Method and device for evaluating a measurement signal emanating from a spin-based quantum system

Abstract

The invention relates to a method and a device for evaluating a measurement signal emanating from a spin-based quantum system, comprising exciting the spin-based quantum system by a pulsed excitation light (320), detecting the measurement signal emanating from the spin-based quantum system, integrating the measurement signal over an integration window (331) containing a signal time of a first light pulse (322) to obtain a signal value, integrating the measurement signal over an integration window (332) containing a signal time of a second light pulse (322) to obtain a reference value, wherein the first and second light pulses are two time-displaced light pulses of the excitation light (320), and determining a measured value from the signal value and the reference value.

Description

The present invention relates to a method and a device for evaluating a measurement signal originating from a spin-based quantum system.BACKGROUND OF THE INVENTIONIn order to measure very small magnetic field strengths, particularly quantum-based or optically pumped magnetometers are suitable as sensors. Such magnetometers often use optically pumped and optically detected magnetic resonance (ODMR). This makes use of the fact that, under the influence of an external magnetic field, the energy levels of certain spin states of unpaired electrons split up, the so-called Zeeman effect. As a result of the splitting of the energy levels, changed transitions in the relaxation result from excited states which can then be measured, for example, by optical excitation and frequency-dependent detection of the resulting fluorescent radiation or by observation of optical properties such as the absorption of light. The magnetic field strength can then again be deduced from the measured optical parameters.For such quantum-based magnetic field sensors, sensor crystals with stimulable defect centers can be used. Typically, diamonds are used that are homogeneously doped with nitrogen hole (NV) negative centers. The quantum state is prepared by optical excitation and the interaction with a static magnetic field and a dynamic magnetic field, i.e. a microwave field.Quantum-based magnetic sensors based on nitrogen defect centers are known, for example, from DE 10 2018 220 234 A1 or DE 10 2018 214 617 A1. In this case, the information stored in the spin system is optically read out by detecting the fluorescence rates of the NV center which are dependent on the spin state. Therefore, the accuracy is limited, among other things, by the signal-to-noise ratio of the optical readout method of the NV center.In general, in spin-based sensor arrangements (also referred to here as quantum sensors), pulsed signals are detected and evaluated, for which a self-referencing method can be used. When evaluating a measurement signal, a contrast can be calculated which indicates the spin state of the electron spin. For this purpose, the measurement signal can be integrated, on the one hand, via an integration window containing a signal time of a light pulse in order to obtain a signal value, and, on the other hand, can be integrated via an integration window containing a reference time of the light pulse in order to obtain a reference value. A measured value can be determined from the signal value and reference value, e.g. as a quotient. The integration window with the reference time usually comprises a time period in the measurement signal at which no useful information is present, but merely a background information or a zero signal or base signal. By contrast, the integration window with the signal time is typically chosen such that the maximum information density is contained therein, i.e. the signal-to-noise ratio is optimum. This allows decoupling of the measurement result from the measured photon current and suppression of the intensity noise.Disclosure of the InventionAccording to the invention, a method and a device for evaluating a measurement signal originating from a spin-based quantum system having the features of the independent patent claims are proposed. Advantageous embodiments are the subject matter of the dependent claims and of the following description.The invention makes it possible to overcome disadvantages in the prior art. If a terminal measuring system operates with an optical excitation power under saturation, the optimal length of the integration window with the signal time is dependent on the optical power density. This leads to the conventional fact that mechanical instabilities and fluctuations in the excitation power lead to a fluctuation in the signal value. This leads to a change or fluctuation in the measured value, which adversely influences the sensitivity, for example. Furthermore, conventionally, no additional information about the signal to be measured is obtained in the integration window with the reference time, which has a negative influence on the sensitivity, since the proportion of time in which information is optically read out from the system is not optimum.The invention now uses the measure of integrating the measurement signal via an integration window containing a signal time of a first light pulse in order to obtain a signal value, on the one hand, and integrating the measurement signal via an integration window containing a signal time of a second light pulse in order to obtain a reference value, on the other hand, for evaluating a measurement signal originating from a spin-based quantum system, wherein the first and the second light pulse are two light pulses of a pulsed excitation light offset in time. The light pulses are expediently readout pulses, furthermore expediently two directly successive readout pulses.A measured value can be determined from the signal value and reference value, e.g. as a quotient or difference, or as a quotient of a difference between the signal value and reference value and a sum of the signal value and reference value. A measured value determined in this way expresses a contrast in the measurement signal.In one embodiment, the spin-based quantum system is furthermore excited by an electromagnetic field which jumps back and forth between two field states at a modulation frequency, wherein the first light pulse temporally coincides with a first of the two field states and the second light pulse temporally coincides with a second of the two field states.This jump back and forth corresponds to a rectangular function having the modulation frequency. Such a modulation can also be referred to as bang-bang modulation.The field states can be both different signal values or field strengths and different frequency values, depending on what is excited or modulated in the spin-based quantum system. As a result, the points having the highest information content are now scanned, and possible measurement points therebetween are not measured.This has the advantage that the useful signal can now be measured fully scaled (i.e. with maximum amplitude). In the case of sine / cosine modulation, as in the case of a conventional lock-in method, the useful signal only scales by a factor of 1⁄2. Thus, the signal / noise ratio is improved by a factor of 2 1 / 2( root 2). When using lock-in technology with trigonometric functions (sine / cosine), the sensor is located a part of the measurement time in a range in which no / little information about the measurement signal is recorded, i.e. a change in the measurement signal has no influence on the measurement value, since the measurement signal does not enter into the signal value at this point in time due to the modulation. 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, can be realized more easily than a sinusoidal frequency profile.Typical frequency values of electromagnetic fields used to excite spin-based quantum systems are in the high frequency range. The abbreviation "HF" is also used below for the term "radio frequency". The frequency of the radio frequency (RF) 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 effected. Typical frequencies can be in the microwave range, for example between 300 MHz and 300 GHz or between 300 MHz and 1 THz. The quantum system may be, for example, an NV center in a diamond, wherein the ground state is a triplet in which (without external magnetic field) there is an energy difference corresponding to 2.87 GHz between spin equal to 0 on the one hand and spin equal to +1 or -1 on the other hand. Such NV centers in diamonds can be used, for example, as quantum sensors.In embodiments of the invention, the two field states are selected such that the measurement signal in the integration window with the signal time of the first light pulse has a slope with a different sign than the measurement signal in the integration window with the signal time of the second light pulse.This allows the shortening of the optical readout pulse to the time duration of the integration window, which directly results in an improved sensitivity of the measurement system, since more measurements can be made in the same time.For example, this results in a calculation rule as follows:contrat differental: measured value+, signal.: signal value or reference valueFurthermore, a variation in the signal value is compensated for by a normalization to the sum of the signal value and the reference value, which leads to an increased robustness with respect to variations in the excitation power density. This increases the robustness in the system and allows a sensitivity of the sensor system comparable to a system with two photodiodes that achieve such fluctuations by balancing the photodiode signal without the need for such a balancing circuit.Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawing.The invention is schematically illustrated in the drawing on the basis of exemplary embodiments and is described below with reference to the drawing.Brief Description of the DrawingsFIG. 1 schematically shows a block diagram of an apparatus for generating a measurement signal from a signal originating from a spin-based quantum system according to an embodiment of the invention. FIG. 2 shows in FIG. a) a sequence for a pulsed ODMR measurement and in FIG. b) time-dependent fluorescence emission of two different spin states. FIG. 3 shows a sequence for a pulsed ODMR measurement according to an embodiment of the invention. FIG. 4 shows schematically details of an embodiment of a method on which the evaluation is based in a block diagram. FIG. 5 schematically shows individual stages of a measurement and evaluation according to an embodiment of the invention in a block diagram.Embodiment(s) of the InventionFIG. 1 schematically shows a block diagram of a device 100 for evaluating a measurement signal 5' originating from a spin-based quantum system 4 according to an embodiment of the invention.The apparatus 100 comprises an excitation light generating device 12 for generating a pulsed excitation light 13 and a field generating device 2 for generating an electromagnetic field 3 and the spin-based quantum system 4 to be excited by the excitation light 13 and the electromagnetic field.In embodiments, the spin-based quantum system 4 can have a sensor crystal with color centers, in particular a diamond with nitrogen defect centers. In other embodiments, the spin-based quantum system 4 can have a vapor cell which contains 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 used advantageously 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), further in particular a microwave field, which changes or jumps back and forth between two field states with a modulation frequency. The two field states can be different field strengths, in particular with field strengths of equal magnitude with different directions, or different frequency values which are equally spaced from a center frequency.The device 100 further comprises a measuring device 6 for detecting the measurement signal 5' emanating from the spin-based quantum system 4 in order to obtain a raw signal 7, which is transmitted to a computing unit 8.In embodiments, the measurement signal 5' emanating from the spin-based quantum system 4 is a fluorescence signal or light signal.Accordingly, the measuring device 6 can have, for example, one or more photodiodes or a photodiode measuring arrangement.The arithmetic unit 8 is configured to evaluate the raw signal 7 and to actuate the field generation device 2.An exemplary measuring process is illustrated in FIG. 2 a, possible measurement signals zw. Raw signals in FIG. 2 b. In this case, FIG. 2 ashows a profile 210 of the microwave field 3, a profile 220 of the excitation light 13 and a profile 230 of integration windows.Pulsed ODMR experiments conventionally comprise an initial light pulse (in particular laser pulse) 221 for spin polarization, a series of resonant microwave pulses 211 for spin manipulation and a light pulse (in particular laser pulse) 222 for spin readout via the fluorescence intensity (readout pulse). An exemplary pulse sequence scheme is illustrated in FIG. 2 a.An exemplary time dependence of fluorescence (photon number n) for NV centers in diamonds is shown in FIG. 2 bfor the initial spin states |m=0> and |m=±1>. With longer illumination (t>500 ns), both curves show the same steady-state intensity, which is determined by the ratios of the different transition rates of the intercombination and is conventionally used for determining the reference value.When the initial state is |m=0>, a higher photoluminescence intensity is first observed, which drops to the intensity in the steady state, since a part of the population is trapped in the metastable singlet state due to the remaining intercombination. For an initial state |m=±1>, the intensity drops rapidly to a low level due to the rapid intercombination into the singlet state. Since the singlet state preferably falls to the ground state |m=0>, the intensity recovers in the steady-state direction (~250 ns) within the lifetime of the metastable state. The difference between both curves (lower graph in Figure 2b) represents the signal photons used to read out the spin state. For the evaluation, the measurement signal can be integrated on the one hand via an integration window 231 containing a signal time of a light pulse in order to obtain a signal value and on the other hand via an integration window 232 containing a reference time of the light pulse in order to obtain a reference value. In order to read out the spin state at the end of the sequence with high accuracy, it is thereby decisive to detect the first fluorescence photons after the laser is switched on before the NV center is repolarized in |m=0>. As can be seen in FIG. 2 b, signal times lie in a range from 0 to approximately 0.5 μs, and reference times are correspondingly after this.A pulse sequence for an evaluation according to an embodiment of the invention is shown in FIG. 3, wherein the components corresponding to those of FIG. 2 ahave a reference number increased by 100, but will not be explained again. FIGS. 3 and 2 adiffer in particular in the curves 230 and 330, wherein in the curve 330 the integration window 332 containing a reference time of the light pulse does not belong to the same light pulse as the integration window 331 containing a signal time of a light pulse, but to two temporally offset light pulses 322, in particular to two directly consecutive readout pulses 322. The duration of one or each readout pulse 322 can thus be reduced to the duration of the integration window and the time interval T of the rising edges of two adjacent light pulses 322, 321 decreases accordingly.Furthermore, it is provided in particular that the integration window 332 containing the reference time of the light pulse of a previous measurement simultaneously represents the integration window 331 containing the signal time of a light pulse of the subsequent measurement.In other words, for continuous output of measurement values, the measurement value at the time (N+1) can be formed from the signal value at the time (N+1) and the signal value at the time (N) with continuously changing signs. The following calculation rule is obtained:Result of the measurement at the time N+1N+1: signal value or reference value at time N+1N: signal value or reference value at time NThis allows a higher bandwidth of measurement data to be provided. Furthermore, the sensitivity of the sensor is improved by a factor up to √2 (root 2).In particular, it is provided to implement a so-called bang-bang modulation in order to alternate the signs of the signal values. A possible functional configuration of such an evaluation module implemented in the computing unit 8 is shown together with the signal generation in FIG. 4 as a block diagram. In this case, the signal generation taking place outside the computing unit 8, specifically in the spin-based quantum system 4, is shown on the left-hand side of FIG. 4.In a bang-bang modulation, the excitation of the spin-based quantum system 4 takes place with an electromagnetic field 3 which jumps back and forth at a modulation frequency between two field states. The electromagnetic field 3 can jump in the field strength or the signal value or the frequency, for example, and is based on a modulation signal 9 or is generated by driving the field generating device 2 with the modulation signal 9. This can be, in particular, a frequency modulation.This excitation has the effect that the measurement signal 5' originating from the spin-based quantum system 4 also carries this corresponding modulation or has the corresponding modulation of a useful information 5.The associated demodulation takes place in the arithmetic unit 8, wherein firstly a phase adaptation and then the multiplication of the measurement signal 7 with the modulation signal 9 take place, if appropriate, in order to generate a modulated measurement signal 7', and subsequently, block 10, the demodulating, 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.Embodiments of the invention can be used advantageously in different spin-based quantum systems, which are described below with reference to the previous figures and FIG. 5.For a quantum sensor, such as an NV defect center in diamond, for example, three schemes result in which this measurement method can be used: 1) The tracking or the detection of a change in a resonance peak: FIG. 5 shows an example of a frequency modulation with a rectangular function. In this case, a peak 301 is depicted on the left by way of example in an ODMR spectrum. The frequency of the exciting microwave field is plotted on the x-axis and the absorbed light intensity is plotted on the y-axis. Here, 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 at modulation frequencies f - and f + and are marked with arrows. Since the slopes each have a different sign, each measurement point contains a full information density. These frequencies are advantageously used for square-wave frequency modulation, as shown below the peak 301. The useful signal 320 at these points yields the change in the frequency with sign (no change in the frequency means a lock-in signal of zero).Since a change in the position of the peak here changes the measured value with opposite signs in each case, this corresponds to the alternating sign as required above. With a square-wave frequency modulation of the excitation field, i.e. the periodic jumping back and forth or changing between the two microwave frequencies f - and f +, the maximum information can be generated and extracted. The resonance peak can either be measured prior to continuous measurement or is given as a system parameter in low magnetic fields (expected resonance shift due to external magnetic fields less than the linewidth of resonance). The resonance curve can then be continuously detected (feed-back loop in which the central frequency (i.e. the center of f - and f +) is adjusted on the basis of the measured signal difference of f + and f - ).If the phase behavior between the measurement and the modulation is not known, the measured signal can also be demodulated analogously using two modulation functions phase-shifted by 90°. From this, the signal and its phase offset can then be determined analogously to the sine / cosine modulation.The middle image in FIG. 5 shows a noisy measurement 310 (corresponding to the raw signal 7) and the underlying (theoretical) original signal 311.The right-hand image in FIG. 5 shows the reconstruction of the useful signal 320 with the original signal 311 as reference. 2) Tracking a change of a phase of a quantum mechanical state with respect to a reference frequency:Another possibility for measuring the shift of a resonant frequency is the use of quantum-mechanical superposition states. A typical protocol here is, for example, free induction decay (FID, also called Ramsey protocol) or Hahn echo. In this case, a quantum-mechanical superposition state is generated, which is developed freely for an evolution time. The condition oscillates at the natural frequency (Lamber frequency) and the phase between this oscillation and the microwave frequency is measured at the end.A typical pulse train (FID) isThe first one thereby prepares the superposition state. A temporal development τ then takes place, in which a phase is collected on the basis of the frequency difference between Lambor precision and microwave. This phase difference is then transferred with the last into a population difference which can then be read out.In order to generate a differential signal in this measurement scheme, the last one can be adjusted. This can be done as follows:• By increasing the pulse by another half rotation, an inversion of the population mapping can be generated. That is, one measurement is performed with a last pulse as and the next measurement is performed with one pulse.• The phase of the last can be adjusted so that there is a 180° phase difference between the individual measurements. This also results in an inverted signal.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 the maximum sensitivity. This allows tracking of the measured displacement not only by adapting the microwave frequency, but also by adapting the phase relationship of the measurement pulses (feed-back loop). This has the advantage that the microwave frequency resolution can be selected to be coarser, which has advantages in terms of costs (cheaper components) and development / application (fewer microwave fields have to be calibrated, or a lower calibration error of the microwave intensity). 3) If no inversion of the measurement signal is possible, a differential signal can be produced in comparison with a blind measurement. That is, a measurement contains the desired information, and a measurement is prepared so that no information is contained. This also enables suppression of signal beats still with half the signal strength.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedDE 10 2018 220 234 A1

[0004] DE 10 2018 214 617 A1

[0004]

Claims

Method for evaluating a measurement signal (5') emanating from a spin-based quantum system (4), comprising the steps of: exciting the spin-based quantum system (4) by a pulsed excitation light (13; 320), detecting the measurement signal (5') emanating from the spin-based quantum system (4), integrating the measurement signal (5') via an integration window (331) containing a signal time point of a first light pulse (322) in order to obtain a signal value, integrating the measurement signal (5') via an integration window (332) containing a signal time point of a second light pulse (322) in order to obtain a reference value, wherein the first and the second light pulse are two time-offset light pulses of the excitation light (13; 320), determining a measurement value from the signal value and the reference value.The method of claim 1, further comprising: exciting the spin-based quantum system (4) by an electromagnetic field (3; 310) that jumps back and forth at a modulation frequency between two field states, wherein the first light pulse (322) temporally coincides with a first of the two field states and the second light pulse (322) temporally coincides with a second of the two field states.Method according to claim 2, wherein the electromagnetic field (3; 310) jumps back and forth at the modulation frequency between two modulation field strengths, wherein the two modulation field strengths are in particular the same as each other.Method according to claim 2 or 3, wherein the electromagnetic field (3; 310) jumps back and forth with the modulation frequency between two modulation frequency values.Method according to one of the preceding claims, wherein the spin-based quantum system (4) has a sensor crystal with color centers, in particular a diamond with nitrogen defect centers, or wherein the spin-based quantum system (4) has a vapor cell which contains a mixture of at least one gaseous alkali metal and at least one gaseous noble gas.Method according to one of the preceding claims, wherein the electromagnetic field (3; 310) is a microwave field.Method according to one of the preceding claims, wherein the measurement signal (5') originating from the spin-based quantum system (4) is a fluorescence signal or a light signal.Method according to one of the preceding claims, wherein the determination of a measured value from the signal value and the reference value comprises: forming a quotient from a difference between the signal value and the reference value and a sum of the signal value and the reference value.Method according to one of the preceding claims, wherein the first and the second light pulse are two directly successive read-out pulses of the excitation light (13; 320).Device (100) for evaluating a measurement signal (5') emanating from a spin-based quantum system (4), comprising: the spin-based quantum system (4), an excitation light generating device (12) for generating a pulsed excitation light (13; 320), a measuring device (6) for detecting the measurement signal (5') emanating from the spin-based quantum system (4), wherein the device (100) is configured to carry out a method according to one of claims 1 to 9.The apparatus (100) according to claim 10, further comprising a field generator (2) for generating an electromagnetic field (3; 310) that 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; 310).

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