CROSS REFERENCES OF RELATED APPLICATIONS

The present application claims the priority of the Chinese patent application No. CN202011168758.8 filed on Oct. 28, 2020. The contents of the above identified applications are incorporated herein by reference in their entireties.

FIELD

The present invention relates generally to the field of atomic clocks, atomic magnetometers, atomic spectroscopy, and specifically to a CPT differential detection technique.

BACKGROUND

Coherent Population Trapping (CPT) is a quantum interference effect. Owing to the advantage of microwave cavity free, the passive CPT configuration is used for implementing compact and chip-scale atomic clocks and atomic magnetometers. The passive CPT based atomic clock and sensor is an ideal choice for applications such as deep space exploration, unmanned aerial vehicles, urban and underground rescue, underwater submarines, surveying and the like, and is also a key unit device in micro-technology based positioning, navigation, and timing system (μPNT).

Taking the passive CPT atomic clock (herein and after denoted as CPT clock) as an example, which usually uses a circularly polarized bichromatic light (e.g. left circular σ+σ+) to pump the atoms into a CPT state, saying that when the frequency difference of the bichromatic light is equal to the hyperfine energy level splitting of the alkali metal atom ground state, a quantum interference effect occurs, and the atoms do not absorb the incident light any more, thus the transmitted light intensity increases. When the frequency difference of the bichromatic light is scanned, a narrow linewidth CPT signal can be obtained. The CPT signal is used as a frequency discrimination signal for a local oscillator. When the local oscillator is locked to the CPT resonance, an atomic clock is realized.

On the one hand, in compact and chip-scale CPT clock configurations, a single circularly polarized bichromatic light is usually adopted to interact with an atomic ensemble and pump atoms into polarization dark state, i.e., the extreme (|mF|=F) Zeeman sublevels, wherein: mF is the magnetic quantum number, and F is the total quantum number, thus leading to a low contrast of the clock transition (|2S1/2, F, mF=0→|2S1/2, F′, m′F=0, abbreviated as |mF=0→|m′F=0), typically around only 1%-5%. The contrast is defined here as C=A/B, where A, B are the amplitude and background of the CPT signal, respectively. The short-term frequency stability of the atomic clock is given approximately by: σy(τ)≈√{square root over (RIN/2)}(4q νhf)−1τ−1/2, wherein RIN, q, νhf are laser relative intensity noise, quality factor and clock transition frequency, respectively, and the quality factor q is defined as: q=C/Δ, where Δ is the linewidth of the CPT signal. It shows that the atomic clock frequency stability is inversely proportional to the CPT signal contrast for a given linewidth. Therefore, the low contrast CPT signal becomes a main limiting factor for improving the performance of the compact CPT clock, which is also the case for the chip-scale CPT clock (with a typical short-term frequency stability of 1˜3 E-10 at 1 second).

On the other hand, in the developing high-performance CPT atomic clock, in order to obtain high contrast, a relatively complex optical path configuration is generally used, increasing the clock's volume greatly, thus sacrificing the main advantages of the CPT atomic clock, namely the capability of miniaturization. Meanwhile, due to the fact that large common-mode noise exists in the bichromatic optical configuration and the detection mode used by the typical CPT clock, such as laser amplitude noise (AM), laser frequency noise to amplitude noise through absorption spectral lines (FM-AM), microwave power noise and the like, thus it is very difficult to further improve the frequency stability of the CPT clock.

For example, a double-modulation CPT (DM CPT) configuration uses a mode of synchronously modulating the relative phase and polarization of coherent bichromatic light, so that a CPT signal with maximum clock transition contrast up to about 10% can be obtained, and it also maintains the advantage of miniaturization. However, due to the detection of light intensity transmitted from the atom ensemble, which has large background, laser AM noise, laser FM-AM noise, as well as microwave power noise, thus it sets limits on the frequency stability of the DM CPT atomic clock.

SUMMARY

In order to overcome the defects of low CPT signal contrast and large common-mode noise in the prior art, the invention discloses a double-modulation CPT differential detection method, which has the advantages of compact structure, high contrast and low common-mode noise.

The method used by the invention for solving the technical problems is a double-modulation CPT differential detection method comprises the following steps:1) Phase modulation. Providing a coherent bichromatic light, wherein the relative phase between the two frequency components of the bichromatic light is switched between ϕ+0 and ϕ+π as requested, wherein ϕ is an initial arbitrary phase.2) Polarization modulation. The polarization of coherent bichromatic light is switched among left circular polarization, right circular polarization and linear polarization according to a set rule.3) DM CPT stage. The double modulation coherent bichromatic light, in which the phase modulation mentioned in step 1) and the polarization modulation between left and right circular polarization are implemented synchronously, interacts with a quantum resonance system, forms the DM CPT and prepares the quantum resonance system into a CPT state with maximum clock transition amplitude.4) Linear polarization light-atom interaction stage. After DM CPT stage, the polarization of coherent bichromatic light is switched from circular polarization to linear polarization, which can be decomposed to the left and right circular polarization components. Both components interact with the CPT state prepared in the previous stage simultaneously, and constructive and destructive quantum interference effects occur respectively.5) Polarization conversion and separation. The polarization of the transmitted light from the quantum resonance system is transformed, in which the linear polarization is converted into left or right circular polarization. Then the coherent bichromatic light is spatially separated into mutual orthogonally and linearly polarized bichromatic light. Detected with two balanced photodetectors, CPT signals are observed with constructive and destructive interference respectively.6) Differential signal. Subtracting the CPT signals of the constructive and the destructive interference observed in 5), a differential CPT signal with high signal-to-noise ratio is obtained.

The light-atom interaction is implemented by a continuous wave laser or pulse laser.

Specifically, for a pulse laser to implement the differential detection of double-modulation coherent population trapping, a no-light period is added after the time sequence of DM CPT stage of step 3) to enable a quantum resonance system to evolve freely, then followed by a detection pulse with linearly polarized coherent bichromatic light to interact with the quantum resonance system, and performed the steps 4) to 6), a Ramsey-CPT signal with differential shape is observed.

The length of no-light period is at the same order of the ground states coherence time, for maximizing the contrast of CPT signal.

In this method, the clock transition of differential CPT signal is applied to a CPT atomic clock, the non-clock transition of differential CPT signal is applied to an atomic magnetometer, and both the clock and the non-clock transition of differential CPT signals can be applied to atomic precision spectroscopy.

The invention also discloses a double-modulation CPT differential detection system for realizing the method, which comprises a direct current source, a coupler (bias-tee), a phase modulator, a microwave signal source, a laser, a half wave plate, a polarization modulator, a quarter wave plate, a quantum resonance system, a second quarter-wave plate, a second half-wave plate, a polarization beam splitter and a subtractor.

The direct current source supplies power to the laser diode through the microwave coupler; the microwave signal source, which is phase modulated through a phase modulator, is coupled to the laser diode by the microwave coupler. Then a coherent bichromatic light is formed in the multicolored output of the laser diode. After passing through a half-wave plate and a polarization modulator, a DM coherent bichromatic light is generated. Then it sequentially passes through a quarter-wave plate, a quantum resonance system, a second quarter-wave plate, a second half-wave plate and a polarization beam splitter. After that, the coherent bichromatic light is spatially separated to mutually orthogonally and linearly polarized light, and then detected with two balanced photodetectors to obtain CPT signals with constructive and the destructive interference respectively. Finally, a differential CPT signals is obtained by subtracting the above-mentioned two CPT signals.

The quantum resonance system comprises a CPT resonance energy level structure from two ground states to the same excited state, and adapts to hydrogen atoms (H), alkali metals (Li, Na, K, Rb, Cs), Hg+, Ca+, Yb+, Ba+ or C-60 particles.

The particles are in a gaseous hot atom, gaseous cold atom, gaseous atomic beam, ion, molecular or plasma state.

The quantum resonance system adopts an active CPT or passive CPT configuration.

The invention has the beneficial effects that:1. By combining a double-modulation (DM CPT) with constructive and the destructive interference detection technique, electromagnetic induction transparency (EIT) and electromagnetic induction absorption (EIA) signals can be obtained by controlling the relative phase and polarization of coherent bichromatic light.2. By combining DM CPT with a differential detection technique, the signal contrast is improved by more than one order of magnitude.3. Through differential detection of CPT, common-mode noise such as laser amplitude noise (AM), laser frequency noise to amplitude noise through absorption spectral lines (FM-AM), microwave power noise and the like, is effectively suppressed.4. Compact structure and potential of miniaturization. The polarization modulator can be realized by a liquid crystal polarization rotator with extremely small volume and power consumption, thus can be applied to a chip-scale CPT atomic clock with improved frequency stability.

