Photodetector

Abstract

To provide a photodetector having an avalanche photodiode that detects luminescence for atoms and ions that emit light upon optical excitation, capable of detecting generally synchronously with the optical excitation by integrating signals generated by the detection.SOLUTION: A photodetector 100 having an avalanche photodiode 111 detecting luminescence for atoms and ions that emit light upon optical excitation is configured to integrate the signal generated by the detection and detect in synchronization with the optical excitation at the timing when the atoms and ions emit light.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a photodetector for detecting atoms and ions that emit light by photoexcitation. The photodetector of the present invention is particularly suitable for detecting extremely weak light where the light to be detected can be measured as photons (photons), or for detecting atomic and ionic luminescence (signals) when the ratio (S / N ratio) of atomic and ionic luminescence detection (signal) to disturbance light detection (noise) is low.

Background Art

[0002] A quantum computer is a computer that performs information processing by utilizing quantum mechanical phenomena such as quantum superposition and quantum entanglement. A quantum bit (q-bit), which is the minimum unit of quantum information, is created by the superposition state of atoms, ions, etc., and the accurate measurement of its state is important in the development of quantum computers. The superposition state of atoms and ions is determined by trapping the atoms and ions with a laser, irradiating the atoms and ions with a laser, and measuring the luminescence due to the photoexcitation. However, in the measurement system, there is disturbance light such as the laser light for trapping atoms and ions and other ambient light, and it is difficult to completely block it with an optical filter or the like.

[0003] Patent Document 1 discloses a method of suppressing the influence of disturbance light and measuring the luminescence due to photoexcitation with high precision by stopping the operation of a light receiving element determined to be receiving disturbance light.

[0004] Patent Document 2 discloses a method of detecting atoms and ions in a state including disturbance light, estimating the detection result based on past detection results, and determining the detection result from the existence probability distribution and the detection light probability distribution.

Prior Art Documents

Patent Documents

[0005]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0006] However, in the technology disclosed in Patent Document 1, when the ambient light is strong with respect to the light emission of atoms or ions, the operation of the light receiving element stops frequently, and it is difficult to apply it to a quantum computer that requires immediate measurement of the superposition state.

[0007] In addition, in the technology disclosed in Patent Document 2, since the states of atoms and ions to be measured are different individually, estimation from past detection results as described in Patent Document 2 cannot be performed.

[0008] When considering application to a quantum computer, measurement of the superposition state of atoms and ions is required to be fast and accurate. However, the influence of disturbing light such as laser light for trapping atoms and ions and other ambient light necessary for measurement with respect to the light emission of atoms and ions is not negligibly small, and in the presence of disturbing light that becomes noise, it is necessary to detect the light emission (signal) from atoms and ions, and measurement under conditions where the probability that the detected light is due to the signal is high is required.

[0009] Therefore, in the present invention, there is provided a photodetector having an avalanche photodiode for detecting light emission from atoms and ions that emit light by photoexcitation, the photodetector having a function of integrating signals generated by detection and detecting in synchronization with photoexcitation.

Means for Solving the Problems

[0010] The photodetector of the present invention is characterized in that the avalanche photodiode is operated in each of the light emission period and the non-light emission period by photoexcitation. It is characterized by including a measurement condition setting unit that sets the integration times and detection times of signals generated by detection based on the detection probability of light emission by excitation and the detection probability in the non-light emission period.

Effects of the Invention

[0011] Since the photodetector of the present invention has an avalanche photodiode, it is possible to detect even extremely weak light emissions of atoms and ions (photons: photons). In addition, it has a function of integrating the signals generated by detection, and can count the number of photons to be detected. Further, since it is possible to detect the light emissions of atoms and ions in synchronization with photoexcitation, it is possible to increase the ratio (S / N ratio) of the light emission detection (signal) of atoms and ions to the external disturbance light detection (noise).

[0012] In addition, since the avalanche photodiode of the photodetector of the present invention operates during the non-light emission period, it is also possible to detect only the light emission caused by external disturbance light (noise). Even when the ratio (S / N ratio) of the light emission detection (signal) of atoms and ions to the external disturbance light detection (noise) is low, it is possible to set measurement conditions (the number of signal integrations and the number of detections) with a high probability that the light emission (signal) from atoms and ions is detected from the detection probability of the light emission from atoms and ions and the detection probability of external disturbance light.

Brief Description of the Drawings

[0013]

Figure 1

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Embodiments for Carrying Out the Invention

[0014] Hereinafter, preferred embodiments of the present invention will be described.

[0015] The photodetector of the present invention has an avalanche photodiode for detecting extremely weak light (photons) of atoms and ions that emit light by optical excitation. Even if there is a single avalanche photodiode, the effect is not impaired, but it is preferably arranged in an array for detecting multiple emissions. Particularly in a quantum computer, in order to improve computing power and for redundancy for error correction, it is necessary to increase the number of bits. With an array, multiple emission states can be measured, so it is highly useful for the development of high-bit quantum computers. Since the avalanche photodiode outputs the detected signal to the outside, the pixel includes a readout circuit and is arrayed in pixel units. Also, the pixel has a function of being able to integrate the signal generated by detection. The integrating function may be configured outside the pixel or built into the pixel. FIG. 1 shows an example of a photodetector 100 including an avalanche photodiode array with the integrating function built into the pixel. The pixels 110 are arranged in an array, and each pixel 110 includes an avalanche photodiode 111, a readout circuit section 112, and an integrating circuit section 113 having an integrating function within the pixel 110. With this configuration, since the photodetector 100 includes a plurality of pixels 110, it becomes possible to detect a plurality of emission states with a single photodetector 100.

[0016] FIG. 2 shows an example of an experimental system 200 for detecting the light emission of atoms and ions in which the photodetector 100 of the present invention is used. The experimental system 200 is part of a system for measuring the superposition state of atoms and ions in order to create the state of qubits (q-bits) necessary for realizing a quantum computer. In the experimental system 200, the light emission 201 of atoms and ions (since the atoms and ions are small, they are not shown in the figure and are only numbered) is imaged onto the photodetector 100 of the present invention using an optical system 202 such as a lens. The objects that emit light are various depending on the atoms and ions, and in the case of atoms, they are often selected from alkali metal elements, for example, rubidium (Rb). The reason for selecting from alkali metal elements is that since the melting point is low, it is easy to maintain the atomic state (vaporized state) at room temperature. Atoms and ions are fixed at the measurement position using a plurality of lasers (203a to 203c) by a technique called an optical tweezer that focuses laser light to the diffraction limit and captures it at the focal point. The fixed atoms and ions are irradiated with an excitation laser 204, and the light (photon) 205 emitted when the excited energy transitions to the ground state is detected by the photodetector 100, and the superposition state of the atoms and ions is measured. In order to reliably measure the superposition state of atoms and ions, it is necessary to detect only the light (photon) caused by the light emission of atoms and ions. However, it is difficult to completely block the light of the lasers (203a to 203c) for fixing the atoms and ions and the ambient light surrounding the experimental system, and such disturbing light becomes noise.

[0017] FIG. 3 shows an example of a timing chart of the emission of atoms and ions serving as signals and the emission of other disturbing light. (a) in FIG. 3 is the emission from the excitation laser 204, (b) in FIG. 3 is the emission from atoms and ions serving as signals, (c) in FIG. 3 is the emission from the lasers (203a to 203c) for fixing atoms and ions, and (d) in FIG. 3 is the emission of ambient light surrounding the experimental system 200. Since the wavelength range of the emission of the atoms and ions to be measured shown in (c) of FIG. 3, the emission of the lasers (203a to 203c) for fixing atoms and ions, and the wavelength band of the emission of the ambient light surrounding the experimental system 200 shown in (d) of FIG. 3 are different, by arranging a narrow-band band-pass filter that transmits only the emission of atoms and ions in front of the photodetector 100, the influence of disturbing light can be suppressed, but it cannot be completely blocked and is detected as noise.

[0018] The photodetector 100 of the present invention has the feature of performing detection in synchronization with photoexcitation. In addition to the light emission of atoms and ions that become signals and the light emission of other disturbing light in FIG. 4, an example of a timing chart when operating the avalanche photodiode 111 for detection in approximate synchronization with photoexcitation is shown. The High state is an exposure standby state because the avalanche photodiode 111 is in the ON state. When light is incident on the avalanche photodiode 111 in the High state, the photodetector 100 indicates that detection has occurred. If no light is incident on the avalanche photodiode 111 even in the High state, detection does not occur. As shown in FIG. 4(e), in the photodetector 100 of the present invention, since the avalanche photodiode 111 becomes the High state in approximate synchronization with photoexcitation, the ratio of the light emission (signal) of atoms and ions included in the detection becomes higher with respect to the disturbing light (noise) (relatively suppressing the noise). Also, as shown in FIG. 4(f), by setting the avalanche photodiode 111 to the High state at a timing excluding the light emission period of the pulse for photoexcitation, it is also possible to suppress the detection by the excitation light. The timing chart shown in FIG. 4 is an example of the operating state of the photodetector 100 of the present invention and is not limiting. For example, even when the pulse width of the excitation light is shortened, only the light emission period of atoms and ions and the period of the High state of the avalanche photodiode 111 are shortened, and similarly, the ratio of the light emission (signal) of atoms and ions included in the detection can be made higher with respect to the disturbing light (noise). Also, when the exposure period is shortened, the amount of received light decreases, but since the photodetector 100 of the present invention has the avalanche photodiode 111, even extremely weak light can be detected. When the exposure period is extremely short, instead of always receiving light, it can be treated as a probabilistic event of detecting or not detecting photons. In that case, since the photodetector 100 of the present invention has a function of integrating the signals generated by detection, photon counting can be performed.

[0019] Fig. 5 shows an example of a timing chart different from that of Fig. 4 when the avalanche photodiode 111 operates. As shown in (g) of Fig. 5, the avalanche photodiode is operated during the light emission period due to optical excitation, and as shown in (h) of Fig. 5, during the non-light emission period. In the case of the timing chart of (g) in Fig. 5, the detected light is the light emission (signal) from atoms and ions and the ambient light (noise). On the other hand, in the case of the timing chart of (h) in Fig. 5, only the ambient light (noise) is detected. When each light is extremely weak and can be treated as a probabilistic event of detecting photons, the detection probability of each is expressed by a binomial distribution, and the respective probabilities are p (signal+noise) when detected during the light emission period shown in (g), and p (noise) when detected during the non-light emission period shown in (h). Then, the detection probability of the number of detections k during the light emission period shown in (g) and the non-light emission period shown in (h) in the integration number n of the avalanche photodiode 111 can be expressed as in Formula (1) and Formula (2).

[0020]

Equation

[0021]

Equation

[0022] FIG. 6 shows an example of the probability distribution of the light emission (signal) of atoms and ions or ambient light (noise) when the number of trials n, which is the number of integration times of the avalanche photodiode 111, is changed. The detection probability takes various values depending on the measurement environment. In order to increase the light emission (signal) from atoms and ions, a method of increasing the output of the excitation laser 204 is also employed, but there are drawbacks such as an increase in the cost and size of the laser 204. Also, as a method of suppressing ambient light (noise), a method of devising the placement position and a method using a band-pass filter are considered, but there is a drawback in that the size of the experimental system 200 increases. This method is highly effective when the ratio (S / N ratio) of the light emission detection (signal) of atoms and ions to the ambient light detection (noise) is low. As an example, the detection probabilities during the light emission period and the non-light emission period are set to p (signal+noise) = 0.3, p (noise) = 0.2, and (S / N ratio 0.5).

[0023] (a) in FIG. 6 shows the case of n = 3. The respective probability distributions overlap, and regardless of the number of detections, the probability of the detection including the light emission (signal) of atoms and ions is not high. (b) in FIG. 6 shows the case of n = 20. The peaks of the probability distributions are separated, and although the overlap is less than that in (a) of FIG. 6, for example, at the probability at the time of detection of k = 6, the detection probability of ambient light (noise) is also high, and the probability of the detection including the light emission (signal) of atoms and ions is not high. (c) in FIG. 6 shows the case of n = 100. The peaks of the probability distribution are more separated than in (a) and (b) of FIG. 6. For example, at the probability at the time of detection of k = 30, the detection probability of ambient light (noise) is sufficiently low with respect to the detection probability of the light emission (signal) of atoms and ions, and it can be seen that the result of 30 detections has a high probability of including the light emission (signal) of atoms and ions. (d) in FIG. 6 shows the case of n = 300. The probability distribution is separated as in (c) of FIG. 6, and it can be seen that the probability of the detection including the light emission (signal) of atoms and ions is high, but there is a side effect that the measurement time becomes long due to an increase in the number of integration times of the avalanche photodiode 111.

[0024] FIG. 7 shows an example different from FIG. 1 of the photodetector 100 of the present invention. The photodetector 100 includes a measurement condition setting unit 700 in addition to the configuration of FIG. 1. In FIG. 7, an example is shown in which the measurement condition setting unit 700 is provided for each pixel, but one measurement condition setting unit 700 may be provided for the output of the array, and the effect is not impaired even in this case. The measurement condition setting unit 700 determines measurement conditions (the integration times and detection times of the avalanche photodiode 111) with a high probability of detecting emissions (signals) of atoms and ions according to the flow shown in FIG. 8. In step 800, as shown in FIGS. 4(e), 4(f), or 5(g), the probability of detection including emissions (signals) of atoms and ions and ambient light (noise) can be measured by setting the avalanche photodiode 111 to the High state in synchronization with the emission by optical excitation. In step 801, since the avalanche photodiode 111 is set to the High state during the non-emission period of atoms and ions as shown in FIG. 5(h), the probability of detection of ambient light (noise) can be measured. From the probabilities measured in steps 800 and 801, in step 802, the initial integration times (exposure times) of the avalanche photodiode 111 are set. The integration times are set according to the values of the detection probabilities. For example, if the detection probability is 10 percent, the integration times are preferably set to 10 to 100 times, and if the detection probability is 1 percent, the integration times are preferably set to 100 to 1000 times, that is, the number of times of detection occurring one or more times is set. In step 803, as shown in FIG. 6, it is determined whether the peaks of the respective probability distributions can be separated from the detection probability distributions during the emission period and the non-emission period. If the peaks cannot be separated as shown in FIG. 6(a), the integration times n are increased and the evaluation of the probability distribution is performed again. If the peaks of the probability distribution can be separated, in step 804, at the detection times of the peak of p (signal+noise) , it is determined whether p (noise) is sufficiently small with respect to p (signal+noise) . As an example, if p (signal+noise) is two digits or less with respect to p (noise) at the detection times of the peak of p (signal+noise) , it can be determined that the probability of detecting emissions (signals) of atoms and ions is high. Therefore, in step 805, the integration times n and the detection times k at that time are set as the measurement conditions for the current measurement.

[0025] According to the present invention, it is possible to detect extremely weak light emission (photons: photons) of atoms and ions, and the ratio of light emission detection (signal) of atoms and ions to disturbance light detection (noise) can be increased. Further, even when detecting light emission due to disturbance light (noise) and the ratio of light emission detection (signal) of atoms and ions to disturbance light detection (noise) (S / N ratio) is low, from the detection probability of light emission from atoms and ions and the detection probability of disturbance light, it is possible to set measurement conditions (the number of signal integration times and the number of detection times) with a high probability that light emission (signal) from atoms and ions is being detected. Therefore, it is applicable to a quantum computer that requires quickly and accurately measuring the superposition state of atoms and ions.

Industrial Applicability

[0026] The present invention can be applied to a quantum computer or the like.

Explanation of Signs

[0027] 100 Photodetector 110 Pixel 111 Avalanche photodiode 112 Readout circuit section 113 Integration circuit section 200 Experimental system 201 Light emission 202 Optical system 203a, 203b, 203c, 204 Laser 205 Light 700 Measurement condition setting section

Claims

1. A photodetector having an avalanche photodiode for detecting light emission from atoms and ions that emit light upon optical excitation, integrating a signal generated by the detection, wherein the detection is performed in synchronization with the optical excitation at the timing when the atoms and ions emit light. The photodetector is characterized by this.

2. The photodetector according to claim 1, wherein the avalanche photodiode is operated during each of the light emission period and the non-light emission period due to the optical excitation.

3. The photodetector according to claim 1 or 2, further comprising a measurement condition setting unit that sets the number of integrations and the number of detections of a signal generated by the detection based on the detection probability of light emission due to the optical excitation and the detection probability during the non-light emission period.

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