Method and device for generating a measurement signal from a signal emitted from a spin-based quantum system

Abstract

The invention relates to a method for generating a useful signal (11) from a signal (5') emitted from a spin-based quantum system (4), said method including exciting the spin-based quantum system (4) by means of an excitation light (13), exciting the spin-based quantum system (4) by means of an electromagnetic field (3) which jumps back and forth between two measurement frequencies at a modulation frequency, the two measurement frequencies having a first measurement frequency distance from an expected resonant frequency, detecting the signal (5') emitted from the spin-based quantum system (4) in order 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 the modulation frequency in order to generate a modulated measurement signal (7'), demodulating the modulated measurement signal (7') in order to generate the useful signal (11), determining a comparison signal, and determining whether the expected resonant frequency value is correct based on the comparison signal.

Description

[0001] R.415348

[0002] - 1 -

[0003] Description

[0004] title

[0005] Method and apparatus for generating a measurement signal from a signal emanating from a spin-based quantum system

[0006] The present invention relates to a method and a device for generating a measurement signal from a signal emanating from a spin-based quantum system.

[0007] Background of the invention

[0008] To measure very small magnetic field strengths, guant-based or optically pumped magnetometers are particularly suitable as sensors. Such magnetometers often utilize optically pumped and optically detected magnetic resonances (ODMR). This method exploits the fact that, under the influence of an external magnetic field, the energy levels of certain spin states of unpaired electrons split—the so-called Zeeman effect. This energy level splitting results in altered relaxation transitions from excited states, which can then be measured, for example, by optical excitation and frequency-dependent 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.

[0009] For such guanine-based magnetic field sensors, sensor crystals with excitable defect centers can be used. Typically, diamonds homogeneously doped with negative nitrogen vacancy centers (NV centers) are employed. The quantum state preparation is carried out via R.415348.

[0010] - 2 - optical excitation and the interaction with a static magnetic field and a dynamic magnetic field, i.e. a microwave field.

[0011] For measuring very small magnetic field strengths, quantum-based magnetic sensors based on nitrogen-vacancy centers are known, for example, from DE 10 2018220234 A1 or DE 102018 214617 A1. In these sensors, the information stored in the spin system is read out optically by detecting the spin-state-dependent fluorescence rates of the nitrogen-vacancy center. Therefore, the accuracy is limited, among other things, by the signal-to-noise ratio of the optical readout method of the nitrogen-vacancy center.

[0012] Spin-based sensor arrangements (also referred to here as quantum sensors) can detect and evaluate frequency-modulated signals, for which a lock-in method can be used.

[0013] DE 102023209 714 relates to a method and a device for generating a useful signal from a signal emanating from a spin-based quantum system, comprising exciting the spin-based quantum system by an electromagnetic field which oscillates between two states at a modulation frequency (so-called bang-bang modulation), detecting the signal emanating from the spin-based quantum system in order to obtain a measurement signal, multiplying the measurement signal with a modulation signal which oscillates between two modulation signal states at the modulation frequency in order to generate a modulated measurement signal, and demodulating the modulated measurement signal in order to generate the useful signal.

[0014] Disclosure of the invention

[0015] According to the invention, a method and a device for generating a useful signal from a signal emanating from a spin-based quantum system are proposed, comprising the features of the independent claims. Advantageous embodiments are the subject of the dependent claims and the following description. R.415348

[0016] - 3 -

[0017] In a conventional lock-in method, a useful signal S is modulated with a sinusoidal modulation signal. By knowing the modulation frequency, the useful signal can be extracted from the measurement signal M even with high ambient noise R (especially with very high 1 / f noise, which is very critical for very low-frequency signals < 1 kHz (near DC)).

[0018] Such a modulation can be represented according to: M(t) = sin(a>t + p) * S(t) + R(t) where M{t) is the measurement signal (i.e., the measured signal), S{t) is the useful signal (i.e., the signal to be measured or of interest (or its AC component)), R{t) is the ambient noise, and p is a phase shift between modulation and measurement.

[0019] To extract the useful signal, a demodulation of the form of the following is then performed:

[0020] T

[0021] S = M(t) ■ sin(ü)t + <p) dt o wobei T das Integrationsfenster ist.

[0022] If the phase relationship p is unknown, demodulation can also be performed with a 90° phase shift, and the useful signal can be reconstructed from both basis vectors.

[0023] Multiple useful signals can be extracted from the same measurement signal if they are modulated with different frequencies (sine / cosine signals of different frequencies are orthogonal to each other).

[0024] However, when using the lock-in technique with trigonometric functions (sine / cosine), the sensor is in a region for part of the measurement time where little or no information about the useful signal is recorded; that is, a change in the useful signal has no effect on the measurement signal, since the modulation only minimally affects the useful signal at that time. R.415348

[0025] - 4 - is addressed. This could be improved, as the ratio of information to measurement time is not used ideally here.

[0026] The invention therefore employs the measure of using a square wave signal as a modulation signal, which oscillates between two modulation signal states at a modulation frequency. Specifically, this involves modulating the electric field such that it oscillates between two frequencies chosen relative to a center frequency that corresponds to an expected resonance line. This ensures that the points with the highest information content are sampled, while potential measurement points in between are not measured.

[0027] This back-and-forth oscillation corresponds to a rectangular function that has the modulation frequency. Such modulation can also be called bang-bang modulation.

[0028] This has the advantage that the useful signal can now be measured fully scaled (i.e., at maximum amplitude). With sine / cosine modulation, the useful signal only scales by a factor. 1 A Thus, the signal-to-noise ratio is increased by a factor of 2 by the invention. 1 / 2 (Square root of 2) improved.

[0029] Another advantage is that a modulated excitation signal, especially a microwave signal that oscillates between two measurement frequency values, is easier to implement than a sinusoidal frequency response.

[0030] For example, the following can be used as a modulation signal for frequency modulation: , if int (2 t / ) is even if int(2tf) is odd with the two modulation signal states 1 and -1.

[0031] The modulation M(t) = rect(f, t) * S(t) + R(t) and demodulation S = ■ rect(f, t) dt (2) R.415348

[0032] - 5 - will then be carried out analogously.

[0033] Typical frequencies of electromagnetic fields used to excite spin-based quantum systems lie in the high-frequency range. The frequency of the high-frequency field depends on the quantum system to be excited, i.e., on the energy difference (corresponding to a frequency) between the quantum mechanical states of the quantum system between which transitions are to be induced. Typical frequencies can be in the microwave range, e.g., between 300 MHz and 300 GHz or between 300 MHz and 1 THz. The quantum system could, for example, be an NV center in a diamond, where the ground state is a triplet in which (without an external magnetic field) there is an energy difference corresponding to 2.87 GHz between spin equal to |ms = 0> on the one hand and spin equal to |ms = ±1> on the other. Such NV centers in diamonds can, for example, be used as quantum sensors.

[0034] When such a measurement method is used for the continuous measurement of magnetic fields, continuous determination (so-called tracking) of the resonance frequency is necessary, which manifests as a dip (signal dip) in the fluorescence signal. However, ambiguous magnetic field values ​​can occur if the measurement points do not lie on the edges of the dip (but, for example, on the baseline, i.e., if the tracking has failed).

[0035] The present invention also describes a method to solve this problem. For this purpose, a reference signal is additionally determined, and the accuracy of the expected resonant frequency is determined based on this reference signal.

[0036] The invention proposes a method for verifying the plausibility of a measurement in order to detect ambiguous measured values ​​of an applied magnetic field. Compared to existing tracking or modulation methods, this method enables an unambiguous determination of the magnetic field while simultaneously utilizing the advantages of bang-bang modulation. These include increased sensitivity, which R.415348

[0037] - 6 -

[0038] Reduction of the readout time and compensation for fluctuations in the incident excitation light.

[0039] According to one embodiment, the expected resonant frequency is determined to be correct if the reference signal differs sufficiently from the useful signal and / or the signal emanating from the spin-based quantum system. Once the tracking has successfully hit the dip, the useful signal will show a reduced intensity compared to the baseline. If the reference signal is determined at at least one other point, it will also exhibit a different value or shape. "Sufficiently" in this context means that, for example, deviations due to noise or tolerances may be permissible within certain limits. According to one embodiment, the at least one other point can be located on the baseline, or at the minimum or center of the dip, i.e., at the expected resonant frequency, or at other points on the edges of the dip.

[0040] According to one embodiment, the expected resonant frequency is deemed incorrect if the reference signal does not differ sufficiently from the desired signal. If the tracking fails to capture the dip and the measurement points lie on the baseline, the desired signal will not show a reduced intensity compared to the baseline. If the reference signal is measured at a different point on the baseline, it will not exhibit a (significantly) different value or shape.

[0041] According to one embodiment, the comparison signal is determined by exciting the spin-based quantum system with an electromagnetic field that oscillates between two further measurement frequencies at a second modulation frequency, wherein at least one of the two further measurement frequencies has a second measurement frequency offset from the expected resonance frequency that differs from the first measurement frequency offset, and by capturing the signal emanating from the spin-based quantum system to obtain the comparison signal. R.415348

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[0043] According to one embodiment, the comparison signal can be derived directly from the signal level at the two other measurement frequencies.

[0044] According to another embodiment, acquiring the signal emanating from the spin-based quantum system to obtain the comparison signal comprises acquiring the signal emanating from the spin-based quantum system to obtain a second measurement signal, multiplying the second measurement signal with a second modulation signal that oscillates between two modulation signal states at the second modulation frequency to generate a second modulated measurement signal, and demodulating the second modulated measurement signal to generate the comparison signal.

[0045] In other words, in the aforementioned embodiments, a second measurement is performed at different frequency values ​​(see Fig. 5b), which are not equidistant from the expected resonant frequency and therefore exhibit different signal values ​​when the dip is encountered. In this case, the reference signal, particularly at the second modulation frequency, oscillates between two signal values, whereas the desired signal essentially oscillates between the same values, i.e., it remains essentially constant. Therefore, if the reference signal deviates sufficiently from the desired signal, as expected, the anticipated resonant frequency is correct. The second modulation frequency can be the same as the first or different from it.

[0046] According to one embodiment, the reference signal is determined by exciting the spin-based quantum system with an electromagnetic field whose frequency (hereinafter also referred to as the third measurement frequency) differs from the two measurement frequencies or has at least a third frequency offset from the expected resonance frequency value that differs from the first frequency offset, and by capturing the signal emanating from the spin-based quantum system to obtain the reference signal. In other words, at least one further measurement point (see fa in Fig. 3a) is selected for the reference signal, in particular on the baseline or in the R.415348

[0047] - 8 -

[0048] Center of the dip. The third frequency interval is therefore either expediently chosen to be large enough so that the third measurement frequency lies next to the dip on the baseline, or it is chosen to be zero so that the third measurement frequency corresponds to the expected resonance frequency.

[0049] According to one embodiment, the reference signal is determined by reading out a baseline intensity from a fluorescence spectrum. The fluorescence spectrum can be acquired, for example, by exciting the spin-based quantum system with excitation light, by exciting the spin-based quantum system with an electromagnetic field whose frequency is continuously or stepwise changed from a lower to an upper frequency value, and by capturing the signal emitted by the spin-based quantum system to obtain the fluorescence spectrum. The reference signal can then be determined as a function of the baseline intensity of the fluorescence spectrum, for example, as an average value.

[0050] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawing.

[0051] The invention is schematically illustrated in the drawing using exemplary embodiments and is described below with reference to the drawing.

[0052] Brief description of the drawings

[0053] Figure 1 schematically shows a device for generating a measurement signal from a signal emanating from a spin-based quantum system according to an embodiment of the invention in a block diagram.

[0054] Figure 2 schematically shows details of the underlying evaluation procedure in a block diagram. R.415348

[0055] - 9 -

[0056] Figure 3 shows in two figures a) and b) a curve of the fluorescence intensity against the measurement frequency for an idealized dip at a resonance frequency.

[0057] embodiment(s) of the invention

[0058] Figure 1 schematically shows a device 100 for generating a measurement signal from a signal 5' emanating from a spin-based quantum system 4 according to an embodiment of the invention in a block diagram.

[0059] The device 100 comprises a field generation device 2 for generating an electromagnetic field 3, an excitation light generation device 12 for generating, for example, pulsed excitation light 13 and the spin-based quantum system 4, which is to be excited by the electromagnetic field and the excitation light.

[0060] The spin-based quantum system 4 can, in various embodiments, comprise a sensor crystal with color centers, in particular a diamond with nitrogen-vacancy centers. In other embodiments, the spin-based quantum system 4 can comprise a vapor cell containing a mixture of at least one gaseous alkali metal and at least one gaseous noble gas. Other types of spin-based quantum systems can also be advantageously used within the scope of the invention.

[0061] The field generation device 2 is configured to generate an electromagnetic field 3, in particular a high-frequency field, and further, in particular a microwave field, which alternates or oscillates between two measurement frequencies at a modulation frequency. The two measurement frequencies can be different frequency values ​​that are equidistant from a center frequency.

[0062] The device 100 further comprises a measuring device 6 for detecting or measuring the R.415348 emanating from the spin-based quantum system 4.

[0063] - 10 -

[0064] Signals 5' to obtain a measurement signal 7 which is transmitted to a computing unit 8.

[0065] The signal 5' emanating from the spin-based quantum system 4 is, in certain configurations, a fluorescence signal or light signal. Accordingly, the measuring device 6 can, for example, comprise one or more photodiodes or a photodiode measuring arrangement.

[0066] The computing unit 8 is designed to evaluate the measurement signal 7 and to control the field generation device 2.

[0067] A possible functional configuration of an evaluation module implemented in the computing unit 8, together with the signal generation, is shown as a block diagram in Figure 2. The signal generation taking place outside the computing unit 8, specifically in the spin-based quantum system 4, is shown on the left side of Figure 2.

[0068] As explained, the spin-based quantum system 4 is excited by an electromagnetic field 3, which oscillates between two measurement frequencies at a modulation frequency. The electromagnetic field 3 is based on a modulation signal 9 or is generated by controlling the field generation device 2 with the modulation signal 9. This can, in particular, be a frequency modulation.

[0069] This excitation causes the signal 5' emanating from the spin-based quantum system 4 to also carry this corresponding modulation, or rather, to exhibit the corresponding modulation of useful information 5. This corresponds to the modulation described above, equation (1).

[0070] The associated demodulation, equation (2), takes place in the processing unit 8, whereby first a phase adjustment if necessary and then the multiplication of the measurement signal 7 with the modulation signal 9 takes place in order to generate the modulated measurement signal 7', and subsequently, block 10, the demodulation, here in R.415348

[0071] - 11 -

[0072] A low-pass filter integration of the modulated measurement signal 7' is performed to generate the useful signal 11.

[0073] Embodiments of the invention can be advantageously employed as described below with reference to the preceding figures and Figures 3a and 3b. In particular, a change in a resonance peak in a fluorescence spectrum can be monitored or tracked, from which a change in an external magnetic field can be determined.

[0074] Figs. 3a and 3b each show an idealized profile 301 of the fluorescence intensity I versus the microwave frequency f, as it can be obtained from a continuous measurement on an NV quantum system as a spin-based quantum system at the position of a resonance frequency fo.

[0075] To obtain such a fluorescence spectrum, the NV quantum system is continuously illuminated (continuous wave, cw) with green light (523 nm) as excitation light, thus polarizing it to the state |ms = 0>. Simultaneously, a microwave field is continuously applied as an electromagnetic field and its frequency is tuned. The fluorescence of the NV quantum system is then detected. When the microwave frequency is resonant with the transition |ms = 0>—>|m s If the magnetic field strength is less than ±1, a power-dependent decrease (dip) in the fluorescence of approximately 30% can be observed. If an external magnetic field is changed, the resonance frequency fo shifts.

[0076] In a measurement, the center frequency of the dip can be determined by measuring a measurement frequency f to the left of the expected resonant frequency and a second measurement frequency f+ to the right of the expected resonant frequency. These can be applied alternately using the bang-bang method described above. The two measurement frequencies f+ have a first measurement frequency difference of 302 from an expected resonant frequency f0.

[0077] If these have the same signal value, the center frequency corresponds to the expected resonant frequency f0. If they do not have the same signal value, R.415348

[0078] - 12 - The expected resonant frequency can be tracked until it again corresponds to the actual resonant frequency f0. However, different signal values ​​only occur if the two measurement frequencies f+, f. are still on the edges of the dip. If they both lie on the baseline I0, they exhibit the same signal value, even though the center frequency is no longer a resonant frequency. This problem can be resolved in embodiments of the invention as follows.

[0079] An additional reference signal is determined, and this signal is used to verify whether the expected resonant frequency is correct. The reference signal can be determined at regular intervals, particularly to check the tracking.

[0080] According to a first embodiment, as shown in Fig. 3b, the comparison signal is determined by exciting the spin-based quantum system by an electromagnetic field which oscillates between two further measurement frequencies f+', f.' at the modulation frequency. One of the further measurement frequencies f.' has a second measurement frequency offset 304 from the expected resonance frequency, which differs from the first measurement frequency offset 302.

[0081] This can be achieved by adding a small offset Af to one of the measurement frequencies f. to obtain the further measurement frequency f': f.' = f. + Af.

[0082] In the example shown, the other of the further measurement frequencies f+ also has a fourth measurement frequency distance 305 from the expected resonance frequency, which differs from the first measurement frequency distance 302.

[0083] This can be achieved by adding the same or a different offset Af to both measurement frequencies and f+. It can be advantageous to choose the offset symmetrically, such that: ' = f + Af. R.415348

[0084] - 13 - f + ' = f + + Af.

[0085] Due to the asymmetrical reading of the two points, the differential signal |signal(f+') - signal( ') | > 0 is always finite and thus uniquely distinguishable from two points on the baseline, where |signal(f+') - signal(f.')| » 0 would hold, and likewise distinguishable from the useful signal, which is formed by two symmetrical measurement frequencies f+, f. on edges of the dip (see Fig. 3a).

[0086] According to a second embodiment, shown in Fig. 3a, the comparison signal is determined by exciting the spin-based quantum system with an electromagnetic field with a third measurement frequency fa, which differs from the two measurement frequencies f+, f.

[0087] The measurement frequency fa has a third frequency difference 303 from the expected resonance frequency value fo, which differs from the first frequency difference 302, and which can also be zero (measurement frequency fa = fo ).

[0088] In the first variant, in addition to the two frequencies f+ and f, the signal is also read out at a third frequency fa, which lies sufficiently far outside the resonance on the baseline, particularly at regular time intervals. The desired signal can then be compared with the reference signal, which is equivalent to a comparison at the three frequencies.

[0089] If signal(f+) » signal(f.) » signal(fG), it is shown that all three points are far away from the resonance and the magnetic field measurement would give a false value.

[0090] In the second variant, the third frequency fa can also be selected at the resonance fo. In this case, too, the signal value must differ from the signal values ​​of the two measurement frequencies. R.415348

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[0092] The embodiment shown in Fig. 3a additionally allows the calculation of a slope. For example, for fa > f+, the slope would be given by: m = signal(fG) - signal(f+) / (fa- f+) » 0.

[0093] The finite slope shows that the two selection points f+ and f. cannot lie on the flat baseline.

[0094] According to a third embodiment, the comparison signal is determined by reading out a baseline intensity Io from a fluorescence spectrum.

[0095] In this simple case, the measured signal level at measurement frequencies f+ and f. is compared with the expected baseline intensity Io, which can be obtained, for example, from a previous calibration measurement. As shown in Fig. 3a, signal(f+) and signal(f.) are equal to signal(fG). If the measured signal levels are exactly at or very close to the baseline signal, this indicates that the tracking has failed.

[0096] Depending on the application, one of the proposed solutions is preferable. In the second embodiment, a compromise is made in the bandwidth to ensure distinguishability, while in the first embodiment, an offset in the readout frequency leads to reduced sensitivity. The trivial third embodiment is only preferable in the ideal case, when the ODMR signal is stable during the measurement. In practice, fluctuations in the excitation light, for example, lead to direct fluctuations in the ODMR signal, thus rendering a comparison of the absolute signal values ​​meaningless.

Claims

R.415348 - 15 - Claims 1. Method for generating a useful signal (11) from a signal (5') emanating from a spin-based quantum system (4), comprising the steps: Excitation of the spin-based quantum system (4) by an excitation light (13; 320), Excitation of the spin-based quantum system (4) by an electromagnetic field (3) which oscillates between two measurement frequencies (f+, f.) at a modulation frequency, wherein the two measurement frequencies (f+, f.) have a first measurement frequency difference (302) from an expected resonance frequency (f0), Capturing the signal (5') emanating from the spin-based quantum system (4) in order to obtain a measurement signal (7), Multiplying the measurement signal (7) with a modulation signal (9) that oscillates between two modulation signal states at the modulation frequency to generate a modulated measurement signal (7'), Demodulating the modulated measurement signal (7') to generate the useful signal (11), Determining a comparison signal, and Determine whether the expected resonant frequency value is correct, using the reference signal.

2. The method of claim 1, wherein determining whether the expected resonance frequency value is correct comprises using the reference signal: Determine that the expected resonance frequency is correct if the comparison signal differs sufficiently from the useful signal (11) and / or the signal (5') emanating from the spin-based quantum system (4). R.415348 - 16 - 3. A method according to claim 1 or 2, wherein determining whether the expected resonance frequency value is correct comprises using the reference signal: Determine that the expected resonance frequency is incorrect if the comparison signal does not differ sufficiently from the useful signal and / or the signal (5') emanating from the spin-based quantum system (4).

4. Method according to any one of the preceding claims, comprising determining a reference signal: Excitation of the spin-based quantum system (4) by an electromagnetic field (3) which oscillates between two further measurement frequencies with a second modulation frequency, wherein at least one of the two further measurement frequencies has a second measurement frequency offset from the expected resonance frequency that differs from the first measurement frequency offset, Capturing the signal (5') emanating from the spin-based quantum system (4) in order to obtain the comparison signal.

5. The method of claim 4, wherein the detection of the signal (5') emanating from the spin-based quantum system (4) in order to obtain the comparison signal comprises: Capturing the signal (5') emanating from the spin-based quantum system (4) in order to obtain a second measurement signal (7), Multiplying the second measurement signal (7) with a second modulation signal (9) that oscillates between two modulation signal states at the second modulation frequency to generate a second modulated measurement signal (7'), Demodulating the second modulated measurement signal (7') to generate the comparison signal (11).

6. A method according to any of the preceding claims, comprising determining a reference signal: R.415348 - 17 - Excitation of the spin-based quantum system (4) by an electromagnetic field (3) which has at least one third measurement frequency (f G) exhibits at least a third frequency interval () that differs from the first frequency interval () from the expected resonance frequency value (fo), Capturing the signal (5') emanating from the spin-based quantum system (4) in order to obtain the comparison signal.

7. Method according to any of the preceding claims, comprising determining a reference signal: Reading out a baseline intensity from a fluorescence spectrum.

8. Method according to any of the preceding claims, wherein the spin-based quantum system (4) comprises a sensor crystal with color centers, in particular a diamond with nitrogen vacancy centers.

9. Method according to any of the preceding claims, wherein the electromagnetic field (3) is a microwave field.

10. Method according to any of the preceding claims, wherein the signal (5') emanating from the spin-based quantum system (4) is a fluorescence signal or a light signal.

11. Device (100) for generating a useful signal (11) from a signal (5') emanating from a spin-based quantum system (4), comprising: the spin-based quantum system (4), a field generation device (2) for generating an electromagnetic field (3) that oscillates between two measurement frequencies at a modulation frequency, wherein the spin-based quantum system (4) is arranged in the electromagnetic field (3), R.415348 - 18 - a measuring device (6) for detecting the signal (5') emanating from the spin-based quantum system (4) in order to obtain a measurement signal (7), and a computing unit (8) configured to control the device so that it performs a method according to one of the preceding claims.

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