Polarization independent beam splitter, decoding end, quantum key distribution system and communication method

Abstract

The invention provides a series of polarization-independent beam splitters, a decoding end, a quantum key distribution system and a communication method, which can be applied to the technical field of quantum communication. The polarization independent beam splitter comprises an input optical channel, an input adiabatic evolution waveguide, a variable-width multimode waveguide, an output adiabatic evolution waveguide and an output optical channel, the variable-width multimode waveguide is in a strip shape and comprises a first end and a second end opposite to the first end, multiple waveguide sections with different width value changes are arranged between the first end and the second end, and the variable-width multimode waveguide is arranged between the first end and the second end. The signal photons are subjected to light field evolution in multiple waveguide sections with variable width values and then are subjected to beam splitting at a second end, and first beam splitting signal photons and second beam splitting signal photons are obtained; the length of the variable-width multimode waveguide is combined with different width values, so that the value of a target function is smaller than a target threshold value, and any beam splitting ratio can be realized on the premise of ensuring polarization independence through the combined asymmetric design.

Description

Technical Field

[0001] This application relates to the field of quantum communication, and more specifically to polarization-independent beam splitters with different splitting ratios, decoding terminals, quantum key distribution systems, and communication methods. Background Technology

[0002] Quantum Key Distribution (QKD) technology, leveraging the superposition and no-cloning principles of quantum mechanics and combined with the unconditionally secure "one-time pad" encryption method, can theoretically achieve absolutely secure key distribution. To improve the utilization of photons in the channel and reduce the system complexity and cost required to recover photon polarization states, designing the beam splitter in the system as polarization-independent is a common solution. Simultaneously, to optimize the code rate, unbalanced basis selection is often required, thus necessitating beam splitters with specific splitting ratios. However, in practical applications, foundry-produced beam splitters typically employ multimode interferometers with large processing tolerances. While their splitting ratios are relatively balanced, they often suffer from high losses, strong polarization dependence, and difficulty in achieving unbalanced splitting ratios. Summary of the Invention

[0003] In view of the above problems, this application provides a polarization-independent beam splitter with scalable beam splitting ratio, a decoding end, a quantum key distribution system, and a communication method.

[0004] According to a first aspect of this application, a polarization-independent beam splitter is provided, comprising: an input optical channel including one or two input ports, the cross-section of which is square, for receiving signal photons; an input adiabatic evolving waveguide connecting the input ports and a first end of a variable-width multimode waveguide, for guiding the input signal photons to the first end; and a variable-width multimode waveguide, which is strip-shaped, including the first end and a second end opposite to the first end, with multiple waveguide segments of varying widths between the first and second ends, wherein the signal photons undergo optical field evolution in the multiple waveguide segments of varying widths and are then split at the second end to obtain a first split signal photon and a second split signal beam. The objective function comprises: a variable-width multimode waveguide with varying lengths and widths to make the objective function less than a target threshold; an output adiabatic evolution waveguide connected to a second end and an output optical channel to guide the first and second split signal photons to the output optical channel; and an output optical channel comprising a first output port and a second output port for outputting the first split signal photon through the first output port and the second split signal photon through the second output port; wherein the objective function includes a splitting ratio deviation; the splitting ratio deviation includes the sum of the squared modulus of the differences between the transmittance of the first output port and the transmittance of the second output port under different polarizations and the corresponding target transmittance.

[0005] According to an embodiment of this application, the objective function further includes a polarization balance term; the polarization balance term includes the squared modulus of the transmittance difference between different polarizations, and the transmittance difference includes the difference in transmittance at the first output port, the difference in transmittance at the second output port, and the difference in total transmittance between different polarizations.

[0006] According to embodiments of this application, it further includes a first connecting waveguide and / or a second connecting waveguide; the first end of the variable width multimode waveguide is connected to the input adiabatic evolution waveguide through the first connecting waveguide; the second end of the variable width multimode waveguide is connected to the output adiabatic evolution waveguide through the second connecting waveguide; the length and width of the first connecting waveguide and / or the second connecting waveguide make the value of the objective function less than the target threshold.

[0007] According to an embodiment of this application, the polarization-independent beam splitter further includes at least one subwavelength slot disposed on both sides of the second end and / or disposed inside the second end; the subwavelength slot is perpendicular to the propagation direction of the signal photon.

[0008] According to an embodiment of this application, the polarization-independent beam splitter further includes at least one subwavelength slot disposed inside the second end; the subwavelength slot is parallel to the propagation direction of the signal photon.

[0009] According to an embodiment of this application, the input optical channel includes two input ports, which are symmetrically distributed relative to the first end.

[0010] According to an embodiment of this application, the variable width multimode waveguide has a notched corner feature at the first end, which is used to induce unbalanced beam splitting.

[0011] According to an embodiment of this application, the input optical channel includes two input ports; at least one phase difference waveguide segment is provided between waveguide segments with multiple width values; wherein, the phase difference waveguide segment can be one of trapezoidal, triangular, arc, fan-shaped or two-armed waveguides of unequal length, and the phase difference waveguide segment is used to introduce a phase difference to the input signal photons of the two input ports.

[0012] The second aspect of this application provides a polarization-independent decoding terminal, comprising: a two-way polarization-independent large-range unbalanced beamsplitter, a two-way polarization-independent balanced beamsplitter, and a one-way polarization-independent small-range unbalanced beamsplitter; the two-way polarization-independent large-range unbalanced beamsplitter is the aforementioned polarization-independent beamsplitter, with an input optical channel including two input ports; at least one phase difference waveguide segment is provided between waveguide segments with multiple width variations; the two-way polarization-independent balanced beamsplitter is the aforementioned polarization-independent beamsplitter, with an input optical channel including two input ports, the two input ports being symmetrically distributed relative to the first end; the one-way polarization-independent small-range unbalanced beamsplitter is the aforementioned polarization-independent beamsplitter, and has one input port, the variable-width multimode waveguide having a corner-cut feature at the first end; the first port is sequentially connected to one input port of the two-way polarization-independent large-range unbalanced beamsplitter via a first polarization-independent end-face coupler and a first waveguide; the second port is sequentially connected to the two-way polarization-independent large-range unbalanced beamsplitter via a second polarization-independent end-face coupler and a second waveguide. Another input port of the unbalanced large-range unbalanced beamsplitter is connected; the third port is connected to the second output port of the two-way polarization-independent large-range unbalanced beamsplitter via the third polarization-independent end-face coupler and the third waveguide; the fourth port is connected to the second output port of the two-way polarization-independent balanced beamsplitter via the fourth polarization-independent end-face coupler and the fourth waveguide; the fifth port is connected to the first output port of the two-way polarization-independent balanced beamsplitter via the fifth polarization-independent end-face coupler and the fifth waveguide; the first output port of the two-way polarization-independent large-range unbalanced beamsplitter is connected to the input port of the one-way polarization-independent small-range unbalanced beamsplitter via the sixth waveguide; the first output port of the one-way polarization-independent small-range unbalanced beamsplitter is connected to one input port of the two-way polarization-independent balanced beamsplitter via the seventh waveguide; the second output port of the one-way polarization-independent small-range unbalanced beamsplitter is connected to the other input port of the two-way polarization-independent balanced beamsplitter via the eighth waveguide; the seventh and eighth waveguides have different lengths.

[0013] A third aspect of this application provides a quantum key distribution system, comprising: a timestamp phase encoding transmitter for generating and encoding signal photons, and attenuating the encoded signal photons to a single-photon state before sending them into an optical fiber channel; an optical fiber channel connecting the timestamp phase encoding transmitter and a receiver; and a receiver for parsing the received signal photons to obtain a quantum key; wherein the receiver comprises the polarization-independent decoding end of claim 9 and a detector.

[0014] The fourth aspect of this application also provides a quantum communication method applied to the quantum key distribution system of claim 10, comprising: performing timestamp-phase encoding at the transmitting end to generate timestamp-phase encoded signal photons; transmitting the signal photons to the receiving end using an optical fiber channel; after passing through a two-way polarization-independent large-range unbalanced beam splitter, the stronger light is output to a detector as a standard basis, and the weaker light enters a one-way polarization-independent small-range unbalanced beam splitter and a two-way polarization-independent balanced beam splitter as diagonal bases; and using the receiving end to analyze the aforementioned signal photons to obtain a quantum key. Attached Figure Description

[0015] The above-mentioned contents, other objects, features and advantages of this application will become clearer from the following description of embodiments with reference to the accompanying drawings, in which:

[0016] Figure 1 A schematic diagram of a polarization-independent beam splitter according to an embodiment of this application is shown.

[0017] Figure 2 A schematic diagram of a polarization-independent balanced beam splitter according to an embodiment of this application is shown.

[0018] Figure 3 A schematic diagram of a polarization-independent balanced beam splitter according to an embodiment of this application is shown.

[0019] Figure 4 A schematic diagram of a polarization-independent small-range unbalanced beam splitter according to an embodiment of this application is shown.

[0020] Figure 5 The schematic diagram illustrates the structure of a polarization-independent wide-range unbalanced beam splitter according to an embodiment of this application;

[0021] Figure 6 This schematically illustrates the structure of another polarization-independent wide-range unbalanced beam splitter according to an embodiment of this application;

[0022] Figure 7 A schematic diagram of a polarization-independent decoding terminal according to an embodiment of this application is shown.

[0023] Figure 8 A schematic diagram of a quantum key distribution system according to an embodiment of this application is shown.

[0024] Figure 9 A comparison graph of simulation results for different beam splitters according to embodiments of this application is shown; and

[0025] Figure 10 The actual spectral test results of different polarization-independent beam splitters according to embodiments of this application are shown.

[0026] In the figure, 110 is the input port; 120 is the input adiabatic evolution waveguide; 130 is the variable width multimode waveguide; 131 is the first end; 132 is the second end; 133 is the trapezoidal region; 134 is the straight waveguide; 135 is the arc waveguide; 140 is the output adiabatic evolution waveguide; 151 is the first output port; 152 is the second output port; 160 is the first connecting waveguide; 170 is the second connecting waveguide; 181-183 are the subwavelength structure; and 190 is the corner-cut feature. Detailed Implementation

[0027] The embodiments of this application will now be described with reference to the accompanying drawings. However, it should be understood that these descriptions are exemplary only and are not intended to limit the scope of this application. In the following detailed description, numerous specific details are set forth to provide a thorough understanding of the embodiments of this application for ease of explanation. However, it will be apparent that one or more embodiments may be implemented without these specific details. Furthermore, descriptions of well-known structures and technologies are omitted in the following description to avoid unnecessarily obscuring the concepts of this application.

[0028] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this application. The terms “comprising,” “including,” etc., as used herein indicate the presence of features, steps, operations, and / or components, but do not exclude the presence or addition of one or more other features, steps, operations, or components.

[0029] All terms used herein (including technical and scientific terms) have the meanings commonly understood by those skilled in the art, unless otherwise defined. It should be noted that the terms used herein are to be interpreted in a manner consistent with the context of this specification, and not in an idealized or overly rigid way.

[0030] When using expressions such as "at least one of A, B and C", they should generally be interpreted in accordance with the meaning that is commonly understood by those skilled in the art (e.g., "a system having at least one of A, B and C" should include, but is not limited to, a system having A alone, a system having B alone, a system having C alone, a system having A and B, a system having A and C, a system having B and C, and / or a system having A, B and C, etc.).

[0031] The embodiments of this application provide a polarization-independent beam splitter, which will be discussed below in conjunction with... Figures 1-6 This polarization-independent beam splitter is described.

[0032] Figure 1 A schematic diagram of a polarization-independent beam splitter according to an embodiment of this application is shown.

[0033] like Figure 1As shown, an embodiment of this application provides a polarization-independent beam splitter, including: an input optical channel 110, an input adiabatic evolution waveguide 120, a variable width multimode waveguide 130, an output adiabatic evolution waveguide 140, and an output optical channel.

[0034] The input optical channel 110 includes one or two input ports, each with a square cross-section, for receiving signal photons. Figure 1 It contains one input port.

[0035] The input adiabatic evolution waveguide 120 is connected to the input port 110 and the first end 131 of the variable width multimode waveguide 130, and is used to guide the input signal photons into the first end 131.

[0036] A variable-width multimode waveguide 130 is strip-shaped, comprising a first end 131 and a second end 132 opposite to the first end 131, with multiple segments of different widths (L1, L2, L3, ...) between the first end 131 and the second end 132. … L n The waveguide segments with varying widths undergo optical field evolution in multiple segments, and then the signal photons are split at the second end to obtain the first and second beams of signal photons. The length D1 of the variable-width multimode waveguide 130 is combined with different width values ​​(L1, L2, L3, ...). … L n The objective function is made to be less than the target threshold, wherein the objective function includes the beam splitting ratio deviation; the beam splitting ratio deviation includes the sum of the squared magnitudes of the differences between the transmittance of the first output port 151 and the transmittance of the second output port 152 under different polarizations and the corresponding target transmittance.

[0037] The output adiabatic evolution waveguide 140 is connected to the second end 132 and the output optical channel, and is used to guide the first and second beam-splitting signal photons to the output optical channel.

[0038] The output optical channel includes a first output port 151 and a second output port 152, for outputting a first beam splitting signal photon through the first output port 151 and a second beam splitting signal photon through the second output port 152.

[0039] The input optical channel uses a square cross-section input port, which ensures that the TE mode and TM mode are subject to the same constraints (same effective refractive index and propagation speed) when entering the waveguide, avoiding polarization differences from the source and laying the foundation for subsequent polarization-independent performance.

[0040] A thermally evolved waveguide (TEW) is a waveguide structure used for signal photon transmission and mode conversion. By allowing key waveguide parameters (such as cross-sectional dimensions) to change slowly and continuously, it prevents optical field reflection, leakage, or mode coupling caused by abrupt parameter changes during transmission, thereby achieving low-loss, high-fidelity transmission of signal photons. In this embodiment, the input thermally evolved waveguide 120 and the output thermally evolved waveguide 140 enable a smooth transition of signal photons between the input port 110, the variable-width multimode waveguide 130, and the output port, avoiding optical field leakage and additional losses caused by abrupt changes in the waveguide cross-section, and further maintaining the transmission consistency of TE / TM modes. The input thermally evolved waveguide can be, for example, trapezoidal, to achieve a slow change in waveguide width.

[0041] like Figure 1 As shown, in the variable-width multimode waveguide 130, the width varies from L1 to L2, L2 to L3, L3 to ... to L n The continuous variation of the edge curve creates an edge curve in the 130 region of the variable-width multimode waveguide. By optimizing the linearity of the edge curve, better transmittance consistency of polarized photons can be achieved, as well as optimized loss, beam splitting ratio balance, and bandwidth characteristics.

[0042] In this embodiment, the objective function can be expressed as follows.

[0043]

[0044] in, Let be the objective function. To reduce the beam splitting ratio deviation, the parameters of the polarization-independent beam splitter can be optimized to minimize or reduce it to below the target threshold. For optimizable parameter vectors, such as those containing the length of the variable-width multimode waveguide 130 and different width values; This represents the transmittance at the first output port in the TE polarization state. This indicates the transmittance at the second output port in the TE polarization state. This represents the transmittance at the first output port in the TM polarization state. This represents the transmittance at the second output port in the TM polarization state. T t1 / T t2 For the target beam splitting ratio, T t1 T represents the target transmittance of the first output port. t2 The target transmittance of the second output port.

[0045] In this embodiment, the target threshold can be a preset value. When the target function value is less than the target threshold, it is considered that the current width parameter has met the beam ratio accuracy requirement (e.g., beam ratio deviation < ±2%), and the optimization stops and the width value is output.

[0046] The variable-width multimode waveguide 130 allows signal photons to complete optical field evolution and beam splitting by setting multiple waveguide segments of different widths. Furthermore, by jointly optimizing the objective function with the length and width values ​​of the variable-width multimode waveguide 130, the beam splitting ratio deviation is controlled within the target threshold.

[0047] This implementation precisely controls the beam splitting effect through multi-segment width variations and uses polarization-dependent constraints in the objective function to ensure that the beam splitting ratios of both the TE and TM modes closely match the target value, ultimately achieving polarization-independent characteristics. This embodiment, through a variable-width multimode waveguide structure design combined with objective function optimization, ultimately possesses advantages such as polarization independence, high beam splitting accuracy, controllable beam splitting ratio deviation, and low transmission loss, making it suitable for scenarios such as quantum communication that require high polarization independence and high beam splitting ratios.

[0048] In some embodiments of this application, the objective function further includes a polarization balance term; the polarization balance term includes the sum of the squared magnitudes of the transmittance differences between different polarizations, and the transmittance differences include the differences in transmittance at the first output port, the differences in transmittance at the second output port, and the differences in total transmittance between different polarizations.

[0049] In some embodiments of this application, the objective function can be expressed as follows.

[0050]

[0051]

[0052] in, Represents the polarization balance term. , , , , , … These are weighting coefficients; different superscripts represent wavelength adaptation terms at different wavelengths, with the nth wavelength being the weighting coefficient. The corresponding weighting coefficient is .

[0053] This embodiment incorporates the squared modulus sum of the differences in transmittance at the first output port, the second output port, and the total transmittance at different polarizations into the objective function. This effectively reduces the deviation of transmittance at each output port and the total transmittance of different polarized light, thereby improving the adaptability and beam splitting consistency of the beam splitter for different polarized light and optimizing its polarization-independent performance.

[0054] In some embodiments of this application, the polarization-independent beam splitter further includes a first connecting waveguide 160 and / or a second connecting waveguide 170; the first end 131 of the variable-width multimode waveguide 130 is connected to the input adiabatic evolution waveguide 120 through the first connecting waveguide 160; the second end 132 of the variable-width multimode waveguide 130 is connected to the output adiabatic evolution waveguide 140 through the second connecting waveguide 170; the length and width of the first connecting waveguide 160 and / or the second connecting waveguide 170 make the value of the objective function less than the target threshold.

[0055] This embodiment connects the two ends of the variable-width multimode waveguide to the input and output adiabatic evolution waveguides via a first and / or a second connecting waveguide. The length and width of the connecting waveguides are optimized to make the objective function value less than the target threshold. This achieves a smooth transition between the variable-width multimode waveguide and the input and output adiabatic evolution waveguides, reducing optical field leakage and transmission loss. It also further optimizes the beam splitting ratio accuracy and polarization independence, ultimately improving the overall performance stability and practicality of the beam splitter.

[0056] In some embodiments of this application, the polarization-independent beam splitter further includes at least one subwavelength slot (see reference) disposed on both sides of the second end and / or disposed inside the second end. Figure 2 , Figures 4-6 The subwavelength slot is perpendicular to the propagation direction of the signal photon.

[0057] This embodiment can utilize the subwavelength structure to modulate the optical field, optimize the mode coupling and interference of signal photons with different polarization states during the beam splitting process, thereby improving the consistency and accuracy of the beam splitter for beams of different polarizations, enhancing its polarization-independent performance, and further helping to reduce losses in optical field beam convergence.

[0058] In some embodiments of this application, the polarization-independent beam splitter further includes at least one subwavelength slot disposed inside the second end, such as... Figure 3 The subwavelength groove is 183; the subwavelength groove is parallel to the propagation direction of the signal photon.

[0059] This embodiment can leverage the unique ability of subwavelength structures to control the optical field, intervene in the evolution and coupling process of different modes of optical fields within the waveguide, optimize the transmission characteristics of signal photons in variable-width waveguides, enable photons of different polarization states to achieve more precise energy distribution and balance the beam splitting ratio when splitting the beam, and also help reduce transmission loss.

[0060] Figure 2 A schematic diagram of a polarization-independent balanced beam splitter according to an embodiment of this application is shown.

[0061] like Figure 2As shown, the input optical channel of the polarization-independent beam splitter in this embodiment includes an input port based on the above embodiments, thus forming a one-to-two polarization-independent balanced beam splitter.

[0062] When designing a polarization-independent balanced beam splitter structure, in order to achieve the effect of polarization independence, the edges of the variable-width multimode waveguide are optimized (L1, L2, L3, ...). … L n The effective refractive index of the multimode waveguide region can be adjusted: Since the width of the multimode waveguide changes with the propagation of light, polarization-independent beam splitting is achieved when the distances between the two light spots formed by the cumulative self-imaging effect of the TE and TM modes and the input port are equal. Since the characteristic length of the self-imaging effect is proportional to the square of the multimode waveguide width, this design can also be understood as adjusting the width to make the characteristic lengths obtained by integrating the TE and TM modes within the multimode waveguide equal. Simultaneously, since the light intensity at the two output ports is also affected by the shape of the multimode waveguide structure, methods including but not limited to particle swarm optimization and genetic algorithms can be used to set appropriate objective functions and optimize the structure by monitoring the balance between loss and beam splitting ratio while satisfying polarization consistency. Specific structures include, but are not limited to, […]. Figure 2 The width of the variable-width multimode waveguide at various points ,length Input adiabatic evolution waveguide and output adiabatic evolution waveguide length Width of the first connecting waveguide and the second connecting waveguide ,length The distance between the first output port and the second output port .

[0063] To further improve performance and adjust the light field distribution, this embodiment may include a subwavelength structure 181. The period and duty cycle of the subwavelength structure 181 can be selected based on the operating wavelength and processing capabilities. The position and length of the subwavelength structure 181... Optimization will be guided by the objective function result, aiming to make it less than the target threshold. The subwavelength structure 181 reduces stray light and lowers device losses. Simultaneously, by manipulating the optical field through reflection, refraction, and adjusting the effective refractive index, the beam splitter's performance, such as the beam splitting ratio, is further optimized. The parameters and dimensions of the waveguides at the input and output ports, as well as the input and output adiabatic evolution waveguides, are also considered. Within the optimization range, and considering the need for balanced beam splitting ratio, the two ports can be designed with symmetrical parameters.

[0064] In this embodiment, the specific objective function can be set as a function of the output port transmittance. For example, the objective is to minimize the sum of the squared magnitudes of the differences between the output port transmittance under different polarizations and 0.5, and to perform multi-generation parameter optimization.

[0065] In this embodiment, the first part of the objective function at a wavelength of 1550nm can be expressed as follows:

[0066]

[0067] Where 0.5 indicates that the target transmittance of both the first and second output ports is 0.5, i.e., the target beam splitting ratio T. t1 / T t2 The ratio is 50:50. A parameter vector that can be optimized, containing the width of the variable-width multimode waveguide at various points. ,length Input and output adiabatic evolution waveguide length Width of the first and second connecting waveguides ,length Spacing between the first and second output ports .

[0068] It is worth noting that the optimized structures will vary depending on the material and wafer thickness, due to differences in effective refractive index and algorithm. However, the above method can achieve polarization-independent balanced beam splitting on materials such as silicon, silicon nitride, and lithium niobate. To ensure its bandwidth characteristics, the above objective function can be integrated with multiple operating wavelengths near the center wavelength according to certain weighting coefficients as an optimization reference.

[0069] In some embodiments of this application, the input optical channel of the polarization-independent balanced beam splitter includes two input ports, which are symmetrically distributed relative to the first end.

[0070] Figure 3 A schematic diagram of a polarization-independent balanced beam splitter according to an embodiment of this application is shown.

[0071] like Figure 3 As shown, the polarization-independent beam splitter of this embodiment is based on the above embodiment. The input optical channel includes two input ports, namely the first input port 111 and the second input port 112. The two input ports are symmetrically distributed with respect to the first end 131, thus forming a two-to-two polarization-independent balanced beam splitter.

[0072] In this embodiment, a section of connecting waveguide with optimized length D2 and width W1 can be connected at the junction of the adiabatic evolution waveguide and the multimode waveguide.

[0073] In this embodiment, the signal photon is within a variable-width multimode waveguide (width values ​​are L1, L2, L3, ...). …The self-imaging effect occurs at L1, ..., L5, D1, D2, D3, W1, W2, where the signal photons are coupled from the multimode waveguide region to the two ports of the output optical channel at a specific spot distribution location. In addition to employing the edge curve optimization method for the variable-width multimode waveguides described above for L1, ..., L5, D1, D2, D3, W1, W2, the polarization-independent balanced beamsplitter in this embodiment can also set a partial subwavelength structure 182 at the edge of the second end of the variable-width multimode waveguide to help with optical field convergence and reduce loss. The position and length of the subwavelength structure 182 are specified. Optimization can be guided by the results of the objective function. The optimized multimode waveguide edge shape is related to the refractive index of the waveguide material and the thickness of the waveguide layer. The period and duty cycle of the subwavelength structure are consistent with actual fabrication capabilities. Ultimately, the optimized parameters result in a two-to-two beam splitter with better insertion loss, polarization adaptability, and wavelength adaptability.

[0074] Considering the asymmetry in the optical field distribution of the 2-to-2 structure, a transversely arranged subwavelength structure 183 can be placed near the output port at the second end to further balance the beam splitting ratio. The position and length of the subwavelength structure 183 are as follows: Optimization is guided by the results of the objective function.

[0075] The polarization-independent balanced beam splitter of this embodiment includes two input ports symmetrically distributed relative to the first end. The square cross-section input port ensures initial polarization consistency. The beam is smoothly introduced into a variable-width multimode waveguide through an input adiabatic evolution waveguide. The width and length of the waveguide are jointly optimized to ensure that the beam splitting ratio deviation is less than a threshold. The beam is then efficiently extracted from the output adiabatic evolution waveguide. The objective function can be further optimized through connecting waveguides, thereby achieving the advantages of consistent beam splitting ratio, high accuracy, and low transmission loss for different polarizations.

[0076] Figure 9 A comparison diagram of simulation results for different beam splitters according to embodiments of this application is shown.

[0077] like Figure 9 As shown, (a) and (b) are simulation results of the TE and TM states of the polarization of the polarization-independent balanced beamsplitter in embodiment 1 of this application; (c) and (d) are simulation results of the TE and TM states of the polarization-independent balanced beamsplitter in embodiment 2 of this application. (e) compares the output port results of the polarization-independent balanced beamsplitter in embodiment 1 of this application (using a variable-width multimode interferometer (MMI)) and the one using a constant optimal width waveguide. (f) compares the results (cross port and through port) of the polarization-independent balanced beamsplitter in embodiment 2 of this application and the one using a constant optimal width waveguide. It is evident that the polarization-independent balanced beamsplitter and the one using a constant optimal width waveguide in the embodiments of this application have advantages in polarization consistency (compatibility) and spectral width.

[0078] According to embodiments of this application, when the received single-photon signal enters the receiving end, the above-mentioned one-to-two polarization-independent balanced beam splitter or two-to-two polarization-independent balanced beam splitter can be combined into a Mach-Zehnder interferometer to perform polarization-insensitive quantum state manipulation through interference.

[0079] In some embodiments of this application, the variable width multimode waveguide has a notch feature at the first end, which is used to induce unbalanced beam splitting.

[0080] Figure 4 A schematic diagram of a polarization-independent small-range unbalanced beam splitter according to an embodiment of this application is shown. In this embodiment, based on the above-mentioned polarization-independent balanced beam splitter, a variable-width multimode waveguide has a 190° notch feature at the first end, thus forming a polarization-independent small-range unbalanced beam splitter.

[0081] To differentiate the output port light intensity ratio, a portion of the multimode waveguide is truncated to form a notch feature 190°, causing asymmetry in the multimode waveguide and thus achieving a difference in the output port light spot intensity. The shape of the truncated portion of the notch feature 190° can be optimized to meet the required port light intensity ratio. For example, it can be a rectangle, square, triangle, trapezoid, irregular polygon, or other irregular shapes.

[0082] When designing a polarization-independent, small-range unbalanced beam splitter structure, multimode waveguide curve optimization and partial subwavelength structures can be used to enhance performance. To achieve intensity differences at the output ports, structural asymmetry can be introduced into the multimode waveguide region, for example, by omitting certain corner sections of the variable-width multimode waveguide. Figure 4 As shown. This missing portion can be rectangular, triangular, or other shapes. Its dimensions (W3, D4) are optimized according to the desired beam splitting ratio, ensuring the objective function value is less than the target threshold. The waveguides at the two exit ports can be asymmetrical to accommodate unbalanced beam splitting. For example, for a 40:60 beam splitting ratio, the objective function can be set as the square of the mode differing from 0.4 in transmittance under different polarizations plus the square of the mode differing from 0.6 in transmittance.

[0083] In this embodiment, the signal photon undergoes a self-imaging effect within the variable-width multimode waveguide. When it evolves to a specific spot distribution, the signal photon is coupled from the multimode waveguide region to the two ports of the output optical channel. Near the edge of the multimode waveguide close to the output optical port, a subwavelength structure for beam convergence can be added to reduce loss.

[0084] In this embodiment, a short waveguide with optimized length D2 and width W1 can be connected to the connection points of the input adiabatic evolution waveguide, the output adiabatic evolution waveguide, and the multimode waveguide, respectively. The distance W2 between the output channels is also an optimized value.

[0085] Simulation results show that this embodiment can achieve any beam splitting ratio within the range of 50:50 to 35:65, while maintaining good polarization independence. Larger beam splitting ratios will increase overall loss. Using this as the first-stage beam splitter in an unequal-arm interferometer can pre-compensate for the difference in transmission loss due to unequal arm lengths, and it has advantages in controlling the bimodal intensity of the time-bin encoded signal, which is beneficial for improving the final interference visibility and achieving better quantum communication performance.

[0086] In some embodiments of this application, the input optical channel of the polarization-independent beam splitter includes two input ports; at least one phase difference waveguide segment is provided between the waveguide segments with multiple width variations of the variable-width multimode waveguide. The phase difference waveguide segment can be one of trapezoidal, triangular, arc-shaped, fan-shaped, or two-armed waveguides of unequal length, and is used to introduce a phase difference into the input signal photons at the two input ports.

[0087] When faced with the need for a large-scale unbalanced beam splitter that is polarization independent of two beams, since simple asymmetric multimode interferometers are difficult to achieve the distribution of two beams with huge differences in light intensity, a splicing method can be used to splice the beam splitters into a compact interferometer to achieve a beam splitter with arbitrary beam splitting ratio.

[0088] Figure 5 The diagram schematically illustrates the structure of a polarization-independent wide-range unbalanced beam splitter according to an embodiment of this application.

[0089] like Figure 5 As shown, this embodiment, based on the aforementioned polarization-independent beam splitter, includes two input ports and two variable-width multimode waveguides (L...). n (Indicating different widths) through an angled... The trapezoidal region 133 is connected, and the tilt angle is optimized by control. To achieve the desired beam splitting, the phase difference of the interference is controlled. The objective function for parameter optimization is described previously. When the D5 value of the trapezoid is 0, it becomes a triangle.

[0090] Phase difference waveguide segments can also be two-armed waveguides of unequal length. Figure 6 The diagram schematically illustrates the structure of another polarization-independent wide-range unbalanced beam splitter according to an embodiment of this application.

[0091] like Figure 6As shown, the phase difference waveguide segment in this embodiment includes two waveguide arms of unequal length, namely a straight waveguide 134 (length D6) and an arc-shaped waveguide 135. The waveguide widths of these unequal arms, as well as the length difference between the straight waveguide 134 and the arc-shaped waveguide 135, can be optimized by an objective function. Signal photons undergo self-imaging within the tilted, variable-width multimode waveguide. Upon evolving to a specific spot distribution, they enter a short-distance region of the arc-shaped waveguide 135, which is horizontally curved. Within this region, due to the different phase differences accumulated during the evolution of the two spots, after entering the inverted, equally variable-width multimode waveguide output, it is equivalent to the interference result of a Mach-Zehnder interferometer. This embodiment is a compact Mach-Zehnder interferometer, and this design avoids the tilting of the multimode waveguide. It is worth noting that the output adiabatic evolution waveguide at the output port can also be further optimized using asymmetric parameters (W4, W5) to achieve the required beam splitting ratio.

[0092] This embodiment optimizes the tilt angle of the multimode waveguide ( Figure 5 ) or differences in the length of the arc-shaped region ( Figure 6 By changing the ratio of the light intensity at the two ports of the final output, it is suitable as a large-scale unbalanced beam splitter. Since both tilted variable-width multimode waveguides are polarization-independent, the overall device also maintains polarization-independent characteristics.

[0093] This embodiment forms a two-in-two polarization-independent large-range unbalanced beam splitter by incorporating at least one phase difference waveguide segment in a variable-width multimode waveguide. Combined with objective function optimization, it can achieve a large-range unbalanced beam splitting ratio while ensuring consistent beam splitting characteristics for different polarizations. With the help of an output adiabatic evolution waveguide and possible connecting waveguides, it achieves the advantages of a wide beam splitting ratio adjustment range, strong polarization independence, high precision, and low transmission loss.

[0094] Figure 10 The actual spectral test results of different polarization-independent beam splitters according to embodiments of this application are shown.

[0095] like Figure 10As shown, (a), (b), (c), and (d) represent the test results of the aforementioned polarization-independent balanced beam splitter (1-to-2), polarization-independent balanced beam splitter (2-to-2), polarization-independent small-range unbalanced beam splitter (1-to-2), and polarization-independent large-range unbalanced beam splitter (2-to-2), respectively. In (a), the curves for the first port TE, first port TM, second port TE, and second port TM coincide, indicating good beam splitting balance and polarization consistency across the entire spectrum. In (b), the curves for the cross port TE, cross port TM, straight-through port TE, and straight-through port TM coincide, indicating good beam splitting balance and polarization consistency across the entire spectrum. (c) shows the transmittance test results for the TE and TM modes at a target transmittance of 60:40 (60:40 beam splitting ratio) and at ports with a target transmittance of 40% (40% transmittance). (d) shows the transmittance test results for the TE and TM modes at a target transmittance of 85:15 (85:15 beam splitting ratio) and at ports with a target transmittance of 15% (15% transmittance). The curves (c) and (d) are separated to indicate that the obtained beam is unbalanced, while the relatively overlapping parts (blue and red, brown and green) indicate good polarization consistency.

[0096] Based on the aforementioned polarization-independent beam splitter, this application also provides a polarization-independent decoding terminal. The following will combine... Figure 7 A detailed description of the polarization-independent decoding end is provided.

[0097] Figure 7 A schematic diagram of a polarization-independent decoding terminal according to an embodiment of this application is shown.

[0098] like Figure 7 As shown, the above beam splitters are used as key components to form a polarization-independent decoding end. The decoding end in this embodiment includes a two-to-two polarization-independent large-range unbalanced beam splitter, a two-to-two polarization-independent balanced beam splitter, and a one-to-two polarization-independent small-range unbalanced beam splitter.

[0099] The first port is connected in sequence to an input port of a polarization-independent large-range unbalanced beam splitter via a first polarization-independent end-face coupler and a first waveguide.

[0100] The second port is connected in sequence to another input port of the two-way polarization-independent large-range unbalanced beam splitter via the second polarization-independent end-face coupler and the second waveguide.

[0101] The third port is connected in sequence to the second output port of the two-way polarization-independent large-range unbalanced beam splitter via the third polarization-independent end-face coupler and the third waveguide.

[0102] The fourth port is connected to the second output port of the two-to-two polarization-independent balanced beam splitter via the fourth polarization-independent end-face coupler and the fourth waveguide.

[0103] The fifth port is connected to the first output port of the two-way polarization-independent balanced beam splitter via the fifth polarization-independent end-face coupler and the fifth waveguide.

[0104] The first output port of the two-to-two polarization-independent large-range unbalanced beam splitter is connected to the input port of the one-to-two polarization-independent small-range unbalanced beam splitter via the sixth waveguide.

[0105] The first output port of the polarization-independent small-range unbalanced beam splitter is connected to one input port of the polarization-independent balanced beam splitter via the seventh waveguide; the second output port of the polarization-independent small-range unbalanced beam splitter is connected to another input port of the polarization-independent balanced beam splitter via the eighth waveguide; the seventh and eighth waveguides have different lengths.

[0106] In this embodiment, the polarization-independent end-face coupler typically consists of an inverted adiabatic evolving waveguide, used to amplify the optical field and match it to the mode field of the optical fiber or fiber lens. The adiabatic evolving waveguide can be optimized for linearity, and subwavelength structures can be added to optimize performance. It is connected on-chip to a wide-range unbalanced beam splitter.

[0107] A polarization-independent, wide-range unbalanced beam splitter is used for bias selection.

[0108] In this embodiment, the one-to-two polarization-independent small-range unbalanced beam splitter and the two-to-two polarization-independent balanced beam splitter can form an unequal-arm Mach-Zehnder interferometer, which includes a first-stage beam splitter and a second-stage beam splitter. The first-stage beam splitter is a one-to-two polarization-independent small-range unbalanced beam splitter, used to balance the loss differences caused by different waveguide lengths. The second-stage beam splitter (the third-stage beam splitter of the polarization-independent decoding end as a whole) is a two-to-two polarization-independent balanced beam splitter, which is the exit point for interference.

[0109] The output ports of the unequal-arm Mach-Zehnder interferometer are connected to the fourth and fifth end-face couplers, serving as the fourth and fifth ports, respectively. The third port is led out from the output waveguide of the two-way polarization-independent wide-range unbalanced beam splitter. The structure of the end-face couplers at the three output ports is consistent with that of the first and second polarization-independent end-face couplers at the input ports, and the output optical signals are connected to the detectors respectively.

[0110] Since beam splitters used for biased base selection generally require a base selection probability of 85:15 or even 95:5 to achieve a better code rate, the aforementioned two-to-two polarization-independent large-range unbalanced beam splitter can be used as the first-stage beam splitter. The optical signal entering the unequal-arm Mach-Zehnder interferometer is split into two parts by the second-stage beam splitter. As mentioned above, the intensity of the optical signal after passing through the two arms reaches the same when it reaches the third-stage beam splitter. Therefore, the second-stage and third-stage beam splitters can respectively adopt the design of the one-to-two polarization-independent small-range unbalanced beam splitter and the two-to-two polarization-independent balanced beam splitter described above. The range of the one-to-two polarization-independent small-range unbalanced beam splitter can basically balance the losses of low-loss silicon nitride and lithium niobate within a delay difference of less than 400 picoseconds. When light is incident from the second polarization-independent end-face coupler, more light enters the unequal-arm Mach-Zehnder interferometer, which facilitates strong light testing of the unequal-arm interferometer and the losses of the two arms of the beam splitter that make up it using an oscilloscope or similar equipment.

[0111] Based on the aforementioned polarization-independent beam splitter, this application also provides a quantum key distribution system. The following will combine... Figure 8 The system is described in detail.

[0112] Figure 8 A schematic diagram of a quantum key distribution system according to an embodiment of this application is shown.

[0113] like Figure 8 As shown, the quantum key distribution system includes: a timestamp phase encoding transmitter, used to generate signal photons using a light source and encode them, and to attenuate the encoded signal photons to a single-photon state before sending them into an optical fiber channel; an optical fiber channel connecting the timestamp phase encoding transmitter and the receiver; and a receiver used to analyze the received signal photons to obtain the quantum key; wherein, the receiver includes the aforementioned polarization-independent decoding end and detectors, and in this embodiment, the detectors include detector 1, detector 2, and detector 3.

[0114] Based on the aforementioned polarization-independent beam splitter, this application also provides a quantum communication method.

[0115] The quantum communication method in this embodiment includes: performing time-bin phase encoding at the transmitting end, generating time-bin-phase encoded signal photons through an unequal-arm interferometer; transmitting the signal photons to the receiving end using an optical fiber channel; after passing through a two-split polarization-independent large-range unbalanced beam splitter, the stronger light is output to the detector as a standard basis (Z basis), and the weaker light enters a one-split polarization-independent small-range unbalanced beam splitter and a two-split polarization-independent balanced beam splitter as a diagonal basis (X basis); and using the receiving end to analyze the above signal photons to obtain the quantum key.

[0116] In this embodiment, for signal photons entering the polarization-independent decoding end, there is no need to consider the unavoidable polarization state disturbances in the channel, and the time overhead and system complexity required to restore the polarization state of the signal photons are eliminated. The polarization independence of each device in the decoding end together ensures the polarization universality of the entire decoding end.

[0117] The quantum communication method section in the embodiments of this application corresponds to the quantum communication system section in the embodiments of this application. The description of the quantum communication method section is specifically referred to in the quantum communication system section, and will not be repeated here.

[0118] Those skilled in the art will understand that the features described in the various embodiments of this application can be combined and / or combined in various ways, even if such combinations or combinations are not explicitly described in this application. In particular, the features described in the various embodiments of this application can be combined and / or combined in various ways without departing from the spirit and teachings of this application. All such combinations and / or combinations fall within the scope of this application.

Claims

1. A polarization-independent beam splitter, characterized in that, include: An input optical channel includes one or two input ports, wherein the cross-section of the input ports is square and is used to receive signal photons; An input adiabatic evolution waveguide is connected to the input port and the first end of a variable width multimode waveguide, used to guide the input signal photons into the first end; A variable-width multimode waveguide, which is strip-shaped, includes a first end and a second end opposite to the first end. Between the first end and the second end are multiple waveguide segments with varying width values. After optical field evolution in the multiple waveguide segments with varying width values, the signal photon is split at the second end to obtain a first split signal photon and a second split signal photon. The length of the variable-width multimode waveguide, combined with the different width values, makes the value of the objective function less than a target threshold. An output adiabatic evolution waveguide is connected to the second end and the output optical channel to guide the first and second beam-splitting signal photons to the output optical channel. as well as An output optical channel includes a first output port and a second output port, used to output a first beam-splitting signal photon through the first output port and to output a second beam-splitting signal photon through the second output port; The objective function includes the beam splitting ratio deviation; the beam splitting ratio deviation includes the squared magnitude of the difference between the transmittance of the first output port under different polarizations, the transmittance of the second output port and the corresponding target transmittance.

2. The polarization-independent beam splitter according to claim 1, characterized in that, The objective function also includes a polarization balance term; The polarization balance term includes the squared modulus of the transmittance difference between different polarizations, and the transmittance difference includes the transmittance difference between different polarizations at the first output port, the transmittance difference at the second output port, and the total transmittance difference between different polarizations.

3. The polarization-independent beam splitter according to claim 1, characterized in that, It also includes a first connecting waveguide and / or a second connecting waveguide; The first end of the variable width multimode waveguide is connected to the input adiabatic evolution waveguide through the first connecting waveguide. The second end of the variable width multimode waveguide is connected to the output adiabatic evolution waveguide through the second connecting waveguide. The length and width of the first connecting waveguide and / or the second connecting waveguide make the value of the objective function less than the objective threshold.

4. The polarization-independent beam splitter according to claim 1, characterized in that, It also includes at least one subwavelength groove disposed on both sides of the second end and / or disposed inside the second end; the subwavelength groove is perpendicular to the propagation direction of the signal photon.

5. The polarization-independent beam splitter according to claim 1, characterized in that, It also includes at least one subwavelength groove disposed inside the second end; the subwavelength groove is parallel to the propagation direction of the signal photon.

6. The polarization-independent beam splitter according to any one of claims 1 to 5, characterized in that, The input optical channel includes two input ports, which are symmetrically distributed relative to the first end.

7. The polarization-independent beam splitter according to any one of claims 1 to 5, characterized in that, The variable-width multimode waveguide has a notched corner feature at the first end, which is used to induce unbalanced beam splitting.

8. The polarization-independent beam splitter according to any one of claims 1 to 5, characterized in that, The input optical channel includes two input ports; at least one phase difference waveguide segment is provided between the waveguide segments with multiple width values; The phase difference waveguide segment can be one of trapezoidal, triangular, arc-shaped, fan-shaped, or two-armed waveguides of unequal length, and the phase difference waveguide segment is used to introduce a phase difference to the input signal photons of the two input ports.

9. A polarization-independent decoding terminal, characterized in that, Includes: a two-way polarization-independent large-range unbalanced beam splitter, a two-way polarization-independent balanced beam splitter, and a one-way polarization-independent small-range unbalanced beam splitter; The polarization-independent wide-range unbalanced beam splitter described in claim 8 is the polarization-independent beam splitter described in claim 8. The polarization-independent balanced beam splitter is the polarization-independent beam splitter as described in claim 6; The polarization-independent small-range unbalanced beam splitter described in claim 7 is a polarization-independent beam splitter with one input port. The first port is connected to one input port of the two-way polarization-independent large-range unbalanced beam splitter via a first polarization-independent end-face coupler and a first waveguide. The second port is connected to another input port of the two-way polarization-independent large-range unbalanced beam splitter via a second polarization-independent end-face coupler and a second waveguide. The third port is connected to the second output port of the two-way polarization-independent large-range unbalanced beam splitter via a third polarization-independent end-face coupler and a third waveguide. The fourth port is connected to the second output port of the two-to-two polarization-independent balanced beam splitter via a fourth polarization-independent end-face coupler and a fourth waveguide. The fifth port is connected to the first output port of the two-way polarization-independent balanced beam splitter via the fifth polarization-independent end-face coupler and the fifth waveguide in sequence. The first output port of the two-to-two polarization-independent large-range unbalanced beam splitter is connected to the input port of the one-to-two polarization-independent small-range unbalanced beam splitter via the sixth waveguide. The first output port of the polarization-independent small-range unbalanced beam splitter is connected to one input port of the polarization-independent balanced beam splitter via a seventh waveguide; the second output port of the polarization-independent small-range unbalanced beam splitter is connected to another input port of the polarization-independent balanced beam splitter via an eighth waveguide; the seventh waveguide and the eighth waveguide have different lengths.

10. A quantum key distribution system, characterized in that, include: The timestamp phase-encoded transmitter is used to generate signal photons and encode them, as well as to attenuate the encoded signal photons to a single-photon state and send them into the optical fiber channel. An optical fiber channel connects the timestamp phase encoding transmitter and receiver; The receiving end is used to analyze the received signal photons to obtain the quantum key; The receiving end includes the polarization-independent decoding end and detector as described in claim 9.

11. A quantum communication method, characterized in that, Applied to the quantum key distribution system of claim 10, comprising: At the transmitting end, timestamp-phase encoding is performed to generate timestamp-phase encoded signal photons; The signal photons are transmitted to the receiving end using the optical fiber channel; After passing through the polarization-independent large-range unbalanced beam splitter, the stronger light is output to the detector as a standard base, while the weaker light enters the polarization-independent small-range unbalanced beam splitter and the polarization-independent balanced beam splitter as diagonal bases. The receiving end analyzes the signal photons to obtain the quantum key.

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