Coherent Population Trapping Signal Research Articles

Microwave phase detection of coherent population trapping resonance in a Cs vapor cell

We report a phase-sensitive microwave detection setup for spectroscopy of coherent population trapping (CPT) resonance in a Cs vapor cell. In this scheme, the optically carried microwave interrogation signal that interacts with atoms is detected by a fast photodiode at the cell output and is then mixed with the local oscillator microwave signal itself. This detection scheme produces at the output of the microwave mixer a dispersive CPT signal that can be directly used for correction of the local oscillator frequency onto the atomic resonance. The evolution of the dispersive CPT resonance signal with the laser power is briefly reported. With this detection scheme, we demonstrate the operation of a CPT atomic clock with a short-term fractional frequency stability of 1 × 10 − 11 at 1 s, which is in good agreement with the slope of the frequency discriminator and the residual microwave phase noise of the setup.

Wafer-Level Filling of MEMS Vapor Cells Based on Chemical Reaction and Evaporation.

Micro-electro-mechanical system (MEMS) vapor cells are key components for sensors such as chip-scale atomic clocks (CSACs) and magnetometers (CSAMs). Many approaches have been proposed to fabricate MEMS vapor cells. In this article, we propose a new method to fabricate wafer-level filling of MEMS vapor cells based on chemical reaction and evaporation. The Cs metals are firstly obtained through the chemical reaction between cesium chloride and barium azide in a reservoir baseplate. Then, the Cs metals are evaporated to the preform through the microchannel plate and condensed on the inner glass surface of the preform. Lastly, the MEMS vapor cells are filled with buffer gas, sealed by anodic bonding, and mechanically diced into three dimensions: 5 mm × 5 mm × 1.2 mm, 4 mm × 4 mm × 1.2 mm, and 3 mm × 3 mm × 1.2 mm. The full width at half maximum (FWHM) linewidth of the coherent population trapping (CPT) signal of the MEMS vapor cells is found to be 4.33 kHz. The intrinsic linewidth is about 1638 Hz. Based on the CPT signal, the frequency stability is 4.41 × 10−12@1000 s. The results demonstrate that the presented method of the wafer-level filling of MEMS vapor cells fulfills the requirements of sensors such as CSACs.

High-performance coherent population trapping atomic clock with direct-modulation distributed Bragg reflector laser

The coherent population trapping (CPT) atomic clock is very promising for use in next-generation spaceborne applications owing to its compactness and high performance. In this paper, we propose and implement a CPT atomic clock based on the direct modulation of a large-modulation-bandwidth and narrow-linewidth distributed Bragg reflector laser, which replaces the usually used external bulk modulator in high-performance CPT clocks. Our method retains the high performance while significantly reducing the clock size. Using this highly compact bichromatic light source and simplest CPT configuration, in which a circularly polarized bichromatic laser interrogates the 87Rb atom system, a CPT signal of clock transition with a narrow linewidth and high contrast is observed. We then lock the local oscillator frequency to the CPT resonance and demonstrate an encouraging short-term frequency stability of 3.6 × 10−13 τ −1/2 (4 s ⩽ τ ⩽ 200 s). We attribute it to the ultralow laser frequency and intensity noise as well as to the high-quality-factor CPT signal. This study can pave the way for the development of compact high-performance CPT clocks based on our scheme.

VIPA-based two-component detection for a coherent population trapping experiment

We demonstrate a two-component detection of a coherent population trapping (CPT) resonance based on virtually imaged phased array (VIPA). After passing through a VIPA, the two coupling lights with different frequencies in the CPT experiment are separated in space and detected individually. The asymmetric lineshape is observed experimentally in the CPT signal for each component, and the comparison with the conventional detection is presented. The shift of the CPT resonant frequency is studied with both the two-component and one-component detections. Our scheme provides a convenient way to further study the CPT phenomenon for each frequency component.

A new method to simultaneously improve the sensitivity and absolute accuracy for CPT magnetometer

This paper proposed a new method to simultaneously improve the sensitivity and absolute accuracy for coherent population trapping (CPT) magnetometer. CPT signals can be measured by scanning the frequency difference of a bichromatic pumping field around the ground-state hyperfine-splitting frequency of 87Rb. Meanwhile, the feedback loop can be used to adjust the pump radiation intensity to maintain a constant level of atomic vapour spontaneous fluorescence. This feedback loop improves the resonance contrast greatly. Then the Larmor frequency is determined by differential processing of CPT signals, which can reduce the effects of common-mode noise, nonlinear Zeeman splitting and atomic collisions.

ULPAC: A Miniaturized Ultralow-Power Atomic Clock

This article introduces a chip-scale ultralow-power atomic clock (ULPAC) in the microwave frequency region. A new suspended quantum package architecture along with a fully integrated frequency probing and locking loop implemented in CMOS technology results in a compact package and ultralow-power consumption. In addition, dedicated low-noise magnetic field and temperature control loops are incorporated for isolating and mitigating the internal and external factors affecting the frequency stability. The output frequency of a crystal oscillator is continuously compensated and stabilized by locking to the peak of a coherent population trapping signal, thereby inheriting the superior frequency stability of the atomic resonance. The proposed ULPAC system achieves a frequency stability of $2.2 \times 10^{-12}$ at an averaging time of 105 s, while consuming 59.9 mW. The frequency probing and locking circuits implemented in a standard 65-nm CMOS process node occupy an area of 2.55 mm2. The prototype of this atomic clock occupies a volume of 15 cm3.

Interference-enhanced and power-reduced CPT atomic realization approach with an external cavity.

A scheme is studied for a coherent population trapping (CPT) atomic clock with counterpropagating left and right circularly polarized lasers, wherein the vertical-cavity surface-emitting laser and the partially reflecting mirror form an external cavity of an established length. With cancellation of spin-polarized dark states and interaction repeated with atoms, a CPT signal is obtained that is eight times larger than that of the conventional scheme. For the improvement of modulation efficiency, CPT resonance still can be excited with a lower microwave power compared with the conventional scheme. The experimental results suggest that it is an alternative for low-power atomic clocks.

Experimental study of the application feasibility of a novel chip-scale atomic clock scheme.

We report on research into a microfabricated 87Rb vapor cell with differential detection. Elliptically polarized light is used to interact with alkali atoms to generate coherent population trapping (CPT) resonance, and the CPT signal is obtained by detecting the Faraday rotation effect with differential detection technology. To move closer to an actual chip-scale atomic clock (CSAC), we reduce the volume of the experimental apparatus and use a divergent laser beam to interact with the atoms. We obtain the short-term frequency stability of the CSAC based on the differential detection scheme and compare it with that of a conventional CSAC. The results show that the frequency stability is more than two times better than that of current commercial CSAC devices with the same power consumption and volume.

Selective detection of polarisation components of a coherent population trapping signal in hot alkali metal atoms

A mathematical model of the interaction of bichromatic laser radiation with alkali metal atoms in an optically dense gas cell at above room temperature is constructed. Within the framework of the model, complete hyperfine and Zeeman structures of alkali metal atom levels are considered, which allows the propagation of radiation polarisation along the cell and the effect of a constant magnetic field to be correctly taken into account. It is found that the selective detection of polarisation radiation components carries additional information in comparison with a signal of total intensity.

Differential detection scheme for compact CPT atomic clocks

A scheme is investigated for a coherent population trapping (CPT) atomic clock, wherein the polarization of a beam produced by a vertical-cavity surface-emitting laser is converted to an elliptically polarized beam that interacts with alkali atoms, where the CPT signal is extracted by differentially detecting the magneto-optically rotated light within the transmitted beam. The scheme eliminates the spin-polarized trap state of the atoms and the unwanted background signal, and suppresses in the CPT signal the noise converted from the laser noise. This result reveals the promise of this scheme for realizing a compact CPT atomic clock possessing a significantly improved frequency stability compared to current compact CPT atomic clock devices, coupled with similar power consumption, volume, and cost therewith.

Investigation of coherent population trapping signals in 87Rb cells with buffer gas

Characteristics of CPT (coherent population trapping) signal are investigated in small-size glass vapor cells containing 87Rb and Ne buffer gas with narrow linewidth laser pumping on D2 line. Parameters of CPT absorption signals are measured using small-size vapor cells with Ne buffer gas pressure in range of 200-400 Torr, cell temperature in range of 65-120 °C and values of laser pumping power of 30-400 μW/cm2. Optimum conditions, under which the minimal value of short-term instability of resonance line is achieved, are obtained in experiments. CPT signals using vapor cells based on integrated technologies containing 87Rb in atmosphere of Ne are also investigated. The CPT signals with typical linewidths of 2-3 kHz and signal-to-noise ratio of 1500 in 1 Hz bandwidth are observed, which allows one to provide relative frequency instability of 10-11 at 100 s.

Coherent population trapping with polarization modulation

Coherent population trapping (CPT) is extensively studied for future vapor cell clocks of high frequency stability. In the constructive polarization modulation CPT scheme, a bichromatic laser field with polarization and phase synchronously modulated is applied on an atomic medium. A high contrast CPT signal is observed in this so-called double-modulation configuration, due to the fact that the atomic population does not leak to the extreme Zeeman states, and that the two CPT dark states, which are produced successively by the alternate polarizations, add constructively. Here, we experimentally investigate CPT signal dynamics first in the usual configuration, a single circular polarization. The double-modulation scheme is then addressed in both cases: one pulse Rabi interaction and two pulses Ramsey interaction. The impact and the optimization of the experimental parameters involved in the time sequence are reviewed. We show that a simple seven-level model explains the experimental observations. The double-modulation scheme yields a high contrast similar to the one of other high contrast configurations like push-pull optical pumping or crossed linear polarization scheme, with a setup allowing a higher compactness. The constructive polarization modulation is attractive for atomic clock, atomic magnetometer, and high precision spectroscopy applications.

Double-modulation CPT cesium compact clock

Double-modulation coherent population trapping (CPT) is based on a synchronous modulation of Raman phase and laser polarization, which allows the atomic population to accumulate in a common dark state. The high contrast signal obtained on the clock transition with a relative compact and robust laser system is interesting as basis of a high performance microwave clock. Here we study the parameters of a double-modulation CPT Cs clock working in cw mode. The optimal polarization modulation frequency and cell temperature for maximum contrast of clock transition are investigated. The parameters of the detection are also studied. With the optimal parameters, we observe a CPT signal with contrast of 10% and linewidth of 492 Hz, which is well suited for implementing a cw atomic clock.

Coherent population trapping in small- and chip-scale 87Rb vapor cells with buffer gas

The characteristics of coherent population trapping (CPT) signal were investigated in small-size glass vapor cells containing [Formula: see text]Rb and Ne buffer gas with narrow line-width laser pumping on D2 line. The parameters of CPT signals were measured using small-size vapor cells with Ne buffer gas pressure in the range of 200–400 Torr, cell temperature in the range of 65–120 [Formula: see text]C and the values of laser pumping power of 30–400 [Formula: see text]W/cm2. Optimal conditions, under which the minimal value of short-term instability of resonance line is achieved, were obtained in experiments. Orientation frequency shifts of CPT resonance using glass [Formula: see text]Rb vapor cells containing buffer gas and anti-relaxation coating were compared. CPT signals using vapor cells based on integrated technologies containing [Formula: see text]Rb in atmosphere of Ne were also investigated. The CPT signals with typical line widths of 2–3 kHz and signal-to-noise ratio of 1500 in 1 Hz bandwidth are observed, which allows one to provide relative frequency instability of 10[Formula: see text] at 100 s.

Magic frequency enabled by quantum interference for a dual atomic device

We present a magic frequency associated with two-photon processes in room temperature atomic vapour at the coherent population trapping (CPT) condition for an elliptically polarized light. It is realized by combining the symmetries of the lineshape functions arising from the atomic coherence. In practice, the magic frequency is synthesized from a polarimetric measurement involving the reflected and transmitted CPT signal at the polarizing beam splitter, which provides an atomic frequency standard for near zero magnetic fields. We further develop a magnetometer, which concurrently measures the magnetic field and its direction from the enhanced reflected signal with significantly improved performance. Additionally, the magic frequency and its dependence on the polarization content provide a handle to precisely locate the single photon resonance.

Microfabricated cells for chip-scale atomic clock based on coherent population trapping: Fabrication and investigation

A universal method for fabrication of miniature cells for frequency standards and quantum magnetometers containing 87Rb atoms in the atmosphere of inert gas neon based on integrated technologies is considered. The results of experimental studies of coherent population trapping signals observed for a series of cells which provided recovery of vapors of an alkali metal from the rubidium dichromate salt with the help of laser radiation are presented. The coherent population trapping signals with a typical linewidth of 2–3 kHz and a signal-to-noise ratio of 1500 in the 1-Hz bandwidth were observed, which allows one to provide a relative frequency stability of atomic clock of 10−11 at 100 s.

Study on characteristics of coherent population trapping spectral line for chip-scale atomic clock

To Obtain an appropriate coherent population trapping (CPT) signal through a miniature atomic vapor cell is one of the key steps to implement a chip-scale atomic clock (CSAC). In the present experiment, the high S/N differential CPT spectral line has been achieved with a miniature atomic vapor cell through modulating the laser and extracting 87Rb atom laser interacting signal using the phase sensitive demodulation technique. With the spectral line, the dependence of CPT signal on the working parameters and the dependence of frequency stability of CSAC on the quality of the CPT signal have been studied; the obtained experimental results agree well with the theoretical prediction, which can be used as the recommended working parameters for CSAC. The methods exploited in the experiment can be implemented in the resource of a CSAC, therefore this experiment explores for CSAC the practical approaches of performance optimization.

CPT Cesium-Cell Atomic Clock Operation With a 12-mW Frequency Synthesizer ASIC

In this paper, we present the design, fabrication, and electrical characterization of a low-power microwave source for interrogation of cesium atomic hyperfine transition frequency using the coherent population trapping (CPT) technique. The 4.6-GHz frequency generation and signal buffering is performed by a single-chip frequency synthesizer ASIC with a frequency tuning resolution of 1 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-13</sup> and a programmable RF output power from -10 to 0 dBm. The circuit was used to modulate the current of a vertical-cavity surface-emitting laser through a dedicated impedance matching network and low thermal conductivity transmission line. Strong modulation sidebands with >60% of carrier amplitude were obtained with an ASIC power consumption of 12 mW. The system was used as optical source for atomic interrogation in an experimental cesium CPT clock. The measured clock stability of 5 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-11</sup> at τ = 1 s, going down to 4.5 x 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-12</sup> at τ = 200 s, is limited by the signal-to-noise ratio of the detected CPT signal.

Limitations of Long-Term Stability in a Coherent Population Trapping Cs Clock

Vapor cell atomic clocks exhibit reduced frequency stability for averaging time between about one hundred and a few thousand seconds. Here we report a study on the impact of the main parameters on the mid-to-long term instability of a buffer-gas vapor cell Cs clock, based on coherent population trapping (CPT). The CPT signal is observed on the Cs D1 line transmission, using a double $\Lambda$ scheme and a Ramsey interrogation technique. The effects on the clock frequency of the magnetic field, the cell temperature, and the laser intensities are reported. We show in particular that the laser intensity shift is temperature dependent. Along with the laser intensity ratio and laser polarization properties, this is one of the most important parameters.

Spectral narrowing of coherent population trapping resonance in laser-cooled and room-temperature atomic gas

We have investigated coherent population trapping (CPT) in laser-cooled as well as room-temperature (with and without buffer gas) rubidium atoms. The characteristic broad signal profile emerging from the two-photon Raman resonance for room-temperature atomic vapour is consistent with the theoretical calculation incorporating associated thermal averaging. The spectral width of the dark resonance obtained with cold atoms is found to be broadened, compared to room-temperature vapour cell, due to the feeble role played by thermal averaging, although the cold atomic sample significantly overcomes the limitation of the transit time broadening. An alternative way to improve transit time is to use a buffer gas, with which we demonstrate that the coherent population trapping signal width is reduced to < 540 Hz.