Signal processing loop and signal processing method

Abstract

The signal processing loop is provided with: a first transmission path that transmits a pulse signal; a SQUID that is provided with a first magnetic field input unit and a second magnetic field input unit that are magnetically coupled to the first transmission path by transmitting a pulse signal through the first transmission path; a second transmission path connected to the current source and the SQUID; and a delay loop that delays the pulse signal, the delay loop being provided in the first transmission path between a portion of the first transmission path that is magnetically coupled to the first magnetic field input unit and a portion of the first transmission path that is magnetically coupled to the second magnetic field input unit. A delay time of the pulse signal set by the delay loop is shorter than a total time of a rise time and a fall time of the pulse signal, and a magnetic field direction in the first magnetic field input unit is opposite to a magnetic field direction in the second magnetic field input unit, whereby a pulse width of the voltage signal transmitted in the second transmission path is equal to the delay time.

Description

Technical Field

[0001] This disclosure relates to a signal processing circuit and a signal processing method. Background Technology

[0002] Superconducting nanostrip single-photon detectors (SNSPDs) offer superior performance compared to conventional single-photon detectors (e.g., avalanche photodiode detectors), including high detection efficiency, high time resolution (time jitter), and low dark count rate. Therefore, they hold promise for applications in various fields such as quantum information communication.

[0003] Here, SNSPDs must operate within a cryostat at approximately 2.3 Kelvin (K). However, operating a multi-pixel SNSPD array with multiple SNSPD light-receiving units within a cryostat increases the number of transmission cables required to read the pulse signals output from the SNSPDs. This can lead to heat penetration from the outside through the cables, potentially making the extremely low-temperature operation of the multi-pixel SNSPD array difficult. Therefore, multi-channel SNSPDs require a system design that can multiplex the SNSPD signals within the cryostat to maintain high readout speeds while reducing the number of cables. Furthermore, the number of SNSPD components that can be installed in a single cryostat is determined by the cryostat's cooling power and the power consumption of the multi-channel SNSPD system. Therefore, the practical application of highly scalable multi-channel SNSPD systems requires the development of low-power, high-performance multiplexing readout methods.

[0004] Hereinafter, the present inventors propose a signal processing circuit (for example, see Patent Documents 1-5, etc.) which has a superconducting single-flux quantum (SFQ) or superconducting quantum interference device (hereinafter, SQUID) for reading signals from a multi-pixel SNSPD array with fewer transmission cables than the number of SNSPDs.

[0005] Existing technical documents: Patent documents: Patent Document 1: Japanese Patent No. 5419122; Patent Document 2: Japanese Patent No. 5846626; Patent Document 3: Japanese Patent No. 5875045; Patent Document 4: Japanese Patent No. 6598302; Patent document 5: International Publication No. 2020 / 179554. Summary of the Invention

[0006] The problem the invention aims to solve: One example of the subject matter disclosed herein is to provide a signal processing circuit and signal processing method that can appropriately process voltage signals generated by current sources and SQUIDs compared to the past.

[0007] Solution methods: One aspect of this disclosure provides a signal processing circuit comprising: a first transmission path for transmitting a pulse signal having a longer fall time from a peak value to a predetermined level compared to a rise time from a predetermined level to a peak value; a SQUID having a first magnetic field input and a second magnetic field input that are magnetically coupled to the first transmission path by the transmission of the pulse signal in the first transmission path; a second transmission path connected to a current source and the SQUID; and a first delay circuit disposed between a portion of the first transmission path magnetically coupled to the first magnetic field input and a portion of the first transmission path magnetically coupled to the second magnetic field input, such that the pulse signal is delayed, the delay time of the pulse signal set by the first delay circuit being shorter than the total time of the rise time and the fall time, and the magnetic field direction in the first magnetic field input being opposite to the magnetic field direction in the second magnetic field input, thereby the pulse width of the voltage signal transmitted in the second transmission path being equal to the delay time.

[0008] One aspect of the signal processing method of this disclosure includes the following steps: transmitting a pulse signal in a first transmission path having a longer fall time from the peak to the predetermined level compared to the rise time from the predetermined level to the peak; transmitting the pulse signal in the first transmission path, thereby magnetically coupling it to the first transmission path via a first magnetic field input section and a second magnetic field input section of a SQUID, respectively; delaying the pulse signal by a delay time shorter than the total time of the rise time and the fall time by means of a delay loop provided between the portion of the first transmission path magnetically coupled to the first magnetic field input section and the portion of the first transmission path magnetically coupled to the second magnetic field input section; and shaping the waveform of the voltage signal such that the pulse width of the voltage signal is equal to the delay time by means of the magnetic coupling, so that when the voltage signal is transmitted in a second transmission path connected to a current source and the SQUID, the magnetic field direction in the first magnetic field input section is opposite to the magnetic field direction in the second magnetic field input section.

[0009] Invention effects: The signal processing circuit and signal processing method of this disclosure can achieve the following effect: compared with the past, it can properly process the voltage signal generated by the current source and SQUID. Attached Figure Description

[0010] Figure 1A This is a diagram illustrating an example of a conventional signal processing circuit; Figure 1B This is a diagram illustrating an example of a pulse signal output from an SNSPD; Figure 2A This is a diagram illustrating an example of the signal processing circuit of the first embodiment; Figure 2B It is shown by Figure 2A A diagram of the time graph of the signal processed by the signal processing loop; Figure 3 This is a diagram illustrating an example of the results of a simulation experiment verifying that the signal waveform of a voltage signal can be properly shaped by canceling the magnetic field input to the SQUID. Figure 4A This is a diagram illustrating an example of the signal processing circuit of the second embodiment; Figure 4B It is shown by Figure 4A A diagram of the time graph of the signal processed by the signal processing loop; Figure 5A This is a diagram illustrating an example of a signal processing circuit of a modified embodiment of the second embodiment; Figure 5B It is shown by Figure 5A A diagram of the time graph of the signal processed by the signal processing loop. Detailed Implementation

[0011] In recent years, research efforts aimed at the practical application of quantum networks and the quantum internet have begun, and it can be predicted that the number of photon detectors required for a single node in a quantum network or quantum internet will increase. Furthermore, as the competition in the development of quantum computers accelerates, the necessity of photon detectors increases. In other words, it can be predicted that with the large-scale deployment of quantum computers, the number of photon detectors required will inevitably increase.

[0012] Here, the present inventors propose a multiplexed readout loop (hereinafter, SQUID multiplexing loop) utilizing SQUID as a signal processing loop for a multi-pixel SNSPD array having multiple SNSPD light-receiving sections (e.g., Patent Document 5). The SQUID multiplexing loop features low power consumption and power consumption independent of the number of pixels in the SNSPD. Therefore, the SQUID multiplexing loop offers superior scalability in terms of the number of pixels in the SNSPD.

[0013] However, in SQUID multiplexing loops, when multiple pixels output signals simultaneously, a problem arises where signals cannot be read. Therefore, for example, if a SQUID multiplexing loop is directly applied as a multi-channel SNSPD signal loop, the signals of other SNSPDs cannot be read until the signal output of the photodetector SNSPD ends, thus limiting the improvement of signal readout speed. This will be explained in detail below.

[0014] like Figure 1A As shown, SQUID24, together with SNSPD21, is installed inside the refrigerator 200, thereby cooling to extremely low temperatures (approximately 2.3 Kelvin (K)). SQUID24 has a Josephson junction pair J1, J2 and functions to convert the pulse signal 10 (current pulse) output from SNSPD21 into a rectangular waveform voltage signal 11 by inputting a magnetic field through inductor 23. Details of the functions within SQUID24 are explained later.

[0015] Here, generally speaking, the pulse signal 10 output from SNSPD21 is as follows: Figure 1B As shown, the fall time Twf from the peak level to the specified level is longer than the rise time Twr from the specified level to the peak level. The pulse width Tw of pulse signal 10 is equivalent to the sum of the rise time Twr and the fall time Twf of pulse signal 10 (Twr+twf). Regarding the pulse signal 10 output from the SNSPD21, for example, the rise time Twr of the pulse signal 10 is about a few picoseconds (ps) to tens of picoseconds, and the fall time Twf of the pulse signal 10 is about tens of nanoseconds (ns). Therefore, the SNSPD21 has high time resolution due to the abrupt rise characteristic of the pulse signal 10.

[0016] In contrast, the falling characteristic of the pulse signal 10 is caused by the inductive part (L) of the SNSPD21, and the falling time Twf of the pulse signal 10 is equivalent to the dead time when the SNSPD21 cannot detect the next photon.

[0017] Furthermore, the existing SQUID multiplexing loop can detect even during the fall time Twf of pulse signal 10, therefore... Figure 1AAs shown, the pulse width Tw of pulse signal 10 is almost equal to the pulse width Tw of voltage signal 11. Therefore, when multiple SNSPDs 21 detect light simultaneously, the waveforms of these voltage signals 11 overlap, resulting in the loss of photon incident timing information based on the abrupt rise characteristic of pulse signal 10. Furthermore, by setting an appropriate delay structure so that multiple SNSPDs 21 do not detect light simultaneously, the reading speed of voltage signal 11 is slowed down when reading pulse signal 10 one by one.

[0018] Here, the inventors, through careful study, discovered that by canceling the magnetic field input to the SQUID, the above-mentioned existing problems could be improved, and thus the following aspects of this disclosure came to mind.

[0019] That is, the signal processing circuit of the first aspect of this disclosure includes: a first transmission path that transmits a pulse signal with a longer fall time from the peak to the predetermined level compared to the rise time from the predetermined level to the peak; a SQUID that includes a first magnetic field input section and a second magnetic field input section that are magnetically coupled to the first transmission path by transmitting the pulse signal in the first transmission path; a second transmission path that is connected to a current source and the SQUID; and a first delay circuit that is disposed between the portion of the first transmission path magnetically coupled to the first magnetic field input section and the portion of the first transmission path magnetically coupled to the second magnetic field input section, such that the pulse signal is delayed, the delay time of the pulse signal set by the first delay circuit is shorter than the total time of the rise time and the fall time of the pulse signal, and the magnetic field direction in the first magnetic field input section is opposite to the magnetic field direction in the second magnetic field input section, thereby the pulse width of the voltage signal transmitted in the second transmission path is equal to the delay time of the pulse signal set by the first delay circuit.

[0020] According to this structure, the signal processing circuit of this aspect can appropriately process the voltage signal generated by the current source and the SQUID compared to the conventional one. Furthermore, in this disclosure, the phrase "the pulse width of the voltage signal transmitted in the second transmission path is equal to the delay time of the pulse signal set by the first delay circuit" includes both the case where the pulse width of the former is exactly equal to the delay time of the latter, and the case where deviations due to various reasons cause the two to be not completely equal but still achieve the same effect. Here, the deviation time is preferably less than 100 picoseconds, and more preferably less than tens of picoseconds.

[0021] Here, the signal processing circuit of the second aspect of this disclosure may also be, in the signal processing circuit of the first aspect, the pulse signal mentioned above is a current signal detected by SNSPD.

[0022] As mentioned above, such as Figure 1BAs shown, the pulse signal output from the SNSPD decreases after approximately tens of nanoseconds, but even within this short decrease time Twf, the magnetic field input continues in the SQUID. Therefore, Figure 1A The pulse width Tw of the voltage signal 11 generated by the current source 22 and SQUID 24 in the existing signal processing circuit shown is about tens of nanoseconds.

[0023] In contrast, the signal processing circuit of this aspect is characterized by appropriately shaping the waveform of the voltage signal generated by the current source and the SQUID based on the delay circuit that delays the pulse signal output from the SNSPD and the cancellation of the magnetic field in the SQUID.

[0024] Specifically, in the signal processing circuit of this aspect, if the SNSPD detects photons, the magnetic field formed by the pulse signal output from the SNSPD is input to the first magnetic field input section of the SQUID, causing the aforementioned voltage signal to rise sharply. Furthermore, when the magnetic field formed by the pulse signal delayed by the delay circuit is input again to the second magnetic field input section of the SQUID, the magnetic field direction in the first magnetic field input section is opposite to that in the second magnetic field input section, thus canceling the magnetic field input to the SQUID. Consequently, the SQUID rapidly loses its perception of the pulse signal output from the SNSPD, resulting in a shorter pulse width for the aforementioned voltage signal compared to the case where the magnetic field input to the SQUID was not canceled.

[0025] By shaping the voltage signal waveform as described above, the signal processing circuit in this aspect can reduce the impact of the dead time when SNSPD cannot detect the next photon compared to the past.

[0026] Furthermore, in the signal processing loop of this aspect, if the photon incident time fluctuates over time for some reason, the rise time of the pulse signal output from the SNSPD also appropriately inherits this fluctuation. Here, the rise time of the voltage signal generated by the current source and the SQUID corresponds to the rise time of the pulse signal output from the SNSPD. As a result, since the fluctuation is appropriately inherited in the rise time of the voltage signal, the voltage signal can maintain the time information of photon incident.

[0027] Furthermore, in the signal processing circuit of this aspect, the pulse width of the voltage signal generated by the current source and SQUID is equal to the delay time of the pulse signal set by the first delay circuit. Therefore, the pulse width of the voltage signal can be easily and appropriately set by using the signal delay time, which is a design parameter of the first delay circuit.

[0028] The signal processing circuit of the third aspect of this disclosure may also be, in the signal processing circuit of the first or second aspect, comprising: a plurality of SNSPDs respectively connected to a plurality of first transmission paths; a plurality of SQUIDs respectively magnetically coupled to the first transmission paths; a plurality of optical transmission paths for transmitting photons emitted from a plurality of light sources to the SNSPDs respectively; and a delay structure for making the time difference between the output times of a pair of voltage signals corresponding to a pair of SQUIDs longer than the delay time of the pulse signal set by the first delay circuit, wherein the SQUIDs are connected in series in the second transmission paths respectively.

[0029] According to this structure, the signal processing circuit of this aspect, since the time difference between the output times of the pair of voltage signals corresponding to a pair of SQUIDs is longer than the delay time of the pulse signal set by the first delay circuit, can appropriately perform multiplexing signal processing in a single second transmission path while maintaining the time information of photons incident from multiple light sources to each SNSPD.

[0030] Furthermore, in the signal processing circuit of this aspect, since the time difference between the output times of the pair of voltage signals corresponding to each pair of SQUIDs is longer than the delay time of the pulse signal set by the first delay circuit, the voltage signal can maintain position information about which SNSPD the photon from the light source is emitted to among the plurality of SNSPDs.

[0031] Furthermore, in this signal processing circuit, since the SQUIDs are connected in series in the second transmission path, the current of the current source is constant regardless of the number of channels in the SNSPD. Therefore, even if the number of channels in the SNSPD increases, the power consumption of this signal processing circuit remains constant, resulting in high scalability. Moreover, when the SNSPD is multi-channeled, the voltage signals of each SQUID corresponding to the SNSPD are less affected by the characteristic differences of each SNSPD, maintaining a constant pulse width.

[0032] The signal processing circuit of the fourth aspect of this disclosure may also be, in the signal processing circuit of the third aspect, the above-mentioned delay structure includes: an optical fiber disposed in an optical transmission path, which delays photons transmitted in the optical transmission path.

[0033] According to this structure, the signal processing loop of this aspect can easily and appropriately set the time difference between the output times of the pair of voltage signals corresponding to a pair of SQUIDs by adjusting the photon delay time by the cable length of the optical fiber provided in the optical transmission path.

[0034] The signal processing circuit of the fifth aspect of this disclosure may also be, in the signal processing circuit of the third aspect, the above-mentioned delay structure includes: a second delay circuit, which is disposed in a second transmission path between adjacent SQUIDs, such that the signal transmitted in the second transmission path is delayed.

[0035] According to this structure, the signal processing loop of this aspect can easily and appropriately set the time difference between the output times of the pair of voltage signals corresponding to the pair of SQUIDs by using the design parameters of the second delay loop of the second transmission path between SQUIDs, namely the signal delay time.

[0036] The signal processing method of the sixth aspect of this disclosure includes the following steps: transmitting a pulse signal with a longer fall time from the peak to the predetermined level compared to the rise time from the predetermined level to the peak level in a first transmission path; transmitting the pulse signal in the first transmission path, thereby magnetically coupling the first transmission path with the first magnetic field input section and the second magnetic field input section of the SQUID respectively; delaying the pulse signal with a delay time shorter than the total time of the rise time and the fall time by means of a delay circuit provided between the portion of the first transmission path magnetically coupled with the first magnetic field input section and the portion of the first transmission path magnetically coupled with the second magnetic field input section; and shaping the waveform of the voltage signal so that the pulse width of the voltage signal is equal to the delay time by means of the magnetic coupling, thereby making the pulse width of the voltage signal equal to the delay time.

[0037] Based on the above, the signal processing method of this aspect, compared with the conventional method, can appropriately process the voltage signal generated by the current source and the SQUID. Furthermore, the details of the effect of the signal processing method of this aspect are the same as those of the signal processing circuit of the first aspect, and therefore are omitted from the description.

[0038] Hereinafter, specific examples of the above aspects of this disclosure will be described with reference to the accompanying drawings. The specific examples described below illustrate any one of the above aspects. Therefore, the shapes, materials, values, structural elements, the arrangement of structural elements, and connection methods shown below are not intended to limit the above aspects unless described in the claims. Furthermore, structural elements not described in the independent claims representing the highest concept of this aspect are described as arbitrary constituent elements. Also, in the accompanying drawings, where the same symbols are used, there are instances where descriptions are omitted. Furthermore, for ease of understanding of the accompanying drawings, various structural elements are shown schematically; therefore, shapes and dimensions may not be accurately shown.

[0039] (First Implementation) Figure 2A This is a diagram illustrating an example of the signal processing circuit of the first embodiment. Figure 2B It is shown by Figure 2A A diagram of the time graph of the signal processed by the signal processing loop.

[0040] exist Figure 2A In the example shown, the signal processing circuit 100 includes SNSPD21, SQUID24, delay circuit 30, and current source 22.

[0041] SNSPD21 is configured to detect photons emitted from optical transmission path 20 one by one. If SNSPD21 detects photons, then in transmission path 25, as... Figure 1B As shown, a pulsed current signal (hereinafter referred to as pulse signal 10) with a longer fall time Twf from the peak level to the specified level compared to the rise time Twr from the specified level to the peak level is generated. For example, the fall time Twf of pulse signal 10 is about tens of nanoseconds, while the rise time Twr is about a few picoseconds to tens of picoseconds. In this case, since the rise time Twr of pulse signal 10 is negligible compared to the fall time Twf of pulse signal 10, the pulse width Tw of pulse signal 10 is almost equal to the fall time Twf.

[0042] Furthermore, since the structure and operation of SNSPD21 are well-known, detailed explanations are omitted.

[0043] Optical transmission path 20 is a path for transmitting photons emitted from a light source (not shown). Transmission path 25 is a path for transmitting pulse signal 10. Optical transmission path 20 and transmission path 25 can be of any structure as long as they can respectively transmit photons and pulse signal 10. Optical transmission path 20 can, for example, be constructed of optical fiber. Transmission path 25 can also be constructed using a wiring pattern.

[0044] As described above, SQUID24 has the function of converting the pulse signal 10 output from SNSPD21 into a rectangular waveform voltage signal 11.

[0045] A bias current is supplied to SQUID24 from current source 22. SQUID24 and SNSPD21 are installed together in the cryostat 200, thus SNSPD21 and SQUID24 are in a superconducting state.

[0046] Here, in SQUID24, when pulse signal 10 is transmitted in transmission path 25, magnetic field input is completed via inductor 23. For example... Figure 2AAs shown, SQUID24 has a Josephson junction pair J1 and J2. In SQUID24, since the current is biased to a level below the critical current through current source 22, the voltage across SQUID24 is 0V when no magnetic field is input. Conversely, if a pulse signal reaches inductor 23, a magnetic field is input to SQUID24, thereby reducing the critical current of SQUID24 below the bias current. Therefore, a potential difference is generated across SQUID24.

[0047] In the signal processing circuit 100 of this embodiment, the SQUID 24 includes a first magnetic field input section 124 and a second magnetic field input section 224 that are magnetically coupled to the transmission path 25 by transmitting a pulse signal 10 output from the SNSPD 21 through the transmission path 25. The first magnetic field input section 124 and the second magnetic field input section 224 may also each include an inductor (not shown). That is, by transmitting the pulse signal 10 through the transmission path 25, magnetic field inputs are respectively received via inductors 23 and 31 through the first magnetic field input section 124 and the second magnetic field input section 224 of the SQUID 24.

[0048] The delay circuit 30 is provided in the transmission path 25 between inductor 23 and inductor 31 to delay the pulse signal. Inductor 23 forms part of the transmission path 25 that is magnetically coupled to the first magnetic field input section 124, and inductor 31 forms part of the transmission path 25 that is magnetically coupled to the second magnetic field input section 224.

[0049] Specifically, the first magnetic field input unit 124 inputs a magnetic field to the SQUID 24 when a pulse signal 10 (solid line) before passing through the delay circuit 30 is transmitted in the transmission path 25, provided that a pulse current flows in the inductor 23. The second magnetic field input unit 224 inputs a magnetic field to the SQUID 24 when a pulse signal 10 (dashed line) after passing through the delay circuit 30 is transmitted in the transmission path 25, provided that a pulse current flows in the inductor 31.

[0050] The delay loop 30 can be of any structure as long as it can delay the pulse signal transmitted in the transmission path 25 by a specified delay time. For example, the delay loop 30 can also be formed by arranging the wiring pattern constituting the transmission path 25 in a meandering shape.

[0051] Here, in the signal processing circuit 100 of this embodiment, the delay time of the pulse signal 10 set by the delay circuit 30 is shorter than the total time (Twr+Twf) of the rise time Twr and fall time Twf of the pulse signal 10. That is, through the delay circuit 30, the pulse signal 10 is delayed by a delay time shorter than the total time (Twr+Twf) of the rise time Twr and fall time Twf of the pulse signal 10. In addition, the delay time of the pulse signal 10 set by the delay circuit 30 is a desired value greater than zero in the actual circuit through various parameters, but it can be longer or shorter than the rise time Twr of the pulse signal 10.

[0052] Furthermore, in the signal processing loop 100 of this embodiment, such as Figure 2A As shown, the direction of the magnetic field in the first magnetic field input section 124 (magnetic field input 1) is opposite to the direction of the magnetic field in the second magnetic field input section 224 (magnetic field input 2). Specifically, by forming the wiring diagram constituting the transmission path 25 in such a way that the direction of the current flowing in the inductor 23 is opposite to the direction of the current flowing in the inductor 31, the direction of the magnetic field in magnetic field input 1 is made opposite to the direction of the magnetic field in magnetic field input 2. As a result, SQUID 24 quickly loses its perception of the pulse signal 10 output from SNSPD 21, and the pulse width Tws of the voltage signal 11 transmitted in the transmission path 26 is equal to the delay time of the pulse signal 10 set by the delay circuit 30. That is, through the above magnetic coupling, when the voltage signal 11 is transmitted in the transmission path 26, the direction of the magnetic field in the first magnetic field input section 124 is opposite to the direction of the magnetic field in the second magnetic field input section 224, thereby achieving the desired effect. Figure 2B As shown, the waveform of voltage signal 11 is shaped so that the pulse width Tws of voltage signal 11 is equal to the aforementioned delay time.

[0053] <Simulation Experiment Verification> Figure 3 This is a diagram illustrating an example of the results of a simulation experiment demonstrating that by canceling the magnetic field input to the SQUID, the signal waveform of the voltage signal can be properly shaped.

[0054] The above simulation experiments were reproduced on a computer using the Josephson circuit simulator JSIM. Figure 2A The signal processing circuit 100 shown is used for this purpose. At this time, the peak current of the pulse signal 10 transmitted in the transmission path 25 is 20μA, and the delay time of the pulse signal 10 set by the delay circuit 30 is 100 picoseconds.

[0055] like Figure 3As shown, simulation experiments have verified that when the magnetic fields in magnetic field input 1 and magnetic field input 2 input to SQUID24 are canceled, the pulse width of the voltage signal becomes approximately 100 picoseconds, which is the same as the aforementioned delay time.

[0056] As mentioned above, Figure 2A The signal processing circuit 100 and signal processing method of this embodiment shown are able to properly process the voltage signal 11 generated by the current source 22 and SQUID 24 compared with the conventional method.

[0057] As mentioned above, such as Figure 1B As shown, the pulse signal 10 output from SNSPD21 decreases after approximately tens of nanoseconds, but even within this decrease time Twf, the magnetic field input continues in SQUID21. Therefore, in Figure 1A In the existing signal processing circuit shown, the pulse width of the voltage signal 11 generated by the current source 22 and SQUID 24 is also around tens of nanoseconds.

[0058] In contrast, the signal processing circuit 100 and signal processing method of this embodiment are characterized in that: based on the cancellation of the magnetic field in the delay circuit 30 that delays the pulse signal 10 output from SNSPD21 and the magnetic field in SQUID24, the waveform of the voltage signal 11 generated by the current source 22 and SQUID24 is appropriately shaped.

[0059] Specifically, in this embodiment, when the SNSPD21 detects photons, the magnetic field formed by the pulse signal output from the SNSPD21 is input to the first magnetic field input section 124 of the SQUID24, causing the voltage signal 11 to rise sharply. Furthermore, when the magnetic field formed by the pulse signal 10 delayed by the delay circuit 30 is input again to the second magnetic field input section 224 of the SQUID24, the magnetic field direction in the first magnetic field input section 124 is opposite to that in the second magnetic field input section 224, thus canceling the magnetic field input to the SQUID24. Consequently, the SQUID24 quickly loses its perception of the pulse signal 10 output from the SNSPD21, resulting in a shorter pulse width Tws of the voltage signal 11 compared to the case where the magnetic field input to the SQUID24 was not canceled.

[0060] By shaping the waveform of the voltage signal 11 described above, the signal processing circuit 100 and signal processing method of this embodiment can reduce the impact of the dead time when the SNSPD21 cannot detect the next photon compared with the past.

[0061] In the signal processing circuit 100 and signal processing method of this embodiment, the case where the time of photon incident fluctuates over time is taken into account. Figure 2B The dashed arrows indicate temporal fluctuations in the photons incident on the SNSPD21. When the photon incident time fluctuates due to some reason, the rise time of the pulse signal 10 output from the SNSPD21 appropriately inherits this fluctuation. Here, the rise time of the voltage signal 11 generated by the current source 22 and the SQUID24 corresponds to the rise time of the pulse signal 10 output from the SNSPD21. As a result, since the fluctuations are appropriately inherited in the rise time of the voltage signal 11, the voltage signal 11 is able to maintain the time information of photon incident.

[0062] Furthermore, in the signal processing circuit 100 and signal processing method of this aspect, the pulse width Tws of the voltage signal 11 generated by the current source 22 and SQUID 24 is equal to the delay time of the pulse signal set by the delay circuit 30. Therefore, the pulse width Tws of the voltage signal 11 can be easily and appropriately set by the signal delay time, which is a design parameter of the delay circuit 30.

[0063] (Second Implementation) Figure 4A This is a diagram illustrating an example of a signal processing circuit according to the second embodiment. Figure 4B It is shown by Figure 4A A diagram of the time graph of the signal processed by the signal processing loop.

[0064] exist Figure 4A In the example shown, the signal processing loop 100 includes multiple SNSPDs 21A and 21B, multiple SQUIDs 24A and 24B, multiple delay loops 30A and 30B, a current source 22, and a delay structure 40. Here, SQUIDs 24A and 24B are connected in series on the transmission path 26 extending from the current source 22.

[0065] Figure 4A The diagram illustrates the structure of the signal processing loop 100 when there are two SNSPDs 21A and 21B and two SQUIDs 24A and 24B. However, the number of SNSPDs and SQUIDs is arbitrary. For example, in the signal processing loop 100, there may sometimes be hundreds of SNSPDs and SQUIDs.

[0066] Multiple SNSPDs 21A and 21B are detectors respectively connected to multiple transmission paths 25A and 25B. That is, the pulse signal 10A output from SNSPD 21A is transmitted in transmission path 25A. The pulse signal 10B output from SNSPD 21B is transmitted in transmission path 25B. SQUID 24A is an element magnetically coupled to inductors 23A and 31A, which constitute portions of transmission path 25A. SQUID 24B is an element magnetically coupled to inductors 23B and 31B, which constitute portions of transmission path 25B. Multiple optical transmission paths 20A and 20B are optical paths for transmitting photons emitted from a light source (not shown) to SNSPDs 21A and 21B, respectively. That is, in this embodiment, they are configured independently so that each of the multiple light sources corresponds one-to-one with each of the multiple SNSPDs 21A and 21B.

[0067] Delay structure 40 makes the time difference Td between the output times of the voltage signals 11A and 11B corresponding to SQUID24A and 24B respectively (refer to...) Figure 4B The delay time of the pulse signal 10 set by the delay circuit 30A (refer to...) Figure 4B For example, the time difference Td between the output times of voltage signals 11A and 11B can be approximately twice the delay time of pulse signal 10. In other words, the time difference Td between the output times of voltage signals 11A and 11B can also be approximately twice the pulse width Tws of voltage signal 11 transmitted in transmission path 26.

[0068] Here, the structures of SNSPD21A, 21B, transmission paths 25A, 25B, SQUID24A, 24B, inductors 23A, 31A, 23B, 31B, optical transmission paths 20A, 20B, and delay circuits 30A, 30B are the same as those in the first embodiment, except for the number of each type, so detailed descriptions are omitted.

[0069] exist Figure 4A In the example shown, the delay structure 40 includes an optical fiber disposed in the optical transmission path 20B, which delays the photons transmitted in the optical transmission path 20B.

[0070] The delay structure 40 can be any structure as long as it can delay the photons propagating in the optical transmission path 20B. For example, the cable lengths of the optical fibers in optical transmission paths 20A and 20B are different, thereby allowing for arbitrary adjustment of the incident times of photons arriving at SNSPD21A and SNSPD21B respectively. As an example, such as Figure 4AAs shown, the cable of the optical fiber in optical transmission path 20B can have two loop-shaped portions extending in a manner that depicts a loop. In contrast, the cable of the optical fiber in optical transmission path 20A does not have the aforementioned loop-shaped portions. Therefore, the cable lengths of optical transmission path 20A and optical transmission path 20B are different from each other, thus... Figure 4B As shown, the incident time of photons incident on SNSPD21A and 21B can be easily adjusted for each optical fiber cable. That is, in the signal processing circuit 100 of this embodiment, the time difference between the incident times of photons incident on SNSPD21A and 21B is equivalent to the time difference Td between the output times of voltage signals 11A and 11B respectively.

[0071] Furthermore, the number of annular sections mentioned above is an example and is not limited to this example. That is, the number of annular sections can be arbitrarily set in the signal processing circuit 100 so that the time difference Td between the output times of voltage signals 11A and 11B becomes the desired value.

[0072] As described above, in this embodiment, the signal processing circuit 100 has a time difference Td between the output times of the voltage signals 11A and 11B corresponding to SQUID 24A and 24B, respectively, which is longer than the delay time of the pulse signal 10 set by the delay circuit 30A. Therefore, Figure 4B As shown, voltage signals 11A and 11B can perform appropriate multiplexing signal processing in a single transmission path 26 while maintaining the time information of photons incident from multiple light sources to SNSPD21A and 21B respectively.

[0073] Furthermore, in this embodiment, the signal processing circuit 100 has a longer time difference Td between the output times of the voltage signals 11A and 11B corresponding to SQUID 24A and 24B, respectively, than the delay time of the pulse signal 10 set by the delay circuit 30A. Therefore, the voltage signals 11A and 11B can maintain position information about which SNSPD the photon from the light source is emitted towards among the plurality of SNSPDs 24A and 24B.

[0074] Furthermore, in this embodiment, since SQUIDs 24A and 24B are connected in series in the transmission path 26, the current of the current source 22 remains constant regardless of the number of channels in the SNSPD. Therefore, the power consumption of the signal processing circuit 100 in this embodiment remains constant even if the number of SNSPD channels increases, thus exhibiting high scalability. Moreover, when the SNSPD is multi-channeled, the voltage signals of each SQUID corresponding to the SNSPD are less affected by the characteristic differences of each SNSPD, and their pulse widths can be maintained at a constant level.

[0075] Furthermore, the signal processing loop 100 of this embodiment can easily and appropriately set the time difference Td between the output times of the voltage signals 11A and 11B corresponding to SQUID 21A and 21B, respectively, by adjusting the photon delay time by the cable length of the optical fiber provided in the optical transmission path 20.

[0076] The signal processing circuit 100 of this embodiment may be the same as the signal processing circuit 100 of the first embodiment, except for the features described above.

[0077] (Modified example) Figure 5A This is a diagram illustrating an example of a signal processing circuit of a variation of the second embodiment. Figure 5B It is shown by Figure 5A A diagram of the time graph of the signal processed by the signal processing loop.

[0078] exist Figure 5A In the example shown, the signal processing loop 100 includes multiple SNSPDs 21A and 21B, multiple SQUIDs 24A and 24B, multiple delay loops 30A and 30B, a current source 22, and a delay structure 50. Here, SQUIDs 24A and 24B are connected in series on the transmission path 26 extending from the current source 22.

[0079] Figure 5A The diagram illustrates the structure of the signal processing loop 100 when there are two SNSPDs 21A and 21B and two SQUIDs 24A and 24B. However, the number of SNSPDs and SQUIDs is arbitrary. For example, in the signal processing loop 100, there may sometimes be hundreds of SNSPDs and SQUIDs.

[0080] Multiple SNSPDs 21A and 21B are detectors respectively connected to multiple transmission paths 25A and 25B. That is, the pulse signal 10A output from SNSPD 21A is transmitted in transmission path 25A. The pulse signal 10B output from SNSPD 21B is transmitted in transmission path 25B. SQUID 24A is an element magnetically coupled to inductors 23A and 31A, which constitute portions of transmission path 25A. SQUID 24B is an element magnetically coupled to inductors 23B and 31B, which constitute portions of transmission path 25B. Multiple optical transmission paths 20A and 20B are optical paths for transmitting photons emitted from a light source (not shown) to SNSPDs 21A and 21B, respectively. That is, in this embodiment, they are configured independently so that each of the multiple light sources corresponds one-to-one with each of the multiple SNSPDs 21A and 21B.

[0081] Delay structure 50 makes the time difference Td between the output times of voltage signals 11A and 11B corresponding to SQUID24A and 24B respectively (refer to...) Figure 5B The delay time of the pulse signal 10 set by the delay circuit 30A (refer to...) Figure 5B For example, the time difference Td between the output times of voltage signals 11A and 11B can be approximately twice the delay time of pulse signal 10. In other words, the time difference Td between the output times of voltage signals 11A and 11B can also be approximately twice the pulse width Tws of voltage signal 11 transmitted in transmission path 26.

[0082] Here, the structures of SNSPD21A, 21B, transmission paths 25A, 25B, SQUID24A, 24B, inductors 23A, 31A, 23B, 31B, optical transmission paths 20A, 20B, and delay circuits 30A, 30B are the same as those in the first embodiment, except for the number of each type, so detailed descriptions are omitted.

[0083] exist Figure 5A In the example shown, the delay structure 50 is a delay loop that is set in the transmission path 26 between adjacent SQUIDs 24A and 24B, so that the signal transmitted in the transmission path 26 is delayed.

[0084] The delay structure 50 can be any structure as long as it can delay the signal transmitted in such an optical transmission path 26. For example, the delay loop described above can also be formed by arranging the wiring diagram constituting the transmission path 26 in a meandering manner. That is, in the signal processing circuit 100 of this modified example, the delay time of the signal set by this delay loop is equivalent to the time difference Td between the output times of the voltage signals 11A and 11B respectively.

[0085] As described above, in this modified example, the signal processing circuit 100, because the time difference Td between the output times of the voltage signals 11A and 11B corresponding to SQUID24A and 24B respectively is longer than the delay time of the pulse signal 10 set by the delay circuit 30A, therefore... Figure 5B As shown, voltage signals 11A and 11B can perform appropriate multiplexing signal processing in a single transmission path 26 while maintaining the time information of photons incident from multiple light sources to SNSPD21A and 21B respectively.

[0086] Furthermore, in this modified example, since the time difference Td between the output times of the voltage signals 11A and 11B corresponding to SQUID24A and 24B is longer than the delay time of the pulse signal 10 set by the delay circuit 30A, the voltage signals 11A and 11B can maintain position information about which SNSPD the photon from the light source is emitted towards among the plurality of SNSPDs 21A and 21B.

[0087] Furthermore, in this modified example, since SQUIDs 24A and 24B are connected in series in the transmission path 26, the current of the current source 22 remains constant regardless of the number of channels of the SNSPD. Therefore, the power consumption of the signal processing circuit 100 in this modified example remains constant even if the number of channels of the SNSPD increases, thus exhibiting high scalability. Moreover, when the SNSPD is multi-channeled, the voltage signals of each SQUID corresponding to the SNSPD in this modified example are less affected by the characteristic differences of each SNSPD, and their pulse widths can be maintained at a constant level.

[0088] Furthermore, the signal processing loop 100 of this variant can easily and appropriately set the time difference Td between the output times of the voltage signals 11A and 11B corresponding to SQUID24A and 24B by using the design parameters of the delay loop of the transmission path 26 between SQUID24A and 24B, i.e., the signal delay time.

[0089] In addition to the features described above, the signal processing circuit 100 of this modified example may also be the same as the signal processing circuit 100 of the first embodiment or the second embodiment.

[0090] Furthermore, the first embodiment, the second embodiment, and variations thereof can be combined with each other as long as they do not exclude each other. Also, those skilled in the art will understand from the above description many improvements or other embodiments of this disclosure. Therefore, the above description should be interpreted as illustrative only, provided for the purpose of teaching those skilled in the art the best mode for performing this disclosure. Substantial changes can be made to the details of its structure and / or function without departing from the spirit of this disclosure.

[0091] Industrial availability: One aspect of this disclosure enables signal processing circuits and methods that can appropriately process voltage signals generated by current sources and SQUIDs compared to the past.

[0092] Symbol explanation: 10: Pulse signal; 11: Voltage signal; 11A: Voltage signal; 11B: Voltage signal; 20: Optical transmission path; 20A: Optical transmission path; 20B: Optical transmission path; 21: SNSPD; 21A: SNSPD; 21B: SNSPD; 22: Current source; 23: Inductor; 23A: Inductor; 23B: Inductor; 24: SQUID; 24A: SQUID; 24B: SQUID; 25: Transmission path; 25A: Transmission path; 25B: Transmission path; 26: Transmission path; 30: Delay loop; 30A: Delay circuit; 30B: Delay loop; 31: Inductor; 31A: Inductor; 31B: Inductor; 40: Delayed structure; 50: Delayed structure; 100: Signal processing loop; 124: First magnetic field input section; 200: Refrigeration unit; 224: Second magnetic field input section; Td: Time difference; Tw: Pulse width; Twf: Descent time; Twr: Ascent time; Tws: Pulse width.

Claims

1. A signal processing circuit, characterized in that, have: The first transmission path transmits a pulse signal with a longer fall time from the peak to the predetermined level compared to the rise time from the predetermined level to the peak. A superconducting quantum interference device, comprising a first magnetic field input section and a second magnetic field input section that are magnetically coupled to the first transmission path by transmitting the pulse signal in the first transmission path; A second transmission path, which is connected to the current source and the superconducting quantum interference device; and A first delay circuit is disposed in the first transmission path between a portion of the first transmission path magnetically coupled to the first magnetic field input section and a portion of the first transmission path magnetically coupled to the second magnetic field input section, thereby delaying the pulse signal. The delay time of the pulse signal set by the first delay circuit is shorter than the total time of the rise time and the fall time, and The magnetic field direction in the first magnetic field input section is opposite to that in the second magnetic field input section, so the pulse width of the voltage signal transmitted in the second transmission path is equal to the delay time.

2. The signal processing circuit according to claim 1, characterized in that, The pulse signal is a current signal detected by a superconducting nanobelt single-photon detector.

3. The signal processing circuit according to claim 1 or 2, characterized in that, It comprises: multiple superconducting nanoribbon single-photon detectors, each connected to multiple first transmission paths; multiple superconducting quantum interference devices (SQUs), each magnetically coupled to one of the first transmission paths; multiple optical transmission paths for transmitting photons emitted from multiple light sources to the superconducting nanoribbon single-photon detectors; and a delay structure for ensuring that the time difference between the output times of a pair of voltage signals corresponding to a pair of SQUs is longer than the delay time. The superconducting quantum interference devices are connected in series in the second transmission path.

4. The signal processing circuit according to claim 3, characterized in that, The delay structure includes: an optical fiber disposed in the optical transmission path, which delays the photons transmitted in the optical transmission path.

5. The signal processing circuit according to claim 3, characterized in that, The delay structure includes: a second delay loop disposed between adjacent superconducting quantum interference devices in the second transmission path, such that the signal transmitted in the second transmission path is delayed.

6. A signal processing method, characterized in that, Includes the following steps: A pulse signal with a longer fall time from the peak level to the specified level compared to the rise time from the specified level to the peak level is transmitted in the first transmission path; The pulse signal is transmitted in the first transmission path, thereby magnetically coupling with the first transmission path through the first magnetic field input section and the second magnetic field input section of the superconducting quantum interference device, respectively. By using a delay loop disposed between a portion of the first transmission path magnetically coupled to the first magnetic field input and a portion of the first transmission path magnetically coupled to the second magnetic field input, the pulse signal is delayed by a delay time shorter than the total time of the rise time and the fall time. as well as Through the magnetic coupling, when the voltage signal is transmitted in the second transmission path connected to the current source and the superconducting quantum interference device, the magnetic field direction in the first magnetic field input section is opposite to the magnetic field direction in the second magnetic field input section, thereby shaping the waveform of the voltage signal so that the pulse width of the voltage signal is equal to the delay time.

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