Multi-axis electric field regulation and control entanglement light source based on quantum dots

Abstract

The invention discloses a quantum dot-based multi-axis electric field regulation entangled light source, which adopts a p-i-n diode structure, and sequentially comprises a GaAs layer, an n-type doped layer, an n-type doped layer, an n-type doped layer, an n-type doped layer, an n-type doped layer, an n-type doped layer, an n-type doped layer and An Al < 0.15 > Ga < 0.85 > As layer and an Al < 0.33 > Ga < 0.67 > As layer are sequentially arranged above the n-type doped layer and jointly serve as a tunnel potential barrier to isolate the quantum dots from the n-type doped layer; the GaAs quantum dot layer is used for growing GaAs quantum dots in the Al < 0.33 > Ga < 0.67 > As layer by adopting a liquid drop etching method; an Al < 0.33 > Ga < 0.67 > As layer covers the quantum dots to serve as a blocking potential barrier, and carrier leakage is inhibited; and the top is a p-type doped layer. The wavelength-adjustable quantum dot entangled light source with high entanglement fidelity is realized, and the requirement of a quantum repeater in a quantum network can be met.

Description

Technical Field

[0001] The present invention relates to the field of quantum optics, and particularly to a multi-axis electric field controlled entangled light source based on quantum dots. Background Art

[0002] Classic communication networks, although having achieved remarkable achievements in the field of information transmission, have inherent limitations in terms of security, transmission rate, and computing power due to their principles based on classical physics. With the exponential growth of computing power, traditional encryption algorithms are at risk of being cracked by quantum computers. In addition, classical communication is limited by the speed of light and cannot meet the future demand for ultra-high-speed and large-capacity information transmission. In contrast, quantum networks are based on quantum mechanics and utilize characteristics such as quantum entanglement and quantum superposition to enable theoretically unconditionally secure quantum key distribution, thereby ensuring the absolute confidentiality of information.

[0003] The construction and expansion of quantum networks have a huge demand for quantum repeaters. As a key component connecting different quantum network nodes, quantum repeaters require entangled light sources with tunable wavelengths and high entanglement fidelity to meet the wavelength matching requirements of different quantum network nodes. Currently, nonlinear crystals and cold atomic systems are relatively mature platforms for preparing entangled light sources. Nonlinear crystals (e.g., periodically poled lithium niobate crystals) generate entangled photon pairs through the process of spontaneous parametric down-conversion, with high entanglement fidelity and a wide wavelength tuning range. However, nonlinear crystals have the problem of low photon generation efficiency, and the generation of entangled photon pairs is non-deterministic (probabilistic), which limits their application in quantum networks. Cold atomic systems generate entangled photons through atomic level transitions, with high photon indistinguishability and long coherence times, but their system complexity is high and the experimental environment requirements are stringent. Among them, semiconductor quantum dots can efficiently and stably generate entangled photon pairs due to their deterministic biexciton cascade radiation process, and have gradually become a research hotspot for entangled light sources that have received much attention. Quantum dot entangled light sources have unique advantages in terms of wavelength tunability, integration, etc., and can provide a reliable light source guarantee for the construction of large-scale quantum networks.

[0004] The principle of generating entangled photon pairs based on quantum dots is as Figure 1As shown. In the ground state, the valence band of the quantum dot is completely filled with electrons. Through two-photon resonant excitation, the quantum dot absorbs two photons simultaneously, and two electrons in the valence band jump to the conduction band, forming a biexciton state. At this time, the quantum dot contains two electron-hole pairs. These two electron-hole pairs recombine sequentially through a cascade radiation process. The two possible radiation paths are indistinguishable in quantum mechanics, and the polarization states of the two photons thus form a quantum entanglement state. However, the inherent fine structure splitting of the exciton state will introduce an energy difference in the two orthogonally polarized radiation paths, which will further cause the phase of the photon wave function to evolve differently over time, destroying the coherence of the entanglement state and thus significantly reducing the entanglement fidelity of the photon pair. Therefore, to achieve a high-fidelity entangled photon source, it is necessary to effectively eliminate the adverse effects of fine structure splitting on the coherence of the quantum state.

[0005] Using quantum dots to construct a quantum repeater requires that the quantum dots have tunable emission wavelengths while maintaining a high entanglement fidelity. The regulation of fine structure splitting is an effective way to achieve high entanglement fidelity, which can be realized by applying external fields such as a magnetic field, a transverse electric field or a stress field. At the same time, the tuning of the emission wavelength of the quantum dot mainly depends on the regulation of the longitudinal electric field. However, there is a coupling effect between the fine structure splitting and the emission wavelength regulation, that is, the wavelength adjustment process will inevitably affect the characteristics of the fine structure splitting. This mutual restriction relationship makes it difficult for a single regulation method to optimize these two key parameters simultaneously. For this reason, a multi-physical field collaborative regulation strategy is generally adopted at present, such as: multi-axis stress regulation or electric field-magnetic field composite regulation and other methods to achieve more precise and effective regulation of the emission characteristics of quantum dots. However, the above existing technologies still have the following limitations:

[0006] 1. Higher device complexity: The combination of multiple regulation means usually requires multiple external devices, resulting in a relatively complex overall device design, which is not conducive to future large-scale integration of optical quantum chips;

[0007] 2. The current scheme cannot achieve the regulation of local quantum dots: The applied stress, electric field or magnetic field will affect the quantum dots on the entire chip, making it difficult to adjust specific local quantum dots; this method is not conducive to the independent regulation of multiple quantum dot entanglement light sources on the chip. Summary of the Invention

[0008] The present invention provides a multi-axis electric field regulated entangled light source based on quantum dots. The present invention realizes the output of wavelength-tunable polarization entangled photon pairs by collaboratively regulating the fine structure splitting and emission wavelength of the quantum dots; the present invention adopts a cross-shaped configuration electrode to apply orthogonal electric fields in the transverse dimension to adjust the fine structure splitting of the quantum dots; a bias electric field is applied in the longitudinal dimension to regulate the exciton recombination energy through the quantum confinement Stark effect, thereby regulating the emission wavelength of the quantum dots, as described in detail below:

[0009] A multi-axis electric field regulated entangled light source based on quantum dots. The light source adopts a p-i-n diode structure, which from bottom to top is successively:

[0010] Substrate layer: providing a lattice-matched substrate to ensure the growth of subsequent epitaxial layers;

[0011] n-type doped layer: composed of silicon-doped Al 0.15 Ga 0.85 As. Above this layer is a barrier layer to isolate the quantum dots from the n-type doped layer;

[0012] Quantum dot layer: Quantum dots are embedded in the p-i-n diode structure. Above this layer is a barrier layer to suppress carrier leakage;

[0013] p-type doped layer: composed of carbon-doped Al 0.15 Ga 0.85 As.

[0014] Among them, through electron beam lithography, orthogonal electrodes are constructed in the p-type doped layer around the target quantum dots, adopting a cross configuration to surround the quantum dots, and the redundant areas are removed by selective etching. The electrodes extend to the edge of the device and are coated with metal gold, and the application of a transverse electric field is realized through independent leads.

[0015] Among them, the orthogonal electrodes realize transverse electric fields in four independent orthogonal directions through independent leads, realizing the biaxial dynamic regulation of the fine structure splitting of quantum dots.

[0016] Among them, the electrodes extend to the edge of the device and are coated with metal gold to suppress the interference of surface metal on the photon polarization state and photon collection efficiency. While maintaining the conductivity of the electrodes, the original polarization information of photons is maintained, and a high photon collection efficiency is maintained.

[0017] Among them, the entangled light source adjusts the fine structure splitting of quantum dots by applying a transverse electric field, and tunes the emission wavelength of quantum dots by using a longitudinal electric field, realizing the wavelength tunability of the quantum dot entangled light source and maintaining a high entanglement fidelity.

[0018] Among them, the entangled light source tunes the emission wavelength of quantum dots while maintaining the fine structure splitting of quantum dots approximately zero through the iterative collaborative regulation of the transverse electric field and the longitudinal electric field.

[0019] The beneficial effects of the technical solution provided by the present invention are:

[0020] 1. By collaboratively regulating the fine structure splitting and emission wavelength of quantum dots, the present invention can realize a quantum dot entangled light source with wavelength tunability and high entanglement fidelity, which can meet the requirements of quantum repeaters in quantum networks;

[0021] 2. The present invention realizes the tuning of the fine structure splitting and the emission wavelength. Through the collaborative regulation of the orthogonal transverse electric field, while keeping the fine structure splitting of the quantum dots approximately zero, the quantum-confined Stark effect of the longitudinal electric field can be used to tune the exciton recombination energy; through iterative collaborative regulation, it is ensured that the entanglement fidelity remains at a high level during the wavelength tuning process, thereby guaranteeing the reliability and stability of the quantum repeater;

[0022] 3. The present invention extends the surface electrode to the edge of the device and evaporates metallic gold, which can effectively suppress the influence of the surface metal on the photon polarization state and the photon collection efficiency. While maintaining the conductivity of the electrode, the original polarization information of the photons is maintained, and a high photon collection efficiency is maintained;

[0023] 4. The present invention can apply transverse electric fields in four independent orthogonal directions to the quantum dots. By controlling the bias voltages of the respective electrodes, any combination configuration of the transverse electric field components can be achieved in the quantum dot region; this multi-axis electric field regulation mechanism can adjust the in-plane electric field distribution, providing a full-degree-of-freedom regulation ability for the fine structure splitting of the quantum dot exciton state;

[0024] 5. The present invention can realize the collaborative regulation of the emission wavelength and the fine structure splitting of the quantum dots only by means of a single physical method of electric field regulation; compared with traditional multiple regulation mechanisms (such as the combined regulation of electric field and magnetic field, etc.), this scheme significantly reduces the structural complexity and the process difficulty of the device, and at the same time improves the stability of generating entangled photon pairs, providing a basis for the expansion of future large-scale quantum networks;

[0025] 6. The present invention can realize the localized regulation of the quantum dots, and this localized regulation ability provides a technical basis for the future parallel manipulation of a multi-quantum dot entangled light source array on a monolithic integrated chip. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 is a schematic diagram of a multi-axis electric field regulated entangled light source based on quantum dots;

[0027] Figure 2a is a schematic structural diagram of a multi-axis electric field regulated entangled light source based on quantum dots;

[0028] Figure 2b is a top view of the device structure;

[0029] Figure 3 is a schematic diagram of the response of the quantum dots to the longitudinal electric field;

[0030] Figure 4 is a schematic diagram of the response of the quantum dots to the transverse electric field. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0031] To make the objectives, technical solutions, and advantages of the present invention clearer, the following further describes the embodiments of the present invention in detail.

[0032] Example 1

[0033] The embodiment of the present invention discloses a design scheme of a multi-axis electric field regulated entangled light source based on quantum dots. The scheme is as follows:

[0034] The multi-axis electric field regulated entangled light source adopts a p-i-n diode structure. Its core functional layer is composed of GaAs quantum dots grown by the droplet etching method. The p-type doping layer is above it, and the n-type doping layer is below it. From bottom to top, they are: GaAs layer: providing a GaAs substrate with lattice matching to ensure the high-quality growth of subsequent epitaxial layers; n-type doping layer: composed of silicon-doped Al 0.15 Ga 0.85 As, and above it are successively Al 0.15 Ga 0.85 As layer and Al 0.33 Ga 0.67 As layer, jointly serving as a tunneling barrier to isolate the quantum dots from the n-type doping layer; GaAs quantum dot layer: growing GaAs quantum dots in the Al 0.33 Ga 0.67 As layer by the droplet etching method; the quantum dots are covered with an Al 0.33 Ga 0.67 As layer as a blocking barrier to inhibit carrier leakage; the p-type doping layer includes three layers: carbon-doped Al 0.15 Ga 0.85 As layer (p+); carbon-doped Al 0.15 Ga 0.85 As layer (p++); carbon-doped GaAs layer (p++). The p-type doping layer and the bottom n-type doping layer form a longitudinal p-i-n diode structure, and a longitudinal bias voltage is applied. At the same time, combined with a transverse electric field, it can effectively compensate for the fine structure splitting of the quantum dots.

[0035] In the device manufacturing process, first, a high-precision quantum dot positioning technology (such as: cathodoluminescence imaging) is used to perform sub-micron spatial positioning on the quantum dots (such as: positioning accuracy <50 nm). Subsequently, through electron beam lithography technology, orthogonal electrodes are constructed in the p-type doping layer around the target quantum dots, adopting a cross configuration to surround the quantum dots, and the redundant areas are removed through selective etching technology. This cross-configuration electrode realizes a transverse electric field in four independent orthogonal directions through independent leads, thereby achieving a biaxial dynamic regulation of the fine structure splitting of the quantum dots. At the same time, by applying a bias voltage to the p-i-n structure to generate an adjustable longitudinal electric field, combined with the quantum-confined Stark effect, continuous tuning of the exciton recombination energy, that is, the emission wavelength of the quantum dots, is realized.

[0036] Example 2

[0037] The entangled light source based on quantum dots and multi-axis electric field regulation proposed in the embodiments of the present invention adjusts the fine structure splitting of quantum dots by applying a transverse electric field, and at the same time tunes the emission wavelength of quantum dots by using a longitudinal electric field, so as to realize the tunability of the wavelength of the quantum dot entangled light source and maintain a high entanglement fidelity.

[0038] Figure 2a The device structure diagram of the embodiments of the present invention is shown. The device structure mainly includes: a quantum dot layer, with a p-type doped layer above it and an n-type doped layer below it. Through electron beam lithography, orthogonal electrodes are constructed in the p-type doped layer around the target quantum dot, adopting a cross configuration to surround the quantum dot, and the redundant areas are removed by selective etching technology. Figure 2b This is the top view of the device structure. Four independent orthogonal transverse electric fields are applied to the quantum dot. By independently controlling the bias voltages of each electrode, any combination configuration of the transverse electric field components can be realized in the quantum dot region. This multi-axis electric field regulation mechanism can adjust the in-plane electric field distribution; at the same time, by applying a bias voltage to the p-i-n structure composed of the p-type doped layer and the n-type doped layer, a longitudinal electric field is generated. Combining with the quantum-confined Stark effect, the regulation of the exciton recombination energy, that is, the emission wavelength of the quantum dot, is realized.

[0039] Figure 3 The emission wavelength regulation mechanism of the quantum dot under the action of an external longitudinal electric field is shown. This regulation process is based on the quantum-confined Stark effect: when no external field is applied, the intrinsic built-in electric field inside the quantum dot causes its energy band to show an asymmetric tilted distribution, leading to a spatial localization shift of the electron and hole wave functions. When a longitudinal DC electric field is applied, the external field and the built-in electric field are vectorially superimposed to form a total composite electric field F z , and its intensity can be continuously adjusted by an external bias voltage. This composite electric field will significantly change the tilt degree of the energy band, resulting in a reverse spatial displacement of the electron and hole wave functions along the electric field direction. This relative displacement of the carrier wave functions causes a Stark shift in the exciton recombination energy, and its mathematical expression is:

[0040]

[0041] where E0 is the intrinsic transition energy when the external electric field is 0, p z is the electric dipole moment inside the quantum dot along the electric field z direction, F z is the intensity of the applied DC electric field, and β is the polarizability of the quantum dot.

[0042] In experimental observations, this energy shift is manifested as a red shift or blue shift of the characteristic peak of the quantum dot fluorescence spectrum, and its wavelength shift shows a quasi-linear dependence on the applied electric field intensity.

[0043] Figure 4Shows the response of quantum dots to a transverse electric field. The heterojunction sample used in the embodiments of the present invention is grown on a

[001] -oriented GaAs substrate by molecular beam epitaxy, and the quantum dots are embedded in a p-i-n diode structure. There are geometric asymmetries and residual stress anisotropies during its growth process. This asymmetry causes the eigenwave function of the quantum dots to evolve into an asymmetric superposition state of electron-hole wave functions, and further produces fine structure splitting in the energy level structure. Applying a transverse electric field in the plane of the quantum dots can adjust the spatial distribution of the electron and hole wave functions. The additional potential energy generated by the electric field effectively compensates for the spatial asymmetry of the wave function introduced by the initial growth, making the wave functions of holes and electrons tend to a more symmetric state, thereby adjusting the fine structure splitting. Different from the quantum-confined Stark effect caused by a longitudinal electric field, the transverse electric field mainly changes the spatial symmetry of the wave function rather than the overall spatial displacement, which makes the influence of the transverse electric field on the emission wavelength of quantum dots extremely weak.

[0044] The specific embodiments described above further elaborate on the purpose, technical solutions, and beneficial effects of the present invention. It should be understood that the above are only specific embodiments of the present invention and are not used to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

[0045] Those skilled in the art can understand that the drawings are only schematic diagrams of a preferred embodiment, and the serial numbers of the embodiments of the present invention above are only for description and do not represent the superiority or inferiority of the embodiments.

[0046] The above is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims

1. A multi-axis electric field-regulated entangled light source based on quantum dots, characterized in that, The entangled light source adopts a pin diode structure, which is as follows from bottom to top: Substrate layer: provides a lattice-matched substrate to ensure the growth of subsequent epitaxial layers; n-type doping layer: formed by silicon-doped Al 0.15 Ga 0.85 As, above which is a barrier layer to isolate the quantum dots from the n-type doping layer; Quantum dot layer: quantum dots are grown in the intrinsic layer using the self-organized growth method, and a barrier layer is placed above this layer to suppress carrier leakage; p-type doping layer: composed of carbon-doped Al 0.15 Ga 0.85 As 2. The multi-axis electric field-regulated entangled light source based on quantum dots according to claim 1, characterized in that, Through electron beam lithography, an orthogonal electrode is constructed in the p-type doped layer outside the target quantum dot. The orthogonal electrode surrounds the quantum dot in a cross configuration, and the redundant area is removed by selective etching.

3. The multi-axis electric field controlled entangled light source based on quantum dots according to claim 2, wherein The orthogonal electrodes realize transverse electric fields in four independent orthogonal directions through independent leads, thereby realizing biaxial dynamic regulation of the fine structure splitting of quantum dots.

4. A multi-axis electric field regulated entangled light source based on quantum dots according to claim 1, characterized in that, A cross-shaped electrode structure is formed by electron beam lithography. The electrode extends to the edge of the device and is evaporated with metal gold to reduce the influence of surface metal on the polarization state of generated photons and the collection efficiency.

5. A multi-axis electric field-regulated entangled light source based on quantum dots according to claim 1, characterized in that, The entangled light source regulates the fine structure splitting of quantum dots by applying a transverse electric field, and uses a longitudinal electric field to tune the emission wavelength of the quantum dots, thereby achieving wavelength tunability of the quantum dot entangled light source and maintaining high entanglement fidelity.

6. A multi-axis electric field regulated entangled light source based on quantum dots according to claim 1, characterized in that, The entangled light source tunes the emission wavelength of the quantum dots by iteratively regulating the transverse electric field and the longitudinal electric field while keeping the fine structure splitting of the quantum dots approximately zero.

7. A multi-axis electric field controlled entangled light source based on quantum dots according to claim 1, characterized in that, Based on the full electric field control scheme, a wavelength-tunable entangled light source is realized, which can achieve localized control and is conducive to the on-chip integration of multiple entangled light sources in the future.

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