An apparatus and method for measuring weak high-frequency alternating magnetic fields based on calcium-40 ions

Abstract

The present invention discloses a device and method for measuring weak high-frequency alternating magnetic fields based on calcium-40 ions. The device includes a vacuum chamber, a magnetic field coil and a vacuum maintenance device arranged on the periphery of the vacuum chamber. The magnetic field coil is used to generate a magnetic field in the vacuum chamber, and the vacuum position device is used to maintain the vacuum in the vacuum chamber. The vacuum chamber is provided with eight windows, including five CF63 windows and three CF35 windows. The size of the CF35 window is smaller than that of the CF63 window. The present invention can achieve high-frequency and high-sensitivity magnetic field measurement. The energy level structure of calcium-40 ions is simple, and the manipulation laser wavelength is friendly. Only by using laser to dissipate the energy of ions can the cooling effect be achieved. Since the whole system is in an ultra-high vacuum state, ions can stably stay in the electromagnetic potential well for a long time and can be accurately manipulated by laser.

Description

Technical Field

[0001] The present invention relates to the field of ion trap magnetometer experiments, and particularly to a device and method for measuring weak high-frequency alternating magnetic fields based on calcium-40 ions. Background Art

[0002] The high-precision measurement technology of weak magnetic fields is widely used in geophysical exploration, earthquake monitoring and prediction, airborne magnetic mapping, geomagnetic matching navigation, archaeology, maritime salvage, etc., and is also widely used in military applications such as anti-submarine, magnetic fuzes, and magnetic navigation. Currently, the magnetometers used for weak magnetic field measurement mainly include mechanical magnetometers, fluxgate magnetometers, optically pumped magnetometers, superconducting quantum magnetometers, etc. However, the frequency measurement ranges of these magnetometers are all concentrated in static magnetic fields or low-frequency magnetic fields. Compared with the precise measurement of low-frequency magnetic fields, the precise measurement of weak high-frequency magnetic fields is a rather difficult task.

[0003] The device and method for measuring weak high-frequency alternating magnetic fields based on 43 Ca + ions disclosed in the patent document CN109814049A are based on calcium-43 ions and are not applicable to calcium-40 ions. Summary of the Invention

[0004] The purpose of the present invention is to overcome the above-mentioned deficiencies of the prior art and provide a device and method for measuring weak high-frequency alternating magnetic fields based on calcium-40 ions, so as to be suitable for precisely measuring very weak alternating magnetic fields in a room temperature environment.

[0005] To achieve the above purpose, the technical solution of the present invention is as follows:

[0006] On the one hand, the present invention provides a device for measuring weak high-frequency alternating magnetic fields based on calcium-40 ions, including a vacuum chamber, a magnetic field coil and a vacuum maintenance device arranged outside the vacuum chamber. The magnetic field coil is used to generate a magnetic field in the vacuum chamber, and the vacuum position device is used to maintain the vacuum in the vacuum chamber.

[0007] The vacuum chamber is provided with eight windows, including four CF63 windows and four CF35 windows. The eight windows are the first CF63 window, the second CF35 window, the third CF63 window, the fourth CF35 window, the fifth CF63 window, the sixth CF35 window, the seventh CF63 window and the eighth CF35 window respectively; the size of the CF35 window is smaller than that of the CF63 window.

[0008] The first CF63 window is used for the incident photoionization laser.

[0009] The second CF35 window is used for the incident state preparation laser.

[0010] The third CF63 window and the seventh CF63 window are used for ion fluorescence collection;

[0011] The fourth CF35 window is for the incident Doppler cooling laser;

[0012] The fifth CF63 window is for the incident repumping laser;

[0013] The sixth CF35 window and the eighth CF35 window are for the incident dressed state driving laser;

[0014] The lower CF35 window is for the incident Doppler cooling laser;

[0015] The upper CF35 window is for the incident state detection laser.

[0016] Further, magnetic field coils are respectively installed outside the second CF35 window and the sixth CF35 window.

[0017] Further, the vacuum maintenance device includes a compound pump, a vacuum gauge and a vacuum angle valve. After the three are connected, they are connected to the vacuum chamber to maintain the vacuum degree in the vacuum chamber at the target value.

[0018] Further, an upper CF150 flange is provided on the top surface of the vacuum chamber, a lower CF150 flange is provided on the bottom surface of the vacuum chamber, a radio frequency feedthrough is installed on the CF150 flange, the radio frequency feedthrough is connected to the radio frequency electrode, and there is a DC feedthrough. The DC feedthrough is respectively connected to the DC electrode and the micro-motion compensation electrode.

[0019] Further, after the compound pump, the vacuum gauge and the vacuum angle valve are connected, they are connected to the vacuum chamber through the fifth CF35 window.

[0020] Further, the target value is 8.0×10 -9 Pa.

[0021] On the other hand, the present invention provides a method for measuring a weak high-frequency alternating magnetic field based on calcium-40 ions. Using the above-mentioned device for measuring a weak high-frequency alternating magnetic field based on calcium-40 ions, the method includes:

[0022] Step 1: Inject photoionization laser from the first CF63 window into the center of the linear ion trap. The photoionization laser interacts with calcium-40 atoms to generate calcium-40 ions;

[0023] Step 2: The trapped calcium-40 ions are simultaneously irradiated by the Doppler cooling laser incident through the fourth CF35 window and the lower CF35 window and cooled to below 1 mK;

[0024] Step 3: Inject state preparation laser from the second CF35 window into the center of the ion confinement area to perform initial state preparation on calcium-40 ions;

[0025] Step 4: The left-handed circularly polarized dressed-state driving laser is incident from the sixth CF35 window and acts simultaneously with the right-handed circularly polarized dressed-state driving laser incident from the eighth CF35 window to prepare the initial state of the calcium-40 ion into the protected qubit space;

[0026] Step 5: Determine the magnetic induction intensity B of the static magnetic field generated by the magnetic field coils installed in the second CF35 window and the sixth CF35 window. The alternating magnetic field to be measured is B g coh(2πv g t). Apply the static magnetic field to generate Zeeman splitting and resonate with the alternating magnetic field to be measured, that is:

[0027]

[0028] Step 6: Use the state detection laser incident from the upper CF35 window and the repumping laser incident from the fifth CF63 window to measure the population probability of the ion in the energy level |d> evolving with time, and determine the dark state Rabi oscillation frequency Ω of the protected qubit space excited by the alternating magnetic field to be measured g .

[0029] Further, in Step 1, there are two wavelengths of the photoionization laser incident from the first CF63 window, which are 375 nm and 423 nm respectively.

[0030] Further, in Step 2, the wavelength of the Doppler cooling laser incident through the fourth CF35 window is 397 nm, and the wavelength of the Doppler cooling laser incident through the lower CF35 window is 866 nm.

[0031] Further, in Step 2 and Step 6, the wavelength of the state detection laser incident from the upper CF35 window is 732 nm

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] The present invention can achieve high-frequency and high-sensitivity magnetic field measurement. In the present invention, the entire ion trap system is at room temperature, the energy level structure of the calcium-40 ion is simple, and the wavelengths of the manipulation lasers are friendly. Only need to use lasers to dissipate the energy of the ions to achieve the cooling effect. Since the whole system is in an ultra-high vacuum state, the ions can stably stay in the electromagnetic potential well for a long time and can be accurately manipulated by lasers. Therefore, it is a quantum system with a simple structure, a pure environment and mature technology, and is very suitable as a measurement device.

[0034] During the measurement process, the static magnetic field is adjusted to make the energy level splitting generated by the ions match the frequency of the weak alternating magnetic field. At the same time, this weak magnetic field causes the system to oscillate between different eigenstates. By measuring the oscillation frequency, the amplitude of the alternating magnetic field can be detected. The present invention adopts an appropriate working region (prepared in the subspace of the protected quantum state) to suppress the deviation of the resonance signal caused by the slight jitter of the bias static magnetic field, ensuring that this solution can work properly under non-ideal conditions.

[0035] The difference between the present invention and the "Device and Method for Measuring Weak High-Frequency Alternating Magnetic Field Based on 43Ca+ Ions" disclosed in the patent document CN109814049A is firstly that the present invention uses calcium-40 ions, while the previous application used calcium-43 ions. Compared with calcium-43 ions, calcium-40 ions have a simpler energy level structure, require fewer lasers, and do not require microwave operation. By designing and constructing the protected quantum state, the detection of weak alternating magnetic fields can also be achieved. The natural abundance of calcium-43 element is extremely low and the price is high, while the natural abundance of calcium-40 element is high and the price is cheap, which is more suitable for reducing costs. The second difference is the further optimization of the vacuum chamber and the vacuum maintenance device, which reduces the volume and improves the efficiency. Description of the Drawings

[0036] Figure 1 Schematic diagram of the overall structure of the device for measuring weak high-frequency alternating magnetic field based on calcium-40 ions provided in Embodiment 1 of the present invention;

[0037] Figure 2 Top view structure diagram of the device for measuring weak high-frequency alternating magnetic field based on calcium-40 ions provided in Embodiment 1 of the present invention;

[0038] Figure 3 Schematic diagram of the ion energy level structure used in the present invention;

[0039] Figure 4 Schematic diagram of the splitting and transition of the ground state of the ion energy level of the present invention under the static magnetic field;

[0040] Figure 5 Schematic diagram of the relationship between alternating magnetic fields of different frequencies and the magnitude of the static magnetic field;

[0041] Figure 6 Schematic diagram of the relationship between alternating magnetic fields of different frequencies and the maximum value of the coupling strength;

[0042] Figure 7 Schematic diagram of the relationship between the measurement sensitivity and the frequency of the alternating magnetic field to be measured.

[0043] In the figure: 1. First CF63 window; 2. Second CF35 window; 3. Third CF63 window; 4. Fourth CF35 window; 5. Fifth CF63 window; 6. Sixth CF35 window; 7. Seventh CF63 window; 8. Eighth CF35 window; 9. Magnetic field coil; 10. Upper CF150 flange; 11. DC feedthrough; 12. RF feedthrough; 13. Upper CF35 window; 14. Vacuum angle valve; 15. Vacuum gauge; 16. Composite pump; 17. Lower CF150 flange; 18. Lower CF35 window. Detailed implementation mode

[0044] In the description of the present invention, it should be noted that unless otherwise clearly specified and defined, the terms "installation" and "connection" should be understood in a broad sense. For example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection, an electrical connection, or a signal connection; it can be directly connected, or indirectly connected through an intermediate medium, and it can be said that the interiors of two components are connected. For those of ordinary skill in the art, the specific meanings of the above terms in the present invention can be understood according to specific situations. The technical solutions of the present invention will be further described below with reference to the drawings and embodiments.

[0045] Embodiment 1:

[0046] Refer to Figure 1 、 2 As shown, the device for measuring weak high-frequency alternating magnetic fields based on calcium-40 ions provided in this embodiment includes a vacuum chamber, a magnetic field coil, and a vacuum maintenance device arranged on the periphery of the vacuum chamber.

[0047] The first CF63 window 1, the second CF35 window 2, the third CF63 window 3, the fourth CF35 window 4, the fifth CF63 window 5, the sixth CF35 window 6, the seventh CF63 window 7, and the eighth CF35 window 8 are evenly distributed on the vacuum chamber. An upper CF150 flange 10 is provided on the top surface of the vacuum chamber, and a lower CF150 flange 17 is provided on the bottom surface of the vacuum chamber. Among them, the size of the CF35 window is smaller than that of the CF63 window. That is to say, compared with the solution of patent document CN109814049A, the volume of the vacuum chamber in this application becomes smaller, and some CF63 windows become CF35 windows with smaller sizes.

[0048] The first CF63 window 1 is for the incident photoionization laser; the second CF35 window 2 is for the incident state preparation laser; magnetic field coils 9 are respectively installed outside the second CF35 window 3 and the sixth CF35 window 6. In this way, during the measurement process, by adjusting the static magnetic field, the energy level splitting generated by the ions is matched with the frequency of the weak alternating magnetic field; the third CF63 window 3 and the seventh CF63 window 7 are for collecting ion fluorescence; the fourth CF35 window 4 is for the incident Doppler cooling laser; the fifth CF63 window 5 is for the incident repumping laser; the sixth CF35 window 6 and the eighth CF35 window 8 are for the incident dressed state driving laser; the lower CF35 window 18 is for the incident Doppler cooling laser, and the upper CF35 window 13 is for the incident state detection laser.

[0049] The above-mentioned upper CF150 flange 10 is equipped with a radio frequency feedthrough 12, the radio frequency feedthrough 12 is connected to the radio frequency electrode, and there is a DC feedthrough 11. The DC feedthrough 11 is respectively connected to the DC electrode and the micromotion compensation electrode.

[0050] Specifically, the above-mentioned vacuum maintenance device includes a compound pump 16, a vacuum gauge 15 and a vacuum angle valve 14. After the three are connected, they are connected to the vacuum chamber through the fifth CF35 window 5 to maintain the vacuum degree in the vacuum chamber at about 8.0×10 -9 Pa. Compared with the solution in the patent document CN109814049A, the present application uses a compound pump 16 to replace the ion pump, the getter pump and the sublimation pump, reducing the device volume and cost.

[0051] Example 2:

[0052] The energy levels of calcium-40 ions will exhibit Zeeman splitting under a static magnetic field, as shown in Figure 5 (Compared with the patent document CN109814049A, the present application uses a magnetic field measurement scheme based on calcium-40 ions. Utilizing the simple and easy-to-control energy level structure of calcium-40 ions, a protected quantum state subspace is constructed, simplifying the magnetic field measurement method). Assume that the alternating magnetic field to be measured has the following form: B g (t) = B g cos(2πv g t), where the frequency v of the alternating magnetic field g is equal to the frequency difference between the adjacent Zeeman sub-level splittings of the static magnetic field at the D 3 / 2 energy level, that is: 2πυ g = g D μ B B, t is the time variable, and the amplitude B of the alternating magnetic field to be measured g is the quantity to be measured.

[0053] Specifically, the method for measuring a weak high-frequency alternating magnetic field based on calcium-40 ions provided in this embodiment is based on the device described in Embodiment 1, and specifically includes the following steps:

[0054] Step 1: The photoionization lasers (375 nm and 423 nm) are vertically incident from the first CF63 window 1 into the center of the linear ion trap. The photoionization lasers interact with calcium-40 atoms to generate calcium ions.

[0055] Step 2: The trapped calcium-40 ions are simultaneously irradiated by the Doppler cooling lasers (397 nm and 866 nm) incident from the fourth CF63 window 4 and the lower CF35 window 18, which are used to cool the calcium-40 ions to below 1 mK.

[0056] Step 3: The state preparation laser vertically incident from the second CF35 window 2 into the center of the ion confinement area prepares the ions to the initial state S 1 / 2 state.

[0057] Step 4: The left-handed circularly polarized dressed state driving laser is incident from the sixth CF35 window 6, and acts simultaneously with the right-handed circularly polarized dressed state driving laser incident from the eighth CF35 window 8 to prepare the calcium-40 ions to the initial state in the protected qubit space; for the energy levels and the protected subspace involved in this application, see Figure 4 and use the 866 nm laser to drive When the intensities of the two laser fields driving the transitions from |d1> and |d3> to |p1> and from |d0> and |d2> to |p0> are the same, a protected subspace is formed. In this way, by adopting an appropriate working region (prepared in the protected quantum state subspace), the deviation of the resonance signal caused by the slight jitter of the bias static magnetic field is suppressed, ensuring that this scheme can work properly under non-ideal conditions and ensuring the accuracy of the measurement results.

[0058] Step 5: To make the alternating magnetic field to be measured resonate with the adjacent Zeeman sublevel transitions of the D 3 / 2 energy level, the magnetic induction intensity of the magnetic field coil 9 installed on the second CF35 window 2 and the sixth CF35 window 6 can be adjusted. The magnetic induction intensity B of the required static magnetic field can be obtained by changing the current applied to the magnetic field coil 9. The Zeeman splitting generated by the static magnetic field applied in the experiment resonates with the frequency of the alternating magnetic field to be measured. The relationship between the alternating magnetic fields of different frequencies and the static magnetic field is as Figure 5 shown. In this way, during the measurement process, by adjusting the static magnetic field, the energy level splitting generated by the ions is matched with the frequency of the weak alternating magnetic field. At the same time, this weak magnetic field causes the system to oscillate between different eigenstates. By measuring the oscillation frequency, the amplitude of the alternating magnetic field can be detected.

[0059] Step 6: By using the state detection laser (732 nm) incident from the upper CF35 window 13 and the repumping laser (866 mm) incident from the fifth CF63 window 5, the population probability of the |d> energy level evolving with time can be measured. After a period of measurement, the Rabi oscillation curve between the sub-energy levels of the |D 2 / 3 > state can be measured, and its Rabi oscillation frequency can be calculated from the Rabi oscillation curve, so as to obtain the magnetic field strength B to be measured g .

[0060] The coupling strength between two adjacent Zeeman energy levels needs to satisfy Under the condition of , the relationship between the alternating magnetic field of different frequencies and the maximum coupling strength is as Figure 6 shown

[0061] As shown by Figure 6 , when the frequency v g of the alternating magnetic field to be measured is in the range of 0 - 100 MHz, the amplitude B g of the alternating magnetic field to be measured can be measured according to the above steps, and the range is of the order of μT. The measurement sensitivity is defined as

[0062]

[0063] where the sensitivity is in the unit of and the relationship between and ν g can be determined, as shown by Figure 7 . If the duration T g of the Rabi oscillation can be guaranteed to reach the order of seconds, the measurement sensitivity can reach For a specific measurement, since the time of the Rabi oscillation can be selected according to the actual situation, the sensitivity can be obtained by dividing by again

[0064] It can be seen that five wavelengths of lasers are used in this method, namely 375 nm, 423 nm, 397 nm, 866 nm and 732 nm. Compared with the scheme of patent document CN109814049A, the number of lasers used in this method is smaller, and no microwave is required, reducing the complexity of the device and also reducing the cost

[0065] The above embodiments are only to illustrate the technical concept and features of the present invention. The purpose is to enable those of ordinary skill in the art to understand the content of the present invention and implement it accordingly, and it should not be used to limit the protection scope of the present invention. Any equivalent changes or modifications made according to the essence of the content of the present invention should be covered within the protection scope of the present invention.

Claims

1. A device for measuring weak high-frequency alternating magnetic fields based on calcium-40 ions, comprising a vacuum chamber, a magnetic field coil and a vacuum maintenance device arranged outside the vacuum chamber. The magnetic field coil is used to generate a magnetic field in the vacuum chamber, and the vacuum maintenance device is used to maintain the vacuum in the vacuum chamber. It is characterized in that, The vacuum chamber is provided with eight windows, including four CF63 windows and four CF35 windows. The eight windows are the first CF63 window, the second CF35 window, the third CF63 window, the fourth CF35 window, the fifth CF63 window, the sixth CF35 window, the seventh CF63 window and the eighth CF35 window respectively; the size of the CF35 window is smaller than that of the CF63 window; an upper CF35 window is arranged at the top of the vacuum chamber, and a lower CF35 window is arranged at the bottom of the vacuum chamber; The first CF63 window is used for incident photoionization laser; The second CF35 window is used for incident state preparation laser; The third CF63 window and the seventh CF63 window are used for ion fluorescence collection; The fourth CF35 window is used for incident Doppler cooling laser; The fifth CF63 window is used for incident repumping laser; The sixth CF35 window and the eighth CF35 window are used for incident dressed state driving laser; The lower CF35 window is used for incident Doppler cooling laser; The upper CF35 window is used for incident state detection laser; Magnetic field coils are respectively installed outside the second CF35 window and the sixth CF35 window; The vacuum maintenance device includes a compound pump, a vacuum gauge and a vacuum angle valve. After the three are connected in communication, they are connected to the vacuum chamber to maintain the vacuum degree in the vacuum chamber at the target value.

2. The device for measuring a weak high-frequency alternating magnetic field based on calcium-40 ions according to claim 1, characterized in that, An upper CF150 flange is arranged on the top surface of the vacuum chamber, and a lower CF150 flange is arranged on the bottom surface of the vacuum chamber. An RF feedthrough is installed on the CF150 flange. The RF feedthrough is connected to an RF electrode, and there is a DC feedthrough. The DC feedthrough is respectively connected to a DC electrode and a micro-motion compensation electrode.

3. The device for measuring a weak high-frequency alternating magnetic field based on calcium-40 ions as claimed in claim 1, wherein The compound pump, the vacuum gauge and the vacuum angle valve are connected in communication and then connected to the vacuum chamber through the fifth CF35 window.

4. The device for measuring a weak high-frequency alternating magnetic field based on calcium-40 ions as claimed in claim 1 or 3, characterized in that The target value is .

5. A method for measuring a weak high-frequency alternating magnetic field based on calcium-40 ions, using the apparatus for measuring a weak high-frequency alternating magnetic field based on calcium-40 ions according to claim 2, characterized in that, The method includes: Step 1: Incident photoionization laser from the first CF63 window to the center of the linear ion trap. The photoionization laser interacts with calcium-40 atoms to generate calcium-40 ions; Step 2: The trapped calcium-40 ions are simultaneously irradiated by the Doppler cooling laser incident through the fourth CF35 window and the lower CF35 window and cooled to below 1 mK; Step 3: Incident state preparation laser from the second CF35 window to the center of the ion confinement area to perform initial state preparation on calcium-40 ions; Step 4: Incident left-handed circularly polarized dressed state driving laser from the sixth CF35 window and act simultaneously with the right-handed circularly polarized dressed state driving laser incident from the eighth CF35 window to prepare the initial state of calcium-40 ions into the protected qubit space; Step 5: Determine the magnetic induction intensity of the static magnetic field generated by the magnetic field coils installed in the second CF35 window and the sixth CF35 window , and the alternating magnetic field to be measured is . Apply a static magnetic field to generate Zeeman splitting and resonance with the alternating magnetic field to be measured, that is: ； Step 6: Using the state detection laser incident from the upper CF35 window and the repumping laser incident from the fifth CF63 window, measure the population probability of the ion evolving with time, and determine the dark state Rabi oscillation frequency in the space of the protected qubit excited by the alternating magnetic field to be measured .​ 6. The method for measuring a weak high-frequency alternating magnetic field based on calcium-40 ions as claimed in claim 5, wherein In step 1, there are two wavelengths of the photoionization laser incident from the first CF63 window, which are 375 nm and 423 nm respectively.

7. The method for measuring a weak high-frequency alternating magnetic field based on calcium-40 ions as claimed in claim 5, wherein In step 2, the wavelength of the Doppler cooling laser incident through the fourth CF35 window is 397 nm, and the wavelength of the Doppler cooling laser incident through the lower CF35 window is 866 nm.

8. The method for measuring a weak high-frequency alternating magnetic field based on calcium-40 ions as described in claim 5, wherein In step 6, the wavelength of the state detection laser incident through the upper CF35 window is 732 nm.

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