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 and a device (100) for generating a useful signal (11) from a signal (5') emitted from a spin-based quantum system (4), having the steps of exciting the spin-based quantum system (4) by means of an electromagnetic field (3) which switches back and forth between two states at a modulation 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 switches back and forth between two modulation signal states at the modulation frequency in order to generate a modulated measurement signal (7'), and demodulating the modulated measurement signal (7') in order to generate the useful signal (11).

Description

[0001] Description

[0002] title

[0003] Method and device for generating a measurement signal from a signal emanating from a spin-based quantum system

[0004] 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.

[0005] Background of the invention

[0006] For measuring very small magnetic field strengths, ion-based or optically pumped magnetometers are particularly suitable as sensors. Such magnetometers often use optically pumped and optically detected magnetic resonance (ODMR). This exploits the fact that the energy levels of certain spin states of unpaired electrons split under the influence of an external magnetic field, the so-called Zeeman effect. The splitting of the energy levels results in altered transitions during the relaxation 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 measured optical parameters can then be used to determine the magnetic field strength.

[0007] Sensor crystals with excitable defect centers can be used for such quantum-based magnetic field sensors. Typically, diamonds homogeneously doped with negative nitrogen vacancy centers (NV centers) are used. The quantum state is prepared via optical excitation and interaction with a static magnetic field and a dynamic magnetic field, i.e., a microwave field.

[0008] For measuring very low magnetic field strengths, quantum-based magnetic sensors based on nitrogen vacancy centers are known, for example, from DE 10 2018 220 234 A1 or DE 10 2018 214 617 A1. The information stored in the spin system is optically read out by detecting the spin-state-dependent fluorescence rates of the NV center. Therefore, the accuracy is limited, among other things, by the signal-to-noise ratio of the optical readout method of the NV center.

[0009] US 9 689 679 B2 discloses a diamond sensor with nitrogen vacancies to measure rotational orientation changes.

[0010] One type of spin-based quantum system or quantum sensor is NMR ("nuclear magnetic resonance") gyroscopes. These evaluate nuclear magnetic resonance signals from atomic nuclei with a non-vanishing magnetic moment. One way to set up a single-axis NMR gyroscope is to provide a vapor chamber with a mixture of an element with a non-vanishing nuclear spin (usually a noble gas, such as xenon (Xe)) and an alkali metal (such as potassium (K), cesium (Cs), or rubidium (Rb)). Using a polarized pump laser beam, the rubidium electron spins in the vapor cell can be polarized. Due to strong coupling between rubidium and xenon, this leads to a polarization of the xenon nuclear spins parallel to the rubidium electron spins. By means of a static magnetic field in the polarization direction, a nuclear spin precession of the xenon nuclear spins around the static magnetic field can be generated.The precession frequency is the Larmor frequency, which depends on the static magnetic field. A coherent precession of all nuclear spins can be achieved by applying an alternating magnetic field whose frequency corresponds to the Larmor frequency and which can be applied perpendicular to the static magnetic field. If a polarized sample or evaluation laser beam is then directed through the vapor cell perpendicular to the static magnetic field, the polarization of the sample laser beam is periodically rotated at the Larmor frequency due to the Faraday effect. Using a polarizer or polarization filter and a detector, an intensity fluctuation modulated by the Larmor frequency can be observed. Rotation of the sensor around an axis of rotation parallel to the static magnetic field leads to a shift in the Larmor frequency proportional to the rotation rate.By evaluating the intensity signal output by the detector, a change in the rotational orientation with a rotation axis parallel to the polarization direction can be determined. US Pat. No. 7,282,910 B1 discloses an example of the application of the described NMR gyroscope technique.

[0011] In general, spin-based sensor arrangements (also referred to here as quantum sensors) detect and evaluate frequency-modulated signals, for which a lock-in method can be used.

[0012] Disclosure of the invention

[0013] 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, having the features of the independent patent claims. Advantageous embodiments are the subject of the dependent claims and the following description.

[0014] 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 in the presence of high ambient noise R (especially in the presence of very high 1 / f noise, which is very critical for very low-frequency signals < 1 kHz (near dc)).

[0015] Such a modulation can be represented according to: M(t) = sin(a>t + p) * S(t) + R(t) where M is the measurement signal (i.e. measured signal), S(t) is the wanted signal (i.e. the signal to be measured or of interest (or its AC component)), R(t) is the ambient noise and <p ein Phasenversatz zwischen Modulation und Messung sind.

[0016] To extract the useful signal, a demodulation is then carried out in the form:

[0017] T

[0018] S = M(t) ■ sin(ü)t + <p) dt o wobei T das Integrationsfenster ist.

[0019] If the phase relationship p is not known, demodulation can also be carried out with a 90° phase shift and the useful signal can be reconstructed from both basis vectors.

[0020] Several useful signals can be extracted from the same measurement signal if they are modulated with different frequencies (sine / cosine of different frequencies are orthogonal to each other).

[0021] However, when using the lock-in technique with trigonometric functions (sine / cosine), the sensor is in a range for part of the measurement time where little or no information about the wanted signal is recorded. This means that a change in the wanted signal has no influence on the measurement signal, since the modulation prevents the wanted signal from being included in the measurement signal at this time. This requires improvement, as the ratio of information to measurement time is not ideally utilized.

[0022] The invention therefore utilizes a modulation signal that jumps or alternates between two field states at a modulation frequency. This can involve different signal values ​​or field strengths as well as different frequency values, depending on what is being excited or modulated in the spin-based quantum system. This allows the points with the highest information content to be sampled, and possible measurement points in between are not measured. This jumping back and forth corresponds to a rectangular function that has the modulation frequency. This type of modulation can also be referred to as bang-bang modulation.

[0023] The invention has the advantage that the useful signal can now be measured fully scaled (ie with maximum amplitude). With a sine / cosine modulation, the useful signal scales only by a factor 1A Thus, the signal-to-noise ratio is increased by a factor of 2 1 / 2 (Root 2) improved.

[0024] A further advantage is that a modulation signal, in particular a microwave signal, which jumps back and forth between two modulation field strengths or two modulation frequency values, is easier to realize than a sinusoidal frequency response.

[0025] For example, the following can be used as a modulation signal for frequency modulation: , if int (2 t / ) is just if int(2tf) is odd)

[0026] The modulation M(t) = rect(f, t) * S(t) + R(t) (1) and demodulation S = ■ rect(f, t) dt (2) are then carried out analogously.

[0027] Where f stands for the modulation frequency. Typical frequency values ​​of electromagnetic fields used to excite spin-based quantum systems are in the high-frequency range. The abbreviation "HF" will also be used for the term "high frequency." The frequency of the high-frequency field (HF 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 can, for example, be an NV center in a diamond, where the ground state is a triplet in which (without an external magnetic field) an energy difference corresponding to 2.87 GHz exists between spin equal to 0 on the one hand and spin equal to +1 or -1 on the other. Such NV centers in diamonds can, for example,be used as a quantum sensor.

[0028] The spin-based quantum system can be a quantum-based magnetic sensor based on nitrogen vacancy centers. This comprises an excitation light generating device for generating, for example, pulsed excitation light, a sensor crystal with color centers, in particular a diamond with nitrogen vacancy 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 designed to generate a microwave field. The measuring device is particularly designed to detect fluorescent radiation emitted by the sensor crystal as a result of the excitation of its color centers with the excitation light.

[0029] In particular, the excitation light generating device generates a laser pulse, which initializes the quantum system to a ground state. Subsequently, the field generating device radiates a pulsed microwave field into the quantum system, which manipulates the quantum system. After a waiting period, the manipulated quantum system is again read out using a laser pulse. Such a pulsed method exhibits an improved signal-to-noise ratio (SNR) compared to methods in which the excitation light generating device generates a continuous excitation light beam and / or the microwave field is continuously radiated.

[0030] The invention also encompasses a method in which the spin-based quantum system is first initialized by a first laser pulse generated by the excitation light generating device. In a further step, the spin-based quantum system is excited by an electromagnetic field, in particular the microwave field, which oscillates between two field states at a modulation frequency. In this way, the spin-based quantum system is manipulated. The actual detection of the magnetic field is performed in this step. The quantum system is irradiated with a second laser pulse for readout, causing it to emit the signal. This is detected by the measuring device, thus obtaining the measurement signal. The measurement signal is multiplied by the modulation signal, which oscillates between two modulation signal states at the modulation frequency.This results in a so-called modulated measurement signal, from which the useful signal can be generated, for example, through low-pass filter integration. In this way, the useful signal S can be separated from the ambient noise signal R. This enables very precise and sensitive detection of the magnetic field.

[0031] In a particularly advantageous embodiment, the first and second laser pulses can also be contained in a single laser pulse. In this case, a first pulse portion of the single laser pulse is used to read out the previously initialized and manipulated quantum system, while a second, later pulse portion of the single laser pulse serves to initialize the quantum system for a further iteration. This enables a particularly efficient method.

[0032] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawings.

[0033] The invention is illustrated schematically in the drawing using exemplary embodiments and is described below with reference to the drawing.

[0034] Brief description of the drawings 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.

[0035] Figure 2 shows schematic details of the procedure underlying the evaluation in a block diagram.

[0036] Figure 3 schematically shows individual stages of a measurement and evaluation according to an embodiment of the invention in a block diagram.

[0037] Figure 4 shows an orthogonal basis of rectangular functions for the separation of different signals based on Walsh functions.

[0038] Embodiment(s) of the invention

[0039] 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.

[0040] The device 100 comprises a field generating device 2 for generating an electromagnetic field 3, an excitation light generating device 12 for generating, for example, a pulsed excitation light 13 and the spin-based quantum system 4, which is to be excited by the electromagnetic field and the excitation light.

[0041] In some embodiments, the spin-based quantum system 4 can 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. The field generating device 2 is configured to generate an electromagnetic field 3, in particular a radio-frequency field (RF field), furthermore in particular a microwave field, which alternates or jumps back and forth between two field states at a modulation frequency.The two field states can be different field strengths, in particular with field strengths of equal magnitude but with different directions, or different frequency values ​​that are equally spaced from a center frequency.

[0042] The device 100 further comprises a measuring device 6 for detecting or measuring the signal 5' emanating from the spin-based quantum system 4 in order to obtain a measurement signal 7, which is transmitted to a computing unit 8.

[0043] In some embodiments, the signal 5' emitted by the spin-based quantum system 4 is a fluorescence signal or light signal. Accordingly, the measuring device 6 can, for example, comprise one or more photodiodes or a photodiode measuring arrangement.

[0044] The computing unit 8 is configured to evaluate the measurement signal 7 and to control the field generating device 2.

[0045] A possible functional design 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 left side of Figure 2 shows the signal generation taking place outside the computing unit 8, specifically in the spin-based quantum system 4.

[0046] As explained, the spin-based quantum system 4 is excited with an electromagnetic field 3, which jumps back and forth between two field states at a modulation frequency. The electromagnetic field 3 can, for example, jump in field strength, signal value, or frequency and 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.

[0047] This excitation causes the signal 5' emanating from the spin-based quantum system 4 to also carry this corresponding modulation, or to exhibit the corresponding modulation of a useful information 5. This corresponds to the modulation described above, equation (1).

[0048] The corresponding demodulation, equation (2), takes place in the computing unit 8, wherein 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 then, in block 10, the demodulation, here in the form of a low-pass filter integration, of the modulated measurement signal 7' takes place in order to generate the useful signal 11.

[0049] Embodiments of the invention can be advantageously used in different spin-based quantum systems, which are described below with reference to the previous figures as well as figures 3 and 4.

[0050] For a quantum sensor, such as the above-mentioned NV vacancy center in diamond, there are, for example, three regimes in which this measurement method can be used:

[0051] 1) Tracking or detecting a change in a resonance peak:

[0052] Using a quantum system based on an NV diamond (a diamond with nitrogen vacancies), the field strength of an external magnetic field can be determined, for example. To do this, the NV diamond can first be irradiated with pulsed excitation light – e.g., a first laser pulse – at a suitable wavelength. This initializes the quantum system, i.e., the system is placed in its ground state. The NV diamond is then irradiated with microwave radiation. If the incident microwave frequency corresponds to one of the resonant frequencies of the quantum system, a so-called resonance peak is created. The position of the resonance frequencies depends on the field strength and direction (relative to the crystal axes or NV directions) of an external magnetic field. If the field strength of the external magnetic field changes, the resonance frequency and thus the position of the peak shifts.

[0053] If one of the resonant frequencies is known, a change in the position of the resonance peak and thus a change in the field strength of the external magnetic field can be detected. To do this, the NV diamond is irradiated with frequency-modulated microwave radiation.

[0054] Figure 3 shows an example of frequency modulation with a rectangular function. On the left, an example peak 301 in an ODMR spectrum is shown. The frequency of the exciting microwave field is plotted on the x-axis, and the absorbed light intensity is plotted on the y-axis. In the frequency modulation regime, the points with the highest information density are the respective edges of the peak in the spectrum. The points with the maximum slope, i.e., signal change dS / df, are located at modulation frequencies f and f+. Since the slopes each have a different sign, each measurement point contains a full information density. These frequencies are advantageously used for rectangular frequency modulation, as shown below peak 301. The useful signal 320 at these points results in the change in the resonant frequency with sign, i.e.,From the useful signal 320, the change in the resonant frequency can be deduced, not only the magnitude but also the direction of the change in the resonant frequency (no change in the resonant frequency means a lock-in signal (i.e., a measurement signal 7) of zero). A shift in the resonant frequency leads to a change in the amplitude of the measurement signal—the greater the shift in the resonance peak, the greater the amplitude.

[0055] To read out the quantum system, which is initially initialized by the first laser pulse and subsequently irradiated with frequency-modulated microwave radiation, a second laser pulse is irradiated onto the quantum system, and the intensity of the fluorescence radiation emitted by the NV diamond as a result of the irradiation is recorded. The amplitude of the measurement signal depends on the degree of shift in the position of the actual resonance peak compared to the position of the resonance peak that would be present at the irradiated microwave frequency.

[0056] A method in which the quantum system is initialized and read out using a laser pulse, and in which the microwave field jumps back and forth between two frequencies, shows a significantly improved signal-to-noise ratio and at the same time significantly increased sensitivity compared to other methods in which the quantum system is initialized and read out, for example, with a cw (continuous wave) excitation light beam.

[0057] Since a change in the position of the peak here changes the measured value with the opposite sign, this corresponds to the alternating sign as defined above in rect f,t). With a rectangular frequency modulation of the excitation field, i.e. the periodic jumping back and forth or alternating between the two microwave frequencies f. and f+, the maximum information can be generated and extracted. The resonance peak can either be measured before the continuous measurement or is given as a system parameter in low magnetic fields (expected resonance shift due to external magnetic fields smaller than the linewidth of the resonance). The resonance curve can then be continuously recorded (feedback loop, in which the central frequency (i.e. the middle of f. and / +) is adjusted based on the measured signal difference between f+ and f).A continuous measurement is therefore understood to be a method in which the shift of the position of the resonance peak is recorded iteratively, i.e. the irradiated frequency of the irradiated microwave radiation (central frequency) is gradually adapted, for example, to the actually present resonance frequency.

[0058] If the phase relationship between the measurement and the modulation is unknown, the measured signal can also be demodulated using two modulation functions that are phase-shifted by 90°. This can then be used to determine the signal and its phase shift, analogous to sine / cosine modulation.

[0059] The middle image in Figure 3 shows a noisy measurement 310 (corresponds to the measurement signal 7) and the underlying (theoretical) original signal 311.

[0060] The right image in Figure 3 shows the reconstruction of the useful signal 320 with the original signal 311 as reference.

[0061] 2) Tracking or detecting a change in the phase of a quantum mechanical state with respect to a reference frequency:

[0062] Another way to measure the shift of a resonant frequency is the use of quantum mechanical superposition states. A typical protocol for this is, for example, free induction decay (FID, also called the Ramsey protocol) or the Hahn echo. This involves generating a quantum mechanical superposition state that evolves freely for a certain period of time. This state oscillates at its natural frequency (Lamor frequency), and the phase between this oscillation and the microwave frequency is measured at the end.

[0063] A typical pulse sequence (FID) is - T -

[0064] The first ^-pulse prepares the superposition state. Subsequently, a temporal evolution T occurs, in which a phase is collected due to the frequency difference between the Larmor precision and the microwave. This phase difference is then converted into a population difference with the last ^-pulse, which can then be read out.

[0065] To generate a differential signal in this measurement scheme, the last ^ pulse can be adjusted. This can be done as follows:

[0066] • By extending the pulse by another half rotation, an inversion of the population mapping can be generated. This means that a measurement is recorded with a last pulse as a - pulse and the next measurement with a — - 2 a 2

[0067] Pulse performed.

[0068] • The phase of the last pulse can be adjusted to create a 180° phase difference between the individual measurements. This also results in an inverted signal.

[0069] In order to always generate the maximum sensitivity of the measurement signal, the phase of the last ^-pulse can be adjusted to always be in the range of maximum sensitivity.

[0070] 3) If inversion of the measurement signal is not possible, a differential signal can be created compared to a blank measurement. This means that one measurement contains the desired information, and the other measurement is prepared so that no information is present. This also allows for the suppression of signal fluctuations while still maintaining half the signal strength.

[0071] In order to extract several signals from the recorded measurement data using several modulation functions, the modulation functions must be orthogonal to each other, ie for example for two modulation functions M1 , M2 the following must apply: J o Ml • M2 dt = 0.

[0072] An example of an orthogonal basis for rectangular functions is the Walsh function. Figure 4 shows an example of an orthogonal basis with four Walsh functions 401 to 404 (wal(1,t) to wal(4,t)). Analogous to sine / cosine modulation, the use of Walsh functions allows the extraction of multiple signals from a single measurement if they were modulated with different orthogonal rectangular functions. Typically, the Walsh function with the highest index still representable by the hardware is selected as the first modulation function, and then the Walsh index is reduced until all required measurement signals are covered.

[0073] In the case of white noise, the application of modulation and demodulation in the form of rectangular excitation (Walsh functions) leads, on the one hand, to an increase in the signal by a factor of 2 because the ratio of information content to measurement time is increased. On the other hand, the noise in the demodulated signal also increases by a factor of approximately root(2), since demodulation with Walsh functions has a significantly higher bandwidth than trigometric demodulation. Effectively, assuming white noise, this results in an improvement in the signal-to-noise ratio (SNR) by a factor of root(2). As already mentioned above, 1 / f noise is typically dominant in the application, so it can be assumed that the SNR will continue to improve.

Claims

Claims 1. A 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 electromagnetic field (3), which jumps back and forth between two field states with a modulation frequency, detecting the signal (5') emanating from 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 the modulation frequency in order to generate a modulated measurement signal (7'), and demodulating the modulated measurement signal (7') in order 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 strengths at the modulation frequency, and wherein 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 strengths and / or the two modulation signal values are opposite.

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

5. Method according to one 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.

6. Method according to one of the preceding claims, wherein the spin-based quantum system (4) comprises a vapor cell containing a mixture of at least one gaseous alkali metal and at least one gaseous noble gas.

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

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

9. Method according to one of the preceding claims, wherein the spin-based quantum system (4) is initialized in a previous step with a first laser pulse and the quantum system (4) excited with the electromagnetic field (3) is read out by a second laser pulse, so that the signal (5') is emitted by the quantum system (4), wherein the first and second laser pulses are in particular contained in a common laser pulse, wherein a first pulse part of the common laser pulse is used as a second laser pulse for reading out the quantum system previously initialized and excited with the electromagnetic field (3) and a second pulse part of the common laser pulse is used as a first laser pulse for initializing the quantum system (4) for a further iteration.

10. 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 generating device (2) for generating an electromagnetic field (3) which jumps back and forth between two field states at a modulation frequency, wherein the spin-based quantum system (4) is arranged in the electromagnetic field (3), 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) which is configured to multiply 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'), and to demodulate the modulated measurement signal (7') in order to generate the useful signal (11).

11. Device (100) according to claim 10, which is arranged to carry out a method according to one of claims 2 to 9.

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