A method and apparatus for detecting atomic group density to determine atomic beam flux

Abstract

The application discloses a method and device for detecting atomic group density to determine atomic beam flux, and belongs to the field of cold atom interference physics. The atomic spatial resolution is obtained by acquiring data of multiple positions of a narrow detection beam which only interacts with a small part of atomic flux. The atomic group density is measured by using the absorption method, the atomic spatial density distribution is acquired within a predetermined height range, thereby obtaining the atomic spatial resolution information, and finally obtaining the atomic beam flux. When measuring the total absorption experienced by the detection beam passing through the flux, there is no depth of field problem, and multiple measurements at different positions can construct the atomic column density profile, effectively avoiding the complication of the measurement process caused by the non-uniformity of the Gaussian detection beam and the instability of the detection light intensity. The application has high detection uniformity, and can greatly improve the physical parameter measurement, signal-to-noise ratio and measurement accuracy in the detection process in the field of cold atom physics.

Description

TECHNICAL FIELD

[0001] The present application relates to a method and device for detecting the atomic beam flux of atomic group density determination, belonging to the field of cold atom interference physics. BACKGROUND

[0002] With the continuous development and progress of laser cooling technology, the field of cold atom interference is increasingly valued. Cold atoms have strong coherence, long wavelength, and other characteristics that facilitate manipulation, and have gradually become the core and key of cold atom physics research. For example, cold atom interferometer gravity meters, gyroscopes, gradient meters, cold atom clocks, and other precision measurement devices are all based on the manipulation of cold atom interference. Therefore, accurate control of the number, density, temperature, and spatial position of cold atom groups has become an effective way to improve precision measurement in the field of cold atom interference.

[0003] Currently, for typical cold atom interference experiments, high-quality cold atom sources are mainly used to load MOT (magnetic optical trap) in a vacuum environment, requiring both rapid loading and long-lived atom capture. Therefore, the characteristics of the cold atom source are usually inferred from the loading rate of the MOT, but measuring the loading rate alone can only provide limited information on the dispersion and spatial distribution. The ideal method for obtaining spatial resolution information of atomic density is to illuminate and image the entire region. Obtaining the atomic density distribution in a single measurement requires the depth of field of the imaging optical system to be greater than the width of the imaged sample.

[0004] Based on the falling tower gravity meter, the atomic group diameter at the measurement position (30 cm from the top) is required to diverge to about 2 cm. According to the approximate formula for the depth of field of an imaging system with numerical aperture NA, for a standard lens with a diameter of 25 mm, a focal length of 2 m should be used. This will result in the collection of only 0.001% of scattered light in the case of absorption imaging, making it impractical to measure the atomic light flux. The initial method for detecting the atomic beam flux through absorption imaging is to change the aperture size of the diaphragm to change the position of the detection light. After the atoms absorb the photons, the change in the detected light intensity determines the spatial density distribution of the atoms. However, due to the non-uniformity of the Gaussian detection beam, the detection light intensity changes when the beam scans the atomic group, making the measurement process complex. SUMMARY

[0005] In order to solve the problems of low collection efficiency, weak detection signal strength and the requirement of depth of field preventing the spatial resolution imaging of atomic density in single excitation of the prior art, the present application aims to provide a method and device for detecting atomic group density to determine atomic beam flux, wherein the spatial resolution of the detection atom is obtained by acquiring data of multiple positions of a narrow detection beam interacting with only a small part of atomic flux; when measuring the total absorption experienced by the detection light beam passing through the flux, there is no depth of field problem, and multiple measurements at different positions can construct the atomic column density profile, improve the detection uniformity, and the present application has the advantages of high light intensity signal collection rate, high detection signal strength, high detection precision and the like.

[0006] The object of the present application is achieved by the following technical solutions:

[0007] The present application discloses a method for detecting atomic group density to determine atomic beam flux, comprising the following steps:

[0008] Step one: detecting the absorption imaging signal at different positions.

[0009] The absorption imaging signal measures the atomic density by the attenuation of the detection light beam caused by stimulated absorption and re-scattering. According to the Beer-Lambert law, the intensity of the detection light beam with initial intensity I0(x,y) passing through the density ρV(x,y,z) area along the z direction is

[0010] I(x,y)=I0(x,y)e -ρA(x,y)σ(x,y)

[0011] The collimated detection light passes through the vacuum window and is focused on the photodiode, and the vertical detection position is changed and adjusted by the adjustable periscope composed of the fixed upper mirror and the micron-adjustable lower mirror.

[0012] Step two: formula fitting of the absorption signal to obtain the column density curve, i.e. the atomic column density profile.

[0013] The absorption signal extracted from the detection data at each different height is converted into column density using the following equation and fitted using the Lorentz function.

[0014]

[0015] Where σ(x,y) is the scattering cross section, ρA(x,y) is The atomic column density is given by ρA(z) = ∫ρV(x, y, z) dx, where Z is the propagation distance of the light through the medium, and the product of ρAand σ defines the optical depth. The atomic density at a point in the beam is ρV(x, y, z), and according to the central limit theorem, the transverse atomic density has a Gaussian distribution. When the transverse atomic density is radially symmetric, the atomic density in Cartesian coordinates is the product of two normal distributions in the x and y directions, both with the same standard deviation σ⊥= σ⊥(z), which varies with axial position z, multiplied by the total column density ρ L

[0016]

[0017] The atomic density along the x direction is integrated and measured over a range of y positions to obtain the spatial resolution of the atomic density at a measurement z position.

[0018] However, the vacuum chamber has a finite width, so if the width of the flux is greater than the width of the chamber, then a portion of the flux is cut off. Thus, for a vacuum chamber cell with a width W along the x direction, the imaged atomic density is the integral from x = -W / 2 to x = +W / 2. When the flux is centered in the vacuum chamber, then x0= 0 and the imaged column density is

[0019]

[0020] Step three: From the column density curve obtained in step two, the total flux of the atomic beam is calculated.

[0021] The spatial distribution of ρA(y, z) = ∫ρV(x, y, z) dx is estimated by measuring ρA(y, z) at a given axial position z. From the width of this profile, the divergence can be inferred, and the integral over all y gives ∫ρA(y, z) dy = ∫ρV(x, y, z) dx dy = ρ L (z), which is the number of atoms per unit length along the source axis, the linear atomic density. When there is no loss of atoms as the beam propagates, then ρ L (z) = ρ L , and the total flux can be determined as follows.

[0022] If Φ0is the total flux of atoms (in atoms / second), then the flux of atoms with longitudinal velocity v z is Φ(v z ) = Φ0f(v z ), where f(v z ) is the normalized distribution of longitudinal velocities. The number of atoms per unit length with velocity v z is Φ(v z ) / v z , so the total number of atoms per unit length, the linear atomic density ρ L is

[0023]

[0024] The linear atomic density p is determined by integrating the lateral atomic density distribution L The total atomic beam flux Φ0 is obtained by dividing the expectation value of 1 / v z The atomic beam flux measurement is realized.

[0025] The application further discloses a device for detecting atomic spatial resolution and determining atomic beam flux, which is used to realize the method for detecting atomic spatial resolution and determining atomic beam flux. The device for detecting atomic spatial resolution and determining atomic beam flux comprises a set of adjustable periscope device units for controlling the position of detection light and a focused collimating photodiode. The application can obtain atomic density in a predetermined height range by adjusting the periscope device multiple times, and the collimated detection light beam passes through the vacuum chamber and is focused on the photodiode, so that the spatial density distribution is inferred to determine the flux of the cold atomic beam flow. The application can effectively avoid the complication of the measurement process caused by the non-uniformity of the Gaussian detection light beam and the instability of the detection light intensity, and has the advantages of stronger signal, higher detection precision and the like. The adjustable periscope is composed of a fixed upper mirror and a micron-adjustable lower mirror.

[0026] The working method of the device for detecting atomic spatial resolution and determining atomic beam flux is as follows: the atoms in the ground state are irradiated by detection light with a frequency resonant with the ground state atoms, the atoms absorb photons and jump from the ground state to the excited state, and the atoms fall back to the ground state by spontaneous emission due to the extremely short lifetime of the atoms in the excited state. Since the probability of spontaneous emission in each direction is the same, the atoms absorb part of the photons, and the light intensity of the detection light along the laser action direction is weakened relative to the case that there is no atom in the detection area. The phototube can detect the fluorescence signal emitted by the atomic group in other directions, and the change of the light intensity is proportional to the number of atoms. The change of the light intensity of the detection light is measured, the atomic spatial density distribution in a predetermined height range is obtained, and the atomic spatial resolution information is obtained to finally obtain the atomic beam flux.

[0027] As preferred, the device for detecting atomic spatial resolution and determining atomic flux mainly comprises a vacuum cavity, a tower cavity window, atoms, collimated probe light, a vacuum detection area, a fixed upper mirror, a micron-adjustable lower mirror, a focusing lens and an integrated photodiode. The atoms captured in the vacuum cavity fall into the detection area through the tower cavity window, the collimated probe light vertically enters the detection area, the vertical detection position is changed by the adjustable periscope composed of the fixed upper mirror and the micron-adjustable lower mirror, the absorption signal is detected from each different height, and finally focused on the photodiode through the focusing lens. In the single-lens absorption imaging device, the probe light will illuminate the entire width of the atomic flux, and the shadow projected by the atom will be imaged by a (usually double-lens) focusing optical system. The resolution and sensitivity increase with the increase of the shadow part collected by the objective lens, the atomic spatial density distribution is obtained within a predetermined height range by measuring the change of the probe light intensity, so that the atomic spatial resolution information is obtained, and finally the atomic beam flux is obtained.

[0028] Advantages:

[0029] 1. The method and device for detecting atomic group density and determining atomic beam flux disclosed by the application realize atomic spatial density measurement by detecting the change of probe light intensity after the free falling atomic group in the vacuum cavity interacts with the resonant probe light. Compared with the fluorescence method, the signal is stronger, the collection rate of light intensity signal is effectively improved, and finally the detection of the atomic beam flux is completed.

[0030] 2. The method and device for detecting atomic group density and determining atomic beam flux disclosed by the application only rely on the passive stability of the polarization maintaining optical fiber to produce power drift, and the amplitude of the driving frequency is adjusted by the feedback obtained from the background signal by the probe light to stabilize the power. The feedback system reduces 1-100Hz noise below the observable level, and reduces 1kHz noise by 20 times.

[0031] 3. The method and device for detecting atomic group density and determining atomic beam flux disclosed by the application obtain the detection of atomic spatial resolution by obtaining the data of multiple positions of a narrow probe beam interacting with only a small part of the atomic flux; when measuring the total absorption experienced by the probe light beam passing through the flux, there is no depth of field problem, and multiple measurements at different positions can construct the atomic column density profile, effectively avoiding the complexity of the measurement process caused by the non-uniformity of the Gaussian probe beam and the instability of the probe light intensity, and adopting the absorption method to measure the atomic group density has the advantages of stronger signal and higher detection precision.

[0032] 4. The method and device for detecting atomic group density to determine atomic beam flux, on the basis of realizing the beneficial effects 1, 2 and 3, can realize that the atomic group diameter diverges to 2cm, and the imaging requirement at the measuring position 30cm away from the top end, the present application has higher detection uniformity, improves the detection precision of the existing fluorescence detection technology, and greatly improves the physical parameter measurement, signal-to-noise ratio and measurement accuracy in the detection process of the field of cold atom physics. BRIEF DESCRIPTION OF DRAWINGS

[0033] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed to be used in the embodiments or prior art description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor under the premise of the drawings.

[0034] Figure 1 The assembly drawing in the method and device for detecting atomic spatial resolution to determine atomic flux provided by the embodiment of the present application;

[0035] Figure 2 The atomic group column density measurement schematic diagram in the method and device for detecting atomic spatial resolution to determine atomic flux provided by the embodiment of the present application;

[0036] Wherein, 1-vacuum cavity; 2-tower cavity window; 3-atom; 4-collimation detection light; 5-vacuum detection area; 6-fixed upper mirror; 7-micron adjustable lower mirror; 8-focusing lens; 9-integrated photodiode. DETAILED DESCRIPTION

[0037] In order to better illustrate the purpose and advantages of the present application, the content of the application will be further described in combination with the drawings and examples.

[0038] Embodiment 1:

[0039] As Figure 1As shown, the embodiment discloses a device for detecting atomic spatial resolution and determining atomic flux, which is composed of a vacuum cavity 1, a tower cavity window 2, atoms 3, collimated probe light 4, a vacuum detection area 5, a fixed upper mirror 6, a micron-adjustable lower mirror 7, a focusing lens 8, and an integrated photodiode 9. Among them, the atoms 3 captured in the vacuum cavity 1 fall into the detection area 5 through the tower cavity window 2, the collimated probe light 4 vertically enters the detection area 5, the vertical detection position is changed by the adjustable periscope composed of the fixed upper mirror 6 and the micron-adjustable lower mirror 7, the absorption signal is detected from each different height, and finally focused on the photodiode 9 through the focusing lens 8. In the single-lens absorption imaging device, the probe light will illuminate the entire width of the atomic flux, and the shadow projected by the atom will be imaged by a one-lens (usually double-lens) focusing optical system. The resolution and sensitivity increase with the increase of the shadow part collected by the objective lens.

[0040] For a typical flux of 10 9 atoms per second, a full-angle scattering of 30 mrad, an average velocity of 15 m / s, the density of the measurement position is about 10 6 atoms per cubic centimeter, and the absorption signal is about 0.1%. At a measurement position of 30 cm, a 30 mrad divergence angle corresponds to a 4.5 mm atomic cluster radius, so the product of the cross-sectional area and the velocity is 0.001 m 3 , which is about 2×10 -12 Φ per second of atomic volume swept, divided by the flux Φ to obtain the atomic density and signal strength, (assuming circularly polarized light, σ0=2.9×10 -9 cm 2 , I=I sat / 10). The collimated probe beam passes through the vacuum cavity system, and the waist radius of the Gaussian beam is w0(1 / e 2 radius), and when the beam propagates a distance equal to the Rayleigh length (z R =πw0 2 / λ), the waist radius w=√2w0. The path length of the probe light passing through the imaging device is about 1 m, which limits the minimum waist to ≥0.5 mm, where z R =1.00 m. The selected probe beam has a power of 30 μW and a 1 / e 2 diameter of 1.5 mm, corresponding to an average intensity of 1.0I sat .

[0041] The probe atomic cluster column density curve is described with reference to Figure 2 The embodiment discloses a method for detecting atomic spatial resolution and determining atomic flux, which is realized by the following steps:

[0042] Step one: detecting the absorption imaging signal at different positions.

[0043] Absorption imaging measures atomic density by the attenuation of a probe beam caused by stimulated absorption and re-emission. The Beer-Lambert law states that a probe beam of initial intensity I0(x,y) passing through a region of density pV(x,y,z) in the z direction has an intensity of

[0044] I(x,y) = I0(x,y)e -ρA(x,y)σ(x,y)

[0045] The collimated probe light passes through a vacuum window and is focused on a photodiode. The vertical probe position is changed by a tunable periscope consisting of a fixed upper mirror and a micron tunable lower mirror.

[0046] Step two: Fit the absorption signal with a mathematical function to calculate the column density profile.

[0047] The absorption signal extracted from the probe data at each different height is converted to column density using the equation below, fitted with a Lorentzian function.

[0048]

[0049] where σ(x,y) is the scattering cross section, pA(x,y) is the atomic column density given, and Z is the propagation distance of the light through the medium. The product of pAand σ defines the optical depth. The atomic density at a point in the beam is pV(x,y,z), which has a Gaussian distribution in the transverse direction according to the central limit theorem. When the transverse atomic density is radially symmetric, the atomic density in Cartesian coordinates is the product of two normal distributions in the x and y directions, both with the same standard deviation σ⊥= σ⊥(z) that varies with the axial position z, multiplied by the total column density p L :

[0050]

[0051] The atomic density is integrated along the x direction and measured over a range of y positions to obtain the spatial resolution of the atomic density measurement at the z position.

[0052] However, the vacuum chamber has a finite width, so if the width of the flux is larger than the width of the chamber, a portion of the flux is cut off. Therefore, for a glass unit with a width W along the x direction, the imaged atomic density is the integral from x = -W / 2 to x = +W / 2. When the flux is centered in the vacuum chamber, x0= 0, then the imaged column density is

[0053]

[0054] Step three: Calculate the total flux of the atomic beam.

[0055] The spatial distribution of ρA(y,z) is estimated by measuring the column density ρA(y,z) = ∫ρV(x,y,z)dx at a given axial position z. The width of this profile gives the divergence, and the integral over all y gives ∫ρA(y,z)dy = ∫ρV(x,y,z)dx dy = ρ L (z), the linear atomic density, i.e., the number of atoms per unit length along the source axis. When there is no loss of atoms as the beam propagates, then ρ L (z) = ρ L , and the total flux can be determined as follows.

[0056] If Φ0is the total flux of atoms (in atoms / sec), then the flux of atoms with longitudinal velocity vzis Φ(v z ) = Φ0f(v z ) where f(v z ) is the normalized distribution of longitudinal velocities. The number of atoms per unit length with velocity v z is Φ(v z ) / v z , and thus the total number of atoms per unit length, the linear atomic density ρ L , is

[0057]

[0058] The total flux Φ0is determined by integrating the transverse atomic density distribution ρ L , and dividing by the expectation value of 1 / v z .

[0059] The above detailed description of the specific description, the purpose, technical solutions and beneficial effects of the invention are further described in detail, should be understood that the above described only for the specific embodiments of the present invention, and not used to limit the scope of the present invention, any modification, equivalent replacement, improvement, etc. within the spirit and principles of the present invention, should be included in the scope of the present invention.

Claims

1. A method for detecting atomic cluster density and determining atomic beam flux, characterized in that: The following steps are included: Step 1: Detect absorption imaging signals at different positions; The absorption imaging signal measures the atomic density through the attenuation of the probe beam caused by stimulated absorption and rescattering. According to the Beer-Lambert law, the probe beam with an initial intensity of I0(x,y) passes through the density ρV(x,y,z) region along the z direction, and its intensity is I(x,y)=I0(x,y)e -ρA(x,y)σ(x,y) The collimated detection light passes through the vacuum window and is focused on the photodiode. The vertical detection position is changed and adjusted by an adjustable periscope consisting of a fixed upper mirror and a micrometer-adjustable lower mirror. Step 2: Fit the absorption signal to obtain the column density curve, i.e. the atomic column density profile; The absorption signal extracted from the detection data at each different height was converted into column density using the following equation, which was fitted using the Lorentzian function; where σ(x,y) is the scattering cross section and ρA(x,y) is The atomic column density is given by ρA, while Z is the propagation distance of light through the medium. The product of ρA and σ is defined as the optical depth. The atomic density at a point in the beam is ρV(x,y,z). According to the central limit theorem, the transverse atomic density has a Gaussian distribution. When the transverse atomic density is radially symmetric, the atomic density in Cartesian coordinates is the product of two normal distributions in the x and y directions, both with the same standard deviation σ⊥=σ⊥(z), which varies with the axial position z, multiplied by the total density ρ which is independent of the longitudinal position. L , The atomic density along the x-direction is integrated and measured over a range of y-positions to obtain the spatial resolution of the atomic density at the z-position. However, the vacuum chamber has a finite width, so if the width of the flux is greater than the width of the chamber, a portion of the flux is cut off; therefore, for a vacuum chamber unit with a width W in the x direction, the imaged atomic density is the integral from x = -W / 2 to x = +W / 2; when the flux is located at the center of the vacuum chamber, the column density imaged at x0 = 0 is Step 3: Calculate the total atomic beam flux based on the column density curve obtained in step 2; The spatial distribution of ρA(y,z) is estimated by measuring the column density ρA(y,z) = ∫ρV(x,y,z)dx for a given axial position z; the divergence can be inferred from the width of the spatial distribution profile, and integration over all y yields ∫ρA(y,z)dy = ∫ρV(x,y,z)dx dy = ρ L (z), which is the number of atoms per unit length along the source axis, i.e., the linear atomic density; when the atomic beam propagates without loss, then ρ L (z) = ρ L , and the total flux can be determined as follows; If Φ0 is the total flux of atoms (in atoms / second), then with longitudinal velocity v z The atomic flux is Φ(v z )=Φ0f(v z )where f(v z ) is the normalized distribution of longitudinal velocity; velocity v z The number of atoms per unit length is Φ(v z ) / v z , so the total number of atoms per unit length, the linear atomic density ρ L for Determine the linear atomic density ρ by integrating the lateral atomic density distribution L , by dividing by 1 / v z The expected value of the total atomic beam flux Φ0 is obtained, that is, the atomic beam flux measurement is realized.

2. A device for detecting atomic spatial resolution and determining atomic beam flux, for implementing the method for detecting atomic cluster density and determining atomic beam flux as claimed in claim 1, characterized in that: The invention comprises a set of adjustable periscope device units for controlling the position of the detection light and a focusing collimating photodiode. By adjusting the periscope device multiple times, the collimated detection light beam passes through the vacuum chamber and is focused onto the photodiode. The atomic density can be obtained within a predetermined height range, from which the spatial density distribution can be inferred to determine the flux of the cold atomic beam. The adjustable periscope consists of a fixed upper mirror and a micrometer-adjustable lower mirror.

3. The device for detecting atomic spatial resolution and determining atomic beam flux according to claim 2, characterized in that: By irradiating atoms in the ground state with detection light whose frequency resonates with that of the ground state atoms, the atoms absorb photons and transition from the ground state to the excited state. Since the lifetime of atoms in the excited state is extremely short, the atoms fall back to the ground state through spontaneous radiation. Since the probability of spontaneous radiation in all directions is the same, the atoms absorb some photons, and the intensity of the detection light along the direction of laser action will be weakened compared to when there are no atoms in the detection area. The phototube can be used to detect the fluorescence signal radiated in other directions of the atomic group, and the change in its intensity is proportional to the number of atoms. By measuring the change in the intensity of the detection light, the atomic spatial density distribution is obtained within a predetermined height range, thereby obtaining the atomic spatial resolution information and ultimately the atomic beam flux.

4. The device for detecting atomic spatial resolution and determining atomic beam flux according to claim 2, characterized in that: It is mainly composed of a vacuum cavity, a tower cavity window, atoms, collimated detection light, a vacuum detection area, a fixed upper reflector, a micron-adjustable lower reflector, a focusing lens, and an integrated photodiode. The atoms captured in the vacuum cavity fall into the detection area through the tower cavity window, and the collimated detection light is vertically incident on the detection area. The vertical detection position is changed by an adjustable periscope composed of a fixed upper mirror and a micron-adjustable lower mirror. The absorption signal is detected from each different height, passes through the focusing lens, and is finally focused on the photodiode. In a single-lens absorption imaging device, the detection light will illuminate the entire width of the atomic flux, and the shadow cast by the atom will be imaged by a focusing optical system. The resolution and sensitivity increase with the increase of the shadow part collected by the objective lens. By measuring the change in the intensity of the detection light, the atomic spatial density distribution is obtained within a predetermined height range, thereby obtaining the atomic spatial resolution information and finally obtaining the atomic beam flux.

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