulation doping atoms has diverse directions. For hillock-shaped quantum dots, the in-plane electric field component can significantly control the spatial distribution of the exciton wave function, reduce its degeneracy, eliminate cross-dephasing, and minimize energy splitting. Furthermore, positioning and bonding the single-mode optical fiber to the quantum dot sample effectively avoids the problem of random on-chip positioning of the quantum dots, which requires scanning and addressing by confocal microscopy. This effectively simplifies the complexity of the optical system and reduces sensitivity to vibration, achieving stable transmission. Direct fiber coupling effectively improves light collection efficiency and offers greater flexibility and ease of use compared to spatial optical paths. Single-mode fiber transmission operates in two mutually perpendicular linear polarization modes, suitable for maintaining the linear polarization of single photons and the polarization correlation of photon pairs. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] FIG1 schematically shows a structural diagram of a device for realizing photon pair emission and optical fiber output based on quantum dots in an embodiment of the present disclosure;

[0019] FIG2 schematically shows a schematic diagram of the principle of single quantum dot generating single and biexciton cascade luminescence in an embodiment of the present disclosure;

[0020] FIG3 schematically shows the test results of polarization-resolved second-order correlation function of quantum dot polarization-correlated photons to quantum sources in an embodiment of the present disclosure;

[0021] FIG4 schematically shows the test results of the polarization-resolved second-order correlation function in one embodiment of the present disclosure;

[0022] FIG5 schematically shows the test results of polarization-resolved spectroscopy and second-order correlation function and a schematic diagram of the principle of polarization-correlated single- and biexciton cascade luminescence in a single path in another embodiment of the present disclosure;

[0023] FIG6 schematically shows a flow chart of a method for realizing photon pair emission and optical fiber output based on quantum dots according to an embodiment of the present disclosure.

[0024] Reference numerals: 1-single quantum dot; 2-microcavity; 3-first Bragg reflector; 4-second Bragg reflector; 5-single-mode optical fiber; 6-fiber coupling assembly; 7-low-temperature cooling stage; 8-first beam splitter; 9-light source output module; 10-fiber output unit; 11-photon pair correlation test unit; 12-filter; 13-second beam splitter; 14-first narrow-line filter; 15-second narrow-line filter; 18-first polarizer; 19-second polarizer; 20-counting measurement module; 16-single exciton luminescence; 17-biexciton luminescence; 21-modulated doping layer; 22-doping atoms; 23-in-plane electric field component; 24-electron-hole overlap degree; 25-exciton fine structure splitting; 26-cross-dephasing; 27-polarization-correlated photon pair HH; 28-polarization-correlated photon pair VV; 29-point defect; 30-double-modulated electric field; 31-"electric quadrupole" biexciton wave function; 32-single HV polarization-correlated photon pair emission. DETAILED DESCRIPTION

[0025] To make the objectives, technical solutions, and advantages of the present disclosure more clearly understood, the present disclosure is further described below in conjunction with specific embodiments and with reference to the accompanying drawings. It is apparent that the embodiments described are only a portion of the embodiments of the present disclosure, not all of them. All other embodiments derived by persons of ordinary skill in the art based on the embodiments of the present disclosure without inventive effort are intended to fall within the scope of protection of the present disclosure.

[0026] The terms used herein are intended only to describe specific embodiments and are not intended to limit the present disclosure. The terms "comprise," "include," and the like as used herein indicate the presence of the described characteristic images, steps, operations, and / or components, but do not preclude the presence or addition of one or more other characteristic images, steps, operations, or components.

[0027] In this disclosure, unless otherwise expressly specified or limited, terms such as "mounted," "connected," "connect," and "fixed" should be understood broadly. For example, they may refer to fixed connections, detachable connections, or integration; mechanical connections, electrical connections, or communication; direct connections or indirect connections through an intermediate medium; and internal communication between two components or interaction between two components. Those skilled in the art will understand the specific meanings of these terms in this disclosure based on specific circumstances.

[0028] In the description of the present disclosure, it should be understood that the terms "longitudinal", "length", "circumferential", "front", "rear", "left", "right", "top", "bottom", "inside", "outside", etc., indicating orientations or positional relationships, are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present disclosure and simplifying the description, and do not indicate or imply that the subsystem or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present disclosure.

[0029] Throughout the drawings, identical elements are represented by identical or similar reference numerals. Conventional structures or configurations are omitted where they may cause confusion in understanding the present disclosure. Furthermore, the shapes, sizes, and positional relationships of the components in the drawings do not reflect actual size, proportion, or actual positional relationships. Furthermore, any reference symbols placed between parentheses in this disclosure should not be construed as limiting the present disclosure.

[0030] Similarly, in order to streamline the present disclosure and aid in understanding one or more of the various disclosed aspects, in the above description of the exemplary embodiments of the present disclosure, the various characteristic images of the present disclosure are sometimes grouped together into a single embodiment, figure, or description thereof. Descriptions with reference to the terms "one embodiment," "some embodiments," "example," "specific example," or "some examples" and the like mean that the specific characteristic images, structures, materials, or features described in conjunction with the embodiment or example are included in at least one embodiment or example of the present disclosure. In this specification, the schematic representations of the above terms do not necessarily refer to the same embodiment or example. Moreover, the specific characteristic images, structures, materials, or features described may be combined in an appropriate manner in any one or more embodiments or examples.

[0031] Furthermore, the terms "first" and "second" are used for descriptive purposes only and should not be understood to indicate or imply relative importance or implicitly specify the number of technical feature images indicated. Therefore, a feature image defined as "first" or "second" may explicitly or implicitly include one or more of the feature images. In the description of this disclosure, "plurality" means at least two, such as two, three, etc., unless otherwise specifically defined.

[0032] FIG1 schematically shows a structural diagram of a device for realizing photon pair emission and optical fiber output based on quantum dots in an embodiment of the present disclosure.

[0033] As shown in FIG1 , the device for realizing photon pair emission and optical fiber output includes: a single quantum dot 1 , a microcavity 2 , an optical fiber coupling component 6 , a light emitting output module, and a photon pair correlation test unit 11 .

[0034] Single quantum dot 1, used to generate single and biexciton cascade luminescence.

[0035] According to an embodiment of the present disclosure, the single quantum dot 1 is an indium arsenide (InAs) self-organized quantum dot with a luminescence wavelength of 900 nm. As a solid-state quantum luminescent material, semiconductor self-organized quantum dots have discrete energy levels and can produce multi-exciton luminescence with sharp and discrete spectral lines. Exciton luminescence is characterized by fine structure splitting, anisotropy, wave function degeneracy, and light-heavy hole symmetry. The exciton state is a dipole formed by the Coulomb attraction of a single electron and a single hole, with H and V double polarization states of luminescence and fluorescence lifetime. The energy splitting of the doublet state depends on the material crystal orientation

[0110] and [1-10].

[0036] The microcavity 2 is formed by a first Bragg reflector 3 and a second Bragg reflector 4 and is used to control the single quantum dot 1 to emit light in a directional manner.

[0037] According to an embodiment of the present disclosure, the microcavity 2 is composed of a first Bragg reflector 3 and a second Bragg reflector 4, arranged in parallel in an up-down manner to form a Fabry-Perot plane microcavity. A single quantum dot 1 is located at the center of the microcavity to achieve optimal cavity enhancement and vertical light emission.

[0038] According to an embodiment of the present disclosure, the thickness of the microcavity 2 is determined based on the emission wavelength of the single quantum dot, that is, the equivalent length of the emission wavelength in the microcavity material.

[0039] Among them, the first Bragg reflector 3 and the second Bragg reflector 4 are multi-layer periodic alternating structures of aluminum gallium arsenide (AlGaAs) and gallium arsenide (GaAs), and the thickness of each layer is one-quarter of the equivalent length of the single quantum dot emission wavelength in GaAs or AlGaAs. The logarithms of the first Bragg reflector and the second Bragg reflector are optimized and designed to present a high reflection band within a certain wavelength range, so that most of the photons in the microcavity 2 are emitted from the first Bragg reflector 3 above.

[0040] The optical fiber coupling component 6 is used to collect the single and biexciton cascade luminescence generated by a single quantum dot.

[0041] According to the embodiments of the present disclosure, the fiber coupling assembly is formed by curing and encapsulating a single-mode optical fiber 5. The single-mode optical fiber 5 utilizes a 900nm single-mode optical fiber that matches the quantum dot emission wavelength. The single-mode optical fiber 5 is positioned directly above the quantum dot sample. The light emitted by a single quantum dot 1 is collected through optical field mode matching coupling. Combined with the vertical light output of the quantum dot sample, the single quantum dot 1 is spatially resolved. Light collection efficiency is optimal when facing directly above the single quantum dot 1. Mode matching coupling effectively improves light collection efficiency.

[0042] According to the embodiments of the present disclosure, a fiber coupling assembly 6 is formed by directly bonding a single-mode optical fiber 5 to a quantum dot sample and then curing and encapsulating the sample with UV glue or AB resin. The cured and encapsulated single-mode optical fiber is plug-and-play, requiring no manual commissioning, making installation and use more convenient and quicker. Bonding the single-mode optical fiber 5 to the quantum dot sample effectively avoids the problem of random distribution of quantum dots on the chip, which makes collection difficult.

[0043] Furthermore, in addition to collecting the light emission from a single quantum dot, the fiber coupling component 6 can also generate point defects in the vicinity of the single quantum dot 1, thereby regulating the single quantum dot. After curing, the AB resin has a large thermal expansion coefficient, which is much larger than that of metallic copper, semiconductors, and quartz. During the cooling-heating process of the fiber coupling component, sufficient thermal stress will accumulate between the AB resin and the quantum dot sample, thereby generating point defects in the vicinity of the single quantum dot 1, and jointly constructing an in-plane double electric field with the doping atoms, generating electric quadrupole biexcitons and single-path HV polarization-correlated photon pairs. Using UV-curing glue as a transition layer between the AB resin and the quantum dot sample can reduce thermal stress, allowing the fiber coupling component 6 to maintain the HH / VV polarization correlation of the single and biexciton cascade luminescence.

[0044] According to the embodiments of the present disclosure, the single-mode fiber 5 transmits two mutually perpendicular linear polarization modes, effectively maintaining linearly polarized single photons during transmission. The low polarization-correlated contrast ratio is due to the collapse of the single-photon polarization state at the mode-field coupling point of the fiber beam splitter 8, requiring precise control of the polarization principal axis to align with the polarization direction of the single quantum dot.

[0045] The light output module is used to collect fluorescence from single quantum dots and output the collected single- and biexciton cascade light in the form of a Gaussian light field. The light output module includes: a first beam splitter 8 and an optical fiber output 10.

[0046] The first beam splitter 8 is used to collect fluorescence from a single quantum dot. The first beam splitter 8 can be, for example, a single-mode optical fiber with a fusion splice or built-in beam splitter, which achieves wavelength splitting or power fractionation. By controlling the polarization field coupling at the optical fiber splitting point, the linear polarization of the photons can be effectively maintained.

[0047] According to an embodiment of the present disclosure, the first beam splitter 8 is a fiber beam splitter with a splitting ratio of >80:20, wherein the first beam splitter (8) is aligned with the polarization principal axis of the single quantum dot (1). Since polarization mode field coupling at the splitting point will cause the polarization state of the single photon to collapse and affect the correlation, it is necessary to precisely adjust the alignment of the polarization principal axis of the beam splitter with the single quantum dot.

[0048] Fiber output 10 is used to output the collected single-exciton and biexciton cascade luminescence as a Gaussian light field. Because Gaussian beams are self-focusing and transmission-invariant, outputting single-exciton and biexciton cascade luminescence as a Gaussian light field effectively overcomes the effects of system vibrations and achieves stable beam transmission.

[0049] The photon pair correlation test unit 11 is used to filter the single and biexciton cascade quantum luminescence and test the polarization correlation based on the filtered single and biexcitons.

[0050] According to an embodiment of the present disclosure, the photon pair correlation test unit includes: a filter 12 , a beam splitter 13 , a narrow line filter group, a polarizer group, and a counting and measuring module 20 .

[0051] The filter 12 is used to eliminate the laser background and filter out the fluorescence of the single quantum dot 1.

[0052] According to an embodiment of the present disclosure, the single- and biexciton cascade luminescence outputted from the optical fiber output 10 is first filtered by the filter 13 to eliminate the laser background and obtain the fluorescence emitted by the single quantum dot.

[0053] The beam splitter 13 is used to evenly split the fluorescent signal into two paths to obtain a first fluorescent signal and a second fluorescent signal.

[0054] The narrow line filter set is used to filter the first fluorescent signal and the second fluorescent signal respectively to obtain single exciton luminescence and biexciton luminescence.

[0055] According to an embodiment of the present disclosure, the narrow line filter set includes a first narrow line filter 14 and a second narrow line filter 15. The first narrow line filter is used to filter the first fluorescent signal to obtain single exciton luminescence. The second narrow line filter 15 is used to filter the second fluorescent signal to obtain biexciton luminescence.

[0056] According to an embodiment of the present disclosure, the first narrow-line filter 14 and the second narrow-line filter 15 are both dielectric film interference filters with a filtering line width of less than 0.5 nm and a transmittance of more than 60%. The filtering wavelength can be tuned by adjusting the angle or cavity length of the first narrow-line filter and / or the second narrow-line filter.

[0057] During the specific implementation, while the first narrow-line filter 14 and / or the second narrow-line filter 15 are precisely adjusted, the rear-end receiving fiber is connected to an imaging spectrometer to facilitate real-time observation of the filtered spectrum and ensure that a single-line spectrum is filtered out. After the filter set is adjusted, the rear-end receiving fiber is disconnected from the imaging spectrometer and connected to a single-photon counter and counting and measurement module 20 to begin testing.

[0058] Among them, the optical beam splitter 13 and the narrow line filter group can also be replaced by optical fiber filtering elements, such as fiber Bragg gratings or dispersion elements (such as gratings combined with knife edges). Technicians can make choices based on actual application scenarios, and this disclosure does not make specific limitations here.

[0059] The polarizer group is used for polarization filtering of single exciton luminescence and biexciton luminescence.

[0060] According to an embodiment of the present disclosure, the polarizer set includes a first polarizer 18 and a second polarizer 19. The first polarizer 18 is connected to the first narrow-line filter 14 and is used to perform polarization filtering on the light emitted by a single exciton, filtering out photons with H polarization; the second polarizer 19 is connected to the second narrow-line filter 15 and is used to perform polarization filtering on the light emitted by a biexciton, filtering out photons with V polarization.

[0061] The counting and measuring module 20 is used to measure the second-order cross-correlation of two photons to test the polarization correlation.

[0062] In addition, the device for realizing photon pair emission and optical fiber output provided by the embodiment of the present disclosure further includes: a low-temperature cooling stage 7 and a light source output device 9.

[0063] The cryogenic refrigeration stage 7 is connected to the optical fiber coupling assembly 6 and is used to control the temperature of the optical fiber coupling assembly 6. The cryogenic refrigeration stage 7 is a low-temperature refrigerator with a refrigeration temperature below 50K and is easy to use and portable, including but not limited to a closed-circuit helium cycle cryostat.

[0064] The light source output device 9 is used to output excitation laser to excite the excitons of the single quantum dots to emit light.

[0065] According to an embodiment of the present disclosure, the light source output device 9 is connected to the first beam splitter 8, and the output excitation laser is coupled to the single quantum dot 1 through the first beam splitter 8 to excite the exciton to emit light.

[0066] According to an embodiment of the present disclosure, the light source output device 9 is equipped with a rotating wheel with an adjustable attenuator. By rotating the wheel, the transmittance of the light beam is changed, thereby achieving continuous and precise adjustment of the optical fiber output power. The maximum power of the light source output device 9 is at least sufficient to saturate the excitons of the single quantum dot 1. A pulsed light source can also be used as the excitation light source, but the power requirement of the pulsed light source is slightly higher. In the embodiment of the present disclosure, the excitation light source generated by the light source output device 9 is a laser with a wavelength less than 800nm.

[0067] FIG2 schematically shows a principle diagram of a single quantum dot generating single and biexciton cascade luminescence in an embodiment of the present disclosure.

[0068] As shown in FIG2 , a modulation doping layer 21 is distributed above or below the quantum dot layer at a distance less than 10 nm.

[0069] According to an embodiment of the present disclosure, the modulation doping layer is δ-doped and is located above or below the quantum dot layer, with a distance from the quantum dot layer of less than 10 nanometers. The doping concentration of the modulation doping layer 21 is determined based on the quantum dot density in the quantum dot layer (0 to 2 per square micron) to ensure that each single quantum dot 1 is occupied by 1 to 2 electrons, thereby generating a built-in electric field of the doped atoms (i.e., a local electric field) and improving the biexciton population.

[0070] According to an embodiment of the present disclosure, the built-in electric field includes multiple doping atoms 22, and the positions of the doping atoms 22 relative to the single quantum dot 1 are random. The doping atoms 22 can reduce the splitting of the exciton fine structure by introducing vertical electric field components. At the same time, since the positions of the doping atoms 22 are random, for the excitons, the electric field components generated by the doping atoms 22 are multi-directional within the plane, which can reduce the splitting of the exciton fine structure while regulating the degeneracy of the dipole wave function of the exciton's dual polarization state, thereby affecting cross-dephasing.

[0071] According to the embodiments of the present disclosure, the electron-hole overlap degree 24 and the exciton fine structure splitting 25 of the hillock-shaped quantum dot (i.e., the single quantum dot 1) can be effectively changed by the in-plane electric field component 23. The in-plane electric field component 23 in a specific direction can cause the exciton wave function to stretch sharply, thereby degenerate with the exciton wave function in another perpendicular direction, reduce cross-dephasing 26, and cause the single and double exciton luminescence to produce two sets of polarization-correlated photon pairs HH and polarization-correlated photon pairs VV that are independent of each other, effectively reducing the generation of HV-correlated photons, and avoiding the signal interference that may be caused by HV-correlated photons during the detection process, so as to ensure the accuracy and reliability of polarization correlation detection. Among them, the polarization-correlated photon pairs HH and polarization-correlated photon pairs VV generated by the single and double exciton luminescence are based on the photon pair correlation test unit 11 to detect polarization correlation, wherein the photon pair correlation test unit can test the polarization correlation through a polarization-resolved second-order correlation test.

[0072] According to an embodiment of the present disclosure, the single quantum dot 1 is a hillock-type InAs quantum dot, which has weak in-plane quantum confinement and whose electron-hole overlap degree 24 and exciton fine structure splitting 25 are easily changed by the in-plane electric field component 23 .

[0073] According to the embodiments of the present disclosure, the excitation power of a single quantum dot must be very low (on the order of hundreds of nW) to ensure that the luminescence signal of a single exciton is stronger than the luminescence signal of a biexciton, thereby effectively reducing the influence of biexciton redistribution and on-band carrier spin scattering on the polarization correlation, so that the polarization-correlated photon pairs are contained in a large number of single-photon sequences of single-exciton luminescence 16, so as to maintain the high concealment and detection sensitivity of the polarization-correlated photon sequence.

[0074] FIG3 schematically shows the test results of the polarization-resolved second-order correlation function of the quantum dot polarization-correlated photon pair quantum source in an embodiment of the present disclosure.

[0075] As shown in FIG3 , a very high correlation count was detected on the HH (VV, not shown) polarization basis, while the correlation count on the HV polarization basis was low, indicating that the present disclosure presents HH / VV polarization correlation, and this polarization correlation contrast can be used to achieve quantum enhanced detection. According to FIG3 , the quantum dot exhibits four exciton peaks in the 920nm band, and the two middle exciton peaks are single exciton luminescence 16 and biexciton luminescence 17, respectively. The in-plane electric field direction is at a 45-degree angle to the material crystal phases

[0110] and [1-10] (reflected by the arrow in the figure, which is the direction of the lowest energy exciton). At this time, the dipole wave functions |H> and |V> of the exciton's dual polarization states are no longer degenerate (as shown in FIG2 ), eliminating cross-dephasing 26. Cross-dephasing can be expressed as ∫ T <H|exp(iFSS*t / h)|V>dt, where T is the fluorescence lifetime of a single exciton, FSS is the fine structure splitting, t is time, h is Planck's constant, and i is the imaginary unit.

[0076] FIG4 schematically shows the test results of the polarization-resolved second-order correlation function in one embodiment of the present disclosure.

[0077] As shown in Figure 4 , the fiber coupling assembly 6 in this embodiment is encapsulated using UV adhesive. The cured encapsulation of the fiber coupling assembly 6 with UV adhesive as a transition layer results in minimal thermal stress accumulation. Testing revealed that the correlation counts for the HH polarization basis are still higher than those for the HV polarization basis, but the correlation contrast is lower than that of the quantum dot sample shown in Figure 3 . This is because the principal axis of the polarization mode field coupling at the first beam splitter 8 deviates from the HV linear polarization direction of the quantum dots, causing the polarization of the single photons transmitted through the fiber to randomly "collapse" along the fiber's birefringence axis, reducing contrast. The circular polarization of the fiber's output luminous intensity (illustrated in the illustration) also reflects this random "collapse" of the single-photon polarization, contrasting with the elliptical polarization shown in Figure 2 .

[0078] According to an embodiment of the present disclosure, when the correlation contrast is lower than that of the quantum dot sample result, it is necessary to perform a calibration operation on the first beam splitter 8 to align the polarization main axis direction of the first beam splitter 8 with the HV linear polarization direction of the quantum dot, control the polarization light field coupling at the splitting point, thereby maintaining the linear polarization of the single photon, so that the polarization correlation of the photon pair is output as shown in Figure 3.

[0079] FIG5 schematically shows the test results of polarization-resolved spectrum and second-order correlation function in another embodiment of the present disclosure.

[0080] As shown in Figure 5, the fiber coupling assembly 6 in this embodiment is encapsulated by curing AB resin. Since the cured AB resin has a large thermal expansion coefficient, the thermal stress accumulated in the fiber coupling assembly 6 during the cooling and heating process can generate point defects 29 in the vicinity of the single quantum dot 1.

[0081] As shown in FIG5 , according to an embodiment of the present disclosure, the built-in electric field (ie, the local electric field) includes point defects 29 in addition to the dopant atoms 22 . The point defects 29 also serve as modulation doping to form the built-in electric field.

[0082] According to the embodiment of the present disclosure, a dual-modulated electric field 30 is jointly generated by the doping atoms 22 and the point defects 29. The dual-modulated electric field 30 can cause the single quantum dot 1 to generate an "electric quadrupole" biexciton wave function 31, which can reduce the spatial overlap 24 between the electron cloud and the hole cloud of the exciton, thereby reducing the exciton binding energy, making the biexciton luminescence 17 and the single exciton luminescence 16 spectrum close, and facilitating the control of the interaction between the single and biexcitons. In addition, only a single HV polarization-associated photon pair emission 32 is allowed to be generated (i.e., the biexciton is V and the single exciton is H). The polarization state of the single HV polarization-associated photon pair is very certain, and the transmission stability can be maintained in a single-mode optical fiber, presenting the polarization-resolved light intensity as shown in the figure.

[0083] According to an embodiment of the present disclosure, a single HV polarization-correlated photon pair emission 32 is also detected by the second-order correlation test. At this time, the second-order correlation function reflects the autocorrelation of the sum of the two, and the actual HV polarization correlation count is higher than the test result in the figure.

[0084] According to an embodiment of the present disclosure, the spectral lines of biexciton luminescence 17 and single exciton luminescence 16 are identified by varying the excitation power spectrum: single exciton luminescence is dominant at low excitation power, and biexciton luminescence is dominant at high excitation power.

[0085] According to the embodiment of the present disclosure, there is no cross-dephasing in the emission 32 of the single HV polarization-associated photon pair, and dual polarization association of HV and VH can be achieved through post-selection (such as a 2×2 fiber splitter forming a Sagnec ring).

[0086] The disclosed embodiments utilize semiconductor self-organized quantum dots to generate biexciton-exciton photon pair emission, reduce exciton fine structure splitting by modulating the local electric field of the doping, and regulate the exciton anisotropy based on the electric field component within the modulated doping electric field, so as to degenerate the exciton wave function and avoid cross-dephasing, so that the cascade transition luminescence of the biexcitons and excitons produces HH / VV linear polarization correlation, and outputs through optical fiber.

[0087] Compared with the prior art, the present invention directly generates photon pairs by exciting quantum dots, without considering the technical issues such as the pump laser focus spot size, crystal temperature control, cavity design, quasi-phase matching required by nonlinear conversion, effectively simplifying the generation process of photon pairs, the generation method of photon pairs is relatively simple, and the photon pair counting rate is high. By integrating the microcavity, the quantum dots are made to emit light in a directional manner, so as to facilitate the collection of photon pairs. The exciton wave function is regulated by the local electric field to achieve the purpose of dedegeneracy and reduce cross-dephasing, thereby generating HH / VV or HV linearly polarized correlated photon pairs, and realizing efficient optical fiber coupling output for easy use. Among them, the photon pair spectrum generated by quantum dots is sharp and can be narrow-line filtered or quantum interference measurement can be performed. The photon pairs generated by the method provided by the present invention are contained in a large number of single-photon sequences, maintaining high concealment and detection sensitivity, and due to the linear polarization of the photons, they can be well maintained in single-mode optical fiber transmission.

[0088] FIG6 schematically shows a flow chart of a method for realizing photon pair emission and optical fiber output based on quantum dots according to an embodiment of the present disclosure.

[0089] As shown in FIG6 , the method for realizing photon pair emission and optical fiber output based on quantum dots includes operations S100 to S400 , including:

[0090] In operation S100 , single and biexciton cascade light emission is generated from a single quantum dot.

[0091] In operation S200 , the exciton luminescence characteristics are regulated by modulating the doping local electric field to generate polarization-correlated photon pairs.

[0092] In some embodiments, the exciton luminescence characteristics include energy splitting, wave function degeneracy, and electric quadrupole biexcitons. The local electric field regulates the single quantum dot to reduce the exciton energy splitting. The in-plane electric field can degenerate the exciton wave function and reduce the doublet cross-vector phase. The in-plane double-doping electric field can generate electric quadrupole biexcitons, realizing HH / VV or HV polarization-correlated photon pairs.

[0093] In operation S300, the single and biexciton cascade light emission is collected based on the light output module;

[0094] In operation S400 , the single and biexciton luminescence is filtered based on the photon pair correlation test unit and polarization correlation is checked.

[0095] The specific embodiments described above further illustrate the technical solutions of the present disclosure in detail. It should be understood that the above description is merely a specific embodiment of the present disclosure and is not intended to limit the present disclosure. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present disclosure should be included in the scope of protection of the present disclosure.

Claims

1. A device for realizing photon pair emission and optical fiber output based on quantum dots, characterized in that: include: A single quantum dot (1) is used to generate single and double exciton cascade luminescence, wherein the exciton state has H and V dual polarization states, wherein the dual polarization states have energy splitting; A microcavity (2) is formed by a first Bragg reflector (3) and a second Bragg reflector (4), and is used to control the single quantum dot (1) to emit light in a directional manner; A fiber coupling assembly (6) is prepared by vertically bonding single-mode optical fibers and then solidifying and encapsulating them, and is used to collect the single- and biexciton cascade luminescence generated by the single quantum dot (1); A local electric field, generated by a single dopant atom (22) in a neighboring modulation doping layer (21), is used to control the single quantum dot to reduce exciton energy splitting (25); A luminescence output module, configured to collect fluorescence from the single quantum dot and output the collected single- and biexciton cascade luminescence in the form of a Gaussian light field; A photon pair correlation test unit (11) is used for filtering the single and biexciton cascade luminescence and testing polarization correlation based on the filtered single and biexcitons.

2. The device according to claim 1, characterized in that The thickness of the microcavity (2) is the equivalent length of the luminescence wavelength of the single quantum dot (1) in the current material.

3. The device according to claim 2, characterized in that The first Bragg reflector (3) and the second Bragg reflector (4) are arranged in parallel in an up-down manner and are made of a material with a periodically alternating distribution of refractive indices.

4. The device according to claim 1, characterized in that The light output module includes: a first beam splitter (8), for exciting the single quantum dot and collecting fluorescence; The optical fiber output unit (10) is used to output the single and biexciton cascade luminescence in the form of a Gaussian light field.

5. The device according to claim 4, characterized in that The first beam splitter (8) is a fiber beam splitter with a splitting ratio greater than 80:20, wherein the first beam splitter (8) is aligned with the polarization principal axis of the single quantum dot (1).

6. The device according to claim 4, characterized in that The device also includes: A light source output module (9) is used to generate an excitation laser to excite a single quantum dot, wherein the excitation laser is output through an optical fiber coupling component (6) and connected to the first beam splitter (8), and the output power of the light source output module (9) is continuously adjustable.

7. The device according to claim 1, characterized in that The photon pair correlation test unit includes: A filter (12) for eliminating laser background and filtering out the fluorescence of the single quantum dot (1); A second beam splitter (13) is used to evenly split the fluorescent signal into two paths to obtain a first fluorescent signal and a second fluorescent signal; A narrow line filter set, consisting of a first narrow line filter (14) and a second narrow line filter (15), for filtering the first fluorescent signal and the second fluorescent signal respectively to obtain single exciton luminescence and biexciton luminescence; A polarizer group, consisting of a first polarizer (18) and a second polarizer (19), for performing polarization filtering on the single exciton luminescence and the biexciton luminescence to obtain H polarization or V polarization; The counting and measuring module (20) is used to measure the second-order cross-correlation of two-path photons to test polarization correlation.

8. The device according to claim 7, characterized in that The first narrow line filter (14) and the second narrow line filter (15) are both filter elements with a filtering line width of less than 0.5 nanometers and a transmittance of more than 60%. The filtering wavelength is tuned by adjusting the angle or cavity length of the first narrow line filter and / or the second narrow line filter.

9. The device according to claim 1, characterized in that The optical fiber coupling component (6) is encapsulated by AB resin to generate thermal stress point defects based on temperature increase and decrease, wherein the thermal stress point defects can serve as a second doping in the local electric field to form an in-plane double electric field.

10. A method for realizing photon pair emission and optical fiber output based on quantum dots, characterized in that: include: Single and double exciton cascade luminescence is generated by a single quantum dot (1); The exciton luminescence properties are regulated by modulating the local electric field of the doping to generate polarization-correlated photon pairs; Collecting the single and biexciton cascade luminescence based on the luminescence output module; The photon pair correlation test unit filters the single and biexciton luminescence and checks the polarization correlation.

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