A Bragg reflection waveguide for directly generating hybrid entangled photon pairs and its applications

Abstract

The present invention discloses a Bragg reflection waveguide for directly generating hybrid entangled photon pairs and its application. The thicknesses of each layer and the effective refractive index of the mode are determined through software simulation calculations, and then an AlGaAs Bragg reflection waveguide is fabricated accordingly. It is used to prepare a device for directly generating hybrid entangled photon pairs, and the device includes: a pump laser, a polarization controller, a single-mode lens optical fiber, a Lagrangian reflection waveguide, a long-pass filter, and a wavelength division multiplexer. The present invention has the advantages of high generation efficiency of entangled photon pairs, a phase matching bandwidth, low insertion loss, a simple and compact optical path structure, stable performance, easy operation and control, etc. It can integrate a laser and a nonlinear process together to form a solution for generating entangled light by electrical drive, providing an effective solution for the integration, miniaturization, and large-scale production of quantum optical chips.

Description

Technical Field

[0001] The present invention belongs to the technical field of quantum information science and technology, and particularly relates to a Bragg reflection waveguide for directly generating hybrid entangled photon pairs and its applications. Background Art

[0002] The structural size of silicon-based electronic chips has approached the limit of chips at the atomic scale, and Moore's law is difficult to continue to hold. With the proposal and application of artificial intelligence and deep learning algorithms, classical silicon-based chips are gradually beginning to fail to meet people's needs. With the continuous development of quantum technology, people pin their hopes on quantum computing chips to bring more powerful computing capabilities. Currently, quantum computing chips are still in the research and development stage, and the materials used each have their own advantages, such as quantum wells, cold atoms, superconducting materials, and so on. Photons are one of the ideal media for realizing quantum information processing. Encoding using photons has the following advantages: First, the interaction between photons and matter is small, enabling photons to well maintain the quantum superposition state, that is, having the ability to resist decoherence; Second, photons have rich degrees of freedom, such as frequency, polarization, mode, angular momentum, etc., and photons have a high transmission speed, so they are ideal flying qubits; Finally, considering the current traditional optoelectronic devices used, photons can be well compatible with existing fiber optic communication networks. These advantages have made quantum information technology with photons as the information carrier widely concerned soon after its proposal. Through the solid-state device, miniaturization, and chip integration of quantum optical paths, realizing large-scale scalable quantum information processing has become an inevitable trend for its wide application.

[0003] Gallium arsenide semiconductor materials and their derivative compounds AlGaAs provide an excellent platform for realizing integrated quantum entanglement chips. Since GaAs materials are direct bandgap semiconductors, compared with Si-based indirect bandgap semiconductors, they can achieve electrical injection, that is, there is no need to introduce a light source outside the quantum optical integrated chip. Therefore, GaAs semiconductors can not only be used as the dielectric waveguide for SPDC, but also as the pump light source in the process of spontaneous parametric down-conversion. In addition, GaAs materials have a large second-order nonlinearity ~ 200 pm / V, a wide transparent window of 1 - 17 µm, a large damage threshold, and low linear propagation loss, etc. However, as a semiconductor material, GaAs has large material dispersion and lack of material birefringence, which will make it difficult to achieve the phase matching condition in the nonlinear process. Summary of the Invention

[0004] Technical problems to be solved: Aiming at the above technical problems, the present invention provides a Bragg reflection waveguide for directly generating hybrid entangled photon pairs and its applications, providing an efficient, stable, and integratable semiconductor quantum entanglement light source for fields such as quantum integrated chips and quantum information processing.

[0005] Technical solution: A Bragg reflection waveguide for directly generating hybrid entangled photon pairs is prepared through the following steps:

[0006] Step 1: Set the guessed thickness values of the core layer and the periodic cladding in the Bragg reflection waveguide, and find the effective refractive index of the current total internal reflection mode TIR through software simulation calculation;

[0007] Step 2: Substitute this value into the program for calculating the effective refractive index of the BRW mode, and calculate the thickness values of each layer that conform to the Bragg reflection waveguide theory;

[0008] Step 3: Then re-enter the calculated thickness values of each layer into the software simulation to obtain the new effective refractive index of the BRW mode and the effective refractive index of the total internal reflection mode TIR;

[0009] Step 4: Repeat steps 2 and 3 until the effective refractive index of the BRW mode obtained by software simulation calculation is between the effective refractive index of the TIR-TE mode and the effective refractive index of the TIR-TM mode, and the thickness values of each layer are consistent with the calculated values, then the thicknesses of each layer and the effective refractive index of the mode of the final design can be determined;

[0010] Step 5: After growing the SiO2 etching mask by PECVD, then fabricate a photoresist strip waveguide mask on the surface of the SiO2 etching mask;

[0011] Step 6: Complete the fabrication of the SiO2 etching mask through ICP etching according to the photoresist mask, and then remove the photoresist mask;

[0012] Step 7: Complete the etching of the AlGaAs ridge waveguide through ICP etching according to the SiO2 etching mask;

[0013] Step 8: Cleave the waveguide end face and fabricate an antireflection film on the end face.

[0014] Preferably, in step 6, the fabrication of the SiO2 etching mask is completed through F-based ICP etching.

[0015] Preferably, in step 7, the etching of the ridge waveguide is completed through Cl-based ICP etching.

[0016] The above-mentioned Bragg reflection waveguide is applied in the device for directly generating hybrid entangled photon pairs.

[0017] Preferably, the device includes:

[0018] A pump laser for generating narrow-bandwidth pump light;

[0019] A polarization controller for controlling the polarization of the pump light when input;

[0020] A single-mode lens optical fiber for coupling a pump beam into an AlGaAs Bragg reflection waveguide;

[0021] A Bragg reflection waveguide for providing mode phase matching conditions for the pump light and generating down-converted photon pairs with polarization and frequency hybrid entanglement through the process of spontaneous parametric down-conversion;

[0022] A long-pass filter for suppressing the pump light and high-frequency noise at the output end;

[0023] Wavelength division multiplexing (WDM) for separating the signal and idler biphotons output by spontaneous parametric down-conversion to obtain photon pairs with polarization and frequency entanglement.

[0024] Preferably, the pump laser includes: a 780 nm tunable laser, a tunable optical attenuator, and a spectrum analyzer.

[0025] Preferably, the long-pass filter is composed of 6 fiber-coupled long-pass filter lenses.

[0026] Preferably, the device further includes a single-photon detector for performing two-photon coincidence counting and bandwidth measurement on the photon pairs with polarization and frequency entanglement separated by the wavelength division multiplexing (WDM).

[0027] Advantageous effects: The present invention makes full use of the second-order optical nonlinear effect of GaAs semiconductors and their derivative compound AlGaAs materials, and generates photon pairs with polarization and frequency hybrid entanglement through the process of mode phase-matched spontaneous parametric down-conversion. The present invention has the advantages of high generation efficiency of entangled photon pairs, phase matching bandwidth, low insertion loss, simple and compact optical path structure, stable performance, easy operation and control, etc. The present invention uses group III-V semiconductors to realize a single-photon source of quantum entanglement. This Bragg reflection waveguide structure does not require any oxidation process, so the laser and the nonlinear process can be integrated together to form a solution for generating entangled light by electrical drive, which is a very good choice for realizing monolithic integration of quantum optical chips, and provides an effective solution for the integration, miniaturization, and large-scale production of quantum optical chips. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 It is a schematic diagram of the ridge structure of the Bragg reflection waveguide of the present invention, which is composed of a core layer in the middle and cladding layers with 6 periods on the upper and lower sides;

[0029] Figure 2 It is a schematic diagram of the physical process in which the pump light participates in the process of spontaneous parametric down-conversion in the Bragg reflection waveguide to generate an entangled photon pair of a signal photon and an idler photon;

[0030] Figure 3 It is a process flow chart for the preparation of a Bragg reflection waveguide in Embodiment 1;

[0031] Figure 4 Schematic diagram of a method for directly generating hybrid entangled photon pairs on a Bragg reflection waveguide according to the present invention;

[0032] Figure 5 Detection result diagram of two-photon coincidence counting for generating polarization and frequency hybrid entanglement quantum light source in Example 1;

[0033] Figure 6 Result diagram of the bandwidth of down-converted photons generated by the polarization and frequency hybrid entanglement quantum light source in Example 1. Detailed implementation manners

[0034] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments.

[0035] Example 1

[0036] As Figure 3 shown, a Bragg reflection waveguide for directly generating hybrid entangled photon pairs is prepared through the following steps:

[0037] Step 1: Set the guessed values of the thicknesses of the core layer and the periodic cladding layer in the Bragg reflection waveguide, and find the effective refractive index of the current total internal reflection mode TIR through software simulation calculation;

[0038] Step 2: Substitute this value into the program for calculating the effective refractive index of the BRW mode, and calculate the thickness values of each layer that conform to the Bragg reflection waveguide theory;

[0039] Step 3: Then re-enter the calculated thickness values of each layer into the software simulation calculation to obtain the new effective refractive index of the BRW mode and the effective refractive index of the total internal reflection mode TIR;

[0040] Step 4: Repeat Steps 2 and 3 until the effective refractive index of the BRW mode obtained through software simulation calculation is between the effective refractive index of the TIR-TE mode and the effective refractive index of the TIR-TM mode, and the thickness values of each layer are consistent with the calculated values, then the thicknesses of each layer and the effective refractive index of the mode of the final design can be determined;

[0041] Step 5: After growing the SiO2 etching mask by PECVD, then fabricate a photoresist strip waveguide mask on the surface of the SiO2 etching mask;

[0042] Step 6: Complete the fabrication of the SiO2 etching mask through F-based ICP etching according to the photoresist mask, and then remove the photoresist mask;

[0043] Step 7: Complete the etching of the AlGaAs ridge waveguide through Cl-based ICP etching according to the SiO2 etching mask;

[0044] Step 8: Cleave the waveguide end face and fabricate an antireflection film on the end face.

[0045] The fabricated ridge-type Bragg reflection waveguide is as Figure 1 shown. The pump laser enters the Bragg reflection waveguide from the front, and in the waveguide, a physical process of spontaneous parametric down-conversion occurs as Figure 2 shown, and a pair of entangled photon pairs, namely signal photons and idler photons, are generated.

[0046] The device for generating hybrid entangled photon pairs using this Bragg reflection waveguide is as Figure 4 shown: a pump laser, which is used to generate a narrow-bandwidth pump light; a polarization controller, which is used to control the polarization of the pump light when input; a single-mode lens fiber, which is used to couple the pump light beam into the AlGaAs Bragg reflection waveguide; the Bragg reflection waveguide, which is used to provide mode phase matching conditions for the pump light and generate down-converted photon pairs with polarization and frequency hybrid entanglement through the process of spontaneous parametric down-conversion; a long-pass filter, which is used to suppress the pump light and high-frequency noise at the output end; a wavelength division multiplexing WDM, which is used to separate the signal and idler two photons output by the spontaneous parametric down-conversion to obtain photon pairs with polarization and frequency entanglement.

[0047] Among them, the pump laser further includes: a 780nm tunable laser, which is used to generate a continuously tunable pump light source; an adjustable attenuator, which is used to control the power of the input laser; a spectrum analyzer, which is used to analyze and determine the frequency and power of the input laser.

[0048] The device also includes a single-photon detector, which is used to perform two-photon coincidence counting and bandwidth measurement on the photon pairs with polarization and frequency entanglement separated by the wavelength division multiplexing WDM.

[0049] The design idea of the above device for generating hybrid entangled photon pairs is as follows:

[0050] Pump light source: The pump light source can use a pulsed laser in the 780nm band or a continuous-wave laser. The pulsed laser has a high instantaneous optical power, a wide spectral coverage, and the pulsed signal can be used as a trigger signal for the single-photon detector, which is beneficial to improving the photon pair production rate and reducing the dark count. However, in our actual use, it is found that the femtosecond laser in the 780nm band of AVEST PERL-PM-HP has leakage in the 1550nm band, which requires adding an additional pre-filtering optical path to the input light and increasing the system complexity. Therefore, we change to a continuous-wave laser, using a MOGLABS 780nm external cavity laser, with a wavelength tuning range of about 770 - 793nm, a pure spectrum, and a maximum fiber output power of 10mW. To meet the requirements of type-II SPDC, a polarization controller needs to be added at the input end to ensure that the pump light is TE input. The polarization controller is composed of a half-wave plate and a quarter-wave plate, and is a polarization controller with fiber input and output.

[0051] Coupling of optical fiber and waveguide chip: Since the width and depth of GaAs / AlGaAs waveguide are 4-5 microns, and the core diameter of 1550nm single-mode optical fiber is usually 9 microns, there is a certain gap between the optical fiber and the waveguide during chip testing. It is necessary to use customized lens optical fiber to focus the optical fiber head mode to a size close to the waveguide size. The input end uses a customized lens optical fiber with a wavelength of 780nm, and the output end uses a customized lens optical fiber with a wavelength of 1550nm.

[0052] Down-conversion photon separation: Down-conversion photon pairs around 1560nm generated by GaAs / AlGaAs waveguide chip SPDC need to be separated into two optical paths as two quantum bits. Since the down-conversion photons themselves have wavelength differences, CWDM can be used for wavelength division multiplexing to separate the two photons.

[0053] Pump filtering: This embodiment designs and customizes two groups of 3 high-quality long-pass filter lenses in series to form a spatial filtering module, providing a total extinction ratio of more than 120dB. The spatial light is coupled to the optical fiber through a collimator solidified in the module.

[0054] Based on the above design ideas, a 780nm tunable laser is selected in this embodiment, the pump light wavelength is 779.69nm, the laser power at the waveguide input end is about 50uW, the overall insertion loss of the waveguide is about 10dB, and the polarization state is adjusted to TE, corresponding to the phase matching point of the type-II spontaneous parametric down-conversion process. After the lens fiber coupling enters the waveguide chip, after the output, the pump light is filtered out by 6 long-pass filter lenses, and the down-converted photon pair enters the CWDM, and is output from the central wavelength 1550nm and 1570nm branches respectively. The bandwidth of each branch is 20nm, and they enter the single photon detector SPD respectively. Each SPD generates a counting signal and inputs it into the time-correlated single photon counter TCSPC, which is labeled with a time label. The time difference between two photon counting events in a certain time interval and the number of events can be obtained through the automatic calculation of TCSPC.

[0055] The single-photon detector uses an InGaAs APD type single-photon detector, which works at room temperature and is relatively small in size. The parameters of the two-way detector in this embodiment are set as follows: the detection efficiency is selected as 10%, and the dead time is selected as 4.3us and 5.3us respectively. Under this condition, the two-photon coincidence counting results of this embodiment are as follows Figure 5 shown.

[0056] Select one of the 1550nm and 1570nm branches of the 20nm bandwidth CWDM, and add a DWDM with a bandwidth of 0.8nm thereafter. By continuously changing the different wavelength channel DWDMs, the bandwidth of the type-II spontaneous parametric down-conversion entangled photons obtained in this embodiment is as follows: Figure 6 shown.

[0057] In summary, the present invention can make good use of the second-order optical nonlinear effect of GaAs semiconductors and their derivative compound AlGaAs materials, generate polarization and frequency hybrid entangled photon pairs through the mode phase-matched spontaneous parametric down-conversion process, and has the advantages of high generation efficiency of entangled photon pairs, phase-matching bandwidth, low insertion loss, simple and compact optical path structure, stable performance, easy operation and control, etc.

Claims

1. A Bragg reflection waveguide for directly generating hybrid entangled photon pairs, characterized in that It is prepared through the following steps: Step 1: Set the guessed values of the thicknesses of the core layer and the periodic cladding layer in the Bragg reflection waveguide, and find the effective refractive index of the current total internal reflection mode TIR through software simulation calculation; Step 2: Substitute this value into the program for calculating the effective refractive index of the BRW mode, and calculate the thickness values of each layer that conform to the Bragg reflection waveguide theory; Step 3: Then re-enter the thickness values of each layer obtained from this calculation into the software simulation to obtain the new effective refractive index of the BRW mode and the effective refractive index of the total internal reflection mode TIR; Step 4: Repeat Steps 2 and 3 until the effective refractive index of the BRW mode obtained from the software simulation is between the effective refractive index of the TIR-TE mode and the effective refractive index of the TIR-TM mode, and the thickness values of each layer are consistent with the calculated values, then the thicknesses of each layer and the mode effective refractive index of the final design can be determined; Step 5: After growing the SiO2 etching mask by PECVD, then fabricate a photoresist strip waveguide mask on the surface of the SiO2 etching mask; Step 6: Complete the fabrication of the SiO2 etching mask through ICP etching according to the photoresist mask, and then remove the photoresist mask; Step 7: Complete the etching of the AlGaAs ridge waveguide through ICP etching according to the SiO2 etching mask; Step 8: Cleave the waveguide end face and fabricate an antireflection film on the end face.

2. A Bragg reflection waveguide for directly generating hybrid entangled photon pairs according to claim 1, wherein In the said Step 6, the fabrication of the SiO2 etching mask is completed through F-based ICP etching.

3. A Bragg reflection waveguide for directly generating hybrid entangled photon pairs according to claim 1, characterized in that, In the said Step 7, the etching of the ridge waveguide is completed through Cl-based ICP etching.

4. Application of the Bragg reflection waveguide described in Claim 1 in the preparation of a device for directly generating hybrid entangled photon pairs.

5. The application according to claim 4, wherein The said device includes: A pump laser, which is used to generate narrow-bandwidth pump light; A polarization controller, which is used to control the polarization of the pump light when input; A single-mode lens fiber, which is used to couple the pump beam into the AlGaAs Bragg reflection waveguide; A Bragg reflection waveguide, which is used to provide mode phase matching conditions for the pump light and generate down-converted photon pairs with polarization and frequency mixing entanglement through the process of spontaneous parametric down-conversion; A long-pass filter, which is used to suppress the pump light and high-frequency noise at the output end; A wavelength division multiplexer WDM, which is used to separate the signal and the idler two photons output by spontaneous parametric down-conversion to obtain photon pairs with polarization and frequency entanglement.

6. The application according to claim 5, characterized in that, The said pump laser includes: a 780nm tunable laser, an adjustable attenuator and a spectrum analyzer.

7. The application according to claim 5, characterized in that, The said long-pass filter is composed of 6 fiber-coupled long-pass filter lenses.

8. The application according to claim 5, wherein The said device also includes a single-photon detector, which is used to perform two-photon coincidence counting and bandwidth measurement on the photon pairs with polarization and frequency entanglement separated by the wavelength division multiplexer WDM.

Images