System and method for increasing the transient bandwidth of a rydberg atom microwave sensor

Abstract

The present application relates to a kind of system and method for improving the transient bandwidth of Rydberg atom microwave sensor, including two lasers, respectively for generating probe light and coupling light;Probe light and coupling light are all through half-wave plate and obtain first vertical polarized light by reflecting through polarizing beam splitter, the light transmitted through polarizing beam splitter is sequentially adjusted into second vertical polarized light by a mirror and a half-wave plate;Two-way probe light and coupling light are respectively in two rubidium atom cell opposite coincident propagation, the probe light of reference rubidium atom cell and polarizing beam splitter and the probe light after rubidium atom cell filled with buffer gas and half-transmission half-reflection mirror are all received by photoelectric detector;Signal microwave and local microwave are respectively radiated to two rubidium atom cell by power divider and microwave horn antenna, after applying microwave electric field, the intensity of beat frequency signal generated by the interference of signal microwave and local microwave is measured by the change of light intensity near EIT resonance.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of weak signal detection, and particularly relates to a system and method for improving the transient bandwidth of a Rydberg atom microwave sensor. BACKGROUND

[0002] Quantum sensors can achieve higher sensitivity and accuracy than traditional microwave sensors. As a microwave receiver to replace traditional microwave sensors, the transient bandwidth of a Rydberg atom microwave electric field sensor is a very important parameter. The transient bandwidth of a Rydberg atom microwave electric field sensor refers to the maximum signal bandwidth that can be modulated within a single transmission pulse. It is usually expressed as the bandwidth at which the signal sensitivity decreases by 3dB, i.e. 3dB bandwidth. Due to the discrete characteristics of Rydberg atom energy levels, when the microwave frequency is resonant with the Rydberg atom energy level transition, very high sensitivity can be obtained. However, this characteristic also leads to a sharp decrease in signal strength within a very small frequency range, resulting in a very small 3dB bandwidth, which is very disadvantageous in practical applications.

[0003] It has been pointed out in the literature that the 3dB bandwidth is limited by the lifetime of the atomic energy level. The literature [2020Nat. Phys. 16911-5] first demonstrated that the transient bandwidth that can be achieved using a single Rydberg atom EIT (electromagnetically induced transparency) in a single atomic vapor cell is plus or minus 150 kHz using heterodyne. Subsequently, the literature [Phys. Rev. Appl. 18014045 (2022)] used heterodyne under the AC Stark effect to achieve a transient bandwidth of 0.8 MHz in the 30 MHz radio frequency operating frequency band. The literature [Phys. Rev. Appl. 18 054003 (2022)] used resonance heterodyne and two-photon AT splitting mechanism to increase this bandwidth to 1.9 MHz. The literature [Phys. Rev. Appl. 15014053 (2021)] used non-resonant heterodyne technology under the action of a super-strong local microwave field to achieve a transient bandwidth of 4 MHz. The literature used multiple Rydberg states to achieve a transient bandwidth of 6.1 MHz [Phys. Rev. Appl. 19014025 (2023)]. The literature [IEEE Transactions on Quantum Engineering, vol. 4, pp. 1-8, 2023, Art no. 3500108] used spatially separated staggered pulse probe lasers to achieve a transient bandwidth of 100 MHz. The literature [EPJ Quantum Technol. 10, 51 (2023)] used double EIT beams to increase the 3dB bandwidth by 1.5 times compared to a single beam, reaching 872 kHz. The literature [Chip 3, 100089 (2024)] used spatially separated RF chip modulation technology to achieve a 100 kHz bandwidth at double operating frequencies, and pointed out that this bandwidth is limited by the EIT linewidth, with an upper limit of about 10 MHz. The literature [AVS Quantum Sci. 4, 035001 (2022);] reduced the probe light beam to 85um, and increased the EIT linewidth by increasing the transit broadening, achieving a 3MHz 3dB transient bandwidth, but the small beam resulted in a sharp decrease in EIT intensity, which also inevitably led to a decrease in sensitivity. The literature [Increased instantaneous bandwidth of Rydberg atom electrometry with an optical frequency comb probe] used an optical comb to achieve a transient bandwidth of 12 plus or minus 1 MHz, but the cost of the optical comb is very high.

[0004] A very good study in the prior art is to use the six-wave mixing process of high-power coupled light to achieve a 10MHz instantaneous bandwidth, although the 3dB bandwidth of some in this study reaches 10MHz, but the 3dB bandwidth of the flat area is only 3MHz. How to use a simpler method to achieve a large instantaneous bandwidth with flat sensitivity is an important research direction. SUMMARY

[0005] In view of the defects in the prior art, the purpose of the present application is to provide a system and method for improving the transient bandwidth of a Rydberg atom microwave sensor. Different pressure buffer gas neon is filled into a rubidium atom cell. By studying the influence of the buffer gas on the Rydberg EIT frequency shift and EIT line width, and measuring the corresponding transient bandwidth, it is found that the EIT frequency shift and the broadening are proportional to the pressure of the buffer gas. The Rydberg EIT broadening caused by buffer gas collision and polarization can greatly improve the transient bandwidth. When the beam diameter is 1mm, 10MHz of transient bandwidth can be achieved after filling neon, and the sensitivity change is less than 1dB within 9MHz, realizing a large instantaneous bandwidth with flat sensitivity. The transient bandwidth of the Rydberg atom sensor is very simply improved, which has great significance in microwave sensing.

[0006] To achieve the above purpose, the technical scheme adopted by the present application is:

[0007] In a first aspect, a system for improving the transient bandwidth of a Rydberg atom microwave sensor is provided. The system includes a first laser for generating 780nm laser light, and a second laser for generating 480nm laser light. The 780nm laser light is the probe light, and the 480nm laser light is the coupling light. A half-wave plate and a polarizing beam splitter prism are sequentially arranged in front of the first laser and the second laser. The probe light and the coupling light pass through the half-wave plate and the polarizing beam splitter prism to obtain first vertical polarized light. The light transmitted through the polarizing beam splitter prism passes through a mirror and a half-wave plate to adjust the light into second vertical polarized light. The second vertical polarized light and the first vertical polarized light are transmitted through two rubidium atom cells in opposite directions. The probe light in the first vertical polarized light is received by a photodetector after passing through a reference rubidium atom cell and a polarizing beam splitter prism. The probe light in the second vertical polarized light is received by a photodetector after passing through a rubidium atom cell filled with buffer gas and a 780nm light transmitting 480nm light reflecting mirror. A signal microwave and a local microwave with a similar frequency are radiated to the two rubidium atom cells through a power divider and a microwave horn antenna, respectively, to realize interference in the two rubidium atom cells. After applying a microwave electric field, the change in light intensity near the EIT resonance is measured by the intensity of the beat frequency signal generated by the interference between the signal microwave and the local microwave.

[0008] Further, the two rubidium atom cells are both cylindrical in shape.

[0009] Further, the two rubidium atom cells are both cylindrical in shape.

[0010] Further, the two rubidium atom cells are both cylindrical in shape.

[0011] Further, the two rubidium atom cells are both cylindrical in shape.

[0012] Further, the two rubidium atom cells are both cylindrical in shape.

[0013] S1, the 780nm probe light and 480nm coupling light are used to propagate in the rubidium atom cell in opposite directions to form electromagnetic induced transparency of the Rydberg atom, and after the application of a microwave electric field, the electromagnetic induced transparency of the Rydberg atom is split into Autler-Townes, and the change of the signal microwave electric field intensity is measured by the change of the transmittance of the probe light at the resonance position of the electromagnetic induced transparency of the Rydberg atom, so as to construct a Rydberg atom microwave electric field sensor;

[0014] S2, the spectrum of the electromagnetic induced transparency of the Rydberg atom is obtained, and the influence of the buffer gas neon on the linewidth and frequency shift of the electromagnetic induced transparency of the Rydberg atom is further obtained;

[0015] S3, the signal microwave electric field and the local microwave electric field form a beat signal;

[0016] S4, the transient bandwidth of the Rydberg atom mixer is measured based on the beat signal;

[0017] S5, the relationship between the EIT spectral peak intensity and the EIT linewidth and the buffer gas pressure is calibrated;

[0018] S6, the relationship between the transient bandwidth of the Rydberg atom mixer and the buffer gas pressure is obtained, and the amount of the buffer gas filled into the rubidium atom cell is determined accordingly.

[0019] Further, in step S1, the 780nm probe light and 480nm coupling light are used to propagate in the rubidium atom cell in opposite directions to form electromagnetic induced transparency of the Rydberg atom, and after the application of a microwave electric field, the electromagnetic induced transparency of the Rydberg atom is split into Autler-Townes, and the change of the signal microwave electric field intensity is measured by the change of the transmittance of the probe light at the resonance position of the electromagnetic induced transparency of the Rydberg atom, so as to construct a Rydberg atom microwave electric field sensor; 87 The Rb atom involves a Rydberg atom composed of four energy levels, which are 5S 1 / 2 (F=2), 5P 3 / 2 (F=3), and 55D 5 / 2(F=4) and 56P 3 / 2 (F=3).

[0020] Further, the Rydberg signal of the reference rubidium atomic cell is measured in each experiment in step S2, a reference absorption spectrum or EIT spectrum is generated to calibrate the frequency of the corresponding spectrum under the buffer gas, and the information of the atomic energy level and the Rydberg EIT spectrum shift and broadening is obtained.

[0021] Further, the transmission of the beat signal is periodic in step S3, and the frequency of the change is the frequency difference of the two microwave electric fields, and the intensity of the periodic signal is proportional to the electric field amplitude of the signal microwave electric field, which is called a Rydberg atomic mixer.

[0022] Further, the frequency of the coupling light is locked on the Rydberg atomic electromagnetically induced transparency peak in step S4, then the intensity of the beat signal corresponding to each beat frequency is measured by changing the frequency difference of the signal microwave electric field relative to the local microwave electric field, and the relationship between the transient bandwidth of the Rydberg microwave electric field sensor and the beat frequency is obtained, so that the transient bandwidth of the Rydberg atomic mixer is obtained based on the intensity of the beat signal.

[0023] The beneficial technical effects of the present application are that: on the basis of the Rydberg atomic mixer, different pressure buffer gas neon is filled into the rubidium atomic cell, the influence of the buffer gas on the Rydberg EIT frequency shift and EIT line width is studied, and the corresponding transient bandwidth is measured. It is found that the EIT frequency shift and the broadening are proportional to the pressure of the buffer gas, and the neon will greatly reduce the intensity of the Rydberg EIT signal. The intensity of the Rydberg EIT signal will cause the sensitivity of the mixer to decrease, and the broadening of the Rydberg EIT will greatly improve the transient bandwidth. When the beam diameter is 1mm, the transient bandwidth of 10MHz can be realized after filling neon, and the sensitivity change is less than 1dB within 9MHz. A very simple method for improving the transient bandwidth of the Rydberg atomic sensor is proposed, which is very meaningful in microwave sensing. BRIEF DESCRIPTION OF DRAWINGS

[0024] Figure 1 A structure schematic diagram of a system for improving the transient bandwidth of a Rydberg atomic microwave sensor is shown for an embodiment of the present application;

[0025] Figure 2 A level schematic diagram of a system for improving the transient bandwidth of a Rydberg atomic microwave sensor is shown for an embodiment of the present application;

[0026] Figure 3a Rydberg EIT in a rubidium vapor cell filled with 0.3, 0.5, 0.7, and 0.9 Torr of neon, respectively;

[0027] Figure 3b A graph of the relationship between the EIT spectrum peak position and the neon gas pressure;

[0028] Figure 3c Figure 6 is a graph of the EIT linewidth versus the neon pressure;

[0029] Figure 3d Figure 7 is a graph of the EIT peak intensity versus the neon pressure;

[0030] Figure 4a Figure 8 is a graph of the beat signal intensity versus the beat signal frequency in the rubidium vapor cell filled with 0.3, 0.5, 0.7, 0.9 Torr neon, respectively;

[0031] Figure 4b Figure 9 is a graph of the 3dB bandwidth versus the EIT linewidth at different neon pressures;

[0032] Figure 4c Figure 10 is a graph of the beat signal intensity and the EIT intensity versus the neon pressure when the mixer is 100 kHz;

[0033] Figure 4d Figure 11 is a graph of the Rydberg atom microwave sensor sensitivity versus the Rydberg EIT height when the mixer is 100 kHz;

[0034] Wherein: 1-polarization beam splitter, 2-half wave plate, 3-rubidium atom cell filled with buffer gas, 4-reference rubidium atom cell, 5-780 nm light reflecting mirror, 6-mirror, 7-photodetector, 8-power divider, 9-microwave horn antenna. DETAILED DESCRIPTION

[0035] The application will be further described below in conjunction with the drawings and specific embodiments.

[0036] Example 1

[0037] As Figure 1As shown in the figure, the embodiment of the present application provides a system for improving the transient bandwidth of a Rydberg atom microwave sensor, which comprises a 780nm laser and a 480nm laser for generating 780nm laser and 480nm laser respectively, the 780nm laser is used as probe light, and the 480nm laser is used as coupling light, a half-wave plate 2 and a polarization beam splitter prism 1 are sequentially arranged in front of the 780nm laser and the 480nm laser, the probe light and the coupling light pass through the half-wave plate 2 and the polarization beam splitter prism 1 to obtain first vertical polarization light, and the light transmitted through the polarization beam splitter prism 1 passes through another half-wave plate 2 to be adjusted into second vertical polarization light. The directions of the reflected light first vertical polarization light and the second vertical polarization light are perpendicular to the direction of the incident light (probe light and coupling light), and the two paths of the probe light and the coupling light formed by the first vertical polarization light and the second vertical polarization light are coincidently propagated in the same size of the buffer gas filled rubidium atom cell 3 and the reference rubidium atom cell 4, and the probe light is received by two photodetectors 7 through a 780nm light transmitting and 480nm light reflecting mirror 5 to receive two paths of probe light signals.

[0038] In the embodiment, the buffer gas filled in the rubidium atom cell 3 is inert gas neon, and the reference rubidium atom cell 4 is a vacuum rubidium atom cell.

[0039] The optical devices in the figure are all fixed in height and the light paths are all in the same horizontal plane, a signal microwave and a local microwave with a frequency close to the signal microwave are respectively radiated to the rubidium atom cells 3 and 4 through a power divider 8 and a microwave horn antenna 9 to realize interference in the rubidium atom cells 3 and 4. The microwave horn antenna 9 is in the same horizontal plane with the rubidium atom cells 3 and 4 and satisfies the far field condition, and the microwave horn antenna 9 is a rectangular microwave antenna.

[0040] The 780nm laser and the 480nm laser are coincidently propagated in the rubidium atom cells 3 and 4 to form electromagnetic induction transparency (EIT) of Rydberg atoms, at this time, after a microwave electric field is applied, the EIT will be split by Autler-Townes, and the microwave electric field intensity can be measured by detecting the light intensity change of the probe light near the EIT resonance.

[0041] In the embodiment of the present application, the probe light power is 1 micro watt, the diameter in the rubidium atom is about 1000 microns, the coupling light power is 210 milliwatts, and the diameter in the rubidium atom is about 1000 microns. Figure 2 As shown in the figure, the probe light is 780nm laser provided by a semiconductor laser, and the frequency is locked to the transition of Rb 87 5S 1 / 2 ,F=2 to 5P 3 / 2 ,F’=3 by using saturated absorption spectrum. The coupling light is 480nm laser provided by a frequency doubling laser, and the wavelength can be locked to the transition of Rb87 5P 3 / 2 F' = 3 to 55D 3 / 2,5 / 2 The Rydberg EIT or EIT-AT spectrum is obtained by scanning between the states to obtain the Rydberg EIT or EIT-AT spectrum, using the frequency difference of 69.16708 MHz between the 55D 3 / 2,5 / 2 state. The above parameters are only used as an example in the embodiments of the present application, and in fact are not limited thereto. In the embodiments, only the principal quantum number n = 55 of the Rydberg state is used as an example, and in fact different working frequencies can be achieved by selecting different principal quantum numbers n of the Rydberg state.

[0042] In the experiment, for comparison, a vacuum rubidium atomic cell 4 and a rubidium atomic cell 3 filled with inert gas neon were used, which have the same size, i.e. a cylinder with a length of 75 mm and a diameter of 35 mm. An avalanche photodetector 7 more sensitive to weak light was used to receive the change in light intensity of the probe light after passing through the rubidium atomic cells 3 and 4.

[0043] In order to study in detail the influence of the buffer gas neon on the atomic energy level and the Rydberg EIT linewidth and frequency shift, the 5S-5P-55D Rydberg EIT spectrum was used. In each experiment, the Rydberg signal of the reference rubidium atomic cell 4 was measured at the same time, to generate a reference absorption spectrum or EIT spectrum to calibrate the frequency of the corresponding spectrum under the condition of filling with neon, and to obtain the information of the atomic energy level and the Rydberg EIT spectrum shift and broadening. A sound light modulator was used to perform sin modulation on the intensity of the coupling light at 30 kHz, and then the signal of the photodetector 7 was connected to a lock-in amplifier to improve the signal-to-noise ratio of the spectrum reading. Then the output signal of the lock-in amplifier was recorded by an oscilloscope, and was averaged 64 times to further improve the signal-to-noise ratio of the Rydberg EIT spectrum.

[0044] The signal microwave and a local microwave with a similar frequency passed through the power divider 8 and the same microwave horn antenna 9 respectively to be radiated to the rubidium atomic cells 3 and 4, and interference was realized in the rubidium atomic cells 3 and 4. It is noted that in the experiment, the microwave horn antenna 9 was used to realize the transmission of the microwave electric field to the rubidium atomic cells 3 and 4, and the microwave horn antenna 9 can provide a very good linearly polarized microwave signal under far field conditions. The beat frequency signal of the interference was measured by the Rydberg atomic microwave electric field sensor, that is, the periodic sinusoidal change of the probe light intensity with time was obtained, and the frequency of the beat frequency was equal to the frequency difference Δ MW When the amplitude E LO of the local microwave electric field is much larger than the amplitude E SIG of the signal microwave electric field, the probe light intensity T p is related to the amplitude of the signal microwave electric field as follows:

[0045] T p ∝ E LO +ESIG sin(Δ MW t) (1)

[0046] Only the amplitude of the probe light is concerned here, so the phase information of the local microwave field and the signal microwave field is ignored. From equation (1), it can be seen that the transmittance of the probe light periodically changes after the probe light interacts with the atoms, the coupling light and the microwaves in the rubidium atomic cells 3 and 4. The frequency of the change is the frequency difference of the two microwave fields, and the strength of the periodic signal is proportional to the field amplitude of the signal microwave field. This device is called a Rydberg atom mixer, and it can be used to measure the field strength, phase and polarization of the signal microwave field.

[0047] The frequency of the coupling light is locked on the Rydberg EIT peak, and then the frequency difference between the signal microwave field and the local microwave field is changed. The photodetector 7 collecting the probe light signal is connected to the frequency spectrometer, and the strength of the beat signal corresponding to each beat frequency is measured to obtain the relationship between the transient bandwidth of the Rydberg microwave field sensor and the beat frequency, thereby obtaining the information of the transient bandwidth. Specifically, the output power of the microwave sources of the signal microwave field and the local microwave field is adjusted to obtain the maximum beat signal at 100 kHz, and the corresponding powers are 4 dBm and 0 dBm respectively. It should be noted that since the neon filling will move the Rydberg EIT, the frequency of the coupling light is adjusted by the acousto-optic modulator to ensure that the frequency of the coupling light is locked on the Rydberg EIT peak in experiments with different neon filling pressures. The output power of the microwave sources of the signal microwave field and the local microwave field is adjusted to obtain the maximum beat signal at 100 kHz, and the corresponding powers are 4 dBm and 0 dBm respectively. Then the frequency of the local microwave field is fixed, and the frequency of the signal microwave field is changed. The frequency spectrometer is used to measure and record the strength of the beat signal corresponding to each beat frequency to obtain the relationship between the transient bandwidth of the Rydberg microwave field sensor and the beat frequency, thereby obtaining the information of the transient bandwidth.

[0048] The pressure of the neon filling in the Rydberg EIT of the rubidium atomic cell 3 filled with buffer gas neon is calibrated by moving the Rydberg EIT. Then the movement and broadening of the Rydberg EIT by neon are studied. By scanning the frequency of the coupling light, the Rydberg EIT in the reference rubidium atomic cell 4 and the rubidium atomic cell 3 filled with 0.3, 0.5, 0.7 and 0.9 Torr of neon marked by the manufacturer are measured respectively under the same experimental parameters such as light intensity and spot diameter, and the frequency is calibrated by the energy level interval (69.16708 MHz) between 55D 3 / 2 and 55D 5 / 2 in the reference rubidium atomic cell 4. The results are shown in Figure 3(a), and the black solid line is the EIT spectrum in the reference rubidium atomic cell 4. The small peak on the left corresponds to 5P 3 / 2 to 55D 3 / 2EIT of the 5P 3 / 2 to 55D 5 / 2 EIT of the 5P

[0049] Figure 3(b) shows the relationship between the EIT peak position and the neon pressure. In the present embodiment, the data of the shift of the nD Rydberg state with the neon pressure reported in the literature

Phys. Rev. A 36, 2134 (1987)

[0050] The mixer output intensity as a function of the beat frequency is measured for the reference rubidium vapor cell 4 and the rubidium vapor cell 3 with 0.40, 0.68, 0.95 and 1.20 Torr neon by changing the frequency difference between the signal microwave field and the local microwave field. The results are shown in Figure 4. The experimental results show that the instantaneous bandwidths in all the vapor cells exhibit similar trends, i.e. slowly decreasing or remaining constant in the range of 0-10 MHz, and rapidly decreasing at more than 10 MHz, which is due to the bandwidth limitation of the avalanche photodiode used.

[0051] The black hollow squares in the figure are the experimental results for the reference rubidium vapor cell 4, and the black dashed line is the 3dB mark for the beat signal intensity at 100 kHz. It can be seen that the 3dB bandwidth is 2.4 MHz. The red hollow circles are the experimental results for the rubidium vapor cell 3 with 0.40 Torr neon, and the red dashed line is the 3dB mark at 100 kHz. It can be seen that the 3dB bandwidth is 9.6 MHz. Compared with the reference rubidium vapor cell 4, the 3dB bandwidth is greatly increased, about 4 times. This increase is positively correlated with the increase of the EIT linewidth by 2 times. Within the 3dB bandwidth of the reference rubidium vapor cell 4, the sensitivity decreases continuously; while within the 3dB bandwidth of 9.6 MHz for the rubidium vapor cell 3 with 0.40 Torr neon, there is a flat region in the range of 0-9.6 MHz, and the sensitivity decreases rapidly only when the frequency is more than 9.6 MHz. More interestingly, the sensitivity changes less than 1 dB in the flat region of 0-9.6 MHz, which is a very good flat region response.

[0052] As the increase of the neon pressure, such as 0.68, 0.95 and 1.20 Torr, as shown in the green hollow triangle, blue hollow triangle and cyan hollow lozenge in Fig. 4(a), it is found that the 3dB bandwidth does not change significantly compared with the 0.4 Torr neon, but the sensitivity gradually decreases with the increase of the neon pressure. Fig. 4(b) shows the 3dB bandwidth and the corresponding EIT linewidth under different neon pressures, it can be seen that the EIT linewidth almost linearly increases with the increase of the neon pressure, but the 3dB bandwidth almost does not change above 0.4 Torr, which means that the nature of the Rydberg state itself limits the improvement of the transient bandwidth. Fig. 4(c) shows the relationship between the 100kHz beat signal intensity and the EIT intensity with the increase of the neon pressure, the sensitivity decreases sharply with the increase of the neon pressure, which is consistent with the rapid decrease of the EIT intensity with the increase of the neon pressure, as shown in Fig. 4(d).

[0053] As can be seen from Fig. 3(c), the buffer gas will increase the linewidth of the Rydberg EIT, which in some sense interferes with the process of the Rydberg EIT reaching the equilibrium state, i.e. interrupts the process of the Rydberg EIT reaching the equilibrium. As can be seen from Fig. 3(d), even a very small buffer gas pressure will greatly reduce the intensity of the Rydberg EIT, which means that the collision of the buffer gas will seriously interfere with the dynamic process of the establishment of the Rydberg EIT, making it difficult to reach the stable state when the collision effect is ignored. These effects greatly shorten the time of the Rydberg EIT to establish the "equilibrium process", which on the one hand brings the increase of the transient bandwidth, as shown in Fig. 4(a).

[0054] Embodiment two

[0055] The embodiment of the present application provides a method for improving the transient bandwidth of a Rydberg atom microwave sensor, which improves the transient bandwidth of the Rydberg atom microwave sensor by the system for improving the transient bandwidth of the Rydberg atom microwave sensor provided in the embodiment one of the present application, and the method comprises the following steps:

[0056] S1, constructing a Rydberg atom microwave electric field sensor

[0057] By using 87 Rb atom, the Rydberg atom involved is composed of four energy levels, which are 5S 1 / 2 (F=2), 5P 3 / 2 (F=3), 55D 5 / 2 (F=4), 56P 3 / 2 (F=3), but not limited to specific atomic energy levels. The 780nm probe light acts on the transition of 5S 1 / 2 (F=2)→5P 3 / 2 (F=3), and the 480nm coupling light acts on the transition of 5P3 / 2 (F=3)→55D 5 / 2 (F=4)→56P 5 / 2 (F=3)→55D 3 / 2 (F=3)→55D. The 780nm probe light and 480nm coupling light counter-propagating in the rubidium vapor cell form electromagnetically induced transparency (EIT) of Rydberg atoms. When a microwave field is applied, the EIT will be split into two peaks, which is called Autler-Townes splitting. The intensity of the microwave field can be measured by the change of the transmission of the probe light at the EIT resonance position.

[0058] In the embodiment of the present application, the power of the probe light is 1 micro-watt, the diameter of the probe light in the rubidium vapor is about 1000 microns, the power of the coupling light is 210 milliwatts, and the diameter of the coupling light in the rubidium vapor is about 100 microns. In fact, the present application is not limited to this.

[0059] S2, the spectrum of the Rydberg EIT is obtained, and the influence of the buffer gas neon on the linewidth and frequency shift of the Rydberg EIT is studied.

[0060] In order to study the influence of the buffer gas neon on the atomic energy level and the linewidth and frequency shift of the Rydberg EIT in detail, the 5S-5P-55D Rydberg EIT spectrum is set. In each experiment, the Rydberg signal of the vacuum rubidium vapor cell 4 is measured at the same time, which is used to generate a reference absorption spectrum or EIT spectrum to calibrate the frequency of the corresponding spectrum under the condition of filling neon, so as to obtain the information of the atomic energy level and the shift and broadening of the Rydberg EIT spectrum. In the embodiment of the present application, a sound light modulator is used to perform 30kHz sin modulation on the intensity of the coupling light, and then the signal of the photodetector 7 is connected to a lock-in amplifier to improve the signal-to-noise ratio of the spectrum reading. Then the output signal of the lock-in amplifier is recorded by an oscilloscope, and the Rydberg EIT spectrum is averaged for 64 times to further improve the signal-to-noise ratio of the Rydberg EIT spectrum.

[0061] S3, the signal microwave field and the local microwave field form a beat signal.

[0062] The signal microwave and the local microwave with a frequency close to the signal microwave are radiated to the rubidium atom vapor cell 3 and 4 through the power divider 8 and the same microwave horn antenna 9 respectively, and interference is realized in the rubidium atom vapor cell. It is noted that the rectangular microwave horn antenna 9 is used to realize the transmission of the microwave electric field to the rubidium atom vapor cell in the experiment, and the rectangular microwave horn antenna 9 can provide a very good linearly polarized microwave signal under the far field condition. By measuring the change of the probe light transmittance when the two microwave electric fields act together, the electric field amplitude, phase and polarization of the signal microwave electric field can be measured.

[0063] S4, measuring the transient bandwidth of the Rydberg atom frequency mixer based on the beat signal.

[0064] The frequency of the coupling light is locked on the Rydberg EIT peak, and then the frequency difference between the signal microwave electric field and the local microwave electric field is changed, the photodetector for collecting the probe light signal is connected to the frequency spectrometer, the intensity of the beat signal corresponding to each beat frequency is measured, the relationship between the transient bandwidth of the Rydberg microwave electric field sensor and the beat frequency is obtained, and thus the information of the transient bandwidth is obtained. Specifically, the output powers of the microwave sources of the signal microwave electric field and the local microwave electric field are adjusted to obtain the maximum beat signal at 100 kHz, and the corresponding powers are 4 dBm and 0 dBm respectively.

[0065] It is noted that, since the neon filling will move the Rydberg EIT, the frequency of the coupling light is locked on the Rydberg EIT peak in different experiments with different neon gas pressures by adjusting the frequency of the coupling light with the acousto-optic modulator. In the experiment, the output powers of the microwave sources of the signal microwave electric field and the local microwave electric field are adjusted to obtain the maximum beat signal at 100 kHz, and the corresponding powers are 4 dBm and 0 dBm respectively. Then the frequency of the local microwave electric field is fixed, the frequency of the signal microwave electric field is changed, the intensity of the beat signal corresponding to each beat frequency is measured and recorded with the frequency spectrometer, the relationship between the transient bandwidth of the Rydberg microwave electric field sensor and the beat frequency is obtained, and thus the transient bandwidth is obtained based on the intensity of the beat signal.

[0066] S5, calibration of the buffer gas neon pressure.

[0067] The pressure of neon buffer gas in the rubidium atomic vapor cell is calibrated by the shift of the Rydberg EIT. Then the shift and the broadening of the Rydberg EIT by neon buffer gas are studied. The Rydberg EIT in the reference rubidium atomic vapor cell 4 and the rubidium atomic vapor cells filled with 0.3, 0.5, 0.7 and 0.9 Torr neon buffer gas are measured by scanning the frequency of the coupling laser, respectively. The frequency is calibrated by the energy level interval (69.16708 MHz) between 5P 3 / 2 and 55D 5 / 2 in the reference rubidium atomic vapor cell 4. The results are shown in Fig. 3(a), the black solid line is the EIT spectrum in the reference rubidium atomic vapor cell 4, the left small peak corresponds to the EIT between 5P 3 / 2 and 55D 3 / 2 , the right large peak corresponds to the EIT between 5P 3 / 2 and 55D 5 / 2 . The red, blue, green and purple lines are the Rydberg EIT in the rubidium atomic vapor cells filled with 0.3, 0.5, 0.7 and 0.9 Torr neon buffer gas, respectively. Fig. 3(b) shows the relationship between the EIT peak position and the neon buffer gas pressure. The data of the shift of the nD Rydberg state by neon buffer gas reported in the literature

Phys. Rev. A 36, 2134 (1987)

[0068] S6: Obtain the relationship between the transient bandwidth of the Rydberg atomic frequency mixer and the buffer gas, and determine the amount of buffer gas filled into the rubidium atomic vapor cell according to the relationship.

[0069] The frequency difference between the signal microwave electric field and the local microwave electric field was changed, and the mixer output intensity of the reference rubidium atomic cell 4 and the rubidium atomic cell 3 filled with 0.40, 0.68, 0.95 and 1.20 Torr of neon were measured respectively, and the relationship between the transient bandwidth of the Rydberg atom mixer and the pressure of the buffer gas was obtained, as shown in FIG. 4. According to the relationship between the transient bandwidth of the Rydberg atom mixer and the pressure of the buffer gas, the amount of the buffer gas filled into the rubidium atomic cell can be determined. As shown in FIG. 4, the transient bandwidths in all the rubidium atomic cells 3 and 4 show similar trends, that is, slowly decreasing or remaining unchanged within 0-10 MHz, and sharply decreasing when greater than 10 MHz, which should be due to the bandwidth limitation of the photoelectric detector 7 used.

[0070] The black hollow squares are the experimental results of the reference rubidium atomic cell 4, and the black dashed line is a marker line for the 3dB drop of the beat signal intensity at 100 kHz, and it can be seen that the 3dB bandwidth is 2.4 MHz at this time; the red hollow circles are the experimental results of the rubidium atomic cell 3 filled with 0.40 Torr of neon, and the red dashed line is a marker line for the 3dB drop at 100 kHz, and it can be seen that the 3dB bandwidth is 9.6 MHz at this time. Compared with the reference rubidium atomic cell 4, the 3dB bandwidth is greatly increased, about 4 times. This increment is positively correlated with the increase of the EIT linewidth by 2 times. Within the 3dB bandwidth of the reference rubidium atomic cell 4, the sensitivity continuously decreases; while within the 3dB bandwidth of 9.6 MHz of the rubidium atomic cell 3 filled with 0.4 Torr of neon, there is basically a flat zone within 0-9.6 MHz, and the sensitivity only sharply decreases when greater than 9.6 MHz. More interestingly, within the flat zone of 0-9.6 MHz, the change of the sensitivity is less than 1 dB, which is a very good flat zone response.

[0071] With the increase of the neon pressure, such as 0.68, 0.95 and 1.20 Torr, as shown in FIG. 4(a) green hollow triangles, blue hollow triangles and cyan hollow lozenges, it is found that compared with 0.4 Torr of neon, the 3dB bandwidth does not change significantly, but the sensitivity gradually decreases with the increase of the neon pressure. FIG. 4(b) shows the 3dB bandwidth and the corresponding EIT linewidth under different neon pressures, and it can be seen that the EIT linewidth almost linearly increases with the increase of the neon pressure, but the 3dB bandwidth almost does not change when the neon pressure is greater than 0.4 Torr, which means that the properties of the Rydberg state itself limit the improvement of the transient bandwidth. FIG. 4(c) shows the relationship between the beat signal intensity at 100 kHz of the mixer and the EIT intensity with the increase of the neon pressure, and the sensitivity sharply decreases with the increase of the neon pressure, which is consistent with the fact that the EIT intensity sharply decreases with the increase of the neon pressure, as shown in FIG. 4(d).

[0072] It can be seen from Fig. 3(c) that the buffer gas increases the line width of the Rydberg EIT, which in a sense interferes with the process of the Rydberg EIT reaching the equilibrium state, i.e. interrupts the process of the Rydberg EIT reaching the equilibrium. It can be seen from Fig. 3(d) that even a very small buffer gas pressure greatly reduces the strength of the Rydberg EIT, which means that the collision of the buffer gas seriously interferes with the dynamic process of the Rydberg EIT to establish, so that it is difficult to reach the stable state when the collision effect is ignored. These effects greatly shorten the time of the Rydberg EIT to establish the "equilibrium process", which on the one hand brings an increase in the transient bandwidth, as shown in Fig. 4(a).

[0073] It can be seen from the above examples that the system and method for improving the transient bandwidth of the Rydberg atomic microwave sensor disclosed in the present application include two lasers for generating probe light and coupling light, respectively; the probe light and the coupling light both pass through a half-wave plate and are reflected by a polarization beam splitter prism to obtain first vertical polarization light, and the light transmitted through the polarization beam splitter prism is adjusted by a mirror and a half-wave plate in sequence to become second vertical polarization light; the two-way probe light and the coupling light are propagated in opposite directions in two rubidium atomic vapor cells, and the probe light passing through the reference rubidium atomic vapor cell and the polarization beam splitter prism and the probe light passing through the rubidium atomic vapor cell filled with buffer gas and the half-transmission half-reflection mirror are both received by a photodetector; a signal microwave and a local microwave are radiated onto the two rubidium atomic vapor cells through a power divider and a microwave horn antenna, respectively, and after the microwave electric field is applied, the change in the light intensity near the EIT resonance is measured by the probe light to measure the microwave electric field strength.

[0074] On the basis of the Rydberg atomic mixer, different pressures of buffer gas neon are filled into the rubidium atomic vapor cell, the influence of the buffer gas on the Rydberg EIT frequency shift and the EIT line width is studied, and the corresponding transient bandwidth is measured. The results show that the EIT frequency shift and the line width are proportional to the pressure of the buffer gas, and the neon greatly reduces the strength of the Rydberg EIT signal. The reduction of the Rydberg EIT signal strength leads to the decrease of the sensitivity of the mixer. The line width of the Rydberg EIT greatly improves the transient bandwidth, and when the beam diameter is 1 mm, the transient bandwidth of 10 MHz can be realized after filling neon, and the sensitivity change is less than 1 dB in 9 MHz. The transient bandwidth of the Rydberg atomic sensor is very simply improved, which has important significance and value in microwave sensing.

[0075] The system and method described in the present application are not limited to the examples described in the specific embodiments, and other embodiments can be derived by those skilled in the art according to the technical solutions of the present application, which also belong to the technical innovation range of the present application.

Claims

1. A system for increasing the transient bandwidth of a Rydberg atom microwave sensor, the system comprising: The system comprises a first laser for generating 780nm laser light; a second laser for generating 480nm laser light; the 780nm laser light is probe light, and the 480nm laser light is coupling light; a half-wave plate and a polarization beam splitter prism are sequentially arranged in front of the first laser and the second laser; the probe light and the coupling light pass through the half-wave plate and the polarization beam splitter prism to obtain first vertical polarization light; the light transmitted through the polarization beam splitter prism passes through a mirror and a half-wave plate to be adjusted into second vertical polarization light; the second vertical polarization light and the first vertical polarization light are transmitted through two rubidium atom cells in opposite directions; the probe light in the first vertical polarization light is received by a photodetector after passing through a reference rubidium atom cell and a polarization beam splitter prism; the probe light in the second vertical polarization light is received by the photodetector after passing through a rubidium atom cell filled with buffer gas and a 780nm light transmitting and 480nm light reflecting mirror; a signal microwave and a local microwave with a similar frequency are radiated to the two rubidium atom cells through a power divider and a microwave horn antenna respectively, and interference is realized in the two rubidium atom cells; after the microwave electric field is applied, the intensity of the beat frequency signal generated by the signal microwave and the local microwave is measured by measuring the change in the intensity of the probe light near the EIT resonance; the transient bandwidth of the Rydberg atom mixer is measured based on the beat frequency signal; the relationship between the EIT spectral peak intensity and the EIT linewidth and the buffer gas pressure is calibrated; the relationship between the transient bandwidth of the Rydberg atom mixer and the buffer gas pressure is obtained; and the amount of the buffer gas filled in the rubidium atom cell is determined accordingly.

2. The system for increasing the transient bandwidth of a Rydberg atom microwave sensor of claim 1, wherein: The reference rubidium atom cell and the rubidium atom cell filled with buffer gas are of the same size; the buffer gas filled in the rubidium atom cell is inert gas neon; and the reference rubidium atom cell is a vacuum rubidium atom cell.

3. The system for increasing the transient bandwidth of a Rydberg atom microwave sensor of claim 2, wherein: The two rubidium atom cells are both cylindrical in shape.

4. The system for increasing the transient bandwidth of a Rydberg atom microwave sensor of claim 1, wherein: The microwave horn antenna is located at a distance from the two rubidium atom cells that satisfies the far-field condition and is located in the same horizontal plane as the two rubidium atom cells; and the microwave horn antenna is a rectangular microwave antenna.

5. The system for increasing the transient bandwidth of a Rydberg atom microwave sensor of claim 1, wherein: The photodetector is an avalanche photodetector.

6. A method of increasing the transient bandwidth of a Rydberg atom microwave sensor, the method comprising: The method for improving the transient bandwidth of a Rydberg atom microwave sensor by using the system for improving the transient bandwidth of a Rydberg atom microwave sensor according to any one of claims 1-5 comprises the following steps: S1, the probe light of 780nm and the coupling light of 480nm are transmitted through the rubidium atom cell in opposite directions to form electromagnetic induction transparency of the Rydberg atom; after the microwave electric field is applied, the electromagnetic induction transparency of the Rydberg atom is split by Autler-Townes; the change in the intensity of the signal microwave electric field is measured by measuring the change in the transmittance of the probe light at the resonance position of the electromagnetic induction transparency of the Rydberg atom; and a Rydberg atom microwave electric field sensor is constructed; S2, the spectrum of the electromagnetic induction transparency of the Rydberg atom is obtained, and the influence of the buffer gas on the linewidth and frequency shift of the electromagnetic induction transparency of the Rydberg atom is further obtained; S3, a beat frequency signal is formed by the signal microwave electric field and the local microwave electric field; S4, the transient bandwidth of the Rydberg atom mixer is measured based on the beat frequency signal; and S5, calibrate the relationship between the EIT peak intensity and the buffer gas pressure; S6, obtain the relationship between the transient bandwidth of the Rydberg atom mixer and the buffer gas pressure, and determine the amount of buffer gas filled into the rubidium atomic cell according to the relationship.

7. The method of increasing the transient bandwidth of a Rydberg atom microwave sensor of claim 6, wherein: In step S1 the use of 87 Rydberg atoms, which are involved, consist of four energy levels, namely 5S 1 / 2 (F=2), 5P 3 / 2 (F=3), 55D 5 / 2 (F=4) and 56P 3 / 2 (F=3).

8. The method of increasing the transient bandwidth of a Rydberg atom microwave sensor of claim 6, wherein: In step S2, the Rydberg signal of the reference rubidium atomic cell is measured in each experiment, and a reference absorption spectrum or EIT spectrum is generated to calibrate the frequency of the corresponding spectrum under the condition of filling buffer gas, so as to obtain the information of the atomic energy level and the shift and broadening of the Rydberg EIT spectrum.

9. The method of increasing the transient bandwidth of a Rydberg atom microwave sensor of claim 6, wherein: In step S3, the transmission rate of the beat signal changes periodically, and the frequency of the change is the frequency difference between the two microwave electric fields. The intensity of the periodic signal is proportional to the electric field amplitude of the signal microwave electric field, which is called the Rydberg atom mixer.

10. The method of increasing the transient bandwidth of a Rydberg atom microwave sensor of claim 9, wherein: In step S4, the frequency of the coupling light is locked on the Rydberg atom electromagnetically induced transparency peak, and then the intensity of the beat signal corresponding to each beat frequency is measured by changing the frequency difference of the signal microwave electric field relative to the local microwave electric field, so as to obtain the relationship between the transient bandwidth of the Rydberg microwave electric field sensor and the beat frequency, and thus obtain the transient bandwidth of the Rydberg atom mixer based on the intensity of the beat signal.

Images