Physical quantity measurement device, physical quantity measurement method, and quantum computer

Abstract

The present invention addresses the problem of providing: a high-speed Coulomb blockade thermometer that can reduce the effects of self-heating and achieve a high S / N; and a quantum computer that uses the Coulomb blockade thermometer.　A preferred embodiment of the present invention is a quantum computer that comprises a quantum bit array chip that forms semiconductor quantum dots, a cooling device that cools the quantum bit array chip, a Coulomb blockade thermometer that is provided inside the cooling device, a temperature control unit that controls a heat source that is inside the cooling device, and a storage unit that stores a conversion table for converting the conductance of the Coulomb blockade thermometer to temperature. The temperature control unit applies a constant voltage to the Coulomb blockade thermometer to measure the conductance, references the conversion table to convert the measured conductance to temperature information, and controls the heat source on the basis of the temperature information.

Description

Physical quantity measuring device, physical quantity measuring method, and quantum computer

[0001] The present invention relates to a device for measuring a physical quantity at low temperatures, a method for measuring a physical quantity, and a quantum computer.

[0002] Quantum computers are believed to be capable of faster information processing for certain problems than existing computers. While existing computers handle only the binary values ​​0 and 1, quantum computers can handle superpositions of these two values.

[0003] To handle superposition states, quantum computers require elements that realize qubits. Qubits can be realized using superconducting elements, cooled atoms, photons, and quantum dots made from semiconductor elements. The basic operations of a quantum computer include initialization, calculation, and readout. Further basic operations include single-qubit gates and two-qubit gates, and it is known that universal quantum computing can be realized by combining these.

[0004] In quantum computers that use electrons (or holes; hereinafter, electrons will be used as an example) as quantum bits, the temperature of the electrons is directly linked to calculation errors, so devices are used that are cooled to low temperatures, for example, 1 K or less. Furthermore, since an increase in electron temperature during calculation can cause calculation errors, the electron temperature must be measured in a low-temperature environment, but a fast and highly accurate method for measuring the electron temperature is an issue. Patent Documents 1 to 3 disclose methods for measuring the electron temperature.

[0005] Special Publication No. 2004-506886 US2022 / 0412812A1EP3477272A1

[0006] Coulomb blockade thermometers are used to measure electron temperatures in extremely low temperature environments. A Coulomb blockade thermometer can be constructed, for example, by connecting a number of metal islands in a row with a thin barrier layer. A tunnel junction is formed between the metals, and when a voltage is applied, electrons pass through the junction. The tunneling frequency, or conductivity, depends on the temperature of the system and the electrostatic energy of the islands. This principle is used to measure the temperature of the system.

[0007] Conventional Coulomb blockade thermometers have the problem of slow measurement speed because the voltage sweep takes time, as the electron temperature is estimated from the conductance waveform obtained by sweeping the voltage.

[0008] In Patent Document 1, electron temperature is measured by measuring the third derivative of the current-voltage characteristics in a Coulomb blockade thermometer. While this method achieves faster temperature measurement than conventional methods, it requires a complex measurement system due to the use of a lock-in amplifier, and requires AC signal drive, which can lead to self-heating. Furthermore, there is also the problem of a low S / N ratio due to the detection of third harmonics.

[0009] In Patent Documents 2 and 3, the electron temperature is measured at high speed in a Coulomb blockade thermometer by measuring the conductance using a radio frequency (RF) wave. However, the RF signal input causes the problem of self-heating, and the gate voltage is swept, which limits how quickly the measurement can be performed.

[0010] Quantum computers must operate continuously and stably. However, quantum chips placed in the cryogenic environment of a dilution refrigerator are subject to heat inflow from various sources, including heat flowing in from wiring, heat from peripheral circuits and ambient thermal radiation, and heat due to the loss of control signals such as RF pulses. This heat can increase the electron temperature and cause calculation errors. Therefore, it is desirable to monitor the electron temperature. To achieve this, real-time electron temperature measurement is required, but as mentioned above, electron temperature measurement using a Coulomb blockade thermometer faces challenges such as speed, self-heating, and signal-to-noise ratio.

[0011] An object of the present invention is to realize a Coulomb blockade thermometer that can reduce the influence of self-heating at high speed and achieve a high S / N ratio, and a quantum computer using the same.

[0012] A preferred aspect of the present invention is a physical quantity measuring device comprising: a monitor unit that measures a physical quantity of a measurement target; electrodes that apply a voltage to the monitor unit; and a storage unit that has a conversion data table that associates an output result of the monitor unit with the physical quantity when a predetermined voltage is applied to the monitor unit, and that measures the physical quantity by comparing the output result with data stored in the conversion data table.

[0013] Another preferred aspect of the present invention is a physical quantity measurement method, comprising: applying a constant voltage to a Coulomb blockade thermometer; obtaining an output signal from the Coulomb blockade thermometer; and converting the output signal into a temperature using a conversion data table that associates the output signal with a temperature.

[0014] Another preferred aspect of the present invention is a quantum computer comprising: a quantum bit array chip comprising semiconductor quantum dots; a cooling device for cooling the quantum bit array chip; a Coulomb blockade thermometer disposed within the cooling device; a temperature control unit for controlling a heat source within the cooling device; and a memory unit for storing a conversion table for converting the conductance of the Coulomb blockade thermometer into temperature, wherein the temperature control unit applies a constant voltage to the Coulomb blockade thermometer to measure the conductance, converts the measured conductance into temperature information by referring to the conversion table, and controls the heat source based on the temperature information.

[0015] It is possible to realize a Coulomb blockade thermometer that can reduce the influence of self-heating at high speed and achieve a high S / N ratio, and a quantum computer using the same.

[0016] 1 is a schematic block diagram of a quantum computer system; 2 is a schematic diagram of a Coulomb Blockade Thermometer (CBT); 3 is a graph showing the change in conductance G in a SET; 4 is a graph showing the change in normalized conductance G; 5 is a graph showing the derivative of the change in conductance G; 6 is a schematic diagram of a temperature measurement system according to an embodiment; 7 is a schematic diagram of a data table according to an embodiment; 8 is a block diagram of a configuration for calibration according to an embodiment; 9 is a flow chart of a calibration method; 10 is a graph showing V for multiple values ​​of Q; ｇA graph of the T-G curve plot. A table showing the relationship between Q and T. A graph showing the T-G relationship obtained in the example. A schematic diagram of the device used in the demonstration experiment for real-time CBT calibration. A graph showing the measurement results of the heat generation amount Q and temperature T. V of the conductance G for each heat generation amount Q of the gate ｇ Graph showing the dependence and measured T results for multiple V for each temperature T of the gate. ｇ 5B. Schematic block diagram of an operation control method using real-time CBT. Explanatory diagram showing an example of operation control by real-time feedback. Graph showing temperature distribution on line A-A' in FIG. 5B. Schematic diagram of a multi-core multi-chip quantum computer. Cross-sectional view of quantum dots for forming a quantum dot array. Plan view showing a two-dimensional quantum dot array in which the quantum dot array is expanded to a two-dimensional structure. Plan view showing an example of a user interface during calibration of CBT. Plan view showing an example of a user interface during temperature monitoring. Graph showing the peak shape of conductance G by CBT of Example 2. Graph showing the derivative with respect to temperature T of Equation 3. Block diagram of a quantum computer of an embodiment.

[0017] The following describes the embodiments in detail with reference to the drawings. However, the present invention should not be construed as being limited to the description of the embodiments shown below. Those skilled in the art will readily understand that the specific configurations of the present invention can be modified within the scope of the concept and purpose of the present invention.

[0018] In the configuration of the invention described below, the same parts or parts with similar functions are denoted by the same reference numerals in different drawings, and redundant explanations may be omitted. When there are multiple elements with the same or similar functions, they may be described with different subscripts. However, in some cases, the subscripts may be omitted.

[0019] The terms "first," "second," "third," and the like used in this specification are used to identify components and do not necessarily limit the number, order, or content thereof. Furthermore, numbers used to identify components are used in different contexts, and numbers used in one context do not necessarily indicate the same configuration in another context. Furthermore, a component identified by a certain number does not preclude consideration of the function of a component identified by another number.

[0020] In order to facilitate understanding of the invention, the position, size, shape, range, etc. of each component shown in the drawings etc. may not represent the actual position, size, shape, range, etc. Therefore, the present invention is not necessarily limited to the position, size, shape, range, etc. disclosed in the drawings etc.

[0021] One of the embodiments described below relates to a method for measuring ambient temperature using electrons (or holes) confined in quantum dots.

[0022] [Quantum Computer System] In this example, a quantum computer system using electron temperature measurement by a high-speed Coulomb Blockade Thermometry (CBT) will be described.

[0023] A schematic diagram of a quantum computer system is shown in Figure 1. Quantum computer 10 cools a quantum bit array chip 120 using a refrigerator 100, such as a dilution refrigerator, that can maintain an extremely low temperature environment of 1 K or less, and performs quantum computations by communicating control signals to this chip through wiring.

[0024] In addition to the qubit array chip 120, a control chip 130 for controlling it may also be placed inside the refrigerator 100. Implementing the control chip 130 inside the refrigerator offers benefits such as low-latency control, real-time feedback operation in quantum computing, low-noise signal generation, and low-noise signal detection. A magnet 150 for generating electron spin qubits is also installed inside the refrigerator 100. In this system, quantum computing can proceed by communication between a control unit 110 outside the refrigerator and the control chip 130 and qubit array chip 120 inside the refrigerator.

[0025] In this quantum computer system, maintaining the quantum bit array chip 120 at an extremely low temperature of 1 K or less is essential for stable execution of quantum computation with low errors. However, temperature rise in the quantum bit array chip 120 becomes a problem due to heat flowing in from the control chip 130 and wiring, heat from peripheral circuits within the chip and ambient thermal radiation, heat due to loss of control signals such as RF pulses, and the like.

[0026] Therefore, it is necessary for the control unit 110 to grasp the situation and control the quantum computing operation by measuring and monitoring the temperature at each position in the refrigerator using temperature sensors 140. In particular, it is desirable to monitor the temperatures around the control chip 130, which is the heat source, and around and inside the quantum bit array chip 120, which is the key to quantum computing.

[0027] These temperature measurement data are transmitted to the control unit 110 in real time. If an abnormality is detected, the quantum computing must be immediately stopped, the heat source must be identified, operation must be stopped, the computation schedule must be adjusted, and other control and countermeasures must be taken. Therefore, real-time temperature measurement is necessary. However, existing temperature sensors have difficulty measuring the temperature of the quantum bit array chip 120, particularly the temperature of the electrons (electron temperature) that constitute the quantum bits, with high speed and precision. [Electron Temperature Measurement Method] Figure 2A shows a schematic diagram of the Coulomb Blockade Thermometer (CBT) used in this example. Two potential barriers 203 and a gate electrode 201 are located between two electrodes, the source 204 and the drain 205, and a single-electron transistor (SET) structure, which functions as a quantum dot 202, is located between them. In other words, the semiconductor quantum dots of a quantum computer can be used as a CBT. Arrow 209 indicates the flow of electrons.

[0028] 2B shows a schematic diagram of the conductance G of the SET. The conductance G of the SET is determined by the Coulomb blockade phenomenon, which is a function of the gate voltage V applied to the gate electrode 201. ｇ In other words, when the bias voltage between the source and drain is fixed, the current value changes as V ｇ A peak of current flows with respect to the sweep of the voltage V. This peak is called a Coulomb peak, and the peak shape is described by Equation 1 or Equation 2.

[0029] ...(Formula 1)

[0030] ...(Formula 2)

[0031] where the electron temperature T of the quantum dot and the Boltzmann constant k Ｂ , a constant α determined by the structure of the SET, and an elementary charge e are introduced, and V ｇ The voltage value at the peak center is set to 0.

[0032] Equation 1 is based on the quantum level spacing of the quantum dot. ＢThis holds true when T is large, and can be used at relatively high temperatures called the classical regime. On the other hand, Equation 2 is true when k is larger than the quantum level spacing of the quantum dot. Ｂ This holds true when T is small, and can be used at relatively low temperatures known as the quantum regime. Equation 1 can also be approximately rewritten into a form similar to Equation 2 using the cosh function.

[0033] From these equations, it can be seen that the peak shape depends on the electron temperature T, as shown by the curves 210 at high temperature indicated by the dotted line and 211 at low temperature indicated by the solid line in Figure 2B. Therefore, by measuring the Coulomb peak shape and half-width, the electron temperature in the quantum dot 202 can be measured, and this method is called Coulomb Blockade Thermometry (CBT).

[0034] In conventional CBTs, V ｇ The temperature was calculated by sweeping the V ｇ There was a delay in temperature measurement due to the sweep time.

[0035] On the other hand, in this embodiment, V ｇ is fixed at point A in FIG. 2B, and the conductance G is monitored by measuring the current, for example. ｇ At point A, the conductance G differs between high and low temperatures, and the current value flowing through the quantum dot changes. Therefore, the electron temperature can be detected by the change in the current value.

[0036] Conductance G = I / V from the measured current value I ｄ (V ｄ : source-drain bias voltage) is known, and theoretically, T can be estimated using Equation 1 and Equation 2. In this way, the method of this embodiment for performing CBT in real time is hereinafter referred to as real-time CBT. Here, the gate voltage V ｇ It is desirable to fix it to

[0037] 2C shows the waveform of conductance G normalized by its maximum value. To consider the temperature measurement sensitivity, the waveforms normalized by their maximum values ​​for the approximation of Equation 1 (classical-ap) and Equation 2 (quantum) are shown as graphs of classical-ap2001 and quantum2002. Here, α = 0.1 and T = 1K.

[0038] Figure 2D also shows the waveforms obtained by differentiating the waveforms of Equation 1 and Equation 2 with respect to T. The graph shows the classical-ap differential 2011 and the quantum differential 2012. The absolute value of this waveform represents the magnitude of the temperature measurement sensitivity. Here, α = 0.1 and T = 1K. In this waveform, V at points B and C ｇ These are the points where the sensitivity is high in both the quantum and classical regions. In the quantum region, the sensitivity is also high at point D. By setting the voltage values ​​at these points, highly sensitive temperature measurement can be performed. Note that the highly sensitive points B and C change depending on the temperature, so highly sensitive temperature measurement can be performed by setting them appropriately according to the temperature region being used.

[0039] In actual devices, due to complex factors, the conductance may not necessarily follow Equation 1 or Equation 2. Therefore, it is effective to obtain the relationship between the current value or conductance and the temperature in advance by performing the calibration described below. Furthermore, the conductance can be measured not only by the current value but also by a method using the reflection characteristics of radio frequency electromagnetic waves, as described in Patent Documents 2 and 3, for example.

[0040] 2E shows a schematic diagram of a temperature measurement system according to an embodiment of the method described above. A CBT thermal monitor 221 is placed around or inside a quantum bit array 222, and the conductance G of the SET is acquired by a method such as measuring a current value. Based on the acquired value of conductance G, a data table 223 is referenced, which holds a relationship between a pre-created current value I and temperature T, to calculate temperature T. This data table 223 may be a table created based on, for example, Equation 1 or Equation 2, or may be a table created by calibration, which will be described later.

[0041] This allows time-series temperature information at multiple locations within the refrigerator to be acquired in real time. In other words, spatial and temporal temperature distribution information can be obtained. By processing this temperature information, the location of a problematic heat source, such as the control circuit 220, can be identified and control can be performed to suppress heat generation. In this way, high-speed real-time CBT can be achieved using the data table 223. The obtained spatial and temporal temperature distribution information can be processed and fed back to the operation, allowing stable quantum computing to be performed. [Calibration Method] FIG. 3A shows a schematic diagram of the data table 223. The data table 223 stores multiple pieces of information on real-time CBT, such as conductance G, current value, and RF wave reflection characteristics (intensity, phase), as well as corresponding temperature information. As an example, the data table 223 in FIG. 3A records temperature T versus conductance G of the SET, which is the output of the CBT.

[0042] This data table can be created based on Equation 1 or 2 for devices that behave close to ideal, or by performing the calibration shown below for devices that behave non-ideally due to complex factors.

[0043] We present a calibration method for devices with non-ideal behavior.

[0044] 3B shows a configuration diagram for calibration. A CBT 303 and a controllable heat source 304 are located inside a refrigerator 301, and each can communicate with a control unit 302. The control unit 302 is connected to a storage unit 300 that can store data such as measurement results.

[0045] For example, when a dilution refrigerator is used as the refrigerator 301, the heat source 304 may be an element that is installed near a mixing chamber plate or a chip and generates Joule heat by passing a current through it. Alternatively, it may be made using wiring resistance within the same chip as the chip on which the CBT 303 is located. The heat source 304 has a known and controllable heat generation amount, and by controlling the heat generation amount, the temperature at the CBT 303 position can be controlled. The CBT 303 is Vｇ "Monitor mode" to fix the ｇ The control unit 302 can switch between at least two different operations in a "calibration mode" that sweeps the signal. The above configuration can be implemented in a quantum computer, which will be described later with reference to FIG.

[0046] The flow of the calibration method is shown in FIG. ｇ The control unit 302 sends a command to set the mode to "calibration mode" in which the laser beam is swept, and switches the mode (s3001).

[0047] Next, the value of the heat generation amount Q of the heat source 304 is set, and heat generation starts (s3002, s3003). After that, after waiting until a state of approximate thermal equilibrium is reached, V ｇ is swept to obtain the conductance G (s3004), and the resulting data is stored in the storage unit 300 (s3005).

[0048] When this measurement is completed, the heat generation is stopped (s3006). The above procedure is repeated multiple times while changing the heat generation amount Q within the measurement range (s3007). Instead of the heat source 304, a temperature increase by controlling the amount of cooling may be used.

[0049] FIG. 3D shows V for multiple values ​​of Q obtained by repeating steps s3002 to s3006. ｇ -G curve plot. After completing the measurement range of the predetermined Q, ｇ From the peak shape of the -G curve plot, the electron temperature T is determined for each value of Q by fitting the formula 1 or 2 or calculating the half-width (s3008). Here, α in formulas 1 and 2 is determined in advance by a method such as Coulomb diamond measurement in CBT.

[0050] FIG. 3E shows the relationship table between Q and T obtained as a result of step s3008 and stored in the storage unit 300.

[0051] Next, V ｇ As shown in Figure 3D (i), (ii), and (iii), multiple V ｇ Set each V ｇThe relationship between the heat generation amount Q and the conductance G is found. Then, Q is converted to T using the Q-T relationship shown in FIG. 3E.

[0052] FIG. 3F shows the V of (i), (ii), and (iii) obtained by converting Q to T. ｇ The relationship between T and G is shown. From the slope of the relationship between T and G, ｇ The sensitivity of G to T can be seen in the figure. The larger the value of this conductance G, the higher the S / N ratio of a temperature monitor can be realized. The electron temperature at which the occurrence rate of calculation errors increases is set as the temperature threshold 310, and the sensitivity or S / N ratio V is set to be the highest near this temperature threshold 310. ｇ The maximum sensitivity point V ｇ opt Here, taking into consideration both the sensitivity and S / N, the optimum V ｇ You may decide.

[0053] After returning the CBT to the monitor mode (s3010), the V calculated above is ｇ opt V ｇ (s3011), and V ｇ opt A data table 223 is created from the T-G relationship (FIG. 3F) in the above and stored in the storage unit (s3012). Here, the data table 223 may be created by interpolating the measurement results of the T-G relationship using a smooth function such as a polynomial. This completes the calibration.

[0054] In this way, at a predetermined temperature, which is typically a threshold value, each V ｇ The voltage to be applied to the Coulomb blockade thermometer is determined based on at least one of the gradient of the graph of the T-G relationship data in 2 and the magnitude of the output signal. Then, data showing the relationship between the output signal and the temperature for the determined voltage is stored as a data table 223.

[0055] Due to the influence of changes over time in the device and complex changes in the surrounding environment, the accuracy of the real-time CBT data table 223 may not be maintained after calibration. Therefore, by performing calibration at an appropriate frequency, it is possible to reduce the deterioration of the accuracy of the data table due to the influence of changes over time and the resulting temperature measurement errors, and perform stable temperature monitoring. This calibration is performed, for example, on V ｇ By adjusting Q in detail, it is possible to increase the accuracy, or by adjusting Q roughly, it is possible to increase the speed. ｇ and Q are precisely adjusted to perform highly accurate calibration, and V ｇ By roughly varying Q and finely correcting the data table at high speed, it is possible to efficiently maintain a highly accurate data table 223 and reduce temperature measurement errors.

[0056] The above calibration used temperatures measured by CBT, but temperature sensors such as commercially available ruthenium oxide resistors, platinum resistors, Cernox resistors, germanium resistors, thermocouples, and silicon diodes may also be used in combination. Such temperature sensors are approximately millimeters in size, making it difficult to measure the temperature of electrons like CBTs. However, they are capable of temperature measurement with a simple configuration, making it possible to measure the temperature of the substrate on which the chip is mounted, the metal plate inside the refrigerator, and the like. By appropriately using and integrating these multiple temperature sensors, calibration can be made more efficient. [Calibration Demonstration Experiment] A demonstration experiment was conducted to demonstrate the feasibility of the above calibration.

[0057] Figure 4A shows a schematic diagram of the device used in the demonstration experiment for real-time CBT calibration. This device has a structure in which multiple polysilicon gate electrodes are formed on a T-shaped Si channel formed by SOI (Silicon on Insulator).

[0058] A total of four gate electrodes, gate 400 to gate 403, cross an Si channel 410, with both sides acting as terminals allowing current 411 to flow. In addition, three gates, gate 404 to gate 406, form an SET structure on the channel extending vertically, forming a CBT 412, which allows electron temperature measurement. In this demonstration experiment, current is passed through each of the four gates, gate 400 to gate 403, to serve as a heat source, and temperature is measured by the CBT 412.

[0059] Figure 4B shows the measurement results of temperature T. It can be seen that increasing the amount of heat generated Q for each gate increases the measured electron temperature T. It can also be seen that the closer the gate is to the CBT, the higher the temperature becomes. This indicates that the thermal resistance between the heat source and the CBT depends on the distance.

[0060] FIG. 4C shows an example of the V of conductance G for each heat generation amount Q of a gate. ｇ The graph shows the dependence of the conductance G on the temperature and the measured T. It can be seen that increasing the calorific value Q increases the maximum value of the conductance G and the width of the peak also tends to broaden. The peak shape of G changes depending on the temperature, and the temperature is measured by fitting the peak shape of G using Equation 1.

[0061] FIG. 4D shows a graph of multiple V ｇ The conductance G at each V ｇ The temperature dependence of the conductance G is different in V ｇ V ｇ opt Specifically, the value of conductance G is set to V ｇ is selected as having a high S / N ratio, and the one with a steeper gradient in the T-G relationship is selected as having a high sensitivity of G to T. In this example, V ｇ It is considered that V = 2.10 can be selected. ｇ opt5A shows a schematic diagram of an operation control method using real-time CBT. Chip 506 placed in low-temperature environment 507 includes a quantum bit array and peripheral circuits, and is connected to voltage control unit 503, RF control unit 504, and conductance measurement unit 502 for initializing, manipulating, and reading out the quantum bits. Alternatively, a quantum bit control chip may be placed separately in the low-temperature environment.

[0062] The conductance measurement unit 502 measures the conductance in the CBT in real time, and the processing unit 505 calculates the electron temperature from the results and the information in the data table created by the above calibration and stored in the data table storage unit 501.

[0063] This temperature information can be displayed on, for example, an electronic temperature display unit 508, allowing the user to check it in real time. If the measured temperature exceeds the temperature threshold 310, which may cause a calculation error, the alert information is fed back (FB) to the control unit 500, and the control unit 500 identifies the problematic heat source and suppresses the heat generation by restricting operation, etc.

[0064] In this way, real-time feedback of temperature information provides a mechanism for preventing calculation errors. In particular, by monitoring the temperature close to the heat source, it is possible to control heat generation before the heat flow reaches the quantum bit array, thereby efficiently preventing calculation errors.

[0065] 5B shows an example of operational control using real-time feedback. A control circuit 513 is located around a quantum bit array 511, and a thermal monitor 512 is installed between them. Here, the thermal monitor 512 refers to a module that has a real-time CBT, measures temperature T in real time, and can communicate the measurement results.

[0066] In addition, a heat sink path 514 made of a metal such as aluminum or copper, which has low thermal resistance, is installed in the control circuit 513 for cooling, and heat flow 515 is discharged from the control circuit 513. Furthermore, SiO, which is a material with high thermal resistance, is installed between the thermal monitor 512 and the quantum bit array 511.２ A heat insulating structure 516 made of an insulator such as an insulating material is provided.

[0067] When unexpected heat generation (abnormal heat generation) occurs in control circuit 513, the temperature at the position of thermal monitor 512 rises, and signs of abnormal heat generation can be obtained by measuring the temperature in real time with thermal monitor 512. The measured temperature T is transmitted in real time to feedback circuit 517, and when such signs appear, feedback circuit 517 instantly issues command C to suppress the operation of the control circuit, thereby suppressing the amount of heat generation from control circuit 513. This makes it possible to prevent a temperature rise in quantum bit array 511.

[0068] Figure 5C shows the temperature distribution along line A-A' in Figure 5B. Due to thermal insulation structure 516, the temperature at the position of quantum bit array 511 is lower than that at the position of thermal monitor 512. As a result, for example, if the temperature at the position of thermal monitor 512 is 1 K, the temperature at quantum bit array 511 will be about 0.1 K. Since CBT has the problem that measurement accuracy decreases if the temperature is too low, this problem is overcome by placing the position of thermal monitor 512 close to the heat source and installing thermal insulation structure 516 between the position of thermal monitor 512 and the position of quantum bit array 511.

[0069] Furthermore, by increasing the thermal resistance between the quantum bit array 511 and the heat-generating component using the thermal insulation structure 516, the time constant of heat conduction between them can be increased. This allows real-time feedback control of the operation before the temperature rise due to abnormal heat generation in the control circuit is transmitted, thereby preventing quantum computing errors associated with temperature rise in the quantum bit array. Even when abnormal heat generation is severe and preventing quantum computing errors is difficult, the time of heat generation can be determined in real time, allowing the calculation results to be discarded and restarted, enabling efficient quantum computing with low error rates. [Multi-Core Multi-Chip] Figure 5D shows a schematic diagram of a multi-core multi-chip quantum computer. If the component including the quantum bit array and peripheral circuitry, in which quantum bits are quantum-coupled, is referred to as a core 520, having multiple cores 520 and performing calculations using these cores 520 may enable efficient execution of quantum algorithms. Alternatively, executing multiple tasks in parallel can shorten the effective computation time. For this reason, multi-core quantum chips with multiple cores 520 on a single chip 521 are expected.

[0070] Furthermore, a multi-chip quantum computer 522 with multiple multi-core quantum chips is expected to increase the parallelism of calculations and enhance integration. These multi-core, multi-chip quantum computers also require more complex control peripheral circuits, raising concerns about increased heat generation. Therefore, by placing thermal monitors on each core, chip, etc. and acquiring the temperature distribution of each core and chip, device temperature control (thermal management) can be performed, reducing calculation errors. For example, chips and cores close to heat sources are prone to calculation errors, so optimization can be performed, such as by adjusting the calculation schedule to minimize the impact of heat. [Temperature Measurement within a Quantum Dot Array] Next, we will describe a thermal monitoring method using a quantum bit array within one core 520. In quantum computers using semiconductor quantum dots such as silicon, one electron is stored in each quantum dot, and the spin state of this electron is used as a quantum bit to perform quantum calculations. An array of quantum dots is called a quantum dot array, and a quantum bit array 601 is created by storing multiple electrons in this quantum dot array to realize an array of quantum bits. As shown in Fig. 2A, because the CBT is based on quantum dots, it can be constructed anywhere on the quantum dot array. This allows temperature information within the quantum bit array 601 to be obtained. Fig. 6A shows the cross-sectional structure of quantum dots 622 used to form the quantum bit array 601. The quantum dots 622 represent an electromagnetic field structure that can hold one or more electrons 624 by controlling the potential 623 using an electromagnetic field, and each quantum dot 622 is separated by a potential barrier 621.

[0071] The quantum bit array 601 is mounted in a chip (quantum bit array chip 120 in FIG. 1) that is kept at an extremely low temperature of several mK to several K. Fig. 6A shows a structure in which gate electrodes 600 based on a semiconductor MOS (Metal-Oxide-Semiconductor) structure are arranged one-dimensionally, and a schematic diagram of potential 623 inside silicon channel 620 with oxide film 610 sandwiched between each gate electrode.

[0072] If the gate electrodes 600 are numbered 1 to 9 from left to right, quantum dots 622 are located directly below odd-numbered gate electrodes, and potential barriers 621 are located directly below even-numbered gate electrodes. In this figure, one electron 624 is held in each quantum dot 622, but in some cases, the number may be zero or more. The quantum dots 622 are formed so that they can hold one or more electrons. By using any quantum dot 622 to form a CBT and lowering the potential barrier 621 so that the other quantum dots become a current path, as shown by the dotted line potential barrier 625, it is possible to measure the temperature at the position of any quantum dot.

[0073] 6B is a plan view showing a two-dimensional quantum dot array 634, which is a two-dimensional extension of the quantum bit array 601 shown in FIG. 6A. In this two-dimensional quantum dot array 634, a quantum dot 633 at an arbitrary position is designated as a CBT 631, and a current path 630 is realized by lowering the potential barrier so that the current passes through the quantum dot. By measuring the current of the CBT 631 through the current path 630, the temperature T can be measured in real time, as described above.

[0074] At this time, since electrons cannot be held in the quantum dot that forms the current path 330, if a quantum bit 632 is present, the electrons that make up the quantum bit may be moved by shuttling or the like to a quantum dot 633 other than the current path 330. This makes it possible to measure the temperature at any quantum dot 633 position.

[0075] In this embodiment, a plurality of semiconductor quantum dots constitute a two-dimensional quantum dot array, and each semiconductor quantum dot is realized by a transistor having a source, a drain, and a gate. At least one semiconductor quantum dot in the two-dimensional quantum dot array can be used as the temperature monitor. When used as a temperature monitor, the gate is used as an electrode to apply a voltage, and the applied voltage is the voltage determined by the above-mentioned calibration.

[0076] In this way, by setting a step of measuring the temperatures of multiple quantum dots 633 using the CBT 631 before quantum computation and determining the temperature distribution of the two-dimensional quantum dot array 634, quantum computation scheduling can be performed, for example, by allocating less computational effort to quantum dot positions with relatively high temperatures and more computational effort to quantum dot positions with relatively low temperatures, thereby reducing errors and enabling efficient quantum computation. [Calibration User Interface] Figure 7A shows an example of a user interface (UI) for CBT calibration. During calibration, the CBT sensor number (700), CBT sensor gate voltage (701), and measurement heat generation range (702) are set. Additionally, the bias voltage of the CBT sensor, the number of measurement points, the measurement frequency, the measurement time interval, and the like may also be set.

[0077] Next, the gate voltage of the selected CBT sensor number is set to a set value, and the heat generation amount Q is changed within a predetermined range to measure the temperature T. As the measurement results, the measurement result (709) of T with respect to Q (710), V with respect to Q or T ｇ -G curve, V ｇ TG curve and differential sensitivity of T (707, 708), V ｇ (704), maximum sensitivity V ｇ The TG relationship (706, 705) at this time is displayed, allowing the user to perform calibration.

[0078] Figure 7B shows an example of a user interface for temperature monitoring. It displays the ratio of the measured temperature of each CBT to the normal temperature (711), temperature time series information (712), temperature spatial distribution information (713), operation information of the control circuit that is the heat source (714), heat generation information, and other information. This information allows the user to perform thermal management of the quantum computer. [Summary] According to the method of this embodiment, the CBT using a data table can monitor electron temperature in real time with high spatial resolution and suppress self-heating.

[0079] In this example, temperature is measured in real time using a CBT using an N-piece tunnel junction array, as described in Patent Document 2. Unlike the semiconductor quantum dots described in Example 1, a CBT using a tunnel junction array generally cannot control the gate voltage. Instead, the bias voltage V is controlled to obtain a peak shape of the conductance G based on the Coulomb blockade phenomenon. This peak shape is expressed, for example, by Equation 3.

[0080] ...(Formula 3)

[0081] Here, G Ｔ is the differential conductance when the bias voltage V is sufficiently large, and the half-width of this curve is 5.439NK B The electron temperature can be calculated using the fact that T / e.

[0082] FIG. 8A shows the peak shape of the conductance G due to the CBT in this example based on Equation 3.

[0083] 8B shows the differentiation of Equation 3 with respect to temperature T. As in Example 1, by fixing the bias voltage V at positions B', C', and D', which are highly sensitive to temperature, a real-time CBT can also be performed using a tunnel junction array using a data table. This enables high-precision, high-speed temperature measurement at extremely low temperatures.

[0084] In this way, a Coulomb blockade thermometer of quantum dots with a metal island structure can be used as the thermal monitor unit. This thermal monitor unit is preferably provided separately from the semiconductor quantum dots used for quantum computing.

[0085] CBT can measure not only temperature but also the effects of photon-assisted tunneling (PAT) caused by RF application and charge noise. These can also be monitored in real time using a data table. PAT is caused by the electric field of RF applied to manipulate quantum bits in a spin quantum computer, and if it occurs during quantum computing, it can cause calculation errors. Charge noise is also caused by various device and environmental factors, and if it occurs during quantum computing, it can also cause calculation errors. Therefore, monitoring the effects of PAT and charge noise is effective in reducing calculation errors.

[0086] The effects of PAT and charge noise can be monitored by installing the real-time CBT described in Example 1 near or inside the quantum bit array and monitoring the time derivative or variance of the conductance change over time or the frequency characteristics. In particular, PAT can be measured by the change in conductance when RF is applied. The effects of PAT and charge noise can also be monitored by calibrating using a method similar to that used for temperature in Example 1 and creating a data table. Therefore, the CBT can be used to monitor not only temperature but also PAT and charge noise.

[0087] 9 is a block diagram of a quantum computer employing real-time CBT according to an embodiment. Quantum computer 10 includes a quantum bit array chip 120 maintained at low temperature within refrigerator 100, and a control chip 130 for controlling quantum bit array chip 120. As already mentioned, semiconductor quantum dots are formed on quantum bit array chip 120 using a transistor configuration created using semiconductor manufacturing technology. Control unit 110, located outside the refrigerator, is configured as a general computer and includes a processing unit 901, an input unit 902, an output unit 903, and a recording unit 900.

[0088] Although each component is configured to be able to communicate, buses and the like for connecting them are omitted. Also, known configurations for operating a quantum computer are omitted, and only temperature control, which is the focus of the embodiment, will be explained.

[0089] The recording unit 900 stores a calibration unit 904, a temperature control unit 905, and a data table 223. The calibration unit 904 and the temperature control unit 905 perform desired operations by having the processing unit 901 execute software, but they may also be configured as hardware.

[0090] The calibration unit 904 executes the processes described in Figures 3A to 4D and creates the data table 223. The temperature sensor 140 used during calibration can be configured using any of the sensors described in Examples 1 to 3, or a combination of these. The temperature measurement principle has been described in Figures 2A to 2E. The timing of the calibration operation is arbitrary, and calibration is performed at a preset timing or according to instructions input from the input unit 902.

[0091] The data table 223 is ｇ opt The data is created from the TG relationship (FIG. 3F) in the above and stored in the recording unit 900. The data format is arbitrary.

[0092] When the first embodiment is applied, the temperature control unit 905 controls the gate voltage of the CBT sensor to V ｇ opt The conductance G is measured, and the conductance is converted to temperature in the data table 223, enabling high-speed temperature measurement. Examples of temperature control have been described with reference to FIGS. 5A to 6B.

[0093] During calibration or temperature control, the temperature of the quantum computer can be easily controlled by displaying a UI such as that described in Figures 7A and 7B on a display or the like that is one of the output units 903.

[0094] 9 shows an example of a quantum computer, and modifications are possible as appropriate. For example, some or all of the processing of the control unit 110 may be executed by the control chip 130. Furthermore, some or all of the processing of the control unit 110 may be executed by another computer.

[0095] According to this embodiment, the electron temperature can be monitored to ensure stable operation in a quantum computer. This makes it possible to realize an efficient quantum computer, which consumes less energy, reduces carbon emissions, prevents global warming, and contributes to the realization of a sustainable society.

[0096] Quantum computer 10, refrigerator 100, control unit 110, quantum bit array chip 120, control chip 130, temperature sensor 140, data table 223, recording unit 900, processing unit 901, input unit 902, output unit 903, calibration unit 904, temperature control unit 905

Claims

1. A physical quantity measuring device comprising: a monitor unit that measures the physical quantity of an object to be measured; electrodes that apply a voltage to the monitor unit; and a memory unit that has a conversion data table that associates the output results of the monitor unit with the physical quantities when a predetermined voltage is applied to the monitor unit, wherein the physical quantity is measured by comparing the output results with data stored in the conversion data table.

2. The physical quantity measuring device according to claim 1, wherein the physical quantity is temperature, and the monitor unit is a Coulomb blockade thermometer.

3. The physical quantity measuring device according to claim 2, wherein the measurement object is an environment in which semiconductor quantum dots are placed, and the monitor unit is a Coulomb blockade thermometer using the semiconductor quantum dots.

4. The physical quantity measuring device according to claim 3, wherein a plurality of the semiconductor quantum dots constitute a two-dimensional quantum dot array, each of the semiconductor quantum dots is realized by a transistor having a gate, and the monitor unit utilizes at least one semiconductor quantum dot in the two-dimensional quantum dot array.

5. The physical quantity measuring device according to claim 4, wherein the gate is used as an electrode for applying a voltage to the monitor section, and the predetermined voltage is a predetermined fixed voltage.

6. The physical quantity measuring device according to claim 2, wherein the measurement target is an environment in which semiconductor quantum dots are placed, a plurality of the semiconductor quantum dots form a two-dimensional quantum dot array, each of the semiconductor quantum dots is realized by a transistor having a gate, the monitor unit is a Coulomb blockade thermometer using a tunnel junction array, and an electrode that applies a voltage to the monitor unit provides a predetermined bias voltage to the tunnel junction array.

7. The physical quantity measuring device according to claim 1, wherein the physical quantity is at least one of the influence of PAT due to RF application and charge noise, and the monitor unit is a Coulomb blockade thermometer.

8. A physical quantity measuring method comprising: applying a constant voltage to a Coulomb blockade thermometer; obtaining an output signal from the Coulomb blockade thermometer; and converting the output signal into a temperature using a conversion data table that associates the output signal with a temperature.

9. The physical quantity measuring method according to claim 8, wherein the conversion data table is created using a mathematical formula for converting conductance based on the output signal of the Coulomb blockade thermometer into temperature.

10. The conversion data table is created by a calibration process, which includes: a first step of applying a predetermined amount of heat to the environment of the Coulomb blockade thermometer; a second step of recording an output signal while sweeping an applied voltage to the Coulomb blockade thermometer; a third step of correlating the predetermined amount of heat in the first step with the output signal in the second step to obtain first relationship data; and repeating the first to third steps a plurality of times by changing the predetermined amount of heat in the first step; a fourth step of calculating a temperature from the output signal of the first relationship data; a fifth step of correlating the predetermined amount of heat with the temperature to obtain second relationship data; a sixth step of determining third relationship data showing the relationship between the predetermined amount of heat and the output signal for a plurality of the applied voltages; and a seventh step of determining fourth relationship data showing the relationship between the output signal and the temperature for a plurality of the applied voltages based on the second relationship data and the third relationship data. an eighth step of determining an applied voltage to be applied to the Coulomb blockade thermometer based on the fourth relationship data, and setting the applied voltage as a reference voltage to be used when creating the conversion data table.

11. The physical quantity measuring method according to claim 10, wherein in the eighth step, the voltage to be applied to the Coulomb blockade thermometer is determined based on at least one of the slope of the graph of the fourth relationship data and the magnitude of the output signal.

12. The physical quantity measuring method according to claim 11, wherein the voltage to be applied to the Coulomb blockade thermometer is determined based on at least one of the slope of the graph of the fourth relationship data at a predetermined temperature that is a threshold value and the magnitude of the output signal.

13. The physical quantity measuring method according to claim 10, wherein data indicating the relationship between the output signal at the reference voltage and the temperature is selected from the fourth relationship data, and the conversion data table is created.

14. A quantum computer comprising: a quantum bit array chip comprising semiconductor quantum dots; a cooling device for cooling the quantum bit array chip; a Coulomb blockade thermometer disposed within the cooling device; a temperature control unit for controlling a heat source within the cooling device; and a memory unit for storing a conversion table for converting the conductance of the Coulomb blockade thermometer into temperature, wherein the temperature control unit applies a constant voltage to the Coulomb blockade thermometer to measure the conductance, converts the measured conductance into temperature information by referring to the conversion table, and controls the heat source based on the temperature information.

15. The quantum computer according to claim 14, wherein the semiconductor quantum dot is used as the Coulomb blockade thermometer.

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