A quantum key distribution encoding optical chip based on silicon and silicon nitride waveguides

Abstract

The utility model provides a kind of quantum key distribution encoding light chip based on silicon and silicon nitride waveguide, it introduces silicon nitride waveguide to realize delay line in unequal arm interferometer, can realize phase stabilization under conventional temperature control condition, reduce system complexity. With this, by high-speed modulator is located in equal arm interferometer and unequal arm interferometer (short arm) inside, can effectively reduce equal arm interferometer loss and unequal arm interferometer long short arm loss difference, effectively control the increase of loss on the basis of introducing silicon nitride waveguide to realize delay line, greatly reduce the application difficulty. The light chip design scheme proposed in the utility model has high integration, mature manufacturing process, and is not harsh to temperature control requirement, can effectively promote the practical application of quantum key distribution encoding light chip.

Description

Technical Field

[0001] This utility model relates to the field of quantum communication, specifically to a quantum key distribution encoding optical chip based on silicon and silicon nitride waveguides. Background Technology

[0002] Quantum key distribution (QKD) is based on the principles of quantum mechanics. Due to the quantum no-cloning and uncertainty principles, it does not rely on computational complexity, is immune to high-powered attacks, and provides a high level of information security. With the development of quantum key distribution technology, small size, low cost, and high stability of the devices have become key considerations. Quantum key distribution often involves complex optical signal encoding and decoding, which is currently often based on combinations of traditional fiber optic devices, resulting in large size and high cost. Implementing optical signal encoding and decoding on optical chips is one of the important solutions for small-size, low-cost, and highly stable quantum key distribution devices. In recent years, various designs and solutions for QKD encoding optical chips have been proposed.

[0003] Figure 1 The present invention illustrates a time-phase encoding chip based on a hybrid integration approach in the prior art, wherein the phase modulator PM and the equal-arm interferometer SMZ are implemented through a lithium niobate waveguide structure, and the unequal-arm interferometer AMZ is implemented through a planar optical waveguide PLC chip based on silicon dioxide material. In order to achieve time-phase encoding, the phase modulator PM needs to adjust four phases: 0, π / 2, π, and 3π / 2.

[0004] Figure 2 Another time-phase encoding module in the prior art is shown, which uses lithium niobate material to make a phase modulation module and PLC material to make an optical waveguide delay module, which is a waveguide segment.

[0005] exist Figure 1 and 2 The existing QKD coded optical chip solutions shown all employ a conventional hybrid integration approach: the modulation unit and the unequal-arm interferometer are designed separately. The modulator is fabricated using lithium niobate material based on proton exchange and titanium diffusion manufacturing processes, while the unequal-arm interferometer with long delay lines is fabricated using silicon dioxide (or planar optical waveguide material). These materials have small refractive index differences and large bending radii, often resulting in large chip sizes. Furthermore, the need to hybridize or encapsulate two different elemental systems of materials leads to high process requirements and costs.

[0006] Although silicon-based integration processes have significant advantages in terms of process maturity and mass production costs, silicon materials have a large thermo-optical coefficient (approximately 2.02*10). -4 / K), based on a silicon waveguide, an unequal-arm interferometer with a delay difference of 1ns between its two arms. A temperature change of 0.05℃ will cause a phase change of π. In practical systems, the phase needs to be stable, and a phase change of π will lead to 100% bit error rate. Therefore, the temperature control accuracy needs to reach the mK level, which is a demanding requirement and makes the application difficult.

[0007] Figure 3 This paper illustrates a heterogeneous integrated chip scheme for quantum key distribution in the prior art. Units 1-5 are, respectively, input unit 1, modulation unit 2, third phase modulation unit 3, intensity modulation unit 4, and attenuation unit 5. Units 1, 2, 4, and 5 can constitute an optical path for phase or time-phase state encoding. In this heterogeneous integration scheme, a silicon nitride waveguide is used, which can reduce the temperature control accuracy requirements. In general reports, silicon nitride is mainly used to manufacture passive devices with very low loss, primarily due to the high-temperature processes that achieve ultra-low loss (on the order of approximately 0.1 dB / cm). However, currently, silicon nitride compatible with active silicon photonics is limited by the doping processes of active silicon photonics. Silicon nitride waveguides require low-temperature processes, which can introduce defects such as voids and nitrogen-hydrogen bonds, resulting in generally higher losses (on the order of approximately 2 dB / cm at the 1550 nm band). Therefore, if based on conventional solutions (e.g.... Figure 1 and 2 As shown, simply changing the unequal-arm interferometer from a silicon waveguide to a silicon nitride waveguide will significantly increase the loss difference between the long and short arms, and the total optical path loss will also increase significantly (for example, in a 625MHz system, the corresponding 800ps delay difference is about 24dB with a silicon nitride waveguide of about 12cm), which will lead to application difficulties.

[0008] In addition, Figure 3 In the heterogeneous integrated chip scheme shown, the repetition frequency of the driving signal of the intensity modulation unit 4 needs to be twice the system repetition frequency, which requires a high repetition rate driving signal. That is, when an optical pulse with a repetition frequency of f is input into input port A, it will be split into two optical pulse components with a specific delay difference after passing through unit 230. Two types of encoding need to be generated in the time state (|t0>, i.e., the first pulse component is extinguished and the second pulse component is lit; |t1>, i.e., the first pulse component is lit and the second pulse component is extinguished). The modulation repetition frequency of the intensity modulation unit 4 is consistent with the two components, which is 2f.

[0009] In conclusion, in Figure 1 In the optical chip implemented using a conventional hybrid integration scheme, as shown in Figure 2, different elemental material systems are required in the modulator and delay line sections, resulting in a large chip size and high process requirements and costs. Directly using silicon-based integration processes, however, requires extremely high temperature control precision due to the high thermo-optical coefficient of silicon, making its application difficult. And as... Figure 3As shown in the diagram, based on the heterogeneous integration scheme, directly replacing silicon waveguides with silicon waveguides in existing optical chip schemes can reduce the temperature control accuracy requirements, but it will introduce a large total loss in the optical path. Furthermore, the loss difference between the long and short arms of the unequal arm interferometer will be large, and the modulation repetition frequency of intensity modulation unit 4 is twice the system repetition frequency, leading to application difficulties. Utility Model Content

[0010] To address the aforementioned problems in existing technologies, this invention proposes a quantum key distribution encoding optical chip based on silicon and silicon nitride waveguides. By introducing a silicon nitride waveguide as a delay line within the unequal-arm interferometer module, phase stabilization can be achieved using conventional temperature control, reducing system complexity. Correspondingly, by placing the high-speed modulator within both the equal-arm and unequal-arm (short arm) interferometers, the losses of the equal-arm and unequal-arm interferometers (long and short arms) can be effectively reduced. This allows for effective control of loss increment while simultaneously implementing a delay line using silicon nitride waveguides. It also increases the thermally induced (refractive index change-induced) optical path change of the unequal-arm (short arm) interferometer, reducing the difference in thermally induced optical path change between the long and short arms. Compared to solutions using only silicon nitride for both arms of the unequal-arm interferometer, this further reduces temperature sensitivity and significantly lowers application difficulty. Therefore, the optical chip design proposed in this invention achieves high integration, has a mature manufacturing process, and is not demanding in terms of temperature control, effectively promoting the practical application of quantum key distribution encoding optical chips.

[0011] Specifically, the quantum key distribution encoding optical chip based on silicon and silicon nitride waveguides of this invention may include an unequal-arm interferometer module, which includes a first optical beam splitter, an optical beam combiner, and a long arm and a short arm connected between the two.

[0012] The long arm includes a first silicon nitride waveguide and first and second mode converters respectively disposed at both ends of the first silicon nitride waveguide;

[0013] The short arm includes a second silicon nitride waveguide and third and fourth mode converters respectively disposed at both ends of the second silicon nitride waveguide;

[0014] The mode converter is configured to achieve optical coupling between the silicon waveguide and the silicon nitride waveguide; and...

[0015] The short arm is also equipped with an adjustable optical attenuator and a first high-speed phase modulator.

[0016] Furthermore, the quantum key distribution encoding optical chip of this utility model also includes a second optical beam splitter, and a first arm and a second arm are provided between the second optical beam splitter and the first optical beam splitter to form an equal-arm interferometer module;

[0017] The first arm is equipped with a second high-speed phase modulator and a low-speed phase modulator;

[0018] A third high-speed phase modulator is provided on the second arm.

[0019] Preferably, the length of the second silicon nitride waveguide does not exceed 10 micrometers.

[0020] Preferably, the optical beam splitter and / or optical beam combiner comprises a silicon-based multimode interferometer or a directional coupler.

[0021] Preferably, the low-speed phase modulator is a thermo-optic modulator.

[0022] Preferably, the high-speed phase modulator is a silicon-based plasma dispersive modulator, or a Mach-Zehnder phase modulator composed of a silicon-based plasma dispersive modulator and a thermo-optic modulator.

[0023] Preferably, the tunable optical attenuator is a Mach-Zehnder interferometer composed of an optical beam splitter and a low-speed phase modulator, or a doped tunable optical attenuator.

[0024] Preferably, the silicon nitride waveguide is deposited on a silicon chip.

[0025] Preferably, the mode converter has a waveguide tapered structure for forming evanescent wave energy coupling between the silicon nitride waveguide and the silicon waveguide optical field.

[0026] Preferably, the quantum key distribution encoding optical chip of this invention is realized using a silicon-based integrated process compatible with CMOS technology. Attached Figure Description

[0027] The specific embodiments of this utility model will be further described in detail below with reference to the accompanying drawings.

[0028] To more clearly illustrate the technical solutions in the embodiments of this utility model or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this utility model. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0029] Figure 1 This illustrates a time-phase encoded optical chip based on a hybrid integration approach in the prior art;

[0030] Figure 2 This illustrates another time-phase encoding module in the prior art;

[0031] Figure 3This illustrates a heterogeneous integrated chip for quantum key distribution in the prior art;

[0032] Figure 4 An example of a quantum key distribution encoding optical chip based on silicon and silicon nitride waveguides according to the present invention is shown, which can realize phase encoding in quantum key distribution;

[0033] Figure 5 Another example of a quantum key distribution encoding optical chip based on silicon and silicon nitride waveguides according to the present invention is shown, which can realize time-phase encoding in quantum key distribution. Detailed Implementation

[0034] In the following description, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. The following embodiments are provided by way of example to fully convey the spirit of the present invention to those skilled in the art. Therefore, the present invention is not limited to the embodiments disclosed herein.

[0035] Figure 4 An example of a quantum key distribution encoding optical chip based on silicon and silicon nitride waveguides according to the present invention is shown. By forming a specific unequal-arm interferometer structure on a silicon-based material (where the functional devices / modules are connected through silicon waveguides), the phase encoding process in quantum key distribution can be realized without significantly increasing system losses and without the need for highly precise temperature control.

[0036] Specifically, the unequal-arm interferometer module according to this utility model may include a first optical beam splitter, an optical beam combiner, and a long arm and a short arm connected between the two.

[0037] The optical chip of this invention employs silicon-based integration technology. Therefore, as an example, a beam splitter can be implemented by means of a multimode interferometer or directional coupler etched on silicon material to achieve beam splitting of light.

[0038] Similarly, as an example, an optical beam combiner can be implemented by means of a multimode interferometer or directional coupler etched onto silicon material to achieve beam combining of light.

[0039] See also Figure 4 An adjustable optical attenuator 2 and a first high-speed phase modulator can also be installed on the short arm of the unequal-arm interferometer.

[0040] The adjustable optical attenuator 2 is used to adjust the attenuation of light, for example, to achieve loss equalization between the long and short arms of an unequal-arm interferometer.

[0041] As an example, the tunable optical attenuator 2 can be a Mach-Zehnder interferometer based on an optical beam splitter and a low-speed phase modulator, or a doped tunable optical attenuator.

[0042] The first high-speed phase modulator is used to achieve high-speed phase modulation of light, to achieve, for example, the desired phase encoding.

[0043] As an example, the high-speed phase modulator of this invention can be a silicon-based plasma dispersion modulator, which is doped in the region next to the silicon waveguide or on the silicon waveguide and led out through metal electrodes. The corresponding phase modulation is achieved by loading an external radio frequency signal into the metal electrodes.

[0044] As another example, the high-speed phase modulator of this invention can also be a Mach-Zehnder type phase modulator composed of a silicon-based plasma dispersive modulator and a thermo-optic modulator.

[0045] Furthermore, the high-speed phase modulator of this invention can be implemented by cascading one or more stages.

[0046] According to this utility model, preferably, an adjustable optical attenuator 1 can also be provided on the long arm of the unequal-arm interferometer module, so as to allow for more convenient and accurate loss balance between the long and short arms of the unequal-arm interferometer by means of the cooperation between the two adjustable optical attenuators on the long and short arms, for example. Figure 4 As shown.

[0047] See also Figure 4 According to this utility model, based on the above-mentioned unequal-arm interferometer module, a first silicon nitride waveguide can be specially set in the long arm as a delay line unit to generate the required delay difference between the two arms in the interferometer.

[0048] In this invention, as an example, silicon nitride waveguides can be deposited on silicon chips by methods such as PECVD.

[0049] Furthermore, a first mode converter 11 and a second mode converter 21 can be respectively provided at both ends of the first silicon nitride waveguide to realize optical coupling between the first silicon nitride waveguide and the silicon waveguide.

[0050] As an example, the mode converter of this invention can achieve evanescent wave energy coupling between silicon nitride waveguides and silicon waveguide optical fields by means of a waveguide tapered structure.

[0051] According to this invention, a second silicon nitride waveguide can be provided on the short arm of the unequal-arm interferometer module, and a third mode converter 12 and a fourth mode converter 22 can be provided at both ends of the second silicon nitride waveguide to achieve optical coupling between the second silicon nitride waveguide and the silicon waveguide. The third and fourth mode converters on the short arm can have the same design as the first and second mode converters on the long arm. This design eliminates the need to separately evaluate the optical path introduced by the mode converters, thus improving the accuracy of the arm length difference in the unequal-arm interferometer.

[0052] In this invention, the second silicon nitride waveguide can have the shortest possible length, for example, 10 micrometers, thereby reducing the unnecessary losses it introduces.

[0053] Figure 5 Another example of a quantum key distribution encoding optical chip based on silicon and silicon nitride waveguides according to the present invention is shown, which adds an equal-arm interferometer module to the above-mentioned optical chip structure for use in conjunction with an unequal-arm interferometer module to realize time-phase encoding in quantum key distribution.

[0054] like Figure 5 As shown, the quantum key distribution encoding optical chip is also equipped with a second optical beam splitter 1. By setting a first arm and a second arm between it and the first optical beam splitter 2, an equal-arm interferometer structure is formed.

[0055] Those skilled in the art will understand that the first and second arms have the same optical path.

[0056] According to this invention, a second high-speed phase modulator 11 and a low-speed phase modulator are also provided on the first arm, and a third high-speed phase modulator 12 is also provided on the second arm. Therefore, by adjusting the bias point using the low-speed phase modulator in the equal-arm interferometer module, for example, when the second high-speed phase modulator 11 provides a modulation phase of π / 2, light will propagate from the short arm of the unequal-arm interferometer (i.e., the |t0> state); when the third high-speed phase modulator 12 provides a modulation phase of π / 2, light will propagate from the long arm of the unequal-arm interferometer (i.e., the |t1> state); when the first high-speed phase modulator 2 in the unequal-arm interferometer provides modulation phases of 0 and π, the X basis vectors can be prepared, i.e., respectively... (|t0>+i|t1> state and (|t0>-i|t1>) state preparation.

[0057] Table 1 below shows examples of modulation phase combinations used when implementing time-phase encoding for quantum key distribution using the optical chip of this invention.

[0058]

[0059] (Table 1)

[0060] In an example of this invention, the low-speed phase modulator can be a thermo-optic modulator, in which titanium nitride metal is disposed above the silicon waveguide or doped silicon is disposed around the silicon waveguide and led out through metal electrodes. The desired phase modulation is achieved by loading an external low-speed or DC signal into the metal electrodes.

[0061] In a preferred example, the input end of the equal-arm interferometer module can use a waveguide as the input waveguide, which can be used to directly connect to other functional modules of the optical chip, or to provide external connection to the optical chip through optical fiber coupling.

[0062] like Figure 5 As shown, the input waveguide can be connected to the common terminal of the second optical beamsplitter 1; the first beam-splitting terminal of the second optical beamsplitter 1 is connected to the input terminal of the low-speed phase modulator, the output terminal of the low-speed phase modulator is connected to the input terminal of the second high-speed phase modulator 11, and the output terminal of the second high-speed modulator 11 is connected to the first input terminal of the first optical beamsplitter 2; the second beam-splitting terminal of the second optical beamsplitter 1 is connected to the input terminal of the third high-speed phase modulator 12, and the output terminal of the third high-speed phase modulator 12 is connected to the second input terminal of the first optical beamsplitter 2; the first output terminal of the first optical beamsplitter 2 is connected to the input terminal of the first mode converter 11. The optical beam combiner is connected as follows: the output of the first mode converter 11 is connected to the first long silicon nitride waveguide, which is then connected to the input of the second mode converter 21. The output of the second mode converter 21 is connected to the first input of the optical beam combiner. The second output of the first optical beam splitter 2 is connected to the input of the adjustable optical attenuator, which is then connected to the input of the first high-speed phase modulator 2. The output of the first high-speed phase modulator 2 is connected to the second input of the optical beam combiner via the third mode converter 12, the second short silicon nitride waveguide, and the fourth mode converter 22. The output of the optical beam combiner can also use a waveguide as its output waveguide for direct connection to other functional modules of the optical chip, or for external connection to the optical chip via fiber coupling.

[0063] Based on the above optical chip structure, this invention can use a silicon-based integration process compatible with CMOS technology to fabricate the optical chip, thereby allowing monolithic integration and achieving higher integration density.

[0064] As discussed above, compared to conventional silicon-based integrated solutions, this invention utilizes silicon nitride waveguides as delay line units to realize an unequal-arm interferometer, enabling phase stabilization through conventional temperature control and reducing system complexity. Furthermore, compared to existing optical chip designs (where the high-speed modulator is located outside the unequal-arm interferometer), this invention innovatively places the high-speed modulator within both the equal-arm and unequal-arm interferometers, particularly on the shorter arm of the unequal-arm interferometer. This effectively reduces losses in the equal-arm interferometer and, when using silicon nitride waveguides to realize delay line units in the unequal-arm interferometer, effectively reduces the loss difference between the long and short arms. Specifically, by placing the high-speed phase modulator inside the unequal-arm interferometer, the maximum modulation phase of the high-speed phase modulator in the equal-arm interferometer can be reduced from 3π / 2 to π / 2. Therefore, under the same driving capability and phase modulation efficiency, the loss of the high-speed modulator in the equal-arm interferometer can be reduced to 1 / 3 of its original value (approximately on the order of 10 dB), thus providing loss margin for the high losses introduced by using silicon-on-silicon nitride long delay waveguides. Simultaneously, by placing the high-speed modulator portion in the short arm of the unequal-arm interferometer, increasing the short arm loss, the loss difference between the long and short arms of the unequal-arm interferometer can be reduced, decreasing the attenuation required by the tunable optical attenuator in the short arm, thereby eliminating the need for, for example... Figure 3 The optical signal conditioning and beam splitting unit is designed separately as shown in the scheme. As mentioned earlier, if the conventional scheme is used to simply change the silicon waveguide to a silicon nitride waveguide, the total optical path loss will increase by approximately 24 dB, and the loss difference between the long and short arms of the unequal-arm interferometer will increase by approximately 24 dB, making it difficult to apply. However, with the special configuration described above in this invention, the loss can be increased by only approximately 10 dB while introducing a silicon nitride waveguide to realize a delay line, greatly reducing the difficulty of application.

[0065] At the same time, compared to, for example Figure 3 The scheme shown uses only silicon nitride material for both the long and short arms of the unequal-arm interferometer. By using a silicon modulator in the short arm of the unequal-arm interferometer, the thermally induced optical path change of the short arm (caused by the change in refractive index) is increased, and the difference in thermally induced optical path change between the long and short arms of the unequal-arm interferometer is reduced, thereby further reducing temperature sensitivity.

[0066] Furthermore, compared to, for example Figure 3 Compared with the prior art, this invention achieves time-state encoding before unequal arm delay, so the modulation repetition frequency of the driving signal can be equal to the repetition frequency of the system input optical pulse, which can reduce the driving repetition frequency by half and reduce the bandwidth requirement of the driving signal.

[0067] In summary, compared with the existing technology, the optical chip design proposed in this utility model has high integration, mature manufacturing process, and less stringent temperature control requirements, which can effectively promote the practical application of quantum key distribution encoding optical chips.

[0068] Although the present invention has been described above with reference to the accompanying drawings and specific embodiments, those skilled in the art will readily recognize that the above embodiments are merely exemplary and used to illustrate the principles of the present invention. They do not limit the scope of the present invention. Those skilled in the art can make various combinations, modifications and equivalent substitutions to the above embodiments without departing from the spirit and scope of the present invention.

Claims

1. A quantum key distribution encoding optical chip based on silicon and silicon nitride waveguides, comprising an unequal-arm interferometer module, wherein the unequal-arm interferometer module includes a first optical beamsplitter, an optical beam combiner, and a long arm and a short arm connected between the two, characterized in that: The long arm includes a first silicon nitride waveguide and first and second mode converters respectively disposed at both ends of the first silicon nitride waveguide; The short arm includes a second silicon nitride waveguide and third and fourth mode converters respectively disposed at both ends of the second silicon nitride waveguide; The mode converter is configured to achieve optical coupling between the silicon waveguide and the silicon nitride waveguide; and... The short arm is also equipped with an adjustable optical attenuator and a first high-speed phase modulator.

2. The quantum key distribution encoding optical chip as described in claim 1, characterized in that... It also includes a second optical beam splitter, and a first arm and a second arm are provided between the second optical beam splitter and the first optical beam splitter to form an equal-arm interferometer module; The first arm is equipped with a second high-speed phase modulator and a low-speed phase modulator; A third high-speed phase modulator is provided on the second arm.

3. The quantum key distribution encoding optical chip as described in claim 1 or 2, characterized in that, The length of the second silicon nitride waveguide does not exceed 10 micrometers.

4. The quantum key distribution encoding optical chip as described in claim 1 or 2, characterized in that, The optical beam splitter and / or optical beam combiner includes a silicon-based multimode interferometer or a directional coupler.

5. The quantum key distribution encoding optical chip as described in claim 2, characterized in that, The low-speed phase modulator is a thermo-optic modulator.

6. The quantum key distribution encoding optical chip as described in claim 1 or 2, characterized in that, The high-speed phase modulator is a silicon-based plasma dispersive modulator, or a Mach-Zehnder phase modulator composed of a silicon-based plasma dispersive modulator and a thermo-optic modulator.

7. The quantum key distribution encoding optical chip as described in claim 1 or 2, characterized in that, The tunable optical attenuator is a Mach-Zehnder interferometer consisting of an optical beam splitter and a low-speed phase modulator, or a doped tunable optical attenuator.

8. The quantum key distribution encoding optical chip as described in claim 1 or 2, characterized in that, The silicon nitride waveguide is deposited on a silicon chip.

9. The quantum key distribution encoding optical chip as described in claim 1 or 2, characterized in that, The mode converter has a waveguide tapered structure for forming evanescent wave energy coupling between the silicon nitride waveguide and the silicon waveguide optical field.

10. The quantum key distribution encoding optical chip as described in claim 1 or 2, characterized in that... This is achieved using a silicon-based integration process compatible with CMOS technology.

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