CN111947705B - Drift calibration and feedback locking method for vertical position of magneto-optical trap - Google Patents
Drift calibration and feedback locking method for vertical position of magneto-optical trap Download PDFInfo
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- CN111947705B CN111947705B CN202010829234.2A CN202010829234A CN111947705B CN 111947705 B CN111947705 B CN 111947705B CN 202010829234 A CN202010829234 A CN 202010829234A CN 111947705 B CN111947705 B CN 111947705B
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Abstract
The application discloses a drift calibration and feedback locking method for a vertical position of a magneto-optical trap, which is suitable for an upper-throwing type and free-falling type cold atom interferometer. The cold atom interferometer comprises one or more cold atom interference physical probes, wherein each cold atom interference physical probe comprises a magneto-optical trap (the magneto-optical trap comprises a pair of reverse Helmholtz coils and six cooling lasers), a pair of bias magnetic field coils, a beam of detection light and a detector. The invention discloses a method for calibrating the drift of the vertical position of a trapping region of a magneto-optical trap according to TOF signal information, which realizes feedback locking of the vertical position of the trapping region of the magneto-optical trap by feeding back the current of a bias magnetic field without adding other complicated devices, thereby solving the problem of reduced interference fringe contrast caused by the drift calibration of the vertical position of the trapping region of the magneto-optical trap in experiments and being beneficial to improving the long-term stability of the interference fringe contrast.
Description
Technical Field
The invention relates to the technical field of cold atom interferometers, in particular to a drift calibration and feedback locking method for the vertical position of a magneto-optical trap.
Background
With the rapid development of cold atom related technology, cold atom interferometers have been widely used in the field of precision measurement physics as a new generation of quantum sensors. The coherent manipulation of atoms by using raman laser is a very important key technology in an atom interferometer, and when the spatial directions of the raman laser are different, the measured physical quantities are different. Currently, the research is carried out more frequently by using an atomic interferometer in a vertical raman laser configuration (the raman laser propagates along the direction of gravitational acceleration) and a horizontal raman laser configuration (the raman laser propagates along the horizontal direction or at a small angle with the horizontal direction).
In the application of the atomic interferometer, the atomic interferometer in a vertical raman laser configuration is important for measuring acceleration (gravimeter), a vertical gravity gradient (vertical gradient) and the like; horizontal raman laser configuration atomic interferometers in horizontal raman laser configuration are now mainly used for measuring earth rotation (gyrocope), inclination angle change (tiltmeter), and horizontal gravity gradient (horizontal gradiometer).
The single experimental procedure of the atomic interferometer includes the following 5 main steps: preparing cold atom cloud by a magnetic optical trap, polishing (or free falling), preparing states, interfering and detecting. For an atom interferometer of a vertical raman laser configuration, the motion trajectory of an atom and the propagation path of a raman laser beam are always spatially coincident, and therefore the position of a magneto-optical trap (MOT) in the vertical direction (gravitational acceleration direction) shifts (by "S") z MOT"represents") hardly affects the effective ratio frequency sensed by the atoms in the subsequent interference process; however, for a horizontal raman laser configuration atom interferometer, the raman laser and the atom flight trajectory are spatially misaligned, perpendicular or at an angle to each other, in which case the position of the magneto-optical trap shifts in the vertical direction ("S")z MOT") directly changes the effective contrast ratio frequency experienced by the atoms in the subsequent interference process, thereby affecting the interference fringe contrast.
Atomic interferometer is a precision measurement of the belonging quantum systemOne of the biggest characteristics of the instrument is that the long-term stability of the measurement index of the instrument can be well made. However, in the actual long-term experiment process, the position of the magneto-optical trap in the vertical direction can shift due to the drift of the intensity, polarization and magnetic field of the cooling laser. In the prior art, the trapped atomic cloud in the magneto-optical trap can be photographed by high-definition CCD (t.farah, p.gillot, b.cheng, a.landragin, s.merlet, and f.pereira Dos Santos, Effective velocity distribution in an atom gravimeter: Effect of the volume with the response of the detection, phys.rev.a 90,023606,2014) in principle, and then the position change of the magneto-optical trap in the vertical direction is obtained by fitting, but this method needs to add an additional high-definition CCD, which increases the system complexity of the interferometer. The project provides a method for realizing S pair according to the information of TOF signals z MOTThe calibration of the method does not need to add other complicated devices, and then the S is calibrated by feeding back the current of the bias magnetic fieldz MOTTo solve the problem of experimental shift by Sz MOTThe problem of the reduction of the interference fringe contrast is brought, and the long-term stability of the interference fringe contrast is improved.
Disclosure of Invention
The invention aims to overcome the defects of the prior art and provides a drift calibration and feedback locking method for the vertical position of a magneto-optical trap, thereby improving the stability of the vertical position of the magneto-optical trap and solving the problem of S in experimentsz MOTThe contrast of the interference fringes is reduced, and the stability of the interference effect of the cold atom interferometer is improved.
A drift calibration and feedback locking method for the vertical position of a magneto-optical trap is suitable for a cold atom interferometer, and the cold atom interferometer can comprise one or more cold atom interference physical probes, wherein each cold atom interference physical probe comprises a magneto-optical trap (the magneto-optical trap comprises a pair of reverse Helmholtz coils and six beams of cooling laser), a pair of bias magnetic field coils, a beam of detection light which propagates along the horizontal direction, and a detector, and the shooting area of the detector is right opposite to the intersection point of the detection light and an atom motion track.
A drift calibration and feedback locking method for the vertical position of a magneto-optical trap comprises the following steps:
step 1, selecting proper time pulse width t of detection lightdet；
Step 2, under the condition of a given sampling rate, the data acquisition card acquires time flight signals, and the total number of data points of the time flight signals is Ptotal；
Step 3, in the nth atomic interferometer experiment, the value of the abscissa corresponding to the fitting peak of the time flight signal is PnThe change value Δ t of the time when the atomic cloud reaches the probe light is C (P)n-P0)，C＝tdet/PtotalN is a natural number, P0The value of the abscissa corresponding to the fitting peak of the time flight signal under the initial atomic interferometer experiment;
step 4, calculating the velocity v of the atomic cloud in the vertical direction when the atomic cloud reaches the detection areaz；
Step 5, calculating the vertical position drift S of the trapping area of the magneto-optical trapz MOT＝C(Pn-P0)vz。
A drift calibration and feedback locking method for the vertical position of a magneto-optical trap further comprises the following steps:
step 6, measuring the current I of the bias magnetic field coil for adjusting the vertical position of the trapping area of the magneto-optical trapbias；
Step 7, calculating the proportionality coefficient f ═ Sz MOT/IbiasAveraging the proportional coefficients obtained by multiple atomic interferometer experiments to obtain a fitting coefficient F;
step 8, calculating the current of the bias magnetic field coil in the next atomic interferometer experiment as I biasMinus the feedback current Δ Ibias，ΔIbias＝(Pn-P0)×F。
Compared with the prior art, the invention has the following beneficial effects:
realize the vertical position drift S of the imprisoned confinement region of the magneto-optical trapz MOTCalibration and feedback locking to improve magneto-optical trap vertical directionThe position stability is improved, so that the stability of the contrast ratio of the interference fringes can be improved, and the stability of the measurement index of the interferometer is finally improved.
Drawings
FIG. 1(a) is a schematic diagram of the magnetic trap and detection structure of a top-polished cold atom interferometer;
FIG. 1(b) is a schematic diagram of the magneto-optical trap and detection structure of a free-fall cold atom interferometer;
FIG. 2 is a schematic diagram of a time-of-flight signal;
in the figure, a-detector; b-detecting light; c1, c 2-a pair of bias magnetic field coils; e-a cold atom cloud; f-magneto-optical trap (comprising a pair of opposing Helmholtz coils d1, d2, and 6 beams of cooling laser light g).
Detailed Description
The present invention will be described in further detail with reference to examples for the purpose of facilitating understanding and practice of the invention by those of ordinary skill in the art, and it is to be understood that the present invention has been described in the illustrative embodiments and is not to be construed as limited thereto.
The embodiment provides a drift calibration and feedback locking method for the vertical position of a magneto-optical trap, which is suitable for a cold atom interferometer, wherein the cold atom interferometer can be an upward-throwing type cold atom interferometer or a free-falling type cold atom interferometer, and can comprise one or more cold atom interference physical probes.
For the top-polished cold atom interferometer, as shown in fig. 1(a), the cold atom interference physical probe includes a magneto-optical trap, the magneto-optical trap includes a pair of reverse helmholtz coils (d1, d2), a pair of bias field coils (c1, c2), and six cooling lasers g, the geometric central axes of the reverse helmholtz coils are along the vertical direction, the geometric central axes of the bias field coils and the geometric central axes of the reverse helmholtz coils are spatially coincident, the geometric centers of the bias field coils and the geometric centers of the reverse helmholtz coils are spatially coincident and serve as the centers of the magneto-optical trap, two pairs of cooling lasers in the six cooling lasers g are in the horizontal plane, the remaining pair of cooling lasers are in the vertical direction, and the cooling lasers include the pumping back light. When the cooling laser is turned off, the atom cloud is thrown out along the vertical direction, a beam of detection laser b is arranged right above the magnetic optical trap and is transmitted along the horizontal direction, a detector a is right opposite to the intersection point of the detection light and the vertical projection track of the atom cloud, and the intersection point of the detection light and the vertical projection track of the atom cloud is a detection area.
For the falling type cold atom interferometer, as shown in fig. 1(b), the cold atom interference physical probe includes a magneto-optical trap including a pair of reverse helmholtz coils (d1, d2), a pair of bias field coils (c1, c2), and six beams of cooling laser light g, the pair of reverse helmholtz coils (d1, d2), the pair of bias field coils (c1, c2), and the six beams of cooling laser light g of the falling type cold atom interferometer are set to be the same as those of the upper throwing type cold atom interferometer, and when the cooling laser light is turned off, the atom cloud makes a free-fall motion, one beam of the detection laser light b is disposed right below the magnetic trap and propagates in a horizontal direction, and one detector a is right at an intersection point of the detection light and a falling trajectory of the atom cloud.
The invention relates to a magneto-optical trap vertical position feedback locking method, which mainly solves the problem of Sz MOTThus in the pair Sz MOTInterference experiments are not performed in the feedback locking process, one feedback locking can be performed after N interference experiments are performed, then the interference experiments are continued, interference is not performed in the feedback locking experiment, and the following description focuses on Sz MOTCalibration and feedback locking.
A drift calibration and feedback locking method for the vertical position of a magneto-optical trap comprises the following steps:
step 1, selecting proper time pulse width t of detection lightdet(by "tdet"in ms") to ensure that the subsequent time width of the cloud of trapped cold atoms in the magneto-optical trap upon interaction with the probe light is measured as a complete time-of-flight signal (TOF signal) as shown in fig. 2.
Step 2, under the condition of a given sampling rate, the time pulse width tdetThe total number of data points of the time flight signal acquired by the internal acquisition card is Ptotal(in "p"), assuming that the abscissa of the fitted peak of the time-of-flight signal (the TOF signal is fitted by gaussian fitting or lorentz fitting) moves by 1p in two different atomic interferometer experimental measurements, the time change value C-t of the atomic cloud reaching the detection light is represented det/Ptotal(ms/p)；
Step 3, assuming an initial atomic interferometer experiment, the value of the abscissa corresponding to the fitting peak of the time-of-flight signal (TOF signal) is P0In the nth atomic interferometer experiment, the value of the abscissa corresponding to the fitting peak of the time-of-flight signal (TOF signal) is PnWhen the nth atomic interferometer experiment is described, the time when the atomic cloud reaches the detection light is advanced or delayed by the change value Δ t, and the following can be obtained through calculation: Δ t ═ C (P)n-P0) Wherein n is the number of atomic interferometer experiments and is a natural number;
step 4, according to the detuning parameter of the laser, the velocity v of the atomic cloud in the vertical direction when the atomic cloud reaches the detection area can be obtained through calculationz(by "vz"represents) so that it can be concluded that the vertical position drift of the atomic cloud at the detection zone is: c (P)n-P0)vz；
And 5, directly determining the distance between the atom cloud and the detection region at the caging position of the cold atom cloud. For the top-polishing type cold atom interferometer, the atom top-polishing speed v is lambda.f, wherein lambda represents the wavelength of the cooling laser, and f represents the frequency conversion amount of the control atom cloud cooling laser. The frequency precision of a signal generator used in the experiment is generally in the mu Hz magnitude, the frequency conversion f of laser in the atom polishing process is generally in the MHz magnitude, and the difference between the frequency conversion f and the frequency conversion f is 12 magnitude magnitudes, so that the frequency precision of the signal generator can be considered not to influence the atom polishing speed; for the falling type cold atom interferometer, atom clouds are released freely after being imprisoned, so that the influence of the frequency control precision of a signal generator on the falling speed of the atom clouds does not need to be considered. In addition, the on-time of the detection light is controlled by a data acquisition card, and the ratio of the time control precision of the data acquisition card to the time sequence duration is 10 -6Order of magnitude, and hence band of accuracy controlled by the time of data acquisition cardThe effect of uncertainty in time from the beginning can also be neglected. Therefore, it can be considered that the vertical drift of the atomic cloud measured at the detection light is caused by the vertical position drift of the confinement region of the magneto-optical trap, and the vertical position drift of the confinement region of the magneto-optical trap "S" is equal in valuez MOT"can be expressed as: sz MOT＝C(Pn-P0)vz。
A drift calibration and feedback locking method for the vertical position of a magneto-optical trap comprises a feedback locking step for the vertical position of the magneto-optical trap, and specifically comprises the following steps:
step 6, measuring the current I of the bias magnetic field coil for adjusting the vertical position drift of the confinement region of the magneto-optical trapbiasIn an actual experiment process, the light intensity, the polarization fluctuation and the magnetic field variation of the pair of cooling lasers in the vertical direction in fig. 1 all cause the vertical position of the caging region of the magneto-optical trap to drift, so that in this embodiment, a pair of bias magnetic field coils is added to the magneto-optical trap region of each physical probe of the cold atom interferometer, the geometric central axis of the bias magnetic field coils passes through the center of the magneto-optical trap in the vertical direction, and the geometric central axis of the bias magnetic field coils is spatially coincident with the geometric central axis of the reverse helmholtz coil and is also coincident with the pair of cooling laser beams in the vertical direction. Thus, by adjusting the current I of the bias field coil biasThe vertical position of the magneto-optical trap can be adjusted in the vertical direction;
step 7, calculating the proportionality coefficient f ═ Sz MOT/IbiasAveraging the proportionality coefficients obtained by multiple atomic interferometer experiments to obtain a fitting coefficient F, and measuring the vertical position drift S of the trapping region of the magneto-optical trap under the modulation currents of different bias magnetic field coilsz MOTThe modulation current I of the bias magnetic field coil can be obtainedbiasVertical position drift S of confinement region with magneto-optical trapz MOTThe open-loop characteristic relationship of (a) is expressed by the following relation: sz MOT＝f(Ibias)；
Step 8, assuming that the fitting peak of the time flight signal (TOF signal) obtained by the nth atomic interferometer experiment measurement corresponds to the fitting peakHas an abscissa of PnThe fitted peak of the initial time-of-flight signal (TOF signal) corresponds to an abscissa of P0In combination with IbiasAnd Sz MOTThe current feedback quantity delta I of the bias magnetic field coil of the next (n +1) atom interferometer experiment is obtainedbias＝(Pn-P0) xF, i.e. subtracting the current feedback quantity delta I from the bias magnetic field coil of the current sub-atomic interferometer experimentbiasAnd as the current of the bias magnetic field coil in the next (n +1) experiment, the feedback locking of the vertical position of the magneto-optical trap can be realized by feeding back the current of the bias magnetic field coil. According to different experimental requirements, feedback can be performed every N (N is a natural number) times of atomic interferometer experiments, and the smaller N is, the more frequent the feedback is.
The specific embodiments described herein are merely illustrative of the spirit of the invention. Various modifications or additions may be made to the described embodiments or alternatives may be employed by those skilled in the art without departing from the spirit or ambit of the invention as defined in the appended claims.
Claims (1)
1. A drift calibration and feedback locking method for the vertical position of a magneto-optical trap is characterized by comprising the following steps:
step 1, selecting proper time pulse width t of detection lightdet；
Step 2, under the condition of a given sampling rate, the data acquisition card acquires time flight signals, and the total number of data points of the time flight signals is Ptotal；
Step 3, in the nth atomic interferometer experiment, the value of the abscissa corresponding to the fitting peak of the time flight signal is PnThe change value Δ t of the time when the atomic cloud reaches the probe light is C (P)n-P0)，C＝tdet/PtotalN is a natural number, P0The value of the abscissa corresponding to the fitting peak of the time flight signal under the initial atomic interferometer experiment;
step 4, calculating the velocity v of the atomic cloud in the vertical direction when the atomic cloud reaches the detection areaz；
Step 5, calculating the vertical position drift S of the trapping area of the magneto-optical trapz MOT＝C(Pn-P0)vz；
Step 6, measuring the current I of the bias magnetic field coil for adjusting the vertical position of the trapping area of the magneto-optical trap bias；
Step 7, calculating the proportionality coefficient f ═ Sz MOT/IbiasAveraging the proportional coefficients obtained by multiple atomic interferometer experiments to obtain a fitting coefficient F;
step 8, calculating the current of the bias magnetic field coil in the next atomic interferometer experiment as IbiasMinus the feedback current Δ Ibias，ΔIbias＝(Pn-P0)×F。
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