Method and apparatus for generating measurement signal from signal emitted by spin-based quantum system

Abstract

The invention relates to a method and an apparatus (100) for generating a useful signal (11) from a signal (5 ') emitted by a spin-based quantum system (4), comprising: exciting the spin-based quantum system (4) by an electromagnetic field (3) that switches between two states at a modulation frequency; detecting a signal (5 ') emitted by the spin-based quantum system (4) to obtain a measurement signal (7); multiplying the measurement signal (7) by a modulation signal (9) which jumps back and forth between two modulation signal states at a modulation frequency to produce a modulated measurement signal (7 '); and demodulating the modulated measurement signal (7 ') to produce a useful signal (11).

Description

Technical Field

[0001] The present invention relates to a method and apparatus for generating a measurement signal from a signal emitted by a spin-based quantum system. Background Technology

[0002] For measuring extremely weak magnetic field strength, quantum- or optically pumped magnetometers are particularly well-suited as sensors. These magnetometers typically utilize optically pumped and optically probed magnetic resonance (ODMR). The principle is based on the Zeeman effect, which occurs when the energy levels of specific spin states of unpaired electrons split under the influence of an external magnetic field. Due to this energy level splitting, the transition paths from the excited state relaxation state change. This can be measured, for example, by optical excitation and subsequent frequency-correlated detection of the resulting fluorescence radiation, or by observing optical properties such as light absorption. The magnetic field strength can then be deduced from the measured optical parameters.

[0003] For this type of quantum-based magnetic field sensor, a sensitive crystal with excitable defect centers can be used. Typically, the crystal used is diamond uniformly doped with negatively charged nitrogen-vacancy defect centers (NV centers). The preparation of the quantum state is achieved through photoexcitation and interaction with static and dynamic magnetic fields (i.e., microwave fields).

[0004] To measure extremely weak magnetic field strengths, quantum magnetic sensors based on nitrogen-vacancy defect centers are known, for example, from DE 10 2018 220 234 A1 or DE 10 2018 214617 A1. In these sensors, information stored in the spin system is optically read out by detecting the fluorescence rate of the NV centers (Farbzentren), which depends on the spin state. Therefore, its accuracy is particularly limited by the signal-to-noise ratio of the NV center optical readout method.

[0005] US 9,689,679 B2 discloses a diamond sensor with nitrogen-vacancy sites for measuring changes in rotational orientation.

[0006] One type of spin-based quantum system, or quantum sensor, is the NMR gyroscope (Nuclear Magnetic Resonance). These gyroscopes analyze and process nuclear spin resonance signals from atomic nuclei with non-zero magnetic moments. One feasible approach to providing a single-axis NMR gyroscope involves setting up a vapor chamber containing a mixture of an element with non-zero nuclear spin (typically an inert gas, such as xenon (Xe)) and an alkali metal (such as potassium (K), cesium (Cs), or rubidium (Rb)). A polarized pump laser beam can be used to polarize the rubidium electron spins in the vapor chamber. Due to the strong coupling between rubidium and xenon, this causes the xenon nucleus spin to become parallel to the rubidium electron spin. By applying a static magnetic field along the polarization direction, the xenon nucleus spins can precess around this static magnetic field. The precession frequency is the Larmor frequency, which depends on the static magnetic field. By using an alternating magnetic field with a frequency consistent with the Larmor frequency and applied perpendicular to the static magnetic field, coherent precession of all nuclear spins can be achieved. If a polarized probe or measurement laser beam is then injected into the vapor chamber perpendicular to the static magnetic field, the polarization plane of the probe laser beam will rotate periodically at the Larmor frequency due to the Faraday effect. Intensity fluctuations modulated by the Larmor frequency can be observed using a polarizer (or polarization beam splitter) and a detector. When the sensor rotates about a rotation axis parallel to the static magnetic field, a frequency shift in the Larmor frequency proportional to the rotation rate occurs. By analyzing and processing the intensity signal output by the detector, the change in the rotation direction of the rotation axis parallel to the polarization direction can be determined. US 7,282,910 B1 discloses an example of applying the above-described NMR gyroscope technology.

[0007] Typically, in spin-based sensor devices (also known as quantum sensors), frequency-modulated signals are detected and analyzed, which can be done using a lock-in method. Summary of the Invention

[0008] According to the present invention, a method and apparatus for generating useful signals from signals emitted by a spin-based quantum system are proposed, having the features of the independent patent claims. The dependent claims and the subject matter described below are advantageous configurations.

[0009] In traditional locking methods, the useful signal S is modulated by a sinusoidal modulation signal. By knowing the modulation frequency, the useful signal can be extracted from the measurement signal M even when the ambient noise R is high (especially in the case of very high 1 / f noise, which is critical for very low frequency signals <1kHz (close to DC)).

[0010] This modulation can be represented as: ,in It is a measurement signal (i.e., the measured signal). It is a useful signal (i.e., the signal to be measured or of interest (or its AC component)). It's environmental noise. It is the phase offset between modulation and measurement.

[0011] To extract the useful signal, demodulation is then performed in the following form: Where T is the integration window.

[0012] If phase relationship It is unknown, but an additional 90° phase shift can be used for demodulation, and the useful signal can be reconstructed from the two basis vectors.

[0013] If multiple useful signals are modulated at different frequencies (sine / cosine functions of different frequencies are orthogonal to each other), these useful signals can be extracted from the same measurement signal.

[0014] However, when using trigonometric function (sine / cosine)-based locking techniques, the sensor is in a region that detects little or no useful signal information for a portion of the measurement time. In other words, changes in the useful signal have no effect on the measurement signal because, through modulation, the useful signal is not incorporated into the measurement signal at this point. This needs improvement because the ratio of information to measurement time is not being utilized ideally.

[0015] Therefore, this invention employs a modulation signal that jumps or switches between two field states at a modulation frequency. This modulation signal may involve different signal values ​​or field strengths, or different frequency values, depending on what is excited or modulated in the spin-based quantum system. In this way, the system samples only those points with the highest information content, while intermediate possible measurement points are not sampled.

[0016] This back-and-forth transition corresponds to a rectangular wave function with a modulation frequency. This modulation can also be called Bang-Bang modulation.

[0017] The advantage of this invention is that the useful signal can now be measured at the full scale (i.e., maximum amplitude). In contrast, under sine / cosine modulation, the amplitude of the useful signal can only be scaled by a factor of 1 / 2. Therefore, with this invention, the signal-to-noise ratio is improved to 2. 1 / 2 (Right now ) times.

[0018] Another advantage is that it is easier to implement than a sinusoidal frequency change curve, especially for modulated signals that jump back and forth between two modulation field intensities or two modulation frequency values.

[0019] For example, for frequency modulation, the following can be used as the modulation signal: Then modulation is performed similarly. (1) reconciliation (2) in This represents the modulation frequency. Typical frequency values ​​for electromagnetic fields used to excite spin-based quantum systems are in the high-frequency range. The abbreviation "HF" will also be used below to denote the term "high frequency." The frequency of a high-frequency field (HF field) depends on the quantum system to be excited, specifically on the energy difference between the quantum mechanical states in the quantum system that should cause transitions (this energy difference corresponds to a frequency). Typical frequencies may be in the microwave range, for example, between 300 MHz and 300 GHz, or between 300 MHz and 1 THz. This quantum system could, for example, be an NV color center in diamond, whose ground state is a triplet state in which (in the absence of an external magnetic field) there exists an energy difference corresponding to 2.87 GHz between the spin-0 state and the spin-+1 or -1 state. Such an NV color center in diamond, for example, can be used as a quantum sensor.

[0020] This spin-based quantum system can be a quantum magnetic sensor based on nitrogen-vacancy defect centers. The sensor includes: an excitation light generating device for generating, for example, pulsed excitation light; a sensing crystal with color centers, particularly diamond with nitrogen-vacancy defect centers; a measuring device; and a field generating device for generating an electromagnetic field. The excitation light generating device can be a pulsed laser. The field generating device is particularly configured to generate a microwave field. The measuring device is particularly configured to detect fluorescence radiation emitted by the color centers in the sensing crystal after excitation light.

[0021] Here, the excitation light generating device specifically generates a laser pulse, which initializes the quantum system to its ground state. Subsequently, the field generating device specifically emits a pulsed microwave field into the quantum system, which manipulates the quantum system. After a waiting period, the manipulated quantum system is read out again using a laser pulse. Compared to schemes that use an excitation light generating device to generate a continuous excitation beam and / or continuously emit microwave fields, this pulsed method has a higher signal-to-noise ratio (SNR).

[0022] The invention also includes a method in which a spin-based quantum system is first initialized by a first laser pulse generated by an excitation light generating device. Next, the spin-based quantum system is excited by an electromagnetic field, particularly a microwave field, which jumps back and forth between two field states at a modulation frequency. The spin-based quantum system is manipulated in this way. The actual detection of the magnetic field is performed in this step. To read the result, the quantum system is irradiated with a second laser pulse, causing it to emit a signal. This signal is detected by a measuring device, thereby obtaining a measurement signal. This measurement signal is multiplied by a modulation signal that also jumps back and forth between two modulation signal states at a modulation frequency. This yields a so-called modulated measurement signal, from which a useful signal can be generated, for example, by low-pass filtering integration. In this way, the useful signal S can be separated from the interference signal R of ambient noise. This method achieves very accurate and sensitive magnetic field detection.

[0023] In one particularly advantageous embodiment, the first and second laser pulses can also be contained within a single common laser pulse. In this case, the first pulse portion of the common laser pulse is used to read the previously initialized and manipulated quantum system, while the second pulse portion, the later pulse, is used to initialize the quantum system for another iteration. This embodiment makes the method particularly efficient.

[0024] Other advantages and configurations of the invention are given in the specification and drawings.

[0025] The present invention is illustrated with reference to the embodiments shown in the accompanying drawings, which will be described below with reference to these drawings. Attached Figure Description

[0026] Figure 1 A device for generating a measurement signal from a signal output from a spin-based quantum system, according to an embodiment of the present invention, is shown in block diagram form.

[0027] Figure 2 The details of the analysis process are shown in the form of a block diagram.

[0028] Figure 3 The various stages of measurement and analysis processing according to an embodiment of the present invention are illustrated in block diagram form.

[0029] Figure 4 The orthogonal basis of rectangular wave functions, based on the Walsh function, is shown for separating different signals. Detailed Implementation

[0030] Figure 1A device 100 according to an embodiment of the present invention is shown in block diagram form. The device is used to generate a measurement signal from a signal 5' emitted by a spin-based quantum system 4.

[0031] The device 100 includes: a field generating device 2 for generating an electromagnetic field 3; an excitation light generating device 12 for generating, for example, pulsed excitation light 13; and a spin-based quantum system 4 that will be excited by the electromagnetic field and the excitation light.

[0032] In some configurations, the spin-based quantum system 4 can have a sensing crystal with a color center, particularly diamond with nitrogen-vacancy defect centers. In other configurations, the spin-based quantum system 4 can have a vapor chamber containing a mixture of at least one gaseous alkali metal and at least one gaseous inert gas. Other types of spin-based quantum systems can also be advantageously used within the framework of this invention.

[0033] The field generating device 2 is used to generate an electromagnetic field 3, especially a high-frequency field (HF field), and further, especially a microwave field, which switches or jumps back and forth between two field states at a modulation frequency. These two field states can be different field strengths, especially field strengths with the same amplitude but different directions, or different frequency values ​​equidistant from the center frequency.

[0034] The device 100 also includes a measuring device 6 for detecting or measuring a signal 5' emitted by the spin-based quantum system 4 to obtain a measurement signal 7, which is transmitted to the computing unit 8.

[0035] In some configurations, the signal 5' emitted by the spin-based quantum system 4 is a fluorescent or optical signal. Accordingly, the measuring device 6 may, for example, have one or more photodiodes, or a photodiode measuring assembly.

[0036] The calculation unit 8 is used to analyze and process the measurement signal 7 and control the field generating device 2.

[0037] Figure 2 A possible functional configuration of the analysis and processing module implemented in computing unit 8, and the signal generation process, are illustrated in block diagram form. Among them, Figure 2 The left side depicts the signal generation process that occurs outside the computing unit 8, specifically in the spin-based quantum system 4.

[0038] As previously stated, the excitation of the spin-based quantum system 4 is achieved via an electromagnetic field 3 that jumps back and forth between two field states at a modulation frequency. The electromagnetic field 3 can, for example, jump in field strength (i.e., signal value) or frequency, and it is based on a modulation signal 9, or rather, generated by controlling the field generating device 2 with the modulation signal 9. This can be, in particular, a frequency modulation.

[0039] This excitation results in the signal 5' emitted by the spin-based quantum system 4 also carrying a corresponding modulation, or in other words, a modulation carrying useful information 5. This corresponds to the modulation described above, i.e., equation (1).

[0040] The corresponding demodulation, i.e., equation (2), is performed in computation unit 8. First, phase matching is performed as needed, and then the measurement signal 7 is multiplied by the modulation signal 9 to generate the modulated measurement signal 7'. Demodulation is then performed in module 10, where the modulated measurement signal 7' is demodulated in the form of low-pass filter integration to generate the useful signal 11.

[0041] Various embodiments of the present invention can be advantageously applied to different spin-based quantum systems, which will be described below in conjunction with the foregoing figures and... Figure 3 and Figure 4 Describe it.

[0042] For quantum sensors, such as the NV defect centers in diamond mentioned above, there are three scenarios where this measurement method can be applied: 1) Tracking or detecting the displacement of resonance peaks: Using a quantum system based on NV diamond (i.e., diamond with nitrogen-vacancy defect centers), the strength of an external magnetic field can be measured, for example. To do this, the NV diamond is first irradiated with pulsed excitation light (e.g., a first laser pulse) at a suitable wavelength. This step initializes the quantum system, placing it in its ground state. Subsequently, the NV diamond is irradiated with microwave radiation. If the frequency of the incident microwave coincides with a resonant frequency of the quantum system, a so-called resonance peak will appear. The location of the resonance frequency depends on the strength and direction of the external magnetic field (relative to the crystal axis or the NV direction). When the strength of the external magnetic field changes, the resonance frequency shifts, and therefore the location of the resonance peak also shifts.

[0043] If one of the resonant frequencies is known, changes in the position of the resonant peak can be detected, thereby detecting changes in the strength of the external magnetic field. For this purpose, NV diamond needs to be irradiated with frequency-modulated microwave radiation.

[0044] Figure 3An example of frequency modulation using a rectangular wavefunction is shown. Peak 301 in the ODMR spectrum is shown exemplary on the left. The x-axis represents the frequency of the excitation microwave field, and the y-axis represents the absorbed light intensity. In frequency modulation mode, the points with the highest information density are located at the slopes on both sides of the peak in the spectrum. The point with the largest slope (i.e., the rate of change of signal dS / df) is located at the modulation frequency. and Since the two slopes have opposite signs, each measurement point contains the full information density. These frequency points are advantageously used for rectangular wave frequency modulation, as shown below peak 301 in the figure. The useful signal 320 at these points gives the amount of resonant frequency change with a sign; that is, the change in resonant frequency can be inferred from the useful signal 320, including not only the magnitude of the change but also the direction of the change (no change in resonant frequency means that the lock signal (i.e., measurement signal 7) is zero). Shifts in the resonant frequency will cause changes in the amplitude of the measurement signal; that is, the greater the displacement of the resonant peak, the greater the amplitude.

[0045] In order to read the quantum system, which is first initialized by a first laser pulse and then irradiated with frequency-modulated microwave radiation, a second laser pulse is specifically incident on the quantum system, and the intensity of the fluorescence radiation emitted by the NV diamond under this irradiation is detected. The amplitude of the measured signal depends on the degree of offset of the actual resonant peak position relative to the resonant peak position corresponding to the incident microwave frequency.

[0046] The method of initializing and reading out a quantum system using laser pulses, with the microwave field switching back and forth between two frequencies, has a significantly improved signal-to-noise ratio and a markedly enhanced sensitivity compared to other methods, such as initializing and reading out a quantum system using a continuous wave excitation beam.

[0047] Because changes in the position of the resonance peak here will cause the measured value to change with the opposite sign, this is consistent with the above. The alternating symbols defined in the text correspond to each other. This is achieved through rectangular wave frequency modulation of the excitation field, i.e., periodic modulation at two microwave frequencies. and By switching back and forth between these parameters, the maximum information can be generated and extracted. Resonance peaks can be determined before continuous measurements or exist as given system parameters under a low magnetic field (where the expected resonance displacement caused by the external magnetic field is less than the resonance linewidth). Subsequently, the resonance curve can be continuously monitored (feedback loop, based on the measured values). and The signal difference is used to match the center frequency, i.e. and (midpoint). Therefore, continuous measurement refers to a method that iteratively detects the displacement of the resonant peak position, i.e., gradually matches the incident frequency (center frequency) of the incident microwave radiation to, for example, match the actual resonant frequency.

[0048] If the phase relationship between measurement and modulation is unknown, the measurement signal can be demodulated similarly using two modulation functions with a 90° phase difference. Thus, by analogy with sine / cosine modulation, the signal and its phase shift can be determined.

[0049] Figure 3 The middle figure shows the noisy measurement curve 310 (corresponding to the measurement signal 7) and the (theoretically) original signal 311 behind it.

[0050] Figure 3 The figure on the right shows the reconstruction result of the useful signal 320, with the original signal 311 as a reference.

[0051] 2) Tracking or detecting the phase change of a quantum mechanical state relative to a reference frequency: Another method for measuring the resonant frequency shift is to utilize quantum mechanical superposition states. Typical protocols include Free Induction Decay (FID, also known as the Ramsey-Protokoll protocol) or Hahn Echo. Here, a quantum mechanical superposition state is generated and allowed to evolve freely for a period of time. This state oscillates at its eigenfrequency (Ramoel frequency), and the phase difference between this oscillation and the microwave frequency is measured at the end.

[0052] A typical pulse sequence (FID) is . The first one The pulse generates a superposition state. Then, the evolution time begins. During this process, a phase difference accumulates due to the frequency difference between the Larmor precession and the microwave. This phase difference is ultimately resolved through the final... The pulses are converted into population differences (Populationsunterschied) so that they can be read out.

[0053] To generate a differential signal in this measurement scheme, the final... Pulse matching can be achieved in the following ways: By extending the pulse by half a rotation cycle, the population map can be inverted. That is, a single measurement uses... The pulse is the last pulse, and the next measurement will use it. pulse.

[0054] It can match the last one The phase of the pulse is such that the phase difference between two separate measurements is 180°, which will also cause the signal to reverse.

[0055] To always obtain the maximum sensitivity of the measured signal, the final... The phase of the pulse is adjusted so that it is always within the most sensitive region.

[0056] If inverting the measured signal is not possible, a differential signal can be generated relative to the blind measurement. That is, one measurement contains the desired information, while another measurement is prepared to contain no information. This can at least suppress signal fluctuations with half the signal strength.

[0057] In order to extract multiple signals from the detected measurement data using multiple modulation functions, the modulation functions must be orthogonal to each other. That is, for example, for two modulation functions M1 and M2, the following must be satisfied: .

[0058] Orthogonal bases for rectangular wave functions include, for example, Walsh functions. Figure 4 An example of an orthogonal basis consisting of four Walsh functions 401 to 404 (wal(1,t) to wal(4,t)) is shown. Using Walsh functions, similar to sine / cosine modulation, allows the extraction of multiple signals from a single measurement when they are modulated by different orthogonal rectangular wave functions. Typically, the highest-order Walsh function that the hardware can still implement is chosen as the first modulation function, and then the order of the Walsh functions is gradually decreased until all the required measurement signals are covered.

[0059] For cases with white noise, modulation and demodulation using rectangular excitation (Walsh function) achieve two advantages: firstly, it increases the information content to measurement time ratio, doubling the signal strength; secondly, Walsh function demodulation offers significantly higher bandwidth than trigonometric function demodulation, resulting in a substantial increase in noise in the demodulated signal. Therefore, under the assumption of white noise, the signal-to-noise ratio is actually improved by a factor of [number missing]. As mentioned earlier, in practical applications, 1 / f noise usually dominates, so the signal-to-noise ratio can be expected to be further improved.

Claims

1. A method for generating a useful signal (11) from a signal (5') emitted from a spin-based quantum system (4), comprising the steps of: The spin-based quantum system (4) is excited by an electromagnetic field (3), which jumps back and forth between two field states at a modulation frequency; The signal (5') emitted by the spin-based quantum system (4) is detected to obtain the measurement signal (7); The measurement signal (7) is multiplied by the modulation signal (9), the modulation signal switching back and forth between two modulation signal states at the modulation frequency, to generate a modulated measurement signal (7'); and The modulated measurement signal (7') is demodulated to generate the useful signal (11).

2. The method according to claim 1, wherein, The electromagnetic field (3) jumps back and forth between two modulation field intensities at the modulation frequency, and the modulation signal jumps back and forth between two modulation signal values ​​at the modulation frequency.

3. The method according to claim 2, wherein, The two modulation field intensities are opposites of each other and / or the two modulation signal values ​​are opposites of each other.

4. The method according to claim 1, wherein, The electromagnetic field (3) and the modulation signal (9) switch back and forth between two modulation frequency values ​​at the modulation frequency.

5. The method according to any one of the preceding claims, wherein, The spin-based quantum system (4) includes a sensing crystal with a color center, particularly diamond with a nitrogen-vacancy defect center.

6. The method according to any one of the preceding claims, wherein, The spin-based quantum system (4) includes a vapor chamber containing a mixture of at least one gaseous alkali metal and at least one gaseous inert gas.

7. The method according to any one of the preceding claims, wherein, The electromagnetic field (3) is a microwave field.

8. The method according to any one of the preceding claims, wherein, The signal (5') emitted by the spin-based quantum system (4) is a fluorescent signal or an optical signal.

9. The method according to any one of the preceding claims, wherein, In the previous step, the spin-based quantum system (4) is initialized with a first laser pulse and the quantum system (4) excited by the electromagnetic field (3) is read with a second laser pulse, such that the quantum system (4) emits a signal (5'), wherein the first laser pulse and the second laser pulse are contained in a common laser pulse, the first pulse portion of the common laser pulse is used as the second laser pulse for reading the previously initialized and excited quantum system by the electromagnetic field (3), and the second pulse portion of the common laser pulse is used as the first laser pulse for initializing the quantum system (4) for another iteration.

10. A device (100) for generating a useful signal (11) from a signal (5') emitted from a spin-based quantum system (4), comprising: Spin-based quantum systems (4); A field generating device (2) is used to generate an electromagnetic field (3), which jumps back and forth between two field states at a modulation frequency; The spin-based quantum system (4) is disposed in the electromagnetic field (3); A measuring device (6) is used to detect a signal (5') emitted by the spin-based quantum system (4) to obtain a measurement signal (7); and The calculation unit (8) is configured to multiply the measurement signal (7) with the modulation signal (9) to generate a modulated measurement signal (7'), and demodulate the modulated measurement signal (7') to generate a useful signal (11), wherein the modulation signal jumps back and forth between two modulation signal states at a modulation frequency.

11. The apparatus (100) according to claim 10, configured to perform the method according to any one of claims 2 to 9.

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