Rydberg atom receiver signal-to-noise ratio enhancement method based on laser array

Abstract

The invention relates to a Rydberg atom receiver signal-to-noise ratio enhancement method based on a laser array, which mainly combines the idea of a microwave antenna array, utilizes the coherence of the laser array, and adds a group of beam splitters and collimators on the basis of the existing experimental design to form a detection laser array. According to the method, a homologous laser array mode is utilized, the power density of detection light is reduced, meanwhile, the activity and transition of photoelectrons are reduced, and suppression of the photocurrent noise level is achieved; by reducing the optical readout noise and the background noise, the signal-to-noise ratio of the Rydberg atom receiver is improved, the detection sensitivity is improved, a new technical scheme is provided for further realizing the Rydberg atom receiver with ultrahigh sensitivity, and the Rydberg atom receiver has high application value.

Description

Technical Field

[0001] The present invention relates to the field of microwave electric field measurement, and particularly to a method for enhancing the signal-to-noise ratio of a Rydberg atom receiver based on a laser array. Based on the Electromagnetic Induced Transparency (EIT) and the Autler-Townes (AT) effect, and combined with the concept of a microwave antenna array, the present invention proposes a method for enhancing the signal-to-noise ratio of a Rydberg atom receiver based on a laser array. This method can reduce the detection optical power density, decrease the activities and transitions of photoelectrons, suppress the photocurrent noise level, thereby improving the signal-to-noise ratio of the atom receiver, and providing a new way for further optimizing the ultra-high sensitivity Rydberg atom receiver. Technical Background

[0002] Microwave electric field measurement technology is widely used in many fields such as radar detection, wireless communication, physical metrology, material analysis, biomedicine, and industrial detection. Although traditional electromagnetic measurement methods are mature, there are still some limitations. Due to the size effect of classical antennas being affected by the Chu limit, the lower the frequency, the larger the required antenna size. Therefore, not only is the intensification and miniaturization of equipment restricted, but also the sensing and measurement of a large working frequency band cannot be achieved, making it difficult to meet the application requirements of high-speed communication and high-information transmission today. In addition, before actual far-field detection, the metal antenna needs to be placed in a standard field for calibration. The "calibration cycle" problem caused by the non-absoluteness of the standard field and Johnson-Nyquist noise (also known as thermal noise, Johnson noise, or Nyquist noise) etc. all limit the sensitivity and make it difficult to break through the μV / cm level, making it difficult to meet the requirements of ultra-high sensitivity for today's precision electric field measurement technology. Therefore, a new measurement method is urgently needed to solve the problems of traditional electric field measurement.

[0003] With the rapid development of quantum information technology, microwave electric field measurement technology based on Rydberg atoms has gradually emerged. Rydberg atoms have significant advantages such as ultra-high sensitivity, large working bandwidth, and easy integration and miniaturization, and are considered to be an atomic receiver with great development potential. In terms of its preparation, the research in this field usually uses two-photon three-level excitation to generate the electromagnetic induced transparency effect. At the same time, under the action of the electromagnetic wave to be measured, the resonance transition from the Rydberg state to the near-Rydberg state will cause the AT splitting phenomenon in the spectrum. By analyzing the EIT-AT spectrum, high-sensitivity measurement of the microwave electric field is achieved.

[0004] Although the microwave electric field measurement technology based on Rydberg atoms is cutting-edge, exploratory, and revolutionary, it still faces challenges in terms of its performance parameters, especially in achieving ultra-high sensitivity, which is far from the theoretical value. Under the standard quantum limit, the theoretical sensitivity of the Rydberg atom electric field meter is The magnitude is far higher than the current best experimental level at home and abroad There is still huge potential for development. The sensitivity of Rydberg atom detection is mainly limited by the technical noise of the system. This noise includes the background noise of the photodetector and oscilloscope, as well as the optical readout noise. The main limiting factor is the optical readout noise generated by the laser. Summary of the Invention

[0005] The technical problem solved by the present invention is: in view of the practical problem that the ultra-high sensitivity theoretical value of the Rydberg atom receiver is difficult to achieve, a new solution for improving the optical path is proposed. In view of the fact that the optical environment of the existing Rydberg atom excitation preparation is complex, and the optical and microwave equipment generate large noise, resulting in a low detection signal-to-noise ratio, which is not conducive to the improvement of the Rydberg atom detection sensitivity, the present invention adds a beam splitting grating and a collimating grating at the pump laser beam to form a 2×2 laser array. By utilizing the coherence of the beam array, the signal-to-noise ratio of the atom receiver is enhanced, and the Rydberg atom detection sensitivity is further improved. Secondly, in view of the problem that the optical paths of multiple groups of arrays of single beam-single gas chamber are complicated, large in size, and difficult to integrate, the present invention adopts a 2×2 detection beam array to irradiate the same cesium atomic gas chamber, intensively designs the optical path, reduces the size of the Rydberg atom receiver system, and improves the operability and versatility of the laser array.

[0006] The technical solution of the present invention is as follows: Aiming at the problem that the low signal-to-noise ratio of the existing Rydberg atom receiver limits the detection sensitivity, the present invention proposes the following technical solutions: 1. Design of irradiating a cesium atomic cell with a laser beam array. The present invention combines the concept of an array antenna. Cesium atoms are filled in a glass vapor cell. A set of beam splitting gratings and collimating gratings are arranged between the detection laser path and the cesium atomic vapor cell. Under the combined action of the 2×2 detection light array and the coupling light, the cesium atoms are excited to the Rydberg state. The transmission signal of the detection laser is detected and analyzed by a single photodetector (PD) and a spectrum analyzer (SA) through a bunching grating. Under the condition of a certain total power, the detection power density is reduced, and lower optical readout noise is achieved. 2. The beam array is equivalent to several Rydberg atom receivers. Compared with the receivers of four groups of single-beam - single-cell - single-photodetector or detecting the transmission signals generated by the laser array through four photodetectors respectively, the present invention adopts the detection method of a 2×2 laser array through the same photodetector, which reduces the background noise brought by the photodetector and further improves the signal-to-noise ratio of the atomic receiver. 3. Intensive optical path and easy operation. The present invention adopts the simplest existing excitation optical path and realizes the reduction of the signal-to-noise ratio only by adding two beam splitting gratings and collimating gratings. It not only realizes highly efficient sensitivity detection in a limited space but also maintains the original structural design, making it more convenient and easier to operate for subsequent researchers to conduct further comparative experiments, etc.

[0007] The principle of the present invention is as follows:

[0008] (1) Basic characteristics of Rydberg atoms

[0009] A Rydberg atom is a highly excited atom with a very large principal quantum number. It has properties such as a long lifetime, a large radius, and a large electric dipole moment that many other neutral atoms do not have. The concept of Rydberg atoms was first proposed by the Swedish physicist Rydberg. Its binding energy follows a simple empirical formula - the Rydberg formula, that is:

[0010]

[0011] where n is the principal quantum number and Ry is the Rydberg constant of the system. In 1913, with the establishment of the atomic Bohr model based on this theory, n was further determined, and the Rydberg constant and the orbital radius of the electron were given according to basic physical constants, that is:

[0012]

[0013]

[0014] where Z is the atomic number, e is the electron charge, m e is the mass of the electron, and ε0 is the permittivity of vacuum. is the reduced Planck constant, and a0 is the Bohr radius.

[0015] For a general alkali metal atom, there is only one electron in the outermost electron orbit. Through optical excitation, this electron can be driven to an energy state with a high principal quantum number (e.g., the principal quantum number), namely the so-called Rydberg state, and these atoms are Rydberg atoms. Usually, rubidium atoms or cesium atoms are used because, due to their large atomic numbers, compared with alkali metal atoms with smaller atomic numbers, the energy level of the outermost electron ground state is higher and it is easier to be excited to a high-excited state. At this time, many characteristics of Rydberg atoms are related to the principal quantum number n, specifically manifested as follows: the atomic orbital radius and the electric dipole moment are proportional to n 2 ; the lifetime is proportional to n 3 , and the lifetime of a highly excited Rydberg atom can even reach the microsecond or even millisecond level; the energy level spacing is proportional to n -3 , the transition frequencies between different Rydberg states can cover the DC to THz band, and at the same time, the transition probability between adjacent energy levels of Rydberg atoms is very high; the van der Waals force is proportional to n 11 ; the atomic polarizability is proportional to n 7 , making Rydberg atoms very sensitive to external fields and extremely vulnerable to the manipulation of external fields; the interaction strength is proportional to n 4 ; the spectral linewidth is inversely proportional to n 3 etc., and the dependence on n m is shown in Table 1.

[0016] Table 1 Dependence of the main characteristics of Rydberg atoms on n m dependence

[0017]

[0018]

[0019] (2) EIT effect

[0020] EIT is a quantum interference phenomenon. It uses an externally applied coherent light field (electromagnetic field) to induce phase cancellation interference in an atomic system to cancel the absorption of a certain light by the medium, thereby generating an ultra-narrow transparency peak. Rydberg atoms detect radio waves through EIT spectroscopy.

[0021] Here, taking a three-level atomic system as an example, the EIT phenomenon and its physical mechanism are described. Taking a ladder-type three-level atomic system as an example, |1> is used as the atomic ground state, and |2> and |3> are used as the atomic excited states respectively. The probe light acts between |1> and |2>, and the coupling light acts between |2> and |3>. In the dipole approximation, the Hamiltonian of this system is expressed as:

[0022] H = H0 + H1 (1)

[0023] Among them, H0 is the free Hamiltonian of the atomic system, and H1 is the Hamiltonian of the interaction between the optical field and the atom:

[0024]

[0025] Ω p represents the Rabi frequency of the probe light, and Ω c represents the Rabi frequency of the coupling light, and they characterize the intensity of the interaction between the optical field and matter.

[0026] To describe the evolution of the system, the evolution of the density matrix is usually used. First, the density matrix ρ is introduced:

[0027]

[0028] Then, based on the von Neumann equation of motion, the evolution of the atomic density operator is given. In an atomic ensemble, the spontaneous radiative decay of the excited state and the incoherent process that causes the atom to jump from a high state to a low state must be considered. Therefore, the expression of the Bloch equation of the system is:

[0029]

[0030] In the formula, L(ρ) is the Lindblad superoperator, and its expression is as follows:

[0031] L(ρ) = L1(ρ) + L2(ρ) (6)

[0032] L1(ρ) characterizes the atomic population change and decoherence caused by spontaneous emission, and L2(ρ) characterizes the decoherence caused by the interaction between atoms. Under the density conditions of the cesium atomic vapor cell, the interaction between Rydberg atoms and low-energy atoms and the interaction between low-energy atoms can be ignored compared with the influence caused by atomic spontaneous emission. Therefore, L2(ρ) can be set to 0, and the spontaneous emission rate of the energy level i→j is represented by Γ ij to obtain the expansion of the incoherent term in the three-level atomic system as:

[0033]

[0034] The relaxation effect of the three-level system will destroy the coherence of the system, and the Rabi oscillation of the actual system will be suppressed, making its atomic population gradually tend to a steady state. Combining the boundary condition ρ 11 + ρ 22 + ρ 33 = 1, and by combining with Equation (5), the steady-state solution of the density motion equation is obtained by using the numerical solution method.

[0035] Since under weak light conditions, the coherence between the |2> and |3> energy levels is very small, that is, ρ 23= 0. Let the frequency detunings of the probe light and the coupling light be Δ p = ω p - ω 12 and Δ c = ω c - ω 23 respectively, and the laser linewidths be γ p and γ c respectively. Δ R = Δ p + Δ c , γ r = γ p + γ c . Then the expression of ρ 12 in the density matrix at this time is:

[0036]

[0037] Since the system is an isotropic linear medium, the expression of its induced polarization intensity is:

[0038]

[0039] where ε0 is the vacuum permittivity. For an atomic ensemble, P(t) represents an average result. Therefore, the polarization intensity can be expressed by the density matrix after representation transformation:

[0040]

[0041] Comparing Equation (9) and Equation (10), the relational expression can be obtained:

[0042] d 12 Nρ 12 = ε0χE p (11)

[0043] Therefore, by combining Equation (8) and Equation (11), the real and imaginary parts of the linear susceptibility χ = Re[χ] + i Im[χ] of the medium can be obtained as follows:

[0044]

[0045] Among them, the real and imaginary parts of the probe light susceptibility correspond to the dispersion characteristics of the probe light and the absorption ability of the medium to the probe light respectively. When the control field strength Ω c is relatively weak, an extremely narrow transparent window appears in the middle of the absorption peak, that is, the EIT effect.

[0046] (3) A-T splitting effect

[0047] The Autler-Townes splitting was proposed by Autler and Townes when they studied the Stark effect of microwave fields. It refers to the phenomenon that when an electromagnetic field resonates or nearly resonates with the transition of an atom or molecule, the corresponding absorption spectral line splits. When an external microwave field is applied to a three-level Rydberg atom system and the external frequency is close to the transition frequency between atomic energy levels, it will resonate with cesium atoms, that is, couple the Rydberg state with another excited state, causing the intermediate energy level of the atom to split into two symmetric energy levels, forming a four-level system. In the phenomenon of electromagnetically induced transparency, it is manifested as the EIT main peak splitting into two transmission peaks, that is, the A-T splitting effect.

[0048] Compared with the existing solutions, the main advantages of the solution of the present invention are as follows:

[0049] (1) This method is novel in technology, adopts a new quantum system, has a simple implementation principle, is easy to calculate, can be traced back to standard physical quantities, and does not require calibration.

[0050] (2) The laser array design reduces the optical readout noise. This method analogizes the concept of an array antenna, sets up a beam splitting grating and a collimating grating to prepare a laser array. Under the condition of a certain total power, it reduces the detection power density, realizes a lower optical readout noise, and improves the signal-to-noise ratio of the atomic receiver.

[0051] (3) The background noise is reduced by the same photodetector. Compared with a receiver with four groups of single-beam - single-chamber - single-photodetector or detecting the transmission signals generated by a laser array through four photodetectors respectively, this method equates the beam array to several Rydberg atom receivers and detects them through the same photodetector, reducing the background noise brought by the photodetector.

[0052] (4) Intensification and easy operation. Based on the original simplest excitation optical path, only adding a beam splitting grating and a collimating grating reduces the signal-to-noise ratio, improves the detection sensitivity, enhances the space utilization rate, makes the detection more efficient, maintains the original structure design, which is conducive to subsequent researchers to conduct further comparative experiments, etc., and has strong operability. Description of the Drawings

[0053] Figure 1 Schematic diagram of the method for enhancing the signal-to-noise ratio of a Rydberg atom receiver by a laser array Specific implementation manners

[0054] First, place a cylindrical glass gas cell 4 filled with cesium atoms on a metal bracket, and fix it at the same height as the laser incidence on the experimental table. Then, place a beam splitter 2 and a collimator 3 in sequence between the beam splitting prism 1 at the 852 nm laser a emission position and the atomic gas cell. Place another beam splitting prism 5, a beam condenser 6, and a photodetector 7 in sequence on the right side of the atomic gas cell 4. Among them, the beam splitting prism 1 forms a 45-degree angle with the incident direction of the 852 nm laser a and the beam splitter 2, and the beam splitting prism 5 forms a 45-degree angle with the incident direction of the 509 nm laser b and the atomic gas cell 4.

[0055] Secondly, assemble and fix the above optical components on the experimental table respectively, and it is necessary to ensure that the optical components are stably fixed and all on the same horizontal line. In particular, determine an appropriate distance between the beam splitter 2 and the collimator 3 according to the focal length of the beam splitter and the diameter of the collimator, which is used to divide the probe light into four beams and calibrate them to form a laser array. Similarly, determine and fix the distance between the beam condenser 6 and the photodetector 7 to ensure that the laser array can be completely detected by the photodetector after passing through the beam condenser. In addition, determine the placement position of the beam splitting prism 5 so that the center of the 509 nm laser just enters the center of the beam splitting prism at a 45-degree angle and is reflected along the reverse direction of the 852 nm laser to the atomic gas cell 4, so that the two pump lights can meet inside the gas cell to form the required pumping effect.

[0056] Finally, prepare the 852 nm laser a and the 509 nm laser b with lasers respectively, and appropriately adjust the output of the lasers to ensure the stability of the intensities of the pump light and the probe light and their stable frequency locking and phase locking. The laser intensity needs to be high enough to ensure that cesium atoms can be effectively excited to the Rydberg state, thereby generating the EIT effect and the A-T effect. At the same time, the intensity needs to be appropriate to facilitate the subsequent detection of the output signal at the photodetector 7 end under the action of the external electric field of the horn antenna 8. In this step, it is also necessary to appropriately amplify and filter the signal of the photodetector to improve the signal-to-noise ratio of the signal and reduce the influence of environmental noise.

[0057] The content not described in detail in this invention book belongs to the prior art well-known to those skilled in the art.

Claims

1. A method for enhancing the signal-to-noise ratio of a Rydberg atom receiver based on a laser array, characterized in that A technical implementation path for enhancing the signal-to-noise ratio (SNR) of a Rydberg atom receiver is proposed. Combining the concept of a microwave antenna array, due to the reduction in the detection optical power density, the activity and transitions of photoelectrons are reduced, thereby suppressing the level of photocurrent noise. By solving the Johnson-Nyquist noise problem, the SNR of the atom receiver is improved, providing a new approach for further realizing an ultra-high-sensitivity Rydberg atom receiver.

2. A method for enhancing the signal-to-noise ratio of a Rydberg atom receiver based on a laser array according to claim 1, characterized in that: Compared with the performance of a single laser beam, this method forms a laser array by adding more homologous laser beams, reducing the power density of the detection light and further enhancing the SNR of the atomic microwave receiver. Compared with the performance of multiple laser beams, since this method uses only the same photodetector, the background noise and optical readout noise are reduced, thus improving the SNR of the atom receiver. At the same time, the detection laser array has coherence, so the SNR can still be improved and the sensitivity of the Rydberg atom receiver can be enhanced when detecting weak target signals in a strong noise and clutter environment.

3. A method for enhancing the signal-to-noise ratio of a Rydberg atom receiver based on a laser array according to claim 2, characterized in that: A structural design of a detection laser array irradiating an atomic cell is proposed. A set of beam splitters and collimators are sequentially placed between the beam splitter at the 852 nm laser emission point and the atomic cell to form a 2×2 laser array. Another dichroic mirror, a focusing lens, and a photodetector are sequentially placed on the right side of the atomic cell. The dichroic mirror makes a 45-degree angle with the incident direction of the 509 nm laser and the atomic cell, and the coherence of the beam array is used to achieve lower optical readout noise and background noise.

4. A method for enhancing the signal-to-noise ratio of a Rydberg atom receiver based on a laser array according to claim 3, characterized in that: Taking advantage of the electromagnetically induced transparency (EIT) effect and the Autler-Townes effect of Rydberg atoms, a 2×2 split 852 nm detection laser array and a 509 nm coupling light beam are irradiated on a cesium atomic cell in opposite directions. The cesium atoms are excited to the Rydberg state through three-level excitation. Under the action of an externally applied electromagnetic field prepared by a horn antenna, phase extinction interference in the atomic system is induced to produce the electromagnetically induced transparency effect. When the externally applied frequency is close to the transition frequency between atomic energy levels, resonance occurs with the cesium atoms, coupling the Rydberg state and another excited state, resulting in the main peak of the EIT effect splitting into two transmission peaks, i.e., the A-T splitting effect occurs, and the intensity of the electromagnetic signal can be deduced from the splitting interval.

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