JP5846626B2 - Superconducting single photon detection system and superconducting single photon detection method - Google Patents

Description

  The present invention relates to a superconducting single photon detection system and a superconducting single photon detection method using a superconducting single photon detector.

  Superconducting single photon detector (SSPD) is a high sensitivity, low noise and high speed operation single photon detector in various fields such as quantum information communication and quantum optics. The use to is expected. In particular, a superconducting single-photon detection system in which SSPD is mounted in a closed cycle refrigerator is a practically effective system because it can stably achieve extremely low temperatures (about 3K) and can operate continuously. (For example, refer nonpatent literatures 1 and 2). A detection element called a nanowire is used as a detection element for detecting a single photon in SSPD. The nanowire uses a niobium nitride wiring made of, for example, niobium nitride (NbN) in a superconducting state, and the light receiving portion is configured by forming the nanowire in a meander shape (meandering shape) on the light receiving surface.

  In such a superconducting single-photon detection system using SSPD, a single pixel element that can obtain one output by using one light receiving unit for one light input is generally used. In such a superconducting single photon detection system using a single pixel element, the photon detection efficiency of the system is about 10 to 20%, the dark count is about 100 Hz, the response speed is about 50 MHz, and the jitter is about 100 ps. Yes.

  Here, in order to achieve high optical coupling efficiency between the incident photons and the SSPD, a sufficiently large light receiving area must be ensured with respect to the spot diameter (about 9 μm) of the light incident on the SSPD, and about 15 μm. A light receiving area of about the corner is required. On the other hand, when the light receiving area of one SSPD is increased, the length of the nanowire is increased, so that the number of defects in the nanowire is increased and the device performance is deteriorated. Further, when the length of the nanowire is increased, the inductance of the nanowire increases. Accordingly, the time constant (inductance / resistance value) depending on the inductance of the nanowire increases, and the speed of the signal transmitted through the nanowire becomes slow. As a result, the response speed decreases.

  As a solution to such a problem, a configuration is known in which a predetermined light receiving area is divided into a plurality of light receiving portions (a plurality of pixels) (see, for example, Non-Patent Document 3). The nanowire length per pixel can be shortened by dividing the light receiving area into multiple parts while ensuring the light receiving area for achieving high optical coupling efficiency. That is, it is possible to prevent deterioration in device performance and response speed while achieving high optical coupling efficiency.

  As described above, in order to operate SSPD with superconductivity, it is necessary to mount SSPD in a refrigerator. In addition, it is necessary to supply a bias current to SSPD from the outside. For this reason, the conventional coaxial cable, which is the AC signal transmission path, is extended from the refrigerator to the outside and connected to the signal readout circuit, and the bias current is applied to the exposed coaxial cable via the bias tee (Bias Tee). The bias path to be supplied is connected. FIG. 8 is a schematic diagram showing a schematic configuration example when a plurality of superconducting single photon detectors are used in a conventional superconducting single photon detection system. As shown in FIG. 8, in the prior art, the same number of coaxial cables as SSPDs 121 to 124 from the refrigerator 105 in which the SSPD array 102 composed of a plurality of SSPDs 121 to 124 irradiated with photons from the photon source 104 is mounted. 141 to 144 are exposed to the outside. The bias tees T101 to T104 include capacitors C101 to C104 connected in series to the respective coaxial cables 141 to 144 and inductors L101 to L104 connected to wirings (bias paths) 151 to 154 branched from the respective coaxial cables 141 to 144. It has. Bias paths 151 to 154 connected to the inductors L101 to L104 are connected to voltage sources 171 to 174 for bias current, and the bias current from the voltage sources 171 to 174 passes through the bias paths 151 to 154 and the coaxial cables 141 to 144. To be supplied to each SSPD 121-124.

  As described above, in the conventional configuration, the plurality of coaxial cables 141 to 144 connecting the plurality of SSPDs 121 to 124 in the cryogenic refrigerator 105 and the signal readout circuit (not shown) at room temperature are cryogenic. -Since it connects between room temperature, the heat | fever inflow into the refrigerator 105 via the said coaxial cables 141-144 will become large. In particular, when the number of SSPDs 121 to 124 is increased, the number of coaxial cables 141 to 144 to be connected increases accordingly, so that the heat inflow amount becomes larger and the cryogenic state in the refrigerator 105 is maintained. It becomes difficult and the SSPD array cannot be brought into a superconducting state.

  Therefore, a system system (ie, a superconducting single flux quantum circuit (hereinafter sometimes abbreviated as a superconducting SFQ circuit)) that can perform a logical operation in a cryogenic environment as a signal readout circuit. , SSPD, coaxial cable, and signal reading circuit) can all be mounted in the refrigerator (see, for example, Patent Document 1). Even in this case, when the conventional connection method as shown in FIG. 8 is followed, it is necessary to attach a bias tee inside the refrigerator.

JP 2009-232311 A

G. Gol'tsman, O. Okunev, G. Chulkova, A. Lipatov, A. Semenov, K. Smirnov, B. Voronov, A. Dzardanov, C. Williams, and R. Sobolewski, “Picosecond superconducting single photon detector, ”Appl. Phys. Lett. 79, 705-707 (2001). S. Miki, T. Yamashita, M. Fujiwara, M. Sasaki, and Z. Wang, “Multichannel SNSPD system with high detection efficiency at telecommunication wavelength,” Opt. Lett., Vol. 35, No. 13, 2133-2135 , 2010 EADauler, AJ Kerman, BS Robinson, JKW Yang, B. Voronov, G. Gol'tsman, SA Hamilton, and KK Berggren, "Multi-element superconducting nanowire single photon detectors," IEEE Tran. Appl Super, 17, pp. 279, 2007

  However, since the size of the bias tee per unit is usually as large as 3 cm square, a practical refrigerator that can drive a plurality of bias tees required for a plurality of coaxial cables with a small AC100V power supply ( For example, it is difficult to mount in a GM refrigerator. On the other hand, when the refrigerator is enlarged, it is difficult to reduce the size and cost of the system. In addition, it is conceivable to form capacitors and inductors constituting a bias tee on an IC chip constituting an SSPD or SFQ circuit, but in order to realize a general bias tee (capacitance of about 0.2 μF and inductance of about 1 mH). Is not realistic because it requires a large area. In the future, in order to realize a large-scale SSPD array, it is an important issue whether the SSPD array and the SFQ circuit can be connected to a small refrigerator having a limited cooling capacity.

  The present invention has been made to solve the above-described problems. A superconducting single-photon detection system and a superconducting system that can miniaturize the system while maintaining a stable cryogenic temperature and achieve high performance. An object is to provide a single photon detection method.

  A superconducting single photon detection system according to an embodiment of the present invention includes a superconducting single photon detector operated by a bias current, and a load resistance element for a detection signal output from the superconducting single photon detector. A superconducting single-flux quantum circuit that converts to a single-flux quantum signal using a converter and performs arithmetic processing, and between the superconducting single-photon detector and the superconducting single-flux quantum circuit Are connected by an AC signal transmission path, a bias current path for supplying the bias current to the superconducting single photon detector is connected to the AC signal transmission path via a high impedance element, and the high impedance element is The impedance at high frequency is configured to be higher than the impedance of the load resistance element of the superconducting single flux quantum circuit.

  According to the above configuration, the bias current path is connected through the high impedance element between the connection between the superconducting single photon detector and the superconducting single flux quantum circuit, and the superconducting single photon detector is operated. Are supplied to the superconducting single photon detector via the bias current path, the high impedance element, and the AC signal transmission path. Here, since the superconducting single photon detector is in a superconducting state during operation, and has an extremely low resistance (about 0Ω), the superconducting single photon detector and the superconducting single flux quantum circuit are Even if no capacitor is provided between them, the bias current flows to the superconducting single photon detector side having a resistance value lower than that of the load resistance element of the superconducting single flux quantum circuit. In addition, the high impedance element prevents the detection signal from the superconducting single photon detector from proceeding to the bias current path because the impedance at high frequency is higher than the impedance of the load resistance element of the superconducting single flux quantum circuit. Can do. In this way, the bias current path can be connected to the superconducting single photon detector with only a simple resistance element without using a bias tee, and the number of pixels can be increased by a plurality of superconducting single photon detectors. . Therefore, the system can be miniaturized while maintaining the cryogenic temperature stably, and high performance (high detection efficiency and high-speed response) can be obtained.

  The superconducting single photon detector, the superconducting single flux quantum circuit, the AC signal transmission path, and the high impedance element may be mounted in a refrigerator. This makes it possible to significantly reduce the number of AC signal transmission paths introduced from room temperature, which can cause heat intrusion, so that a stable cryogenic temperature can be realized regardless of the number of superconducting single photon detectors. Can do.

  The superconducting single photon detection system includes a plurality of superconducting single photon detectors, a plurality of superconducting single flux quantum circuits provided corresponding to the plurality of superconducting single photon detectors, and A plurality of the AC signal transmission paths and a plurality of the high impedance elements provided corresponding to the plurality of AC signal transmission paths may be provided. According to this, the number of pixels can be increased by a plurality of superconducting single photon detectors.

  Further, the bias current path may be configured to supply the bias current from one current source to the plurality of high impedance elements. Thus, even when a plurality of superconducting single photon detectors are provided, one bias current path is introduced into the refrigerator, and is connected to the high impedance element corresponding to each superconducting single photon detector in the refrigerator. The route can be branched. Therefore, the inflow of heat into the refrigerator can be further suppressed.

  The superconducting single-photon detector may include a superconducting nanowire detection element that detects a single photon, and the high-impedance element may be composed of a nanowire having a narrower line width than the superconducting nanowire detection element. As a result, the high impedance element can be manufactured by the same process as that for manufacturing the superconducting single photon detector, so that the manufacturing cost and the number of manufacturing steps can be reduced. Also, a high impedance element can be realized with a small area.

  According to another aspect of the present invention, there is provided a superconducting single photon detection method comprising: detecting a single photon by supplying a bias current to a superconducting single photon detector through a bias current path; The single photon is output as a detection signal, transmitted to the superconducting single flux quantum circuit through the AC signal transmission path, converted into a single flux quantum signal using a converter having a load resistance element, and then processed. The bias current is supplied to the superconducting single photon detector through a bias current path connected to the AC signal transmission path via a high impedance element, the high impedance element comprising: The impedance at high frequency is higher than the impedance of the load resistance element of the superconducting single flux quantum circuit.

  According to the above method, the bias current path is connected to the AC signal transmission path between the superconducting single photon detector and the superconducting single flux quantum circuit via the high impedance element, and the superconducting single photon detector Is supplied to the superconducting single photon detector via the bias current path, the high impedance element, and the AC signal transmission path. Here, since the superconducting single photon detector is in a superconducting state during operation, and has an extremely low resistance (about 0Ω), the superconducting single photon detector and the superconducting single flux quantum circuit are Even if no capacitor is provided between them, the bias current flows to the superconducting single photon detector side having a resistance value lower than that of the load resistance element of the superconducting single flux quantum circuit. In addition, the high impedance element prevents the detection signal from the superconducting single photon detector from proceeding to the bias current path because the impedance at high frequency is higher than the impedance of the load resistance element of the superconducting single flux quantum circuit. Can do. In this way, the bias current path can be connected to the AC signal transmission path between the superconducting single photon detector and the superconducting single flux quantum circuit by using only a simple resistance element without using a bias tee. The number of pixels can be increased by a plurality of superconducting single photon detectors. Therefore, the system can be downsized while maintaining a very low temperature, and high photon detection accuracy can be obtained.

  The present invention is configured as described above, and produces an effect that the system can be miniaturized while maintaining a very low temperature and high performance (high detection efficiency and high speed response) can be obtained.

FIG. 1 is a schematic diagram showing a schematic configuration example of a superconducting single photon detection system according to the first embodiment of the present invention. FIG. 2 is a circuit diagram showing a configuration example of the SSPD and SFQ circuit of the superconducting single photon detection system shown in FIG. FIG. 3 is a schematic diagram showing a schematic configuration example of a superconducting single photon detection system according to the second embodiment of the present invention. FIG. 4 is a schematic diagram showing a schematic configuration example of a superconducting single photon detection system according to the third embodiment of the present invention. FIG. 5 is a schematic diagram showing a schematic configuration example of a superconducting single photon detection system according to the fourth embodiment of the present invention. FIG. 6 is a graph showing measurement results of changes in photon detection efficiency and dark count with respect to the SSPD bias current in the example of the present invention. FIG. 7 is a graph showing measurement results for jitter in the example of the present invention. FIG. 8 is a schematic diagram showing a schematic configuration example when a plurality of superconducting single photon detectors are used in a conventional superconducting single photon detection system.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the following description, the same or corresponding elements are denoted by the same reference symbols throughout the drawings, and redundant description thereof is omitted.

<First Embodiment>
First, the superconducting single photon detection system according to the first embodiment of the present invention will be described. FIG. 1 is a schematic diagram showing a schematic configuration example of a superconducting single photon detection system according to the first embodiment of the present invention. As shown in FIG. 1, the superconducting single photon detection system 1 of the present embodiment includes a superconducting single photon detector (SSPD) 21 to 24 operated by a bias current and superconducting single photon detectors 21 to 21. 24 includes a superconducting single flux quantum circuit (SFQ circuit) 30 that converts the detection signal output from 24 into single flux quanta by the converters 31 to 34 and performs signal processing by the output conversion circuit 35.

  In the present embodiment, the SSPD chip 2 is provided with a plurality (four) of SSPDs 21 to 24 to form an SSPD array 20. The superconducting single photon detection system 1 includes a photon source 4 that emits photons toward the SSPD array 20. As described above, since the SSPD array 20 formed on one SSPD chip 2 can receive photons from the photon source 4, a multi-pixel light receiving element is formed for one photon input. Can do. Therefore, the light receiving area per pixel can be reduced, and the detection efficiency and response speed can be increased. In addition, since the multi-pixel arrangement can enhance the spatial resolution and the ability to discriminate the number of photons (the pseudo number of photons input), it can be expected to be used for quantum optics experiments and quantum information communication.

  The SFQ circuit 30 is provided on the SFQ chip 3. Specifically, the SFQ circuit 30 is provided corresponding to each of the SSPDs 21 to 24, and converts the detection signals output from the superconducting single photon detectors 21 to 24 into SFQ pulse signals, respectively (MC− DC / FSQ converter (Mutual Coupling-DC / FSQ Converter) 31-34, output conversion circuit 35 for converting weak SFQ pulse signals converted by converters 31-34 into observable pulse signals, and output conversion circuit And a SQUID driver 36 that generates an output signal by increasing the pulse width of the voltage pulse output from the S35.

  The SSPDs 21 to 24 and the converters 31 to 34 of the SFQ circuit 30 are connected by the number of AC signal transmission paths (high-frequency signal transmission paths) corresponding to the SSPD. Specifically, coaxial lines (SSPD coaxial lines) 241 to 244 provided on the SSPD chip 2 are connected to the SSPDs 21 to 24. In addition, coaxial lines (SFQ coaxial lines) 341 to 344 provided in the SFQ chip 3 are connected to the converters 31 to 34 of the SFQ circuit 30. The coaxial cables 41 to 44 are connected between the SSPD coaxial lines 241 to 244 and the SFQ coaxial lines 341 to 344. That is, the AC signal transmission path is a concept including coaxial lines 241 to 244 and 341 to 344 and coaxial cables 41 to 44 provided on the chips 2 and 3.

  In the AC signal transmission path (SFQ coaxial lines 341 to 344 in the present embodiment), bias current paths (DC paths) 51 to 54 for supplying a bias current to the SSPDs 21 to 24 are respectively connected via the high impedance elements 61 to 64. Connected. That is, one end of the high impedance elements 61 to 64 is connected to the SFQ coaxial lines 341 to 344, and the other end of the high impedance elements 61 to 64 is connected to the bias paths 51 to 54. The high impedance elements 61 to 64 are configured such that the impedance at high frequency is higher than the impedance of the load resistance element Rr provided in each of the converters 31 to 34 of the SFQ circuit 30. The impedance at high frequency specifically means the resistance value in the resistance element or the inductance in the inductor. The high impedance elements 61 to 64 are provided on the SFQ chip 3. Bias current sources 71 to 74 are connected to the bias current paths 51 to 54 via resistance elements 81 to 84. Specifically, a bias current determined by a voltage applied from the bias current sources 71 to 74 to the resistance elements 81 to 84 is supplied to the SSPDs 21 to 24.

  The SSPD chip 2 and the SFQ chip 3 are mounted in the refrigerator 5. That is, the SSPD array 20, the SFQ circuit 30, the AC signal transmission path, and the high impedance elements 61 to 64 are mounted in the refrigerator 5. The bias current paths 51 to 54 extend from the refrigerator 5 to the outside and are connected to resistance elements 81 to 84 and current sources 71 to 74 provided outside the refrigerator 5. As the refrigerator 5, for example, a small GM (Gifford-McMahon) refrigerator is preferably used.

  According to the above configuration, the bias current paths 51 to 54 are connected to the coaxial cables 41 to 44 between the SSPDs 21 to 24 and the SFQ circuit 30 via the high impedance elements 61 to 64, respectively. A bias current is supplied to the SSPDs 21 to 24 via the bias current paths 51 to 54, the high impedance elements 61 to 64, and the coaxial cables 41 to 44. Here, since the SSPDs 21 to 24 are in a superconducting state during operation, they have extremely low resistance (about 0Ω). Therefore, even if a capacitor is not provided between the SSPDs 21 to 24 and the SFQ circuit 30, the bias current flows to the SSPDs 21 to 24 side having a lower resistance value than the load resistance element Rr of the SFQ circuit 30. In addition, since the high impedance elements 61 to 64 have an impedance at a high frequency higher than the impedance of the load resistance element Rr of the SFQ circuit 30, the detection signals from the SSPDs 21 to 24 are prevented from proceeding to the bias current paths 51 to 54. A detection signal can be efficiently transmitted to the circuit 30. As described above, by using the simple resistive elements 61 to 64 in the SFQ chip 3 or the SSPD chip 2 without using a bias tee, it is possible to cause the bias current to flow through the SSPDs 21 to 24. Multiple pixels can be achieved.

  Further, as described above, the SSPDs 21 to 24 and the SFQ circuit 30 can be connected in the refrigerator 5 through the AC signal transmission path. As a result, the number of coaxial cables introduced from room temperature can be reduced, and heat inflow from the room temperature side can be dramatically reduced. Although the bias current paths 51 to 54 extend outside the refrigerator 5, the refrigerators via the lead wires in the bias current paths 51 to 54 that are direct current paths are compared with the coaxial cables 41 to 44. The penetration of heat into 5 is sufficiently small. Therefore, a stable cryogenic temperature can be realized regardless of the number of SSPDs 21 to 24. As described above, the system can be downsized while maintaining a very low temperature, and high photon detection accuracy can be obtained.

  In addition, the high impedance elements 61 to 64 in the present embodiment are formed by resistance elements using metal thin films that can be manufactured by a conventional process. However, the present invention is not limited to this as long as it is an element having a higher impedance with respect to the detection signals output from the SSPDs 21 to 24 than the load resistance element Rr, and may be configured with, for example, an inductor.

  When the impedance of the load resistance element Rr of the SFQ circuit 30 is 50Ω, the impedance of the high impedance elements 61 to 64 may be about 5 kΩ, for example. The impedance of the resistance elements 81 to 84 is, for example, about 100 kΩ.

  Hereinafter, more specific configurations of the SSPDs 21 to 24 and the SFQ circuit 30 will be described. FIG. 2 is a circuit diagram showing a configuration example of the SSPD and SFQ circuits of the superconducting single photon detector array system shown in FIG. In FIG. 2, only one SSPD 21 is clearly shown, and the other SSPDs 22 to 24 and corresponding configurations are not shown.

  The SSPD 21 includes a superconducting nanowire detection element 7 that detects a single photon. The superconducting nanowire detection element 7 is formed by stacking niobium nitride (NbN) in a meander shape (meandering shape) on the surface of a magnesium oxide (MgO) substrate. For example, it is formed at a predetermined pitch with a line width of several tens to several hundreds of nanometers, cooled using the refrigerator 5, and used in a superconducting state. The light receiving area of the light receiving surface including the superconducting nanowire detection element 7 is appropriately determined depending on the intended use of the SSPD 21 and the number of SSPDs constituting the SSPD array 20. For example, if the light receiving area of the light receiving surface is 5 × 5 μm square, photon detection efficiency can be increased.

  The current source 71 and the resistance element 81 are set to have a current value and a resistance value so that a predetermined bias current slightly below the critical current flows through the superconducting nanowire detection element 7. In such a configuration, when a photon P (single photon) is incident on the superconducting nanowire detection element 7, energy exceeding the energy gap is supplied at the location where the photon P is incident, and a normal conduction region (high resistance region) called a hot spot. Will occur. In this case, at this location, current flows intensively on both sides in the width direction of the hot spot in the superconducting nanowire detection element 7 so as to bypass the hot spot.

  Then, the current flowing around the hot spot exceeds the critical current, and both side portions in the width direction of the hot spot are in a normal conduction state. That is, a normal conduction region is temporarily formed over the entire width direction of the superconducting nanowire detection element 7 at the location. In this way, the resistance of the superconducting nanowire detection element 7 changes in response to the incidence of the photon P on the superconducting nanowire detection element 7. At this time, the bias current from the current source 71 flows to the converter 31 side of the SFQ circuit 30. Accordingly, since the voltage applied to the SSPD 21 changes based on such resistance change, the incident photon P is appropriately detected as a detection signal (voltage signal). The detection signal is transmitted to the converter 31 of the SFQ circuit 30 via the coaxial cable 41.

  The high-frequency impedance of the high impedance elements 61 to 64 is set to a value higher than that of the load resistance element Rr of the SFQ circuit 30.

  The converter 31 is configured to convert the detection signal output from the SSPD 21 into a single magnetic flux quantum and output it. Specifically, a primary coil L1 connected in series to the AC signal transmission path (coaxial cable 41 and coaxial lines 241 and 341) and the load resistance element Rr, and a secondary coil arranged so as to be mutually inductible with the primary coil L1. L2. A pair of Josephson junctions J1 and J2 are connected to the secondary coil L2, and the pair of Josephson junctions J1 and J2 constitute a superconducting quantum interference photon (SQUID).

  The detection signal output from the SSPD 21 is mutually induced from the primary coil L1 to the secondary coil L2, and converted into magnetic flux. The SQUID is biased to a level below the critical current by the current source 6 and the first resistance element R1. Therefore, when no detection signal is transmitted, no magnetic flux is generated in the SQUID, so the voltage generated at both ends of the SQUID is 0V. On the other hand, when a detection signal is transmitted and a magnetic flux is generated in the SQUID by mutual induction from the primary coil L1 to the secondary coil L2, the current flowing through the Josephson junction J2 exceeds the critical current value, and the flux quantum (SFQ: Single Flux Quantum) ) Occurs in the left and right SQUID loops (including coils L2 and L3) including Josephson junction J2. The SFQ generated in the SQUID loop including the coil L3 switches the Josephson junction J5 to disappear, and further generates the SFQ in the SQUID loop including the right coils L4 and L5. Hereinafter, the SFQ propagates in the circuit by the Josephson junction repeating the switches in a ladder manner. An SFQ propagation circuit in which the SQUID loops are cascade-connected is generally called a Josephson Transmission Line (JTL). The propagation of magnetic flux quantum always involves a Josephson junction switch, which is electrically equivalent to the propagation of a voltage pulse (SFQ pulse) having a width of 4 to 5 ps and a voltage intensity of 0.4 to 0.5 mV. It is.

  The SFQ pulse S1 output from the converter 31 is input to the output conversion circuit 35. The output conversion circuit 35 has a ring oscillator 9 that oscillates with the SFQ pulse S 1 output from the converter 31 as a trigger, and a counter 8 that counts a plurality of SFQ pulses oscillated from the ring oscillator 9. ing. The counter 8 is composed of, for example, a plurality of T flip-flops. The ring oscillator 9 is configured by connecting JTLs in a ring shape. Outputs (SFQ pulses S2 to S4) of converters (not shown) corresponding to the other SSPDs 22 to 24 are also input to the output conversion circuit 35 (the outputs S1 to S4 of a plurality of converters are one output conversion). Input to the circuit 35).

  The SFQ circuit 30 also has an arithmetic circuit (not shown) that performs a predetermined arithmetic processing by collecting the observable voltage pulse signals converted by the output conversion circuit 35 into one. The configuration of the arithmetic circuit is appropriately determined depending on an apparatus to which the superconducting single photon detection system 1 is applied. For example, in an apparatus to which the superconducting single photon detection system 1 is applied for the purpose of quantum key distribution, an OR logic circuit that merges a plurality of inputs into one output is applied as an arithmetic circuit. Further, when applied to an imaging sensor, it may be configured to include a storage unit that temporarily holds the result of the counter 8 as an arithmetic processing circuit, and a reading circuit that reads serially at a predetermined timing.

  The SFQ pulse output from the output conversion circuit 35 is input to the SQUID driver 36 for obtaining about 2 mV as the output voltage level. The SQUID driver 36 is composed of, for example, an RS flip-flop magnetically coupled to a SQUID loop in which a plurality of SQUIDs are connected in series. In the present embodiment, the output of the converter 31 is input to the set terminal S of the SQUID driver 36, and the output of the counter 8 is input to the reset terminal R. Currents are supplied from the individual current sources 6 to the converters 31 to 34, the output conversion circuit 35, and the SQUID driver 36, and each circuit is driven by these currents. The current source 6 may be a common current source.

  In the present embodiment, the output of the converter 31 is directly input to the set terminal S of the SQUID driver 36 to simplify the description, but the present invention is not limited to this. For example, the output of the signal processing circuit as described above may be input to the set terminal S.

  The SFQ pulse S1 output from the converter 31 is very weak (has a short pulse width and a small voltage level), so that it can be output as a signal that can be used in subsequent circuits and systems. The output conversion circuit 35 and the SQUID driver 36 perform amplification (both pulse width and voltage level are amplified). For example, the SFQ pulse S1 having a pulse width of 4 to 5 ps and a voltage level of about 0.4 to 0.5 mV (peak value) is amplified to a pulse width of about 1 ns and a voltage level (peak value) of about 2 mV.

  Specifically, when the SFQ pulse S1 is input to the set terminal S of the SQUID driver 36, an output signal start trigger is generated (the output signal rises). Thereafter, when a predetermined number of SFQ pulses from the ring oscillator 9 are counted in the counter 8, the output from the counter 8 is input to the reset terminal R of the SQUID driver 36, which serves as a termination trigger for the output signal (falling of the output signal). Thus, the output signal output from the SQUID driver 36 is output outside the refrigerator 5 via the coaxial cable 37 (see FIG. 1).

  As described above, the coaxial cable introduced into the refrigerator 5 is only the coaxial cable 37 that transmits the output signal of the SQUID driver 36. The number (one) of the coaxial cables 37 is not related to the number of SSPDs 21 to 24 (number of pixels). For example, if an attempt is made to realize a 10,000 pixel SSPD array in the conventional configuration, 10,000 coaxial cables will extend outside the refrigerator, but according to this embodiment, only one coaxial cable 37 is frozen. It will extend out of the machine 5. Therefore, even if the number of pixels is increased, it is possible to effectively prevent an increase in the amount of heat entering the refrigerator 5.

Second Embodiment
Next, a superconducting single photon detection system according to a second embodiment of the present invention will be described. FIG. 3 is a schematic diagram showing a schematic configuration example of a superconducting single photon detection system according to the second embodiment of the present invention. In the present embodiment, the same components as those in the first embodiment are denoted by the same reference numerals and description thereof is omitted. The superconducting single photon detection system 1B of this embodiment is different from the superconducting single photon detection system 1 of the first embodiment in that a bias current path 50 is provided from one current source 70 as shown in FIG. The bias current is configured to be supplied to the plurality of high impedance elements 91 to 94. More specifically, after the bias current path 50 is integrated into one in the refrigerator 5, the bias current path 50 extends to the outside of the refrigerator 5 and is connected to a current source 70 at room temperature.

  Accordingly, even when a plurality of SSPDs 21 to 24 are provided, one bias current path 50 is introduced into the refrigerator 5 and is connected to the high impedance elements 91 to 94 corresponding to the respective SSPDs 21 to 24 in the refrigerator 5. The route can be branched. Therefore, the inflow of heat into the refrigerator 5 can be further suppressed.

  Further, in the present embodiment, high impedance elements 91 to 94 are provided in the SSPD chip 2B. That is, the high impedance elements 91 to 94 are connected to the SSPD coaxial lines 241 to 244. In this case, the high impedance elements 91 to 94 may be configured by nanowires having a narrower line width than the superconducting nanowire detection element 7. For example, by making the line width of the high impedance elements 91 to 94 thinner than the line width of the superconducting nanowire detection elements 7 of the SSPDs 21 to 24, the critical current value of the nanowires used in the high impedance elements 91 to 94 is increased. It becomes smaller than the bias current flowing through the superconducting nanowire detection elements 7 of the SSPDs 21 to 24 via the impedance elements 91 to 94. That is, since a current exceeding the critical current always flows through the nanowires in the high impedance elements 91 to 94 by flowing a bias current, the nanowire is in a normal conduction state even at an extremely low temperature. Therefore, the nanowire can be used as a resistance element having a relatively high sheet resistance. For example, when the line width of the superconducting nanowire detection element 7 of the SSPDs 21 to 24 is 100 nm, the line width of the nanowires of the high impedance elements 91 to 94 may be about 50 nm. When the sheet resistance of such a nanowire is 500Ω (for example, an NbN thin film having a thickness of 4 mm), a nanowire having a length of 10 μm can be formed and used as a resistance element of 100 kΩ. When the nanowire is manufactured in a meander shape, a high impedance element of 100 kΩ can be realized within an area of 2 μm × 0.45 μm (2 μm length is folded five times). Thus, the high impedance elements 91 to 94 having a very small area and a high resistance component can be manufactured on the chip 2B.

  Thus, since it becomes possible to produce the high impedance elements 91-94 by the same process as the process of producing the SSPD 21-24 on the SSPD chip 2, it is possible to reduce the manufacturing cost and the number of manufacturing steps.

  Moreover, the high impedance elements 91-94 are comprised as variable resistance instead of providing the resistive elements 81-84 in 1st Embodiment.

  The SSPDs 21 to 24 are formed of nanowires (superconducting nanowire detection element 7) as described in the first embodiment. When the nanowire is formed, an error occurs in the optimum current bias value of the nanowire depending on the yield in the manufacturing process. Therefore, the bias current supplied to the SSPDs 21 to 24 may be set to a different value for each of the SSPDs 21 to 24. Therefore, the high impedance elements 91 to 94 are configured as variable resistors so that different bias currents can be set by one current source 70. Further, in this case, since the impedance of the high impedance elements 91 to 94 needs to set a bias current with its own impedance, it is preferable to have an impedance higher than that of the high impedance elements 91 to 94 in the first embodiment. For example, when the load resistance element Rr of the SFQ circuit 30 is 50Ω, the high impedance elements 91 to 94 preferably have an impedance based on about 100 kΩ.

<Third Embodiment>
Next, a superconducting single photon detection system according to a third embodiment of the present invention will be described. FIG. 4 is a schematic diagram showing a schematic configuration example of a superconducting single photon detection system according to the third embodiment of the present invention. In the present embodiment, the same components as those of the second embodiment are denoted by the same reference numerals and description thereof is omitted. The superconducting single photon detection system 1C of this embodiment is different from the superconducting single photon detection system 1B of the second embodiment, as shown in FIG. 4, in which the SSPD array 20, the SFQ circuit 30, and the high impedance element 91 are used. ˜94 are provided on one chip 2C. For this reason, the SSPDs 21 to 24 and the converters 31 to 34 of the SFQ circuit 30 are connected only by the coaxial lines 241 to 244 formed on the chip 2C. As a result, the entire superconducting single photon detection system 1C is formed on one chip 2C, and a coaxial cable between the SSPDs 21 to 24 and the SFQ circuit 30 is not required, thereby making the manufacturing process more efficient and simplified. Can be In addition, the superconducting single photon detection system itself can be further miniaturized.

<Fourth embodiment>
Next, a superconducting single photon detection system according to a fourth embodiment of the invention will be described. FIG. 5 is a schematic diagram showing a schematic configuration example of a superconducting single photon detection system according to the fourth embodiment of the present invention. In the present embodiment, the same components as those in the third embodiment are denoted by the same reference numerals and description thereof is omitted. The superconducting single photon detection system 1D of this embodiment is different from the superconducting single photon detection system 1C of the third embodiment as shown in FIG. A variable resistance element 91 </ b> D to 94 </ b> D for setting a bias current is provided separately from the same configuration and the same reference numeral. That is, the high impedance elements 61 to 64 have an impedance of about 5 kΩ as in the first embodiment, and the variable resistance elements 91D to 94D have an impedance based on about 100 kΩ.

  Furthermore, in this embodiment, the variable resistance elements 91 </ b> D to 94 </ b> D are provided in the refrigerator 5. The SSPD array 20, the SFQ circuit 30, and the high impedance elements 61 to 64 are provided on one chip 2D, and the variable resistance elements 91D to 94D are provided separately from the chip 2D.

  According to the configuration of the present embodiment, the high impedance elements 61 to 64 are arranged on the same chip 2D as the SSPD array 20 and / or the SFQ circuit 30 as elements having a relatively low impedance of about 5 kΩ as in the first embodiment. Once formed, the variable resistance elements 91D to 94D having a relatively high impedance for setting the bias current can be provided as separate chips or separate circuit configurations, and the miniaturization of the chip 2D can be realized more easily. can do. Also in this embodiment, in order to generate a bias current corresponding to a plurality of SSPDs 21 to 24 from one current source 70, the superconducting nanowire detection of each SSPD 21 to 24 is performed by making the resistance elements 91D to 94D variable. The optimum current value of the element 7 can be set appropriately.

  In the present embodiment, the variable resistance elements 91 </ b> D to 94 </ b> D are provided in the refrigerator 5, but may be provided outside the refrigerator 5.

  Although the embodiments of the present invention have been described above, the present invention is not limited to the above-described embodiments, and various improvements, changes, and modifications can be made without departing from the spirit of the present invention. For example, the components in the plurality of embodiments may be arbitrarily combined. Specifically, the high impedance element in the first embodiment, the third embodiment, and the fourth embodiment may be configured using the high impedance element made of nanowires in the second embodiment. On the other hand, the high impedance element according to the second embodiment may be configured as a resistance element or inductor made of a general metal thin film.

  In the above embodiment, the four SSPDs 21 to 24 are described based on the SSPD array 20 formed on one chip. However, the present invention is not limited to this as long as one or more SSPDs are provided. Absent. Further, when a plurality of SSPDs are provided, the plurality of SSPDs may be formed on a plurality of chips. The configuration of the SFQ circuit 30 is not limited to the configuration described in the above embodiment, and various improvements, changes, and modifications are made according to the apparatus or system to which the superconducting single photon detection system of the present invention is applied. Configuration is applicable.

  A superconducting single photon detection system according to the present invention was fabricated, and performance evaluation of photon detection efficiency, dark count, and timing jitter in this system was performed. In this example, two SSPDs were used with the same configuration as in the first embodiment. In this embodiment, instead of using an SSPD array including two SSPDs, only two SSPDs are mounted and the two SSPDs are operated simultaneously. As a performance evaluation method, photon detection efficiency, dark count, and jitter are measured from the SSPD output, while photon detection efficiency, dark count, and jitter are measured from the output of the SFQ circuit connected to the SSPD via a coaxial cable. The two were compared. Thereby, it was verified whether or not each index of the output of the SFQ circuit was deteriorated with respect to the output of only the SSPD.

  FIG. 6 is a graph showing measurement results of changes in photon detection efficiency and dark count with respect to the SSPD bias current in the example of the present invention. FIG. 7 is a graph showing measurement results for jitter in the example of the present invention. As shown in FIG. 6, the photon detection efficiency and the dark count at the output of the SFQ circuit in the present embodiment hardly change in characteristics with respect to the output of only the SSPD. Thus, it can be seen that the signal processing can be executed effectively without causing signal degradation by the connection method between the SSPD-SFQ circuits in this embodiment.

  Further, as shown in FIG. 7, as a result of measuring jitter with only SSPD, the half width was about 40 ps. In contrast, the jitter at the output of the SFQ circuit has a half width of about 50 ps. Thus, although a slight increase in jitter was seen compared to the output of only SSPD, it was suppressed to an increase of about 10 ps, and a sufficiently practical jitter value was obtained.

  The superconducting single photon detection system and superconducting single photon detection method of the present invention are useful for miniaturizing the system while maintaining a very low temperature and obtaining high photon detection accuracy.

1,1B, 1C, 1D Superconducting single photon detection system 2,2B SSPD chip 2C, 2D chip 3 SFQ chip 4 Photon source 5 Refrigerator 6,70-74 Current source 7 Superconducting nanowire detector 8 Counter 9 Ring oscillator 20 SSPD array 21 SSPD (superconducting single photon detector)
30 SFQ circuit (superconducting single flux quantum circuit)
31 converter 35 output conversion circuit 36 SQUID driver 37, 41 to 44 coaxial cable (AC signal transmission path)
50 to 54 Bias current path 61 to 64, 91 to 94 High impedance element 81 to 84 Resistance element 91D to 94D Variable resistance element 241 to 244 SSPD coaxial line (AC signal transmission path)
341 to 344 SFQ coaxial line (AC signal transmission path)
J1 to J5 Josephson junction L1 primary coil L2 secondary coil L3 to L5 coil R1, R2 resistance element Rr load resistance element

Claims (6)

A superconducting single photon detector operated by a bias current;
A superconducting single flux quantum circuit that converts a detection signal output from the superconducting single photon detector into a single flux quantum signal using a converter having a load resistance element and performs arithmetic processing, and
The superconducting single photon detector and the superconducting single flux quantum circuit are connected by an AC signal transmission path without a capacitor ,
A bias current path for supplying the bias current to the superconducting single photon detector is connected to the AC signal transmission path via a high impedance element,
The high-impedance element is a superconducting single photon detection system in which an impedance at a high frequency is higher than an impedance of the load resistance element of the superconducting single flux quantum circuit.   The superconducting single-photon detection system according to claim 1, wherein the superconducting single-photon detector, the superconducting single-flux quantum circuit, the AC signal transmission path, and the high-impedance element are mounted in a refrigerator. . A plurality of said superconducting single photon detectors;
A plurality of superconducting single flux quantum circuits and a plurality of alternating current signal transmission paths provided corresponding to the plurality of superconducting single photon detectors;
The superconducting single photon detection system according to claim 1, further comprising a plurality of the high impedance elements provided corresponding to the plurality of AC signal transmission paths.   The superconducting single photon detection system of claim 3, wherein the bias current path is configured to supply the bias current from a current source to the plurality of high impedance elements. The superconducting single photon detector comprises a superconducting nanowire detection element that detects a single photon,
The superconducting single photon detection system according to claim 1, wherein the high impedance element is configured by a nanowire having a narrower line width than the superconducting nanowire detection element. A detection step of detecting a single photon by supplying a bias current to the superconducting single photon detector through a bias current path;
And outputs the detected single photon as a detection signal, the superconducting single-photon detector and the superconducting single through AC signal transmission path between are connected not through the capacitor between the superconducting single flux quantum circuits A processing step of performing an arithmetic process after being transmitted to a single flux quantum circuit and converted into a single flux quantum signal using a transducer having a load resistance element,
The bias current is supplied to the superconducting single photon detector through a bias current path connected via a high impedance element connected to the AC signal transmission path,
The high-impedance element is a superconducting single photon detection method, wherein an impedance at a high frequency is higher than an impedance of the load resistance element of the superconducting single flux quantum circuit.

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