A time entangled two-photon generation system and a generation method

Abstract

The application discloses a time entanglement two-photon generation system and a generation method. The generation system comprises a laser source, an equal-arm MZ interferometer, a circulating waveguide, an upper computer, a controller, a second-order nonlinear crystal and a light splitting module. The equal-arm MZ interferometer is composed of a first beam splitter, a second beam splitter and a phase modulator. The upper computer calculates the light intensity percentage of the sub-pulse output from the second beam splitter after the light pulse output by the laser source passes through the phase modulator based on the input parameters, and calculates the light intensity percentage of the sub-pulse output from the second beam splitter after the light splitting pulse in each cycle passes through the phase modulator. The controller correspondingly time-modulates the phase modulator based on the light intensity percentages, so that the light splitting pulse entering the circulating loop is output from the output lower port of the second beam splitter at a corresponding light intensity ratio. After multiple cycles, the sub-pulse sequence output is subjected to a spontaneous parametric down-conversion process by the second-order nonlinear crystal to generate entangled two photons.

Description

Technical Field

[0001] This application belongs to the field of quantum information technology, and specifically relates to a time-entangled two-photon generation system and method. Background Technology

[0002] Quantum entanglement is a core resource in quantum information systems, playing a crucial role in their performance. It is widely applied in sub-fields such as quantum key distribution, quantum secure communication, quantum teleportation, quantum precision measurement, and quantum cryptography. Typically, quantum entanglement can be achieved through physical systems based on nonlinear processes, utilizing the laws of energy conservation, linear momentum conservation, and angular momentum conservation in nonlinear processes. Quantum entanglement can be established on different degrees of freedom of photons, such as polarization entanglement, path entanglement, temporal entanglement, and orbital angular momentum entanglement. Among these, polarization-entangled photon sources and temporally entangled photon sources are suitable for the field of quantum communication.

[0003] Currently, optical fibers are the primary carrier for information transmission in the field of communications. When polarization-entangled photons propagate in optical fibers, changes in the external environment, such as fiber perturbations, significantly affect the degree of polarization entanglement, thus limiting the transmission distance. In contrast, time-entangled photon pairs are entangled based on a time relationship; their entanglement degree is unaffected by transmission distance, making them more suitable for long-distance communication and the most widely used encoding method in practical quantum key distribution applications.

[0004] Currently, the most common method for generating temporally entangled photon pairs is downconversion techniques, such as... Figure 1 As shown, a system consisting of a laser, an unequal-arm MZ interferometer, nonlinear materials, a beam splitter, and a coupler is used to generate entangled photons. The laser provides the pump light, and the unequal-arm MZ interferometer, typically employing an fiber-coded optical path, time-splits the pump light to form early and late pulses, which are then pumped. Temporarily entangled photon pairs are generated using the nonlinear material, and then split and coupled into an optical fiber by the beam splitter and coupler. However, this scheme can only generate one pair of two-dimensional time-entangled photons. To achieve higher-dimensional quantum time entanglement, more devices and delay paths are needed, inevitably leading to problems such as large space requirements, high resource consumption, and high costs. Summary of the Invention

[0005] To address the aforementioned problems, this application provides a time-entangled two-photon generation system and method. The optical pulses output from the laser source are repeatedly cyclically formed into a sub-pulse sequence on an equal-arm MZ interferometer and a cyclic waveguide. This sub-pulse sequence is then input into a second-order nonlinear crystal to generate time-entangled two-photons. The system is simple in structure and saves resources and space. The specific scheme is as follows:

[0006] In a first aspect, this application discloses a time-entangled two-photon generation system, including a laser source, an equal-arm MZ interferometer, a cyclic waveguide, a host computer, a controller, a second-order nonlinear crystal, and a beam splitting module;

[0007] The laser source is used to output optical pulses;

[0008] The equal-arm MZ interferometer and the circulating waveguide form a circulating loop, which is used to time-split the optical pulses and form a sub-pulse sequence. The equal-arm MZ interferometer consists of a first beam splitter, an upper interferometer arm, a lower interferometer arm, a phase modulator, and a second beam splitter. The two ends of the upper interferometer arm are respectively connected to the upper output port of the first beam splitter and the upper input port of the second beam splitter. The two ends of the lower interferometer arm are respectively connected to the lower output port of the first beam splitter and the lower input port of the second beam splitter. The phase modulator is disposed on the upper interferometer arm. The lower input port of the first beam splitter is used to receive the optical pulses output by the laser source. The two ends of the circulating waveguide are respectively connected to the upper input port of the first beam splitter and the upper output port of the second beam splitter, and are used to input the split optical pulses output from the upper output port of the second beam splitter to the upper input port of the first beam splitter. The lower output port of the second beam splitter is used to output the sub-pulse sequence.

[0009] The host computer is used to input parameters for generating entangled two-photons and, based on these parameters, calculates the percentage of light intensity R0 of the sub-pulses output from the lower output port of the second beam splitter after the light pulse output from the laser source is modulated by the phase modulator. It also calculates the percentage of light intensity R1, R2…R0 of the sub-pulses output from the lower output port of the second beam splitter after the split light pulse in each cycle is modulated by the phase modulator. N Wherein, the light intensity percentage is the light intensity of the sub-pulse output from the lower output port of the second beam splitter / the light intensity of the pulse input from the lower input port or the upper input port of the first beam splitter, and N is the number of cycles;

[0010] The controller is connected to both the phase modulator and the host computer, and is used to receive the light intensity percentages R0, R1, R2...R output by the host computer. N And based on R0, R1, R2...R N Accordingly, the phase modulator is time-modulated;

[0011] The second-order nonlinear crystal is connected to the lower output port of the second beam splitter to perform a spontaneous parametric downconversion process on the subpulse sequence and generate entangled two photons.

[0012] The beam splitting module is connected to the second-order nonlinear crystal and is used to separate the entangled two photons.

[0013] Furthermore, the parameters include: a preset number of loops N, and probability amplitudes a0, a1, a2…a N In this sequence, the sub-pulse output from the lower output port of the second beam splitter after the optical pulse output from the laser source is modulated by the phase modulator is named the 0th sub-pulse. In each cycle, the sub-pulses output from the lower output port of the second beam splitter are respectively named the 1st sub-pulse, the 2nd sub-pulse, ..., the Nth sub-pulse. The 0th sub-pulse, the 1st sub-pulse, the 2nd sub-pulse, ..., the Nth sub-pulse constitute the sub-pulse sequence. Then, a0 represents the probability amplitude of detecting an entangled photon pair at the time position of the 0th sub-pulse; a1 represents the probability amplitude of detecting an entangled photon pair at the time position of the 1st sub-pulse, and so on. N This represents the probability amplitude of detecting an entangled photon pair at the Nth sub-pulse time position.

[0014] Furthermore, the controller is connected to the laser source. When the laser source generates an optical pulse based on the driving electrical signal, the laser source simultaneously feeds back the driving electrical signal to the controller, and the controller starts timing modulation of the phase modulator based on the driving electrical signal.

[0015] Furthermore, the system also includes a third beam splitter and a photodetector. The input of the third beam splitter is connected to the laser source, one output of the third beam splitter is connected to the photodetector, and the other output is connected to the lower input port of the first beam splitter. The photodetector is connected to the controller. The third beam splitter is used to split the light pulses output by the laser source and transmit a portion of the split light pulses to the photodetector. The photodetector is used to convert the received light pulses into electrical pulse signals and transmit the electrical pulse signals to the controller. The controller starts timing modulation of the phase modulator based on the electrical pulse signals.

[0016] Preferably, the beam splitting module is an arrayed waveguide grating or a wavelength division multiplexer.

[0017] Furthermore, the system also includes a filter disposed between the second-order nonlinear crystal and the beam splitting module, for filtering out noise photons output from the second-order nonlinear crystal that have a frequency different from that of the entangled two photons.

[0018] Furthermore, the system also includes an adjustable attenuator, which is disposed on the transmission path between the laser source and the lower input port of the first beam splitter and connected to the controller, for attenuating the intensity of the light pulses output by the laser source.

[0019] Secondly, this application discloses a method for generating time-entangled two-photons, which is applied to the aforementioned time-entangled two-photon generation system. The method includes:

[0020] The parameters for generating entangled two-photon pairs are input into the host computer. Based on the input parameters, the host computer calculates the intensity percentage R0 of the sub-pulse output from the lower port of the second beam splitter after the light pulse from the laser source is modulated by the phase modulator, and also calculates the intensity percentages R1, R2...R of the sub-pulse output from the lower port of the second beam splitter after the split light pulse in each cycle is modulated by the phase modulator. N ;

[0021] A laser source inputs a light pulse to the lower input port of the first beam splitter. The controller adjusts the phase modulator according to the light intensity percentage R0, so that the light pulse output from the laser source is output from the lower output port of the second beam splitter with an intensity ratio of R0, and output from the upper output port of the second beam splitter to the first loop with an intensity ratio of 1-R0. The controller then adjusts the phase modulator according to R1, so that the split light pulse entering the first loop is output from the lower output port of the second beam splitter with an intensity ratio of R1, and output from the upper output port of the second beam splitter to the second loop with an intensity ratio of 1-R1. This process is repeated... The controller adjusts the phase modulator according to R... N Adjust the phase modulator so that all the split optical pulses entering the Nth loop are output from the lower output port of the second beam splitter, thus completing the timing modulation of the phase modulator by the controller within one entangled state generation cycle;

[0022] Multiple sub-pulses output from the lower output port of the second beam splitter form a sub-pulse sequence. The sub-pulse sequence undergoes a spontaneous parametric downconversion process through a second-order nonlinear crystal and generates entangled two-photons.

[0023] Entangled two photons are separated and output after passing through a beam splitter.

[0024] Furthermore, when the controller is connected to the laser source, the method further includes:

[0025] When the laser source generates optical pulses based on the driving electrical signal, the laser source simultaneously feeds back the driving electrical signal to the controller, and the controller starts to perform timing modulation on the phase modulator based on the driving electrical signal.

[0026] Furthermore, when the system further includes a third beam splitter and a photodetector, wherein the input of the third beam splitter is connected to the laser source, one output of the third beam splitter is connected to the photodetector, and the other output is connected to the lower input port of the first beam splitter, and the photodetector is connected to the controller, the method further includes:

[0027] The third beam splitter splits the light pulses output from the laser source and transmits a portion of the split light pulses to the photodetector. The photodetector converts the received light pulses into electrical pulse signals and transmits the electrical pulse signals to the controller. The controller then starts timing modulation of the phase modulator based on the electrical pulse signals.

[0028] In summary, compared with the prior art, the above-described technical solutions conceived in this application can achieve the following beneficial effects:

[0029] This application provides a time-entangled two-photon generation system and method. The generation system includes a laser source, an equal-arm MZ interferometer, a cyclic waveguide, a host computer, a controller, a second-order nonlinear crystal, and a beam splitting module. The equal-arm MZ interferometer consists of a first beam splitter, a second beam splitter, and a phase modulator disposed between the two beam splitters. The host computer calculates the light intensity percentage R0 of the sub-pulse output from the second beam splitter after the light pulse output from the laser source is modulated by the phase modulator, based on the input parameters, and calculates the light intensity percentages R1, R2…R of the sub-pulse output from the second beam splitter after the split light pulse in each cycle is modulated by the phase modulator. N The controller modulates the phase modulator in a corresponding time sequence based on the percentage of light intensity, so that the split light pulses entering the loop are output from the lower output port of the second beam splitter with corresponding light intensity ratios. After multiple cycles, the multiple sub-pulses output from the lower output port of the second beam splitter form a sub-pulse sequence. The sub-pulse sequence undergoes a spontaneous parametric downconversion process through a second-order nonlinear crystal and generates entangled two-photons. This application obtains the entangled state of the temporally entangled photon pairs of the corresponding dimension by modulating the phase modulator on the equal-arm MZ interferometer in each process according to the preset number of cycles and the preset probability amplitude of sub-pulses formed in each process. It is independent of the number of equal-arm MZ interferometers and has the advantage of adaptive adjustment. Moreover, the loop structure of this application saves resources and space. Attached Figure Description

[0030] To more clearly illustrate the technical solutions in this embodiment or the prior art, the drawings used in the description of the embodiment or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0031] Figure 1 This is a schematic diagram of the structure of the prior art in this application;

[0032] Figure 2 A schematic diagram of a time-entangled two-photon generation system provided in one embodiment of this application;

[0033] Figure 3This is a timing diagram of the sub-pulse sequence output from the lower port of the second beam splitter in one embodiment of this application;

[0034] Figure 4 This is a timing diagram of the sub-pulse sequence output from the lower port of the second beam splitter in another embodiment of this application;

[0035] Figure 5 A schematic diagram of a time-entangled two-photon generation system provided in another embodiment of this application;

[0036] Figure 6 A schematic diagram of the structure of a time-entangled two-photon generation system provided in another embodiment of this application;

[0037] Figure 7 A schematic diagram of a time-entangled two-photon generation system provided in another embodiment of this application;

[0038] Figure 8 A schematic diagram of the structure of a time-entangled two-photon generation system provided in another embodiment of this application;

[0039] Figure 9 This application is based on Figure 7 and Figure 8 A schematic diagram of a time-entangled two-photon generation system is provided.

[0040] Figure 10 This application is based on Figure 6 and Figure 9 A schematic diagram of a time-entangled two-photon generation system is provided. Detailed Implementation

[0041] To make the above-mentioned objectives, features, and advantages of this application more apparent and understandable, the embodiments of this application will be further described in detail below with reference to the accompanying drawings and specific implementation methods. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0042] Many specific details are set forth in the following description in order to provide a full understanding of this application. However, this application may also be implemented in other ways different from those described herein. Those skilled in the art can make similar extensions without departing from the spirit of this application. Therefore, this application is not limited to the specific embodiments disclosed below.

[0043] To facilitate understanding and explanation of the technical solutions provided in the embodiments of this application, the background technology of this application will be described first.

[0044] Currently, the most common method for generating temporally entangled photon pairs is downconversion techniques, such as... Figure 1 As shown, a system consisting of a laser, an unequal-arm MZ interferometer, nonlinear materials, a beam splitter, and a coupler is used to generate entangled photons. The laser provides the pump light, and the unequal-arm MZ interferometer, typically employing an fiber-coded optical path, time-splits the pump light to form early and late pulses, which are then pumped. Temporarily entangled photon pairs are generated using the nonlinear material, and then split and coupled into an optical fiber by the beam splitter and coupler. However, this scheme can only generate one pair of two-dimensional time-entangled photons. To achieve higher-dimensional quantum time entanglement, more devices and delay paths are needed, inevitably leading to problems such as large space requirements, high resource consumption, and high costs.

[0045] Based on this, this application provides a time-entangled two-photon generation system, such as... Figure 2 As shown, it includes a laser source, an equal-arm MZ interferometer, a circulating waveguide, a host computer, a controller, a second-order nonlinear crystal, and a beam splitting module.

[0046] The laser source is used to output optical pulses. Specifically, the laser source can be a free-space laser source device such as a distributed feedback laser, a distributed Bragg reflector laser, or a semiconductor laser diode, or it can be an on-chip integrated laser source device such as a hybrid integrated III-V semiconductor laser diode or a vertical-cavity surface-emitting laser.

[0047] An equal-arm MZ interferometer and a circulating waveguide form a circulating loop, which is used to time-split optical pulses and form a sub-pulse sequence. Specifically, the equal-arm MZ interferometer consists of a first beam splitter, an upper interferometer arm, a lower interferometer arm, a phase modulator, and a second beam splitter. The two ends of the upper interferometer arm are connected to the upper output port of the first beam splitter and the upper input port of the second beam splitter, respectively. The two ends of the lower interferometer arm are connected to the lower output port of the first beam splitter and the lower input port of the second beam splitter, respectively. The phase modulator is located on the upper interferometer arm. The lower input port of the first beam splitter is used to receive the optical pulses output from the laser source. The two ends of the circulating waveguide are connected to the upper input port of the first beam splitter and the upper output port of the second beam splitter, respectively, and are used to input the split optical pulses output from the upper output port of the second beam splitter to the upper input port of the first beam splitter. The lower output port of the second beam splitter is used to output the sub-pulse sequence.

[0048] For ease of distinction and understanding, as described above, the pulse output from the upper output port of the second beam splitter is named the splitting pulse, and the pulse output from the lower output port of the second beam splitter is named the sub-pulse. Specifically, the sub-pulse output from the lower output port of the second beam splitter after the laser source output pulse is modulated by the phase modulator is named the 0th sub-pulse. In each cycle, the sub-pulses output from the lower output port of the second beam splitter are respectively named the 1st sub-pulse, the 2nd sub-pulse, ..., the Nth sub-pulse. The 0th sub-pulse, the 1st sub-pulse, the 2nd sub-pulse, ..., the Nth sub-pulse form a sub-pulse sequence.

[0049] The host computer is used to input the parameters for generating entangled two-photon pairs and, based on these parameters, calculate the percentage of light intensity R0 of the sub-pulses output from the lower output port of the second beam splitter after the light pulse from the laser source is modulated by the phase modulator. It also calculates the percentage of light intensity R1, R2…R0 of the sub-pulses output from the lower output port of the second beam splitter after the split light pulse in each cycle is modulated by the phase modulator. N Wherein, the light intensity percentage is the light intensity of the sub-pulse output from the lower output port of the second beam splitter / the light intensity of the pulse input from the lower input port or the upper input port of the first beam splitter, and N is the number of cycles.

[0050] Specifically, the light intensity percentage R0 is the light intensity of the 0th sub-pulse output from the lower output port of the second beam splitter divided by the light intensity input from the lower input port of the first beam splitter. The light intensity percentage R1 is the light intensity of the 1st sub-pulse output from the lower output port of the second beam splitter in the first cycle divided by the light intensity of the split light pulse input from the upper input port of the first beam splitter in the first cycle. The light intensity percentage R2 is the light intensity of the 2nd sub-pulse output from the lower output port of the second beam splitter in the second cycle divided by the light intensity of the split light pulse input from the upper input port of the first beam splitter in the second cycle, and so on. N The intensity of the Nth sub-pulse output from the lower output port of the second beam splitter in the Nth cycle is the intensity of the split light pulse input from the upper input port of the first beam splitter in the Nth cycle.

[0051] In this application, the parameters for generating entangled two photons include a preset number of cycles N and probability amplitudes a0, a1, a2…a N Where a0 represents the probability amplitude of detecting an entangled photon pair at the 0th sub-pulse time position; a1 represents the probability amplitude of detecting an entangled photon pair at the 1st sub-pulse time position, and so on, a N This represents the probability amplitude of detecting an entangled photon pair at the time position of the Nth sub-pulse. It should be noted that for probability amplitudes a0, a1, a2…a… NSome values ​​can be set to 0 as needed. For example, if a1 is set to 0, it means that all the split light pulses in the first cycle are modulated by the phase modulator and output from the upper output port of the second beam splitter into the second cycle, while no sub-pulse is output from the lower output port of the second beam splitter, that is, no first sub-pulse is output. Obviously, R1 is 0 accordingly.

[0052] In this application, the host computer includes a parameter setting module, a data processing module, and a data transmission module connected in sequence. The parameter setting module is used to input parameters for generating entangled two photons, such as the number of cycles N, probability amplitudes a0, a1, a2…a N The data processing module calculates the percentage of light intensity R0 of the sub-pulses output from the lower output port of the second beam splitter after the light pulse from the laser source is modulated by the phase modulator, based on the set parameters. It also calculates the percentage of light intensity R1, R2…R0 of the sub-pulses output from the lower output port of the second beam splitter after the split light pulse in each cycle is modulated by the phase modulator. N The data transmission module is used to transfer R0, R1, R2…R N Transmitted to the controller.

[0053] The controller is connected to both the phase modulator and the host computer, and is used to receive the percentage light intensity R0, R1, R2…R from the host computer. N And based on R0, R1, R2...R N Correspondingly, a timing modulation phase modulator.

[0054] The controller is based on the percentage of light intensity R0, R1, R2...R fed back from the host computer. N Phase modulation of the phase modulator is performed sequentially according to the time sequence. Specifically, the controller modulates the phase of each light intensity percentage R0, R1, R2…R… N The controller calculates the initial process of the laser source outputting a sub-pulse from the lower output port of the second beam splitter, as well as the modulation voltage or current of the phase modulator corresponding to each subsequent cycle. The controller sequentially completes the phase modulation of the initial process and each cycle according to the preset phase modulation period (the holding time of each modulation voltage or current).

[0055] The specific process is as follows: The light pulse output from the laser source is input from the lower input port of the first beam splitter. The controller modulates the driving voltage or current of the phase modulator to make the light pulse output from the laser source output from the lower output port of the second beam splitter with an intensity ratio of R0. Then, the light pulse is input from the upper output port of the second beam splitter into the first loop with an intensity ratio of 1-R0. The split light pulse entering the first loop is transmitted through the loop waveguide to the upper input port of the first beam splitter and then enters the equal-arm MZ interferometer for transmission. In the first loop, the controller modulates the driving voltage or current of the phase modulator to make the split light pulse entering the first loop output from the lower output port of the second beam splitter with an intensity ratio of R1. Then, the light pulse is output from the upper output port of the second beam splitter into the second loop with an intensity ratio of 1-R1. The split optical pulses entering the second loop are transmitted through a circulating waveguide to the upper input port of the first beam splitter and then to the equal-arm MZ interferometer. In the second loop, the controller modulates the driving voltage or current of the phase modulator, causing the split optical pulses entering the second loop to be output from the lower output port of the second beam splitter at an intensity ratio of R². Correspondingly, they are output from the upper output port of the second beam splitter to the third loop at an intensity ratio of 1-R². This cycle continues until the preset cyclic process of timing modulation of the phase modulator within one entangled state generation cycle is completed.

[0056] The second-order nonlinear crystal is connected to the lower output port of the second beam splitter to perform a spontaneous parametric downconversion process on the sub-pulse sequence and generate entangled two-photons. Here, the second-order nonlinear crystal can be any of the following: periodically polarized lithium niobate, periodically polarized potassium titanium phosphate, periodically polarized barium metaborate, etc., capable of spontaneously downconverting under the action of the sub-pulse sequence to generate entangled two-photons. The entangled state that generates entangled two-photons is represented as:

[0057] |φ>=a0|00>+a1|11>+a2|22>+…+a N |NN>

[0058] Assuming the probability P of a single subpulse in a subpulse sequence generating entangled two-photon pairs through a second-order nonlinear crystal is P, then the probability Pt of each subpulse in the subpulse sequence generating entangled photon pairs through a second-order nonlinear crystal is Pt. n =P n P n This is a very small value and can be ignored, meaning that the probability of each sub-pulse in the sub-pulse sequence generating entangled photon pairs through a second-order nonlinear crystal is negligible. Furthermore, if m sub-pulses (m≥2) in the sub-pulse sequence simultaneously generate multiple entangled two-photons, these will be filtered out during the subsequent selection process, selecting only the sub-pulse sequence that generates only one entangled two-photon within a single entangled state generation period.

[0059] In one embodiment of this application, a preset cycle of 3 times is set, with probability amplitudes corresponding to a0, a1, a2, and a3. Specifically, a0 and a2 are both set to 0. Setting a0 to 0 indicates that the optical pulses output from the laser source, after being modulated by the phase modulator, all enter the first loop from the upper output port of the second beam splitter, and the 0th sub-pulse is not output from the lower output port of the second beam splitter. Setting a2 to 0 indicates that the split optical pulses entering the second loop, after being modulated by the phase modulator, all enter the third loop from the upper output port of the second beam splitter, and the 2nd sub-pulse is not output from the lower output port of the second beam splitter. Based on these parameter settings, the entangled state of the entangled two photons is represented as: |φ>=a1|11>+a3|33>, and the corresponding sub-pulse sequence timing diagram is as follows: Figure 3 As shown, no sub-pulse 0 is generated within the phase modulation period corresponding to time t0, and no sub-pulse 2 is generated within the phase modulation period corresponding to time t2. The first and third sub-pulses are output within the phase modulation periods corresponding to time periods t1 and t3, respectively. The probability amplitude of the first sub-pulse generating entangled two-photon through the second-order nonlinear crystal is a1, and the probability amplitude of the third sub-pulse generating entangled two-photon through the second-order nonlinear crystal is a3.

[0060] In another embodiment of this application, a preset cycle of 4 times is set, with probability amplitudes corresponding to a0, a1, a2, a3, and a4, and none of these five probability amplitudes are 0. Based on these parameter settings, the entangled state of the entangled two photons is represented as follows:

[0061] |φ>=a0|00>+a1|11>+a2|22>+a3|33>+a4|44>

[0062] The timing diagram of the corresponding sub-pulse sequence is as follows Figure 4 As shown, the output sub-pulse 0, sub-pulse 1, sub-pulse 2, sub-pulse 3, and sub-pulse 4 are respectively generated within the phase modulation period corresponding to time periods t0, t1, t2, t3, and t4. The probability amplitudes of the 0th, 1st, 2nd, 3rd, and 4th sub-pulses generating entangled two-photon pairs through the second-order nonlinear crystal are a0, a1, a2, a3, and a4, respectively.

[0063] The beam splitter module is connected to a second-order nonlinear crystal to separate entangled two-photon pairs. Specifically, the beam splitter module is an arrayed waveguide grating or a wavelength division multiplexer.

[0064] The laser source outputs one optical pulse within the pulse period. When the pulse period of the laser source is greater than or equal to the entanglement state generation period, it can be guaranteed that only one optical pulse is input to the lower input port of the first beam splitter within one entanglement state generation period. Therefore, it is necessary to modulate the pulse frequency of the laser source or set an optical switch on the transmission path between the laser source and the lower input port of the first beam splitter. By controlling the opening and closing of the optical switch, it is guaranteed that only one optical pulse is input to the lower input port of the first beam splitter within one entanglement state generation period.

[0065] In this application, in order to obtain the timing of the laser source's output light pulse and the timing when the controller begins modulating the phase modulator, the controller is connected to the laser source, such as... Figure 5 As shown, when the laser source generates optical pulses based on the driving electrical signal, the laser source simultaneously feeds the driving electrical signal back to the controller. The controller then starts timing modulation of the phase modulator based on the driving electrical signal. That is, the moment when the controller receives the driving electrical signal is taken as the starting point for modulation of the phase modulator. Then, the initial process and the phase modulation of each cycle are completed sequentially according to the preset phase modulation period.

[0066] To more accurately determine the time when the controller begins modulating the phase modulator and reduce errors, in another embodiment of this application, a third beam splitter and a photodetector are also provided in the time-entangled two-photon generation system, such as... Figure 6 As shown, the input of the third beam splitter is connected to the laser source, one output of the third beam splitter is connected to the photodetector, and the other output is connected to the lower input port of the first beam splitter. The photodetector is connected to the controller. The third beam splitter is used to split the light pulses output by the laser source and transmit a portion of the split light pulses to the photodetector. The photodetector is used to convert the received light pulses into electrical pulse signals and transmit the electrical pulse signals to the controller. The controller starts timing modulation of the phase modulator based on the electrical pulse signals.

[0067] It should be noted that, in this embodiment, in order to ensure that the majority of the energy of the laser source's output light pulse is input into the equal-arm MZ interferometer, the beam splitting ratio of the third beam splitter is preferably set to 90:10. The light pulse output from the laser source is energy-divided by the third beam splitter, with 10 energy light pulses input to the photodetector and 90 energy light pulses input to the equal-arm MZ interferometer. The photodetector converts the received light pulses into electrical pulse signals and transmits the electrical pulse signals to the controller. The controller starts timing modulation of the phase modulator based on the electrical pulse signals, that is, the moment when the controller receives the electrical pulse signals is taken as the modulation start point of the phase modulator, and then the initial process and the phase modulation of each cycle are completed sequentially according to the preset phase modulation period.

[0068] To reduce noise interference, in one embodiment of this application, the time-entangled two-photon generation system further includes a filter, such as... Figure 7 As shown, the filter is placed between the second-order nonlinear crystal and the beam splitter module to filter out noise photons with frequencies different from those of the entangled two photons output from the second-order nonlinear crystal.

[0069] Entangled two-photons consist of a signal photon and an idler photon. The entangled two-photons generated by the second-order nonlinear crystal may contain noise photons, such as sub-pulses, with frequencies different from those of the entangled two-photons. The noise photons are filtered out from the entangled two-photons by a filter.

[0070] In another embodiment of this application, the time-entangled two-photon generation system further includes an adjustable attenuator, such as... Figure 8 As shown, the adjustable attenuator is set on the transmission path between the laser source and the lower input port of the first beam splitter and is connected to the controller to attenuate the intensity of the light pulse output by the laser source.

[0071] The adjustable attenuator is connected to the controller. Specifically, the adjustment parameters of the adjustable attenuator can be set on the host computer. The host computer feeds back the intensity adjustment parameters to the controller. Based on the received intensity adjustment parameters, the controller controls the adjustable attenuator to attenuate the light pulse output by the laser source to a suitable intensity in order to reduce the noise output by the second-order nonlinear crystal.

[0072] based on Figure 7 and Figure 8 This application also provides a time-entangled two-photon generation system, such as... Figure 9 As shown, the time-entangled two-photon generation system includes an adjustable attenuator and a filter. The adjustable attenuator is located on the transmission path between the laser source and the lower input port of the first beam splitter and is connected to the controller. The filter is located between the second-order nonlinear crystal and the beam splitting module.

[0073] based on Figure 6 and Figure 9 This application provides another time-entangled two-photon generation system, such as... Figure 10 As shown, the time-entangled two-photon generation system includes a third beam splitter, a photodetector, an adjustable attenuator, and a filter. The light pulses output from the laser source are input to the third beam splitter for beam splitting. The adjustable attenuator is positioned between the third and first beam splitters. Both the photodetector and the adjustable attenuator are connected to a controller. A portion of the light pulses after beam splitting by the third beam splitter is transmitted to the photodetector, which converts the received light pulses into electrical pulse signals and transmits these signals to the controller. The remaining light pulses are attenuated by the adjustable attenuator before being transmitted to the first beam splitter. The filter is positioned between the second-order nonlinear crystal and the beam splitting module.

[0074] As can be seen from the above schemes, the host computer calculates the percentage of light intensity R0 of the sub-pulses output from the second beam splitter after the light pulse output from the laser source is modulated by the phase modulator, based on the input parameters, and calculates the percentage of light intensity R1, R2...R of the sub-pulses output from the second beam splitter after the split light pulse is modulated by the phase modulator in each cycle. N The controller modulates the phase modulator in a corresponding time sequence based on the percentage of light intensity, so that the split light pulses entering the loop are output from the lower output port of the second beam splitter with corresponding light intensity ratios. After multiple cycles, the multiple sub-pulses output from the lower output port of the second beam splitter form a sub-pulse sequence. The sub-pulse sequence undergoes a spontaneous parametric downconversion process through a second-order nonlinear crystal and generates entangled two-photons. This application obtains the entangled state of the temporally entangled photon pairs of the corresponding dimension by modulating the phase modulator on the equal-arm MZ interferometer in each process according to the preset number of cycles and the preset probability amplitude of sub-pulses formed in each process. It is independent of the number of equal-arm MZ interferometers and has the advantage of adaptive adjustment. Moreover, the loop structure of this application saves resources and space.

[0075] Based on the time-entangled two-photon generation system provided in the above embodiments of this application, this application also provides a time-entangled two-photon generation method, the method comprising:

[0076] S11: Input the parameters for generating entangled two photons on the host computer. Based on the input parameters, the host computer calculates the percentage of light intensity R0 of the sub-pulse output from the lower port of the second beam splitter after the light pulse output from the laser source is modulated by the phase modulator, and calculates the percentage of light intensity R1, R2...R of the sub-pulse output from the lower port of the second beam splitter after the split light pulse is modulated by the phase modulator in each cycle. N .

[0077] In S11, the parameters for generating entangled two photons include: the preset number of cycles N, and the probability amplitudes a0, a1, a2…a N The host computer calculates the input values ​​a0, a1, a2…a N Calculate and obtain the light intensity percentages R0, R1, R2…R of the sub-pulses output from the lower port of the second beam splitter in each process. N The controller is based on the percentage of light intensity R0, R1, R2…R fed back from the host computer. N The corresponding timing modulation phase modulator is as described in step S12.

[0078] S12: A laser source inputs a light pulse to the lower input port of the first beam splitter. The controller adjusts the phase modulator according to the light intensity percentage R0, so that the light pulse output from the laser source is output from the lower output port of the second beam splitter with a light intensity ratio of R0, and output from the upper output port of the second beam splitter to the first loop with a light intensity ratio of 1-R0. The controller adjusts the phase modulator according to R1, so that the split light pulse entering the first loop is output from the lower output port of the second beam splitter with a light intensity ratio of R1, and output from the upper output port of the second beam splitter to the second loop with a light intensity ratio of 1-R1. This cycle continues... The controller adjusts the phase modulator according to R... N Adjust the phase modulator so that all the split optical pulses entering the Nth loop are output from the lower output port of the second beam splitter, thus completing the timing modulation of the phase modulator by the controller within one entangled state generation cycle.

[0079] In S12, the controller uses the percentage of light intensity R0, R1, R2…R fed back from the host computer. N Phase modulation of the phase modulator is performed sequentially according to the time sequence. Specifically, the controller modulates the phase of each light intensity percentage R0, R1, R2…R… N The controller calculates the modulation voltage or current of the phase modulator corresponding to the initial process of the light pulse output from the laser source outputting a sub-pulse from the lower output port of the second beam splitter, as well as the modulation voltage or current corresponding to each subsequent cycle process. The modulation voltage or current corresponding to each process is maintained for a certain period of time, which is the preset phase modulation period. The controller completes the phase modulation of the initial process and each cycle process in sequence according to the preset phase modulation period.

[0080] S13: Multiple sub-pulses output from the lower output port of the second beam splitter form a sub-pulse sequence. The sub-pulse sequence undergoes a spontaneous parametric downconversion process through a second-order nonlinear crystal and generates entangled two-photons.

[0081] S14: Entangled two photons are separated and output after passing through the beam splitter module.

[0082] Based on the time-entangled two-photon generation method provided in the above embodiments of this application, further, when the controller is connected to the laser source, the method further includes:

[0083] When the laser source generates optical pulses based on the driving electrical signal, the laser source simultaneously feeds back the driving electrical signal to the controller, and the controller starts to perform timing modulation on the phase modulator based on the driving electrical signal.

[0084] The laser source outputs optical pulses under the action of a driving electrical signal. The frequency of the driving electrical signal is the same as the frequency of the output optical pulses from the laser source. Simultaneously, the laser source feeds back the driving electrical signal to the controller. The laser source outputs only one optical pulse per cycle. By modulating the pulse frequency of the laser source or setting an optical switch, it is ensured that only one optical pulse is input to the lower input port of the first beam splitter within one entangled state generation cycle. The controller begins timing modulation of the phase modulator based on the driving electrical signal; that is, the controller uses the moment it receives the driving electrical signal as the starting point for modulation of the phase modulator, and then sequentially completes the initial process and the phase modulation of each cycle according to the preset phase modulation period.

[0085] Based on the temporally entangled two-photon generation method provided in the above embodiments of this application, further, when the temporally entangled two-photon generation system further includes a third beam splitter and a photodetector, wherein the input end of the third beam splitter is connected to a laser source, one output end of the third beam splitter is connected to the photodetector, and the other output end is connected to the lower input port of the first beam splitter, and the photodetector is connected to a controller, the method further includes:

[0086] The third beam splitter splits the light pulses output from the laser source and transmits a portion of the split light pulses to the photodetector. The photodetector converts the received light pulses into electrical pulse signals and transmits the electrical pulse signals to the controller. The controller then starts timing modulation of the phase modulator based on the electrical pulse signals.

[0087] In this embodiment, the controller takes the moment when it receives the electrical pulse signal as the modulation start point of the phase modulator, and then completes the initial process and the phase modulation of each cycle process in sequence according to the preset phase modulation period.

[0088] It should be noted that the various embodiments in this specification are described in a progressive, parallel, or progressive and parallel manner. Each embodiment focuses on the differences from other embodiments, and the same or similar parts between the various embodiments can be referred to each other.

[0089] Furthermore, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Moreover, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that an article or apparatus comprising a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such an article or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the article or apparatus that includes the aforementioned element.

[0090] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A temporal entangled two-photon generation system, comprising: The system comprises a laser source, an equal-arm MZ interferometer, a circulator, a host computer, a controller, a second-order nonlinear crystal and a light splitting module. The laser source is configured to output optical pulses. The equal-arm MZ interferometer and the circulator constitute a circulation loop, which is configured to time-split the optical pulses and form a sub-pulse sequence; the equal-arm MZ interferometer comprises a first beam splitter, an upper arm of interference, a lower arm of interference, a phase modulator and a second beam splitter; two ends of the upper arm of interference are connected to an output upper port of the first beam splitter and an input upper port of the second beam splitter, respectively; two ends of the lower arm of interference are connected to an output lower port of the first beam splitter and an input lower port of the second beam splitter, respectively; the phase modulator is arranged on the upper arm of interference; the input lower port of the first beam splitter is configured to receive the optical pulses output by the laser source; two ends of the circulator are connected to the input upper port of the first beam splitter and the output upper port of the second beam splitter, respectively, and are configured to input the split optical pulses output from the output upper port of the second beam splitter into the input upper port of the first beam splitter; and the output lower port of the second beam splitter is configured to output the sub-pulse sequence. The upper computer is used for inputting parameters for generating entangled two-photon and calculating a light intensity percentage R0 of a sub-pulse output from a lower port of the second beam splitter after a light pulse output by the laser source passes through a phase modulator and is modulated, and calculating a light intensity percentage of a sub-pulse output from the lower port of the second beam splitter after a split light pulse in each cycle passes through the phase modulator and is modulated, which corresponds to R1, R2…R N respectively; wherein the light intensity percentage is a light intensity of the sub-pulse output from the lower port of the second beam splitter / the pulse light intensity input from the lower input port or the upper input port of the first beam splitter, and N is a cycle number. The controller is connected with the phase modulator and the host computer respectively, and is used for receiving each light intensity percentage R0, R1, R2…Rn output by the host computer N , and outputting the corresponding control signal to the phase modulator based on R0, R1, R2…Rn N , and correspondingly timing modulating the phase modulator; The second-order nonlinear crystal is connected to the output lower port of the second beam splitter and is configured to generate spontaneous parametric down-conversion of the sub-pulse sequence and generate entangled two photons. The light splitting module is connected to the second-order nonlinear crystal and is configured to separate the entangled two photons.

2. The temporal entangled two-photon generation system of claim 1, wherein, The parameters include: preset cycle number N, probability amplitude a0, a1, a2…a N ; wherein the light pulse output by the laser source is modulated by the phase modulator, and the sub-pulse output from the lower port of the second beam splitter is named as the 0th sub-pulse; the sub-pulse output from the lower port of the second beam splitter in each cycle corresponds to the 1st sub-pulse, the 2nd sub-pulse, …, and the Nth sub-pulse, respectively; the 0th sub-pulse, the 1st sub-pulse, the 2nd sub-pulse, …, and the Nth sub-pulse form the sub-pulse sequence; a0 represents the probability amplitude of detecting the entangled photon pair in the 0th sub-pulse time position; a1 represents the probability amplitude of detecting the entangled photon pair in the 1st sub-pulse time position; and a N 2 represents the probability amplitude of detecting the entangled photon pair in the 2nd sub-pulse time position, and so on, and aN represents the probability amplitude of detecting the entangled photon pair in the Nth sub-pulse time position.

3. The temporal entangled two-photon generation system of claim 1, wherein, The controller is connected to the laser source, and when the laser source generates optical pulses based on a driving electrical signal, the laser source simultaneously feeds back the driving electrical signal to the controller; and the controller starts timing modulation of the phase modulator based on the driving electrical signal.

4. The temporal entangled two-photon generation system of claim 1, wherein The system further comprises a third beam splitter and a photodetector; an input end of the third beam splitter is connected to the laser source; one output end of the third beam splitter is connected to the photodetector; the other output end of the third beam splitter is connected to the input lower port of the first beam splitter; the photodetector is connected to the controller; the third beam splitter is configured to split the optical pulses output by the laser source and transmit a part of the split optical pulses to the photodetector; the photodetector is configured to convert the received optical pulses into electrical pulse signals and transmit the electrical pulse signals to the controller; and the controller starts timing modulation of the phase modulator based on the electrical pulse signals.

5. The temporal entangled two-photon generation system of claim 1, wherein, The light splitting module is an arrayed waveguide grating or a wavelength division multiplexer.

6. A temporal biphoton generation system according to any one of claims 1-5, wherein, The system further comprises a filter arranged between the second-order nonlinear crystal and the light splitting module, which is configured to filter out noise photons with different frequencies from the entangled two photons output by the second-order nonlinear crystal.

7. A temporal biphoton generation system according to any one of claims 1-5, wherein, The system further comprises an adjustable attenuator arranged in a transmission path between the laser source and the input lower port of the first beam splitter and connected to the controller, which is configured to attenuate the intensity of the optical pulses output by the laser source.

8. A method for time entangled two-photon generation, comprising: The method is applied to the time entangled two photon generation system of any one of claims 1 to 7, and the method comprises: The parameters for generating entangled photons are input on the host computer, and the host computer calculates the light intensity percentage R0 of the sub-pulse output from the lower port of the second beam splitter after the light pulse output by the laser source passes through the phase modulator and is modulated based on the input parameters, and calculates the light intensity percentages R1, R2…R of the sub-pulse output from the lower port of the second beam splitter after the split light pulse in each cycle passes through the phase modulator and is modulated. N ; The laser source inputs an optical pulse to the input lower port of the first beam splitter, and the controller adjusts the phase modulator according to the light intensity percentage R0, so that the optical pulse output by the laser source is output from the output lower port of the second beam splitter at a light intensity percentage of R0, and is output from the output upper port of the second beam splitter at a light intensity percentage of 1-R0 into the first circulation loop; the controller adjusts the phase modulator according to R1, so that the split optical pulse entering the first circulation loop is output from the output lower port of the second beam splitter at a light intensity percentage of R1, and is output from the output upper port of the second beam splitter at a light intensity percentage of 1-R1 into the second circulation loop; and so on, the controller adjusts the phase modulator according to R N The phase modulator is adjusted so that the split optical pulse entering the Nth circulation loop is all output from the output lower port of the second beam splitter, and the time sequence modulation of the controller on the phase modulator in one entangled state generation period is completed. The multiple sub-pulses output from the lower end port of the second beam splitter form a sub-pulse sequence, and the sub-pulse sequence undergoes a spontaneous parametric down-conversion process through the second-order nonlinear crystal and generates entangled two photons. The entangled two photons are separated and output after passing through the light splitting module.

9. The method of claim 8, wherein, When the controller is connected with the laser source, the method further comprises: When the laser source generates the light pulse based on the driving electrical signal, the laser source simultaneously feeds back the driving electrical signal to the controller, and the controller starts to perform the timing modulation on the phase modulator based on the driving electrical signal.

10. The method of claim 8, wherein, When the system further comprises a third beam splitter and a photodetector, an input end of the third beam splitter is connected with the laser source, one output end of the third beam splitter is connected with the photodetector, and the other output end is connected with the lower end port of the input of the first beam splitter, and the photodetector is connected with the controller, the method further comprises: The third beam splitter splits the light pulse output by the laser source and transmits a part of the split light pulse to the photodetector, the photodetector converts the received light pulse into an electrical pulse signal and transmits the electrical pulse signal to the controller, and the controller starts to perform the timing modulation on the phase modulator based on the electrical pulse signal.

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