Bell state projection measurement device and projection measurement method

Abstract

The application discloses a Bell state projection measurement device and a projection measurement method, which comprise four incident ports, three MZ interferometers, four single-photon detectors, a control chip and a coincidence counter, and realize projection measurement on Bell states based on path encoding; one MZ interferometer realizes path switching of input photons under the action of the control chip, and the other two MZ interferometers realize H gate operation on the input photons; four single-photon detectors detect and respond to the two photons subjected to the H gate operation, and transmit signals to the coincidence counter; and the control chip determines the Bell state of the two photons according to the response result fed back by the coincidence counter. The application has the advantages of simple and compact structure, stable optical path and easy on-chip integration, so that the process difficulty and cost of Bell state projection measurement are reduced; and compared with polarization encoding, the application has a higher extinction ratio parameter, so that the projection measurement efficiency and precision are ensured.

Description

Technical Field

[0001] This application belongs to the field of quantum communication technology, and specifically relates to a Bell state projection measurement device and projection measurement method. Background Technology

[0002] Bell states are a set of quantum states used to describe the entanglement between two particles, and are the simplest two-body quantum entangled states. Bell states include the following four entangled states:

[0003]

[0004]

[0005] Bell state measurement is one of the most fundamental requirements in the fields of quantum information and quantum computing. Devices for Bell state projection measurement are used in systems such as quantum computers and measurement-device-independent quantum key distribution, and are also one of the core components of these devices and systems.

[0006] Current Bell state projection measurement devices are basically implemented using phase or polarization encoding. However, phase encoding requires high-speed modulation devices, is costly, and has a difficult manufacturing process. On-chip polarization devices have low extinction ratio and other parameters, poor polarization performance, and on-chip polarization modulators and technologies are not mature. Therefore, this application proposes a Bell state projection measurement device and projection measurement method based on path encoding. Summary of the Invention

[0007] Based on the above, this application provides a Bell state projection measurement device and method. The method involves path encoding of a pair of input photons in a Bell state, switching the path of the input photons using an MZ interferometer, and performing H-gate operations on the received photons using two MZ interferometers to achieve resolution of the input Bell state, thus realizing projection measurement. The specific scheme is as follows:

[0008] In a first aspect, this application discloses a Bell state projection measurement device, comprising four incident ports, a first MZ interferometer, a second MZ interferometer, a third MZ interferometer, a control chip, four single-photon detectors, and a coincidence counter; the four incident ports are respectively a first incident port, a second incident port, a third incident port, and a fourth incident port, and the four single-photon detectors are respectively a first single-photon detector, a second single-photon detector, a third single-photon detector, and a fourth single-photon detector;

[0009] The incident ports are used to receive a pair of photons in the Bell state. One photon is input through the first incident port or the second incident port, and the other photon is input through the third incident port or the fourth incident port. The first incident port and the third incident port are used to receive the |0> state photon, and the second incident port and the fourth incident port are used to receive the |1> state photon.

[0010] The first MZ interferometer is used to output the received photons in a cross-state or a straight-through state. The upper input of the first MZ interferometer is connected to the first incident port, the lower input is connected to the second incident port, the upper output is connected to the upper input of the second MZ interferometer, and the lower output is connected to the upper input of the third MZ interferometer.

[0011] Both the second and third MZ interferometers are used to perform H-gate operations on photons passing through their paths, causing the quantum state of the received photon to evolve as follows: The lower input of the second MZ interferometer is connected to the third incident port, the upper output of the second MZ interferometer is connected to the first single-photon detector, and the lower output of the third MZ interferometer is connected to the second single-photon detector. The lower input of the third MZ interferometer is connected to the fourth incident port, the upper output of the third MZ interferometer is connected to the third single-photon detector, and the lower output of the third MZ interferometer is connected to the fourth single-photon detector.

[0012] The first and second single-photon detectors are used to detect photons output by the second MZ interferometer. The third and fourth single-photon detectors are used to detect photons output by the third MZ interferometer. The output terminals of the four single-photon detectors are all connected to a coincidence counter. The coincidence counter is used to count the photons detected by two of the single-photon detectors within a window time and to record the response results of the corresponding single-photon detectors. At the same time, the counting results and the response results of the corresponding single-photon detectors are fed back to the control chip.

[0013] The three MZ interferometers and the coincidence counter are all connected to the control chip, which is used to modulate the three MZ interferometers and analyze and judge the information fed back from the coincidence counter.

[0014] Furthermore, the Bell state projection measurement device also includes four electrical delay modules. The input terminals of the four electrical delay modules are connected to the output terminals of the four single-photon detectors respectively, and the output terminals of the four electrical delay modules are all connected to the coincidence counter. The electrical delay modules delay the electrical signals output by the corresponding single-photon detectors so that the electrical signals output by each single-photon detector arrive at the coincidence counter simultaneously.

[0015] Furthermore, the Bell state projection measurement device also includes two optical delay modules, namely a first optical delay module and a second optical delay module. The first optical delay module is set on the transmission path between the third incident port and the lower input of the second MZ interferometer, and the second optical delay module is set on the transmission path between the fourth incident port and the lower input of the third MZ interferometer. The two optical delay modules respectively delay the photons passing through them, so that a pair of photons in the Bell state arrive at the corresponding single-photon detector at the same time.

[0016] Preferably, all four single-photon detectors are photon number-resolved detectors.

[0017] Furthermore, each of the three MZ interferometers includes a first 50:50 beam splitter, an upper interferometer arm, a lower interferometer arm, a second 50:50 beam splitter, and a phase modulator. The two ends of the upper interferometer arm are respectively connected to the upper output of the first 50:50 beam splitter and the upper input of the second 50:50 beam splitter. The two ends of the lower interferometer arm are respectively connected to the lower output of the first 50:50 beam splitter and the lower input of the second 50:50 beam splitter. The phase modulator is located on either the upper or lower interferometer arm. The phase modulators in all three MZ interferometers are connected to the control chip.

[0018] Furthermore, when the phase of the phase modulator in the first MZ interferometer is set to 0, the first MZ interferometer outputs the received photons in a direct state; when the phase of the phase modulator in the first MZ interferometer is set to π, the first MZ interferometer outputs the received photons in a cross-state.

[0019] Furthermore, the phase modulation of the phase modulators in the second and third MZ interferometers is π / 2.

[0020] Secondly, this application discloses a Bell state projection measurement method, which is applied to a Bell state projection measurement device, and the method includes:

[0021] The incident port receives a pair of photons in the Bell state. One photon is input through the first or second incident port, and the other photon is input through the third or fourth incident port. The first and third incident ports receive the |0> state photon, and the second and fourth incident ports receive the |1> state photon.

[0022] The control chip modulates three MZ interferometers, causing the first MZ interferometer to output the received photons in one of two states: a straight-through state and a cross-through state, and causing the second and third MZ interferometers to perform H-gate operations on the photons passing through their paths.

[0023] The first and second single-photon detectors detect and respond to the photons output by the second MZ interferometer, while the third and fourth single-photon detectors detect and respond to the photons output by the third MZ interferometer.

[0024] The coincidence counter counts the photons detected by two single-photon detectors within a window time and records the corresponding response results of the single-photon detectors. At the same time, the counting results and the corresponding response results of the single-photon detectors are fed back to the control chip.

[0025] The control chip analyzes and judges the information fed back by the counter. If the analysis shows that both photons are detected by the same single-photon detector, the control chip modulates the first MZ interferometer again, so that the first MZ interferometer outputs the received photons in another state, either a straight-through state or a cross-through state. After repeating the above process, the projection measurement ends and the Bell state of the two photons is determined.

[0026] In summary, compared with the prior art, the above-described technical solutions conceived in this application can achieve the following beneficial effects:

[0027] This application provides a Bell state projection measurement device and method. Based on path encoding, it achieves Bell state projection measurement by setting up three MZ interferometers, four single-photon detectors, and a coincidence counter. One MZ interferometer, under the control of a control chip, switches the path of the input photon. The other two MZ interferometers perform H-gate operations on the input photon. The four single-photon detectors detect and respond to the two photons after the H-gate operation and transmit the signals to the coincidence counter. The control chip determines the Bell state of the two photons based on the response results fed back from the coincidence counter. This application has a simple and compact structure, stable optical path, and is easy to integrate on a chip, thus reducing the process difficulty and cost of Bell state projection measurement. Moreover, compared with polarization encoding, this application has a higher extinction ratio parameter, ensuring projection measurement efficiency and accuracy. Attached Figure Description

[0028] 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.

[0029] Figure 1 This is a schematic diagram of the structure of a Bell state projection measurement device provided in an embodiment of this application;

[0030] Figure 2 A schematic diagram of the structure of a Bell state projection measurement device provided in another embodiment of this application;

[0031] Figure 3 A schematic diagram of the structure of a Bell state projection measurement device provided in another embodiment of this application;

[0032] Figure 4 Based on Figure 1 A schematic diagram of a Bell state projection measurement device is provided.

[0033] Figure 5 Based on Figure 1Another schematic diagram of a Bell state projection measurement device is provided;

[0034] Figure 6 A flowchart of a Bell state projection measurement device provided in an embodiment of this application. Detailed Implementation

[0035] 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.

[0036] 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.

[0037] 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.

[0038] Bell states include four entangled states. Measuring Bell states is the most fundamental requirement in the fields of quantum information and quantum computing. Current Bell state projection measurement devices are basically implemented using phase or polarization encoding. However, phase encoding requires high-speed modulation devices, is costly, and has a difficult manufacturing process. On-chip polarization devices have low extinction ratio and other parameters, poor polarization performance, and on-chip polarization modulators and technologies are not yet mature.

[0039] Based on this, this application provides a Bell state projection measurement device, such as... Figure 1 As shown, it includes four incident ports, a first MZ interferometer, a second MZ interferometer, a third MZ interferometer, a control chip, four single-photon detectors, and a coincidence counter; the four incident ports are the first incident port, the second incident port, the third incident port, and the fourth incident port, and the four single-photon detectors are the first single-photon detector, the second single-photon detector, the third single-photon detector, and the fourth single-photon detector.

[0040] The incident ports are used to receive a pair of photons in the Bell state. One photon is input through the first or second incident port, and the other photon is input through the third or fourth incident port. The first and third incident ports are used to receive the |0> state photon, and the second and fourth incident ports are used to receive the |1> state photon.

[0041] For ease of understanding, a pair of photons in a Bell state can be named the first photon and the second photon. The first photon is input through either the first or second incident port, depending on its quantum state. If the first photon is in the |0> state, it is input through the first incident port; if it is in the |1> state, it is input through the second incident port. The second photon is input through either the third or fourth incident port. Similarly, the specific port used depends on its quantum state: if it is in the |0> state, it is input through the third incident port; if it is in the |1> state, it is input through the fourth incident port.

[0042] The first MZ interferometer is used to output the received photons in a cross-state or a straight-through state. The upper input of the first MZ interferometer is connected to the first incident port, the lower input is connected to the second incident port, the upper output is connected to the upper input of the second MZ interferometer, and the lower output is connected to the upper input of the third MZ interferometer.

[0043] Both the second and third MZ interferometers are used to perform H-gate operations on photons passing through their paths, causing the quantum state of the received photon to evolve as follows: The lower input of the second MZ interferometer is connected to the third incident port, the upper output of the second MZ interferometer is connected to the first single-photon detector, and the lower output of the third MZ interferometer is connected to the second single-photon detector. The lower input of the third MZ interferometer is connected to the fourth incident port, the upper output of the third MZ interferometer is connected to the third single-photon detector, and the lower output of the third MZ interferometer is connected to the fourth single-photon detector.

[0044] Based on the above, it can be seen that the three MZ interferometers do not perform the same functions. Specifically, the first MZ interferometer performs path adjustment of the input photon, while the second and third MZ interferometers perform H-gate operations on the input photon. The three MZ interferometers in this application have identical structures, as described above. Figure 1 Each of the three MZ interferometers includes a first 50:50 beam splitter, an upper interferometer arm, a lower interferometer arm, a second 50:50 beam splitter, and a phase modulator. The two ends of the upper interferometer arm are connected to the upper output of the first 50:50 beam splitter and the upper input of the second 50:50 beam splitter, respectively. The two ends of the lower interferometer arm are connected to the lower output of the first 50:50 beam splitter and the lower input of the second 50:50 beam splitter, respectively. The phase modulator is located on either the upper or lower interferometer arm. The phase modulators in all three MZ interferometers are connected to the control chip.

[0045] The phase modulator in the MZ interferometer is adjusted by the control chip. Different phases of the phase modulator in the MZ interferometer result in different functions. When the phase of the phase modulator in the first MZ interferometer is set to 0, the first MZ interferometer outputs the received photons in a direct state; when the phase of the phase modulator in the first MZ interferometer is set to π, the first MZ interferometer outputs the received photons in a cross-state.

[0046] Specifically, when the phase modulator is adjusted to 0, the first MZ interferometer outputs the received photons in a direct-through state, meaning photons input from the upper input end of the first MZ interferometer are output from the upper output end, and photons input from the lower input end are output from the lower output end. When the phase modulator is adjusted to π, the first MZ interferometer outputs the received photons in a cross-connected state, meaning photons input from the upper input end of the first MZ interferometer are output from the lower output end, and photons input from the lower input end are output from the upper output end. Adjusting the phase modulators of the second and third MZ interferometers to π / 2 causes them to perform H-gate operations on the received photons, resulting in the evolution of the quantum state of the received photons.

[0047] The first and second single-photon detectors are used to detect photons output from the second MZ interferometer. The third and fourth single-photon detectors are used to detect photons output from the third MZ interferometer. The outputs of all four single-photon detectors are connected to a coincidence counter. The coincidence counter is used to count the photons detected by two of the single-photon detectors within a window time and to record the response results of the corresponding single-photon detectors. At the same time, the counting results and the response results of the corresponding single-photon detectors are fed back to the control chip.

[0048] Specifically, the first single-photon detector is connected to the upper output of the second MZ interferometer for detecting photons output from the upper output of the second MZ interferometer; the second single-photon detector is connected to the lower output of the second MZ interferometer for detecting photons output from the lower output of the second MZ interferometer; the third single-photon detector is connected to the upper output of the third MZ interferometer for detecting photons output from the upper output of the third MZ interferometer; and the fourth single-photon detector is connected to the lower output of the third MZ interferometer for detecting photons output from the lower output of the third MZ interferometer. In this application, to improve the accuracy of photon detection, the four single-photon detectors are preferably photon number-resolved detectors.

[0049] When two photons are output from two different output terminals (the two output terminals of the second MZ interferometer and the two output terminals of the third MZ interferometer, for a total of four output terminals), two single-photon detectors will respond accordingly. Each single-photon detector detects the corresponding two photons and outputs an electrical signal, which is simultaneously transmitted to a coincidence counter. The coincidence counter receives the electrical signals output from the two single-photon detectors. If both electrical signals are within a window of the coincidence counter, the counter counts and feeds the count result back to the control chip. When two photons are output from the same output terminal, the coincidence counter does not perform a coincidence count; it only records the response results of the single-photon detectors that received the two photons and feeds the response result back to the control chip.

[0050] The three MZ interferometers and the coincidence counter are all connected to the control chip, which is used to modulate the three MZ interferometers and analyze and judge the information fed back from the coincidence counter.

[0051] In this application, the phase modulators in the second and third MZ interferometers are modulated by a control chip to ensure that their phases are both π / 2, enabling the second and third MZ interferometers to perform H-gate operations on the received photons. For the first MZ interferometer, its phase modulator can be modulated to 0, allowing the received photons to be output in a direct-through state. In this state, if two photons are detected by different single-photon detectors, the Bell state of the two photons can be directly determined based on the corresponding single-photon detector. If the two photons are detected by the same single-photon detector, the Bell state of the two input photons cannot be determined and may be |Φ + >or |Φ - The control chip then modulates the phase of the phase modulator in the first MZ interferometer again to π, and measures the pair of photons in the Bell state input in the next cycle again. Since the pair of photons input in the next cycle and the pair of photons input in the previous cycle are in the same Bell state, the two photons input in the next cycle will be detected by different single-photon detectors, thus determining the Bell state of the two photons. Alternatively, the phase of the phase modulator of the first MZ interferometer can be modulated to π first; this application does not limit the modulation order of the first MZ interferometer.

[0052] The control chip in this application can be a microcontroller or a host computer, and has the functions of information processing and control command transmission.

[0053] In another embodiment of this application, the Bell state projection measurement device further includes four electrical delay modules, such as... Figure 2As shown, the input terminals of the four electrical delay modules are connected to the output terminals of the four single-photon detectors respectively, and the output terminals of the four electrical delay modules are all connected to the coincidence counter. The electrical delay modules delay the electrical signals output by the corresponding single-photon detectors so that the electrical signals output by each single-photon detector arrive at the coincidence counter simultaneously.

[0054] Because the two photons travel along different paths from the input to the single-photon detector, an optical path difference exists. To reduce the impact of this optical path difference and ensure that the electrical signals output by the two single-photon detectors reach the coincidence counter within a window of the counter, an electrical delay module is installed between each single-photon detector and the coincidence counter. By delaying the received electrical signals, the electrical signals output by the two single-photon detectors arrive at the coincidence counter simultaneously, thus ensuring that the arrival times of the two electrical signals are within a window of the counter. In this application, the electrical delay module is preferably an adjustable electrical delay chip.

[0055] In another embodiment of this application, such as Figure 3 As shown, the Bell state projection measurement device also includes two optical delay modules, namely a first optical delay module and a second optical delay module. The first optical delay module is set on the transmission path between the third incident port and the lower input of the second MZ interferometer, and the second optical delay module is set on the transmission path between the fourth incident port and the lower input of the third MZ interferometer. The two optical delay modules respectively delay the photons passing through them, so that a pair of photons in the Bell state arrive at the corresponding single-photon detector at the same time.

[0056] In this embodiment, by setting optical delay modules on the transmission paths of the third incident port and the lower input of the third MZ interferometer, and on the transmission paths of the fourth incident port and the lower input of the third MZ interferometer, the transmission optical path difference of the two photons that are input at the same time is reduced, so that the two photons arrive at the corresponding single-photon detectors at the same time. That is, it is ensured that the time for the electrical signals output by the two single-photon detectors that detect the photons to be transmitted to the coincidence counter is within a window time of the coincidence counter.

[0057] To make this application clearer, the following will be combined with Figure 4 and Figure 5 The working principle of the Bell state projection measurement device is explained in detail.

[0058] For ease of description and understanding, the pair of photons in the Bell state are named photon a and photon b. Photon a is input from either the first or second input port, and photon b is input from either the third or fourth input port. The quantum state of the photon may be either |0> or |1>. The input ports for photons in different quantum states are also different. Specifically, the first and third input ports are used to receive |0> state photons, and the second and fourth input ports are used to receive |1> state photons.

[0059] Based on the above photon naming, the transmission paths between the first incident port and the upper input of the first MZ interferometer, the second incident port and the lower input of the first MZ interferometer, the upper output of the first MZ interferometer and the upper input of the second MZ interferometer, and the lower output of the first MZ interferometer and the upper input of the third MZ interferometer are all denoted as path a, representing the possible transmission path of photon a; the transmission paths between the third incident port and the lower input of the second MZ interferometer, and the fourth incident port and the lower input of the third MZ interferometer are all denoted as path b, representing the possible transmission path of photon b.

[0060] Since it is impossible to determine whether the photons output from the second and third MZ interferometers are photons a or b, for ease of understanding, the transmission path from the upper end of the second MZ interferometer output to the first single-photon detector is denoted as 0c, and the transmission path from the lower end of the second MZ interferometer output to the second single-photon detector is denoted as path 1c. Path 0c and path 1c together form path c. Similarly, the transmission path from the upper end of the third MZ interferometer output to the third single-photon detector is denoted as path 0d, and the transmission path from the lower end of the third MZ interferometer output to the fourth single-photon detector is denoted as path 1d. Path 0d and path 1d together form path d.

[0061] There are four possible Bell states input to the device of this application. Based on the above description, the quantum state of a pair of photons input in a Bell state is represented as follows:

[0062]

[0063]

[0064] It should be noted that this application is based on a path-encoded Bell state projection measurement device, where the subscripts of each quantum state in this formula and the following formulas represent paths.

[0065] If the phase of the phase modulator of the first MZ interferometer is 0, then the first MZ interferometer will output the received photons in a direct-through state, such as... Figure 4 As shown, the input quantum state remains unchanged.

[0066] When the second and third MZ interferometers perform H-gate operations on the input photons, the corresponding quantum state evolution is as follows:

[0067]

[0068]

[0069] After passing through the second and third MZ interferometers, the four possible Bell state evolutions are as follows:

[0070]

[0071]

[0072]

[0073]

[0074] For four single-photon detectors, the responses of different single-photon detectors correspond to different input Bell states. The specific correspondence between the response states of the four single-photon detectors and the input Bell states is shown in the table below:

[0075]

[0076] In the table, D1 represents the first single-photon detector, D2 represents the second single-photon detector, D3 represents the third single-photon detector, and D4 represents the fourth single-photon detector.

[0077] Here, D1-D3 represent a pair of input photons, one photon is detected by the first single-photon detector and the other photon is detected by the third single-photon detector, meaning that the evolved quantum state contains |0> c |0> d The Bell state in this case is |ψ + >; D2-D4 represent a pair of input photons, one of which is detected by the second single-photon detector and the other by the fourth single-photon detector. That is, the evolved quantum state contains |1> c |1> d In this case, the Bell state is also |ψ + Therefore, in summary, when one photon is detected by the first single-photon detector and the other by the third single-photon detector, or when one photon is detected by the second single-photon detector and the other by the fourth single-photon detector, the Bell state of the input pair of photons in these two cases is |ψ + >

[0078] Similarly, D2-D3 represents a pair of input photons, one of which is detected by the second single-photon detector and the other by the third single-photon detector, meaning that the evolved quantum state contains |1> c |0> d The Bell state in this case is |ψ - >; D1-D4 represent a pair of input photons, one photon detected by the first single-photon detector and the other photon detected by the fourth single-photon detector, meaning the evolved quantum state contains |0>. c |1> d In this case, the Bell state is also |ψ - Therefore, in summary, when one photon is detected by the second single-photon detector and another photon by the third single-photon detector, or when one photon is detected by the first single-photon detector and another photon by the fourth single-photon detector, the Bell state of the input pair of photons in these two cases is |ψ - >

[0079] D1-D2 represents one photon being detected by the first single-photon detector and the other photon being detected by the second single-photon detector, indicating that the evolved quantum state contains |0> c |1> c Comparing the four possible Bell state evolution formulas above, it can be seen that |0> c |1> c That is, if the input is a pair of photons in a Bell state, the D1-D2 case will not occur. Similarly, the D3-D4 case will not occur either.

[0080] For D1-D1, it means that both input photons in the Bell state were detected by the first single-photon detector, i.e., the evolved quantum state contains |0> c |0> c Comparing the four possible Bell state evolution results above, it can be seen that when both photons are detected by the first single-photon detector, their Bell state is |Φ + >or|Φ - Similarly, analysis of D2-D2, D3-D3, and D4-D4 shows that when both photons are detected by the same single-photon detector, their Bell state is |Φ + >or|Φ - Of course, it is impossible to determine which specific state it is.

[0081] When two photons are detected by the same single-photon detector, to determine the specific Bell state, the control chip needs to readjust the phase of the phase modulator of the first MZ interferometer to π, and then detect the pair of photons in the Bell state input in the next cycle. Specifically, the first MZ interferometer outputs the received photons in a cross-state, such as... Figure 5 As shown, if photon a is in the |0> state, it is output from the lower output of the first MZ interferometer; if photon a is in the |1> state, it is output from the upper output of the first MZ interferometer. Compared to the direct output state, this case involves flipping the input quantum state, and the quantum state evolves to: |0> a →|1> a ,|1> a →|0> a Therefore, the four possible Bell states of an input pair of photons undergo the following changes:

[0082] |ψ + >→|Φ + >,|ψ - >→|Φ - >,|Φ + >→|ψ + >,|Φ - >→||ψ - >

[0083] After the H-gate operations of the second and third MZ interferometers, since the Bell state of the input pair of photons was already determined when the first MZ interferometer was outputting in a direct-through state, the state of the input pair of photons is |Φ. + >or|Φ - Therefore, |ψ is no longer considered in the crossover output. + > and |ψ - Then the two possible Bell state evolutions are:

[0084]

[0085] Analysis shows that if, within this period, one of the two input photons is detected by the first single-photon detector and the other by the third single-photon detector; or one photon is detected by the second single-photon detector and the other by the fourth single-photon detector, then the input Bell state can be determined to be |Φ + If, within this period, one of the two input photons is detected by the second single-photon detector and the other by the third single-photon detector; or one photon is detected by the first single-photon detector and the other by the fourth single-photon detector, then the input Bell state can be determined to be |Φ. - >

[0086] It's important to note here that for the modulation of the phase modulator in the first MZ interferometer, the phase can be initially set to 0, or it can be modulated to π. If the phase modulator of the first MZ interferometer is initially set to 0, and the input Bell state is |ψ... + >or|ψ - Then, the input Bell state can be directly confirmed as |ψ| based on the response result of the single-photon detector. + >or confirmed as |ψ - >, the projection measurement of the Bell state can be completed without further modulation of the phase modulator in the first MZ interferometer; if the input Bell state is |Φ + >or|Φ - Since the input pair of photons is detected by the same single-photon detector, it is impossible to confirm that the input Bell state is |Φ. + > and |Φ - To determine which quantum state is in the input pair of photons, the phase of the phase modulator in the first MZ interferometer needs to be modulated again to make its phase π, so as to further distinguish the Bell state of the input pair of photons.

[0087] Similarly, if the phase of the phase modulator in the first MZ interferometer is first set to π, and the input Bell state is |Φ + >or|Φ - Then, the input Bell state can be directly confirmed as |Φ based on the response of the single-photon detector. + >or confirmed as |Φ - >, the projection measurement of the Bell state can be completed without further modulation of the phase modulator in the first MZ interferometer; if the input Bell state is |ψ + >or|ψ - When a pair of input photons is detected by the same single-photon detector, it is not possible to directly determine which Bell state it is. It is necessary to modulate the phase of the phase modulator in the first MZ interferometer again to make its phase 0, so as to further distinguish the Bell state of the input entangled photon pair.

[0088] In summary, if the input pair of photons is detected by different single-photon detectors, the Bell state of the input pair of photons can be directly determined. If the input pair of photons is detected by the same single-photon detector, the phase of the phase modulator in the first MZ interferometer needs to be adjusted again to further distinguish which Bell state the entangled photon pair is.

[0089] In summary, this application achieves projection measurement of Bell states based on path encoding. The entire system has a simple and compact structure, stable optical path, and is easy to integrate on-chip, reducing the process difficulty and cost of Bell state projection measurement. Moreover, compared with polarization encoding, this application has a higher extinction ratio parameter, ensuring the efficiency and accuracy of projection measurement.

[0090] In relation to the Bell state projection measurement device provided in this application, this application also provides a Bell state projection measurement method, such as... Figure 6 As shown, the method includes:

[0091] S11: The incident port receives a pair of photons in the Bell state. One photon is input through the first or second incident port, and the other photon is input through the third or fourth incident port. The first and third incident ports receive the |0> state photon, and the second and fourth incident ports receive the |1> state photon.

[0092] S12: The control chip modulates three MZ interferometers, causing the first MZ interferometer to output the received photons in one of two states: a through state and a cross state, and causing the second and third MZ interferometers to perform H-gate operations on the photons passing through their paths.

[0093] In step S12, the control chip modulates the phase modulators in both the second and third MZ interferometers to a phase of π / 2. The second and third MZ interferometers then perform H-gate operations on photons passing through their paths. For the first MZ interferometer, the control chip can either first modulate its phase modulator to 0, causing the received photons to be output in a direct-through state; or it can first modulate its phase modulator to π, causing the received photons to be output in a cross-connected state.

[0094] S13: The first and second single-photon detectors detect and respond to the photons output by the second MZ interferometer, and the third and fourth single-photon detectors detect and respond to the photons output by the third MZ interferometer.

[0095] Specifically, the first single-photon detector detects and responds to photons output from the upper output of the second MZ interferometer; the second single-photon detector detects and responds to photons output from the lower output of the second MZ interferometer; the third single-photon detector detects and responds to photons output from the upper output of the third MZ interferometer; and the fourth single-photon detector detects and responds to photons output from the lower output of the third MZ interferometer. If no photons are input to the single-photon detector, the single-photon detector does not respond.

[0096] S14: The coincidence counter counts the photons detected by two single-photon detectors within a window time and records the response results of the corresponding single-photon detectors. At the same time, the counting results and the response results of the corresponding single-photon detectors are fed back to the control chip.

[0097] Because only one pair of photons in the Bell state is input within one cycle, at most two of the four single-photon detectors will respond within that cycle. That is, when two photons are input to different single-photon detectors, both detectors will respond and output electrical signals. If the time it takes for the two electrical signals to reach the coincidence counter is within one window of the counter, then the counter will perform coincidence counting. If the optical path difference between the two photons is small, then the time it takes for the two electrical signals to reach the coincidence counter will definitely be within one window of the counter. In this application, to reduce the impact of the optical path difference, an electrical delay module can be set between each single-photon detector and the coincidence counter to delay the electrical signal output from the single-photon detector, ensuring that both electrical signals arrive at the coincidence counter simultaneously, i.e., ensuring that the arrival time of the two electrical signals at the coincidence counter is within a window. Alternatively, optical delay modules can be set on the transmission paths between the third incident port and the lower input of the second MZ interferometer, and on the transmission paths between the fourth incident port and the lower input of the third MZ interferometer. These optical delay modules delay the photon transmission, reducing the optical path difference between the two photons, allowing both photons to arrive at the corresponding single-photon detector simultaneously, i.e., ensuring that the arrival time of the two electrical signals at the coincidence counter is within a window. If both photons are transmitted to the same single-photon detector, only one single-photon detector will respond, and the coincidence counter will only record the response result of the corresponding single-photon detector without performing coincidence counting.

[0098] S15: The control chip analyzes and judges the information fed back by the coincidence counter. If the analysis shows that both photons are detected by the same single-photon detector, the control chip modulates the first MZ interferometer again, so that the first MZ interferometer outputs the received photons in another state between the direct state and the cross state. After repeating the above process, the projection measurement ends and the Bell state of the two photons is determined.

[0099] If, in step S12, the control chip modulates the first MZ interferometer to output the received photons in a direct-through state, and the control chip analyzes and finds that both photons are detected by the same single-photon detector, then the control chip modulates the phase modulator of the first MZ interferometer again, causing the first MZ interferometer to output the received photons in a cross-through state, thereby distinguishing the Bell states of the two photons. Similarly, if, in step S12, the control chip modulates the first MZ interferometer to output the received photons in a cross-through state, and the control chip analyzes and finds that both photons are detected by the same single-photon detector, then the control chip modulates the phase modulator of the first MZ interferometer again, causing the first MZ interferometer to output the received photons in a direct-through state, thereby distinguishing the Bell states of the two photons.

[0100] In step S15, if the control chip analyzes and obtains that the two photons are detected by the two single-photon detectors respectively, it can directly confirm the Bell state of the input pair of photons without the need for the control chip to modulate the phase modulator in the first MZ interferometer again.

[0101] In a more detailed embodiment, the steps of a Bell state projection measurement method specifically include:

[0102] The incident ports receive a pair of photons in the Bell state. One photon is input through the first or second incident port, and the other photon is input through the third or fourth incident port. The first and third incident ports receive the |0> state photon, and the second and fourth incident ports receive the |1> state photon.

[0103] The control chip modulates the phase modulators in the three MZ interferometers, making the phase of the phase modulator in the first MZ interferometer 0, and the phase of the phase modulators in the second and third MZ interferometers π / 2. Then, the first MZ interferometer outputs the received photons in a direct state, and the second and third MZ interferometers perform H-gate operations on the photons passing through their paths.

[0104] The first and second single-photon detectors detect and respond to photons output by the second MZ interferometer, while the third and fourth single-photon detectors detect and respond to photons output by the third MZ interferometer.

[0105] The coincidence counter counts the photons detected by two single-photon detectors within a window time and records the response results of the corresponding single-photon detectors. At the same time, the counting results and the response results of the corresponding single-photon detectors are fed back to the control chip.

[0106] The control chip analyzes and judges the information fed back by the counter. If the analysis shows that the two photons are detected by different single-photon detectors, the Bell state of the input pair of photons can be directly determined and the projection measurement ends. If the analysis shows that the two photons are detected by the same single-photon detector, the control chip modulates the phase modulator in the first MZ interferometer again to make its phase π, so that the first MZ interferometer outputs the received photons in a cross state. That is, it projects the entangled photon pair input in the next cycle in a cross output state, and then distinguishes and determines the Bell state of the two photons.

[0107] In another embodiment, the phase of the phase modulator in the first MZ interferometer can be set to π first, and the process is basically the same as in the above embodiment, so it will not be repeated here. Similarly, if the control chip analyzes and finds that the two photons are detected by different single-photon detectors, the Bell state of the input pair of photons can be directly determined and the projection measurement ends; if the analysis finds that the two photons are detected by the same single-photon detector, the control chip modulates the phase modulator in the first MZ interferometer again, making its phase 0, so that the first MZ interferometer outputs the received photons in a direct-through state, that is, it performs projection measurement on the entangled photon pair input in the next cycle in a direct-through output state, and then distinguishes and determines the Bell state of the two photons.

[0108] The various embodiments in this specification are described in a progressive, parallel, or combined manner. Each embodiment focuses on the differences from other embodiments, and the same or similar parts between the embodiments can be referred to each other.

[0109] It should be noted that, 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. Furthermore, 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 limitations, 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.

[0110] 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 Bell-state projection measurement device, characterized in that, It includes four incident ports, a first MZ interferometer, a second MZ interferometer, a third MZ interferometer, a control chip, four single-photon detectors, and a coincidence counter; the four incident ports are the first incident port, the second incident port, the third incident port, and the fourth incident port, and the four single-photon detectors are the first single-photon detector, the second single-photon detector, the third single-photon detector, and the fourth single-photon detector; The incident ports are used to receive a pair of photons in the Bell state. One photon is input through the first incident port or the second incident port, and the other photon is input through the third incident port or the fourth incident port. The first incident port and the third incident port are used to receive the |0> state photon, and the second incident port and the fourth incident port are used to receive the |1> state photon. The first MZ interferometer is used to output the received photons in a cross-state or a straight-through state. The upper input of the first MZ interferometer is connected to the first incident port, the lower input is connected to the second incident port, the upper output is connected to the upper input of the second MZ interferometer, and the lower output is connected to the upper input of the third MZ interferometer. Both the second and third MZ interferometers are used to perform H-gate operations on photons passing through their paths, causing the quantum state of the received photon to evolve as follows: The lower input of the second MZ interferometer is connected to the third incident port, the upper output of the second MZ interferometer is connected to the first single-photon detector, and the lower output of the third MZ interferometer is connected to the second single-photon detector. The lower input of the third MZ interferometer is connected to the fourth incident port, the upper output of the third MZ interferometer is connected to the third single-photon detector, and the lower output of the third MZ interferometer is connected to the fourth single-photon detector. The first and second single-photon detectors are used to detect photons output by the second MZ interferometer. The third and fourth single-photon detectors are used to detect photons output by the third MZ interferometer. The output terminals of the four single-photon detectors are all connected to a coincidence counter. The coincidence counter is used to count the photons detected by two of the single-photon detectors within a window time and to record the response results of the corresponding single-photon detectors. At the same time, the counting results and the response results of the corresponding single-photon detectors are fed back to the control chip. The three MZ interferometers and the coincidence counter are all connected to the control chip, which is used to modulate the three MZ interferometers and analyze and judge the information fed back from the coincidence counter.

2. The Bell state projection measuring device according to claim 1, characterized in that, It also includes four electrical delay modules. The input terminals of the four electrical delay modules are connected to the output terminals of the four single-photon detectors respectively. The output terminals of the four electrical delay modules are all connected to the coincidence counter. The electrical delay modules delay the electrical signals output by the corresponding single-photon detectors so that the electrical signals output by each single-photon detector arrive at the coincidence counter simultaneously.

3. The Bell state projection measurement device according to claim 1, characterized in that, It also includes two optical delay modules, namely the first optical delay module and the second optical delay module. The first optical delay module is set on the transmission path between the third incident port and the lower input of the second MZ interferometer, and the second optical delay module is set on the transmission path between the fourth incident port and the lower input of the third MZ interferometer. The two optical delay modules respectively delay the photons passing through them, so that a pair of photons in the Bell state arrive at the corresponding single-photon detector at the same time.

4. The Bell state projection measuring device according to claim 1, characterized in that, All four single-photon detectors are photon number-resolved detectors.

5. A Bell-state projection measuring device according to any one of claims 1-4, characterized in that, Each of the three MZ interferometers includes a first 50:50 beam splitter, an upper interferometer arm, a lower interferometer arm, a second 50:50 beam splitter, and a phase modulator. The two ends of the upper interferometer arm are connected to the upper output of the first 50:50 beam splitter and the upper input of the second 50:50 beam splitter, respectively. The two ends of the lower interferometer arm are connected to the lower output of the first 50:50 beam splitter and the lower input of the second 50:50 beam splitter, respectively. The phase modulator is located on either the upper or lower interferometer arm. The phase modulators in all three MZ interferometers are connected to the control chip.

6. The Bell state projection measuring device according to claim 5, characterized in that, When the phase of the phase modulator in the first MZ interferometer is set to 0, the first MZ interferometer outputs the received photons in a direct state; when the phase of the phase modulator in the first MZ interferometer is set to π, the first MZ interferometer outputs the received photons in a cross state.

7. The Bell state projection measuring device according to claim 5, characterized in that, The phase modulation of the phase modulators in the second and third MZ interferometers is π / 2.

8. A Bell state projection measurement method, characterized in that, The method is applied to a Bell-state projection measurement device, and the method includes: The incident port receives a pair of photons in the Bell state. One photon is input through the first or second incident port, and the other photon is input through the third or fourth incident port. The first and third incident ports receive the |0> state photon, and the second and fourth incident ports receive the |1> state photon. The control chip modulates three MZ interferometers, causing the first MZ interferometer to output the received photons in one of two states: a straight-through state and a cross-through state, and causing the second and third MZ interferometers to perform H-gate operations on the photons passing through their paths. The first and second single-photon detectors detect and respond to the photons output by the second MZ interferometer, while the third and fourth single-photon detectors detect and respond to the photons output by the third MZ interferometer. The coincidence counter counts the photons detected by two single-photon detectors within a window time and records the corresponding response results of the single-photon detectors. At the same time, the counting results and the corresponding response results of the single-photon detectors are fed back to the control chip. The control chip analyzes and judges the information fed back by the coincidence counter. If the analysis shows that both photons are detected by the same single-photon detector, the control chip modulates the first MZ interferometer again, so that the first MZ interferometer outputs the received photons in another state, either a straight-through state or a cross-through state. After repeating the above process, the projection measurement ends and the Bell state of the two photons is determined.

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