CN112242843B - Method and device for realizing high-contrast CPT (coherent population trapping) inverse detection - Google Patents
Method and device for realizing high-contrast CPT (coherent population trapping) inverse detection Download PDFInfo
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Abstract
The invention provides a method for realizing high-contrast CPT reverse phase detection, which comprises the steps of firstly providing a beam of coherent bicolor light with a relative phase phi, and preparing atoms into a CPT state by incidence of the coherent bicolor light into an atomic gas chamber and interaction of the coherent bicolor light with the atoms; when atoms are prepared to a CPT steady state, the relative phase of the coherent bi-color light is switched to an inverse phi + pi state, so that the CPT dark state is changed to a CPT bright state; and collecting the transmitted light of the atomic air chamber to obtain CPT signals in electromagnetic induction absorption mode. The invention can effectively improve the frequency stability of the CPT atomic clock.
Description
Technical Field
The invention belongs to the field of atomic clocks and atomic magnetometers, and particularly relates to a CPT (coherent population trapping) inversion detection technology.
Background
Coherent Placement Trapping (CPT) is a quantum interference effect. Based on the effect, accurate measurement devices such as atomic clocks, magnetometers and the like can be realized.
Taking the application of atomic clocks as an example, a passive CPT atomic clock (hereinafter referred to as CPT clock) based on coherent layout trapping can be miniaturized or even chipped because of the characteristic of no microwave cavity, has great advantages in terms of volume, power consumption and weight, and can be applied to the fields of micro satellite formation networking, unmanned aerial vehicles, portable GNSS receivers, underwater surveying and the like.
In order to reduce the volume, power consumption and weight of the system, the traditional small-sized and chip CPT clock generally adopts a relatively simple CPT scheme prepared by the interaction of single circularly polarized coherent bi-color light and an atomic system. In this scheme, the two hyperfine energy levels (Zhong Yue transition states) of the base state of the alkali metal atom are coupled to the same excitation state by the coherent bi-color light, when the bi-color light frequency difference is tuned to Zhong Yueqian resonance frequency of the two states, the atom is prepared to be in a coherent dark state, i.e., CPT state, at which time the atom no longer absorbs incident light and appears as a very narrow bright line in transmitted light intensity. The CPT resonance spectral line can be utilized to carry out frequency discrimination on the local oscillator, and finally, the frequency standard is output, so that the atomic clock is realized.
However, due to the existence of the polarized dark state in the traditional single circularly polarized light scheme, namely when single circularly polarized coherent bi-color light interacts with alkali metal atoms, the atomic population leaks to the polarized dark state, so that the atomic population number participating in Zhong Yue migration is greatly reduced, and the CPT signal contrast of clock transition is only 1-5%. Contrast (C) is defined herein as the ratio of the amplitude of the CPT resonance line to the background amplitude.
According to the formula of the frequency stability of the CPT atomic clockWherein τ represents integration time, RIN is relative intensity noise of laser, v hf is frequency difference of two hyperfine energy levels of experimental atomic ground state, q value is quality factor, q=C/delta, C represents contrast of CPT signal, and is defined as ratio of amplitude of CPT resonance spectral line to background amplitude; delta is the Full Width Half Maximum (FWHM) of the CPT signal. It can be seen from the formula that the frequency stability is inversely proportional to the CPT signal contrast (or q-value) with other parameters. Thus, the lower contrast (or q-value) in conventional schemes employing single circularly polarized coherent bi-color light limits further improvement in CPT clock frequency stability.
Disclosure of Invention
In order to overcome the defects of the prior art, the invention provides a method for realizing high-contrast CPT reverse phase detection, which is characterized in that a microwave phase modulation is added on the basis of a miniaturized traditional CPT atomic frequency standard scheme, and atoms prepared to a CPT steady state are subjected to the action of coherent bi-color light in reverse phase with the relative phase by controlling the relative phase of the coherent bi-color light, so that CPT resonance spectral lines in an Electromagnetic Induction Absorption (EIA) form are obtained, thereby obtaining frequency discrimination signals with high contrast and high q value, and effectively improving the frequency stability of a CPT atomic clock.
The technical scheme adopted by the invention for solving the technical problems comprises the following steps:
1) Providing a beam of coherent bi-color light having a relative phase phi, wherein phi is any phase;
2) Making coherent bi-color light incident into the atomic gas chamber and interacting with the atoms to prepare the atoms into CPT state; when the atom is prepared to a CPT steady state by coherent bi-color light with the relative phase phi, the relative phase phi of the coherent bi-color light is switched from phi to an opposite phase phi, namely phi+pi, so that the CPT dark state in the steady state is converted into a CPT bright state, and then a new CPT steady state is gradually entered;
3) And collecting the transmitted light of the atomic air chamber to obtain CPT signals in electromagnetic induction absorption mode.
The duration of the preparation phase of the CPT dark state is tau 1, the duration of the inversion phase is tau 2, the duration tau 1 takes a value of a few milliseconds, and the duration tau 2 takes a value of the order of milliseconds to sub-milliseconds.
According to the invention, 2 detection windows are adopted, the window duration is t w1 and t w2,tw1 respectively, the detection windows are positioned before the end of the CPT dark state preparation stage, the t w2 detection windows are positioned after the start of the inversion stage, the time delay of the 2 detection windows is t d1 and t d2,td1 respectively, the time duration of t d2 is less than or equal to 500 mu s; the t w1 detection window is used for judging whether the CPT dark state has entered a steady state, and the t w2 detection window is used for detecting and obtaining CPT signals in an electromagnetic induction absorption form with high q value.
The invention also provides a device for realizing the high-contrast CPT reverse phase detection, which comprises a direct-current power supply, a microwave source, a phase modulator, a microwave coupler, a laser, a quarter wave plate, an atomic gas chamber and a detector.
The direct current power supply supplies power to the laser through the microwave coupler; the microwave source realizes the modulation of the phase through a phase modulator and provides modulated microwaves for the laser through a coupler; the linear polarization coherent bicolor light emitted by the laser passes through a quarter wave plate to obtain circular polarization coherent bicolor light, and the coherent bicolor light enters an atomic air chamber to interact with atoms so as to prepare the atoms into a CPT state; after atoms are prepared to a CPT steady state by coherent bi-color light with the same relative phase, the relative phase of the coherent bi-color light is switched from the same phase to a reverse phase state through a phase modulator, so that the CPT dark state of the coherent bi-color light prepared to the steady state in the same phase is changed to a CPT bright state, and the transmitted light of an atomic air chamber is detected by a detector, so that a CPT signal in an electromagnetic induction absorption form is obtained.
The microwave frequency of the microwave source is 1/n of the frequency difference v hf of the two hyperfine energy levels of the ground state of the experimental atom, and n is any positive integer.
The beneficial effects of the invention are as follows:
1. by adopting proper microwave phase modulation and working time sequence design, CPT resonance spectrum line in Electromagnetic Induction Absorption (EIA) form can be obtained, and the contrast and q value of CPT signal are obviously improved. The invention is applied to atomic clocks, can further improve the frequency stability of CPT atomic clocks, can also be applied to magnetometers, and improves the magnetic field detection sensitivity of the magnetometers.
2. The device of the present invention has only an electronically added phase modulator with small volume and power consumption compared to the conventional single circularly polarized light scheme. The frequency stability of the CPT atomic clock is improved, the advantages of the traditional scheme in terms of volume and power consumption are maintained, and the CPT atomic clock and the chip magnetometer with high performance can be realized.
Drawings
Fig. 1 is a schematic diagram of the present invention for realizing high contrast CPT.
Fig. 2 is a diagram of a physical system device for implementing high-contrast CPT according to the present invention.
Fig. 3 is a timing diagram of operation of implementing a high-contrast CPT signal according to the present invention.
FIG. 4 is a schematic representation of a typical higher contrast Electromagnetic Induction Absorption (EIA) format CPT signal obtained by the present invention, wherein a conventional method is used to obtain an Electromagnetic Induction Transparency (EIT) format CPT signal as a control.
In the figure, a 1-direct current power supply, a 2-microwave source, a 3-phase modulator, a 4-microwave coupler Bias-Tee, a 5-DBR laser, a 6-quarter wave plate, a 7-atomic gas cell and an 8-detector.
Detailed Description
The invention will be further illustrated with reference to the following figures and examples, which include but are not limited to the following examples.
The invention provides a method for realizing high-contrast CPT inversion detection, which comprises the following steps:
1) A beam of coherent bi-color light is provided having a relative phase phi, where phi is any initial phase.
2) Making coherent bi-color light incident into the atomic gas chamber and interacting with the atoms to prepare the atoms into CPT state; after preparing atoms to CPT steady state by coherent bi-color light with relative phase phi, switching the relative phase of the coherent bi-color light from phi to an opposite phase, namely phi+pi, so that CPT dark state with the previous relative phase phi prepared to steady state is changed to CPT bright state;
3) And collecting the transmitted light of the atomic air chamber to obtain CPT signals in electromagnetic induction absorption mode.
The duration of the preparation phase of the CPT dark state is tau 1, the duration of the inversion phase is tau 2, the duration tau 1 takes a value of a few milliseconds, and the duration tau 2 takes a value of the order of milliseconds to sub-milliseconds.
According to the invention, 2 detection windows are adopted, the window duration is t w1 and t w2,tw1 respectively, the detection windows are positioned before the end of the CPT dark state preparation stage, the t w2 detection windows are positioned after the start of the inversion stage, the time delay of the 2 detection windows is t d1 and t d2,td1 respectively, the time duration of t d2 is less than or equal to 500 mu s; the t w1 detection window is used for judging whether the CPT dark state has entered a steady state, and the t w2 detection window is used for detecting and obtaining CPT signals in an electromagnetic induction absorption form with high q value.
The invention also provides a device for realizing the high-contrast CPT reverse phase detection, which comprises a direct-current power supply, a microwave source, a phase modulator, a microwave coupler, a laser, a quarter wave plate, an atomic gas chamber and a detector.
The direct current power supply supplies power to the laser through the microwave coupler; the microwave source realizes the modulation of the phase through a phase modulator and provides modulated microwaves for the laser through a coupler; the linear polarization coherent bicolor light emitted by the laser passes through a quarter wave plate to obtain circular polarization coherent bicolor light, and the coherent bicolor light enters an atomic air chamber to interact with atoms so as to prepare the atoms into a CPT state; after atoms are prepared to a CPT steady state by coherent bi-color light with the same relative phase, the relative phase of the coherent bi-color light is switched from the same phase to a reverse phase state through a phase modulator, so that the CPT dark state of the coherent bi-color light prepared to the steady state in the same phase is changed to a CPT bright state, and the transmitted light of an atomic air chamber is detected by a detector, so that a CPT signal in an electromagnetic induction absorption form is obtained.
The microwave frequency of the microwave source is 1/n of the frequency difference v hf of the two hyperfine energy levels of the ground state of the experimental atom, and n is any positive integer.
The present invention will be described in detail by taking an atomic gas chamber filled with 87 Rb alkali metal atoms and inert gas as an example.
As shown in fig. 2, the direct current power supply 1 and the microwave of the microwave source 2 modulated by the phase modulator 3 are coupled by a microwave coupler Bias-Tee4 to drive the DBR laser 5 to generate coherent bi-color light. The microwave frequency can be the frequency difference v hf between two ultra-fine energy levels of the ground state of the experimental atom, and can also be v hf/2、νhf/3、νhf/4 …, and v hf/2 is adopted in the embodiment. The phase modulator is used for carrying out periodic transformation from 0 to pi/2 on the microwave phase, and the phase difference of the corresponding coherent bi-color light is also periodically switched between 0 and pi. DBR lasers produce linear polarized coherent double polychromatic light, with the ±1-level sidebands being the coherent polychromatic light required for the experiment. After passing a quarter wave plate, it is converted into circularly polarized light, here exemplified by left circularly polarized light. The left-handed circularly polarized light enters the atomic gas chamber 7 to interact with 87 Rb atoms to prepare the atoms into CPT state.
After atoms are prepared to CPT steady state by coherent bi-color light with relative phase phi (initial phase), the relative phase of the coherent bi-color light is rapidly switched from phi to inverse phi + pi state through a phase modulator, so that the CPT dark state of the coherent bi-color light with the previous relative phase phi is changed to CPT bright state, namely, the CPT dark state is changed to Electromagnetic Induction Absorption (EIA) by Electromagnetic Induction Transparency (EIT). The left circularly polarized light after the interaction with the atoms is detected and received by a detector 8, and a CPT signal in the form of Electromagnetic Induction Absorption (EIA) is obtained.
The operation timing diagram shown in fig. 3 includes a frequency step-and-scan trigger timing, a phase modulation timing, and a probe timing. Each cycle has a length T cycle, which comprises two parts: the 1 st stage is a preparation stage of CPT dark state, the duration is τ 1, and the microwave phase is set to a certain fixed value(Schematically shown in the figure asDegree) for preparing atoms to a steady state of CPT dark state, and thus a time long enough, on the order of milliseconds. The 2 nd stage is a reverse phase detection stage, the duration is tau 2, and the microwave phase rapidly jumps toThe relative phase corresponding to coherent bi-color light changes from phi to the inverted phi + pi state (where phi is the initial phase) for changing the steady state CPT dark state to the CPT bright state. The CPT signal due to transient effects caused by phase jumps at this stage will evolve into a new CPT dark state with increasing interaction time, resulting in a restoration of the CPT contrast to a lower level. Thus, the period of time τ 2 may be shorter than the period of time τ 1 for preparation to CPT steady state, τ 2 may be as short as milliseconds or even sub-milliseconds, so that it does not enter the new CPT dark state, while the modulation period time T cycle may be shortened relative to τ 1=τ2. The modulation period duration T cycle is reduced, the atomic clock is facilitated to use a modulation and demodulation technology with a higher frequency, 1/f noise is reduced, and the signal to noise ratio of CPT is improved.
The invention adopts 2 detection windows: t w1 and t w2, which are respectively located at the end of the preparation phase of the CPT dark state and the beginning of the reverse detection phase, and have a delay of t d1 and t d2, respectively, wherein t d1 is several milliseconds, t d2 ranges from 0 to 500 μs, the former is used for judging whether the CPT dark state has entered a steady state, and the latter is used for detecting and obtaining the CPT signal in the electromagnetic induction absorption form with higher q value.
In the CPT resonance spectrum diagram shown in FIG. 4, windows t w1 and t w2 are detected to obtain CPT resonance spectra in the form of Electromagnetic Induction Transparency (EIT) and Electromagnetic Induction Absorption (EIA), respectively. Wherein the middle EIT transmission peak and EIA absorption peak are CPT Zhong Yue migration, i.e., |m F=0>→|m′F =0 >, and wherein m F and m' F are the number of magnons of the two ground state sub-levels, the contrast ratio of the latter being much higher than the former. By measuring the q value of the frequency stability of the CPT atomic clock, it can be seen that the CPT signal in the form of Electromagnetic Induction Absorption (EIA) is improved by at least more than 2 times relative to the conventional CPT signal. Therefore, the invention can greatly improve the contrast and q value of CPT signals only through an electronic phase modulator and time sequence control on the basis of not increasing the optical physical devices of the prior small-sized and chip CPT clocks, which effectively improves the frequency stability of CPT atomic clocks.
It can also be seen from fig. 4 that the contrast of two CPT signals other than clock transitions is also greatly improved, they are |m F≠0>→|m′F +.0 > non-Zhong Yue transitions, because their spacing is proportional to the magnetic field strength over a range, which can be used to measure magnetic fields, implementing a magnetometer. The invention obtains the magnetic sensitive transition signal in the EIA form with greatly improved q value, thus being applicable to a magnetometer and obviously improving the sensitivity of magnetic field detection.
The foregoing description is only of the preferred embodiments of the present application and is not intended to limit the scope of the application. Any modifications, equivalent substitutions and improvements made by those skilled in the art, which are within the spirit and principles of the present application, are intended to be included within the scope of the present application as set forth in the appended claims.
Claims (5)
1. A method for implementing high contrast CPT inversion detection, comprising the steps of:
1) Providing a beam of coherent bi-color light having a relative phase phi, wherein phi is any phase;
2) Making coherent bi-color light incident into the atomic gas chamber and interacting with the atoms to prepare the atoms into CPT state; after preparing atoms to CPT steady state by coherent bi-color light with relative phase phi, switching the relative phase of the coherent bi-color light from phi to an opposite phase, namely phi+pi, so that CPT dark state with the previous relative phase phi prepared to steady state is changed to CPT bright state;
3) And collecting the transmitted light of the atomic air chamber to obtain CPT signals in electromagnetic induction absorption mode.
2. The method for achieving high contrast CPT inversion detection according to claim 1, wherein: the duration of the preparation phase of the CPT dark state is tau 1, the duration of the inversion phase is tau 2, the duration tau 1 takes a value of a few milliseconds, and the duration tau 2 takes a value of the order of milliseconds to sub-milliseconds.
3. The method for achieving high contrast CPT inversion detection according to claim 1, wherein: adopting 2 detection windows, wherein the window duration is t w1 and t w2,tw1 respectively, the detection windows are positioned before the end of the CPT dark state preparation stage, the t w2 detection windows are positioned after the start of the inversion stage, the time delay of the 2 detection windows is t d1 and t d2,td1 respectively, and the time duration of t d2 is less than or equal to 500 mu s; the t w1 detection window is used for judging whether the CPT dark state has entered a steady state, the t w2 detection window is used for detecting and obtaining CPT signals in an electromagnetic induction absorption form with high q value, and q represents a quality factor.
4. An apparatus for implementing the method of claim 1 to implement high contrast CPT inversion detection, comprising a dc power supply, a microwave source, a phase modulator, a microwave coupler, a laser, a quarter wave plate, an atomic gas cell, and a detector, wherein: the direct current power supply supplies power to the laser through the microwave coupler; the microwave source realizes the modulation of the phase through a phase modulator and provides modulated microwaves for the laser through a coupler; the linear polarization coherent bicolor light emitted by the laser passes through a quarter wave plate to obtain circular polarization coherent bicolor light, and the coherent bicolor light enters an atomic air chamber to interact with atoms so as to prepare the atoms into a CPT state; after atoms are prepared to a CPT steady state by coherent bi-color light with the same relative phase, the relative phase of the coherent bi-color light is switched from the same phase to a reverse phase state through a phase modulator, so that the CPT dark state of the coherent bi-color light prepared to the steady state in the same phase is changed to a CPT bright state, and the transmitted light of an atomic air chamber is detected by a detector, so that a CPT signal in an electromagnetic induction absorption form is obtained.
5. The apparatus for achieving high contrast CPT inversion detection according to claim 4, wherein: the microwave frequency of the microwave source is 1/n of the frequency difference v hf of the two hyperfine energy levels of the ground state of the experimental atom, and n is any positive integer.
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