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 (5') emanating from a spin-based quantum system (4), comprising exciting the spin-based quantum system (4) by a pulsed excitation light (13; 220), alternately exposing the spin-based quantum system (4) to a first and a second series of magnetic field pulses (211; 212), wherein one of the first and second series of magnetic field pulses (211; 212) consists of two π 2 − Pulse contains, and the other of the first and second magnetic field pulse sequence (211; 212) a π 2 − Puls 2 and one − π 2 − Puls includes, a detection of the measurement signal (5') emanating from the spin-based quantum system (4), an integration of the measurement signal (5') over an integration window (231) containing a signal time of a light pulse (222) of the pulsed excitation light (13; 320) between the first magnetic field pulse sequence (211) and the second magnetic field pulse sequence (212) and between the second magnetic field pulse sequence (212) and the first magnetic field pulse sequence (211) to obtain a first and a second signal value, and a determination of a measured value from the first and the second signal value.

Description

[0001] The present invention relates to a method and a device for evaluating a measurement signal emanating from a spin-based quantum system. Background of the invention

[0002] To measure very small magnetic field strengths, quantum-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.

[0003] For such quantum-based magnetic field sensors, sensor crystals with excitable defect centers can be used. Typically, diamonds are used that are homogeneously doped with negative nitrogen vacancy centers (NV centers, nitrogen vacancy). 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.

[0004] For measuring very small 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. 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.

[0005] One measurement method that can be used in this context is based on quantum mechanical superposition states. A typical protocol is, for example, free induction decay (FID, also known as the Ramsey protocol) or the Hahn echo. In this method, a quantum mechanical superposition state is generated using a microwave pulse and is freely evolved for one evolutionary time. This state oscillates around an existing magnetic field at its resonant frequency (Lamor frequency), and the phase between this oscillation and the microwave, which provides information about the magnetic field, is measured at the end.

[0006] A typical pulse pattern (FID) is π2−τ−π2. A laser pulse first initializes a spin state |m s = 0>. The first π2−pulse prepares the superposition state 1 / √2 (|m s = 0> + |m s= ±1 >). Subsequently, a time evolution τ takes place, in which a phase is collected due to the frequency difference between Lamor precision and microwave. This phase difference is then compared with the last π2−pulse transferred into a 2 population difference, which can then be selected. A π2−pulse is a microwave pulse that shifts the Bloch vector in the Bloch sphere by π2=90∘ rotates around an axis, the y-axis.

[0007] For such pulsed measurement protocols in quantum sensors, the contrast of a measurement can be increased by inverting the measured signal in a second measurement. This can be achieved by extending the second measurement. π2−pulses about a half rotation (π2→3π2) This can be achieved. Since it is now possible to calculate the difference between the signals, this is one way to double the contrast.

[0008] However, the quality (fidelity) of the π2− and the 3π2−Pulses Different, i.e., the additional rotation by π reduces the achievable contrast in the measurement signal, making a direct comparison of the measurement signals difficult and reducing the performance gain. The extension of the second pulse to 3π2 This leads to a lengthening of the measurement sequence and thus to a deterioration of sensitivity. Furthermore, the different lengths of the measurement sequences either result in additional dead time (if the lengths of the measurement sequences are adjusted to each other) or the different duty cycles can lead to problems in the signal output as well as to instabilities in the hardware (e.g., different laser behavior in the individual measurement sequences due to different thermal equilibrium being established). Disclosure of the invention

[0009] According to the invention, a method and a device for evaluating a measurement 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.

[0010] Specifically, exciting the spin-based quantum system with pulsed excitation light involves alternately exposing the spin-based quantum system to a first and a second series of magnetic field pulses, where one of the first and second series of magnetic field pulses consists of two π2−pulses contains and the other of the first and second magnetic field pulse sequence a π2−pulse and one −π2−pulse contains. The −π2−pulse in the Bloch sphere the Bloch vector by −π2=−90∘ around the y-axis, thereby transforming the x-component of the Bloch vector into the -z-component and vice versa; the y-component remains unchanged.

[0011] The magnetic field pulses are pulses of a magnetic field oscillation. Typical frequency values ​​of such oscillations, which are used to excite spin-based quantum systems, lie in the high-frequency range. The abbreviation "HF" is used hereafter for "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 lie 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) an energy difference corresponding to 2.87 GHz exists between spin zero (|m). s = 0>) on the one hand and spin equal to +1 or -1 (|m sOn the other hand, such NV centers in diamonds can be used, for example, as quantum sensors.

[0012] The method further comprises acquiring the measurement signal emanating from the spin-based quantum system, integrating the measurement signal over an integration window containing a signal time point of a light pulse of the pulsed excitation light between the exposure of the spin-based quantum system of the first magnetic field pulse sequence and the exposure of the spin-based quantum system of the second magnetic field pulse sequence to obtain a first signal value, and integrating the measurement signal over an integration window containing a signal time point of a light pulse of the pulsed excitation light between the exposure of the spin-based quantum system of the second magnetic field pulse sequence and the exposure of the spin-based quantum system of the first magnetic field pulse sequence to obtain a second signal value. In other words, a signal value is obtained after each magnetic field pulse sequence and before the next magnetic field pulse sequence.

[0013] Finally, a measured value is determined from the first and second signal values.

[0014] The invention makes it possible to reduce the disadvantages described above for differential measurements using two magnetic field pulse sequences. With a −π2−pulse, i.e., a rotation operation with the opposite sign, can have the same effect as with a 3π2−pulse achieved, but with the same quality and length of the first π2−Pulses. By replacing the second π2−Pulses. in one of the two magnetic field pulse sequences by a −π2−pulse This avoids extending the measurement sequence and thus reducing sensitivity. Furthermore, the first and second magnetic field pulse sequences have the same length, preventing variations in the measurement sequence length. This eliminates operating cycle problems, such as fluctuations in the light source due to differing cool-down times. The overall sequence length is minimized because no additional time is required for inversion (typically, a 3π2−pulse three times longer than a π2−pulse). Each measurement step has the same length, which allows for constant-frequency data output without additional pauses. Due to the identical pulse length, the quality factor of the inverted pulse is identical to that of the original pulse. π2−pulse, which allows for simple subtraction of the numbers without an offset. Furthermore, in general, a π2−pulse a higher quality factor than longer pulses, since inhomogeneities in the external magnetic field (background field) and the magnetic field pulse lead to an exponential decrease in quality factor with longer pulse duration.

[0015] In embodiments of the invention, the −π2−pulse with a phase shift of the magnetic field oscillation by an angle Δφ to the π2−pulse generated where Δφ = 180° or where Δφ is specified depending on the measured value, in particular such that the measured value or the contrast is maximized.

[0016] In the Bloch sphere diagram, the phase shift changes the axis around which the Bloch vector is rotated. Assuming that a π2−pulse the Bloch vector π2=90° When it rotates around the y-axis, a 180° phase-shifted spherical plane rotates. π2−pulse (notation chosen here:) π2180°−pulse ) the Bloch vector around the -y-axis. Furthermore, for example, a π290°−pulse the Bloch vector around the -x-axis and a π2−90°−pulse the Bloch vector around the x-axis.

[0017] A phase shift of 180° is technically easy to provide and essentially corresponds to the phase shift required to generate an opposite direction of rotation in the Bloch sphere.

[0018] On the other hand, to always generate the maximum sensitivity of the measurement signal, the phase of the −π2−pulses The frequency response can be finely adjusted to always remain within the range of maximum sensitivity. This allows the determination of the magnetic field-induced frequency shift, and thus the magnetic field itself, not only by adjusting the microwave frequency but also by adjusting the phase relationship of the measurement pulses (feedback loop). This has the advantage that the microwave frequency resolution can be chosen to be coarser, which offers benefits in terms of costs (cheaper components) as well as development / application (fewer microwave fields need to be calibrated, or a smaller calibration error in the microwave intensity).

[0019] Advantageously, a π2−pulse and / or a −π2−pulse The spin-based quantum system switches between two spin states selected from a state |m s = 0>, a state |m s = ±1 > and a superposition state 1 / √2 (|m s = 0> + |m s= ±1 >).

[0020] In embodiments of the invention, the −π2−pulse generated using a phase shifter as a discrete component, a switch network that selects individual phase paths (one fixed phase per path) via switches, or by direct synthesis of the RF pulses (or by mixing a synthesized signal with an RF source in an (IQ) mixer network, where the phase of the pulses is set directly during synthesis).

[0021] In embodiments of the invention, the −π2−pulse This is generated using a setup with a high-frequency source and one or more mixers (especially so-called IQ mixers, short for in-phase and quadrature), which modulate a modulation frequency onto the signal from the high-frequency source. By changing the phase of the modulation frequency, the phase of the generated signal can ultimately be adjusted.

[0022] In embodiments of the invention, a measured value is determined from the first and second signal values ​​by forming a quotient of the difference between the first and second signal values ​​and the sum of the first and second signal values. This allows for increased contrast.

[0023] For example, this results in a calculation rule as follows: contrastdifferental=2(signal+−signal−) / (signal++signal−) with contrast differental : Measured value signal + , signal - : first and second signal value

[0024] Furthermore, normalization to the sum of the first and second signal values ​​compensates for fluctuations in the signal value, leading to increased robustness against variations in the excitation power density. This enhances the system's robustness and allows the sensor system to achieve a sensitivity comparable to a system with two photodiodes, which achieves such fluctuations by balancing the photodiode signal, without requiring a dedicated balancing circuit.

[0025] Further advantages and embodiments of the invention will become apparent from the description and the accompanying drawing.

[0026] The invention is schematically illustrated in the drawing using exemplary embodiments and is described below with reference to the drawing. Brief description of the drawings Fig. 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. Fig. Figure 2 shows a sequence for a pulsed ODMR measurement according to an embodiment of the invention. embodiment(s) of the invention

[0027] Fig. Figure 1 schematically shows a device 100 for evaluating a measurement signal 5' emanating from a spin-based quantum system 4 according to an embodiment of the invention in a block diagram.

[0028] The device 100 comprises an excitation light generation device 12 for generating a pulsed excitation light 13 and a field generation device 2 for generating a magnetic field 3 and the spin-based quantum system 4, which is to be excited by the excitation light 13 and the magnetic field.

[0029] The spin-based quantum system 4 can, in its various embodiments, comprise a sensor crystal with color centers, in particular a diamond with nitrogen-vacancy centers. Other types of spin-based quantum systems can also be advantageously used within the scope of the invention.

[0030] The field generation device 2 is designed to generate a magnetic field 3, in particular a high-frequency field (HF field), and further in particular a microwave field with a desired frequency and pulse length.

[0031] The device 100 further comprises a measuring device 6 for capturing 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.

[0032] The measurement 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 measurement arrangement.

[0033] The computing unit 8 is designed to evaluate the raw signal 7 and to control the field generation unit 2.

[0034] An example measurement process is in Fig. 2 illustrated. This shows Fig. 2 a course 210 of the microwave field 3, a course 220 of the excitation light 13 and a course 230 of integration windows.

[0035] Pulsed ODMR experiments conventionally include an initial light pulse (especially a laser pulse) 221 for spin polarization (especially in the |m state). s= 0>), a magnetic field pulse sequence 211 for spin manipulation and a light pulse (especially laser pulse) 222 for spin readout via the fluorescence intensity (readout pulse).

[0036] The fluorescence (photon number n) for NV centers in diamonds is time-dependent and varies for the spin states |m s The curves differ between |m = 0> (so-called bright state) and |m = ±1 > (so-called dark state). 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 can conventionally be used to determine a reference value.

[0037] If the initial state |m sFor an initial state |m s = ±1> the intensity drops rapidly to a low level due to the fast intercombination into the singlet state. Since the singlet state preferentially transitions to the ground state |m s As the intensity drops below 0, it recovers towards the steady-state value within the lifetime of the metastable state (~250 ns). The difference between the two curves, or a normalized difference, can be conveniently used to read out the spin state.

[0038] In order to determine such a difference, a measurement with |m is necessary. s = 0> and a measurement with |m s= ±1 > necessary. These measurements are made possible in embodiments of the invention by different magnetic field pulse sequences 211 and 212, wherein a first (here 211) of the first and second magnetic field pulse sequences performs a first π2−pulse, a magnetic field to be measured that is present during a time evolution τ and a second π2−pulse, and the other (here 212) of the first and second magnetic field pulse sequence the π2−pulse, the magnetic field to be measured that is present during the temporal evolution τ, and the −π2−pulse contains.

[0039] In the Bloch sphere image in the Rotating Frame Approximation, the time evolution τ causes the coherent superposition state to perform a rotation in the equatorial plane, where the angle of the rotation corresponds to the time integral between 0 and τ of the product of detuning Δν and time t.

[0040] The detuning Δν is here the frequency difference between the frequency of the applied microwave field ν. MW and the resonance frequency ν 0→1 to the transition from bright state |m s = 0> in dark state |m s = ±1 >. Δv=v0→1−vMW

[0041] This frequency is determined by the Zeman effect through the acting magnetic field.

[0042] The last pulse of each of the different magnetic field pulse sequences flips the spin state either to |m s = 0> or after |m s = ±1 >, so that both states can be measured alternately and calculated together to obtain measured values.

[0043] For evaluation, the measurement signal can be integrated on the one hand via an integration window 231 containing a signal time point of a light pulse to obtain a signal value, and on the other hand via an integration window 232 containing a reference time point of the light pulse to obtain a reference value. In order to read out the spin state at the end of the sequence with high accuracy, it is crucial to detect the first fluorescence photons after switching on the laser, before the NV center in |m s = 0> is repolarized.

[0044] Furthermore, it is specifically provided that the integration window containing the reference time of the light pulse of a previous measurement simultaneously represents the integration window containing the signal time of a light pulse of the subsequent measurement.

[0045] In other words, for a continuous output of measured values, the measured value at time (N+1) can be calculated from the signal value at time (N+1) and the signal value at time (N) with continuously alternating signs. The following calculation formula results: contrastdifferental(N+1)=[(−1)NsignalN+(−1)N+1signalN+1] / (signalN+signalN+1) with: contrast differental (N+1): Measurement value at time N+1 signal N+1 : Signal value or reference value at time N+1 signal N : Signal value or reference value at time N

[0046] This allows for a higher bandwidth of measurement data to be provided. Furthermore, the sensor's sensitivity is improved by a factor of up to √2 (square root of 2). Normalization to (signal N + signal N+1 ) is optional and can be omitted.

[0047] The magnetic field-induced frequency shift, and thus the magnetic field itself, can be adjusted not only by changing the microwave frequency, but also by changing the phase relationship between the π2−pulse and the −π2−pulse to be determined.

[0048] Through a phase adjustment of the −π2−pulses Any phase can be sensitively read out. Here it is advantageous to position the axis of rotation in the Bloch sphere for the purpose of... −π2−pulse to adjust the caused rotation so that it is rotated by 90°+Δφ relative to the first π2−pulse is shifted to ensure maximum sensitivity to phase changes. This also has the advantage that the differential signal (contrast) differental(N+1)) represents a change in phase with a sign, i.e., in the differential measurement scheme, the measurement signal is 0 when the phase is exactly compensated (difference between two individual measurements, each with a π290°+Δφ−Puls and a −π290°+Δφ−pulse, (if the phase shift corresponds to Δφ).

[0049] This is a measurement using difference calculation. The difference allows us to calculate the deviation of the phase in the measurement from the specified phase φ. This then allows us to adjust the specified phase φ of the last pulse.

[0050] A deviation in phase creates a sign-preserving deviation in the measurement signal and can therefore be used as a feedback input for controlling and tracking the resonant frequency.

[0051] A typical measurement might proceed as follows: • Measurement of the differential measurement signal with phase φ • Adjustment of the phase φ+90° of the readout pulse to correct the zero deviation of the differential signal by a factor: φ Neu = φ alt *Signal differential • Only in cases of large frequency deviations (reduction of pulse performance due to excessive frequency difference between microwave and resonance) does the microwave frequency need to be readjusted.

[0052] The 90° phase shift is needed to obtain a maximum change in the measurement signal to a phase change (the phase from the detuning is translated into the measurement signal with -cos(), which is why a phase shift of every second π / 2 pulse by 90° is advantageous, since then a change in the detuning leads to a maximum signal change at this operating point).

[0053] The second π / 2 pulse in every second single measurement (S (N+1)) is additionally phase-shifted by 180° (corresponding to a negative sign) to invert the influence of a detuning on the measured single signal.

[0054] It is also possible to establish a coherent state between |m s = +1> and |m s = -1> (double quantum state (DQ)). This has several advantages. However, the method mentioned above can also be applied to such coherences.

[0055] A typical pulse sequence in this context is: π20°DQ−τ−π2φDQ

[0056] The first pulse generates the DQ (|m s = 0> → |m s = +1> + |m s = -1 >). Subsequently, a temporal evolution τ occurs, in which a phase is accumulated due to the. This phase difference is then combined with the last pulse in the |m s The = 0> state is transmitted, which can then be read out.

[0057] Here, the phase of the second pulse can be adjusted for both the differential measurement scheme and phase tracking. Typical implementations of such π2DQ−pulses include: - Composite pulses (π / 2 pulse on one transition (e.g. |m s = 0> → |m s = +1> and a π-pulse on the other transition (e.g. |m s = 0> → |m s = -1 >)). - Simultaneous and equally strong activity of both transitions QUOTES INCLUDED IN THE DESCRIPTION

[0000] This list of documents cited by the applicant was automatically generated and is included solely for the reader's convenience. The list is not part of the German patent or utility model application. The DPMA accepts no liability for any errors or omissions. Cited patent literature

[0000] DE 10 2018 220 234 A1

[0004] DE 10 2018 214 617 A1

[0004]

Claims

[1] Method for evaluating a measurement signal (5') emanating from a spin-based quantum system (4), comprising the steps: Excitation of the spin-based quantum system (4) by a pulsed excitation light (13; 220), alternating exposure of the spin-based quantum system (4) of a first and a second magnetic field pulse sequence (211; 212), wherein one of the first and second magnetic field pulse sequences (211; 212) two π2−pulses contains, and the other of the first and second magnetic field pulse sequence (211; 212) a π2−pulse and one −π2−pulse contains Detection of the measurement signal (5') emanating from the spin-based quantum system (4), Integrating the measurement signal (5') over an integration window (231) containing a signal time of a light pulse (222) of the pulsed excitation light (13; 320) between the exposure of the spin-based quantum system (4) of the first magnetic field pulse sequence (211) and the exposure of the spin-based quantum system (4) of the second magnetic field pulse sequence (212) to obtain a first signal value, Integrating the measurement signal (5') over an integration window (232) containing a signal time of a light pulse (222) of the pulsed excitation light (13; 320) between the exposure of the spin-based quantum system (4) of the second magnetic field pulse sequence (212) and the exposure of the spin-based quantum system (4) of the first magnetic field pulse sequence (211) to obtain a second signal value, Determining a measured value from the first and second signal values. [2] Method according to claim 1, wherein the −π2−pulse with a phase shift of 180° to π2−pulse is generated. [3] Method according to claim 1, wherein the −π2−pulse with a phase shift to π2−pulse is generated, which depends on the specific measured value. [4] Method according to any one of the preceding claims, wherein the − π2−pulse This is generated using a phase shifter or a switch network that disconnects individual phase paths via switches. [5] Method according to any one of the preceding claims, wherein the − π2−pulse This is generated by changing the phase of a modulation frequency that is based on the π2−pulse and the − π2−pulse is modulated. [6] 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. [7] Method according to any of the preceding claims, wherein the magnetic field of the first and second magnetic field pulse sequence is a microwave field. [8] Method according to any of the preceding claims, wherein the measurement signal (5') emanating from the spin-based quantum system (4) is a fluorescence signal or a light signal. [9] Method according to any of the preceding claims, comprising determining a measured value from the first and the second signal value: Forming a quotient of the difference between the first and second signal values ​​and the sum of the first and second signal values. [10] 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 field generating device (2) for generating a magnetic field (3; 310), wherein the spin-based quantum system (4) is arranged in the magnetic field (3; 310), 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 the preceding claims.

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