Coherent Population Trapping Resonance Research Articles

Nonlinear frequency shift caused by asymmetry of the multipeak coherent population trapping resonance

Nonlinear frequency shift caused by asymmetry of the multipeak coherent population trapping resonance

Contrast of Ramsey-CPT Fringes in Quenching and Depolarizing Gases

Molecular nitrogen is often used as a buffer gas in cells with alkali metals due to its known ability to quench resonant fluorescence. It is widely believed that the suppression of spontaneous emission decreases the width of coherent population trapping resonance. However, our recent results have not confirmed this positive action of molecular nitrogen within the typical range of 87Rb concentrations and buffer gas pressures. On the opposite, we have observed the negative influence of quenching, the decrease in contrast of the coherent population trapping resonance in $${{\sigma }^{ + }}$$–$${{\sigma }^{ + }}$$ configuration. In this work, we further confirm these results implementing the Ramsey spectroscopy, and compare the characteristics of the central fringe in nitrogen and neon, and show that the latter provides a significantly better contrast-to-width ratio.

Light shift of Ramsey coherent population trapping resonance using drive current modulation

We investigated the light shift of Ramsey coherent population trapping (CPT) resonance by drive current modulation (DCM), which can be realized with low power consumption in a small system, to improve the performance of a CPT atomic clock. The light shift makes the greatest contribution to the degradation of long-term frequency stability of a CPT atomic clock. We also devised digitizer-based DCM to reduce the uncertainty in the frequency of light shift due to wavelength variation at the time of observation. The results show that light shift with DCM is reduced compared with cw excitation and is consistent with theoretical calculations. The estimated frequency uncertainty of the light shift due to wavelength variation using the proposed method is 2.1 × 10−13, which is approximately 165 times better than that of the conventional method without a digitizer, thus demonstrating the effectiveness of the proposed method for future small atomic clocks.

Single-shot vector magnetic measurements using synchronous coherent population trapping resonance in a feedback compensation system

We demonstrate a real-time, single-shot vector magnetic measurement technique using synchronous coherent population trapping (SCPT) in an apparatus consisting of a small rubidium vapor cell. Vector modality of our magnetometer is enabled by designing a feedback system based on performing a peak-lock on a particularly strong 2ΩL magnetic resonance produced in the SCPT spectrum and compensating the external magnetic field via a three-axis field coil. With its current design, this magnetometer exhibits high sensitivities of approximately 155, 129, and 57pT/Hz in measuring the magnetic field vector components along the (x,y,z) axes. Sensitivities closer to the shot-noise limit can be achieved in the future by reducing the technical noise of our equipment and by employing a differential detection and polarization rotation measurement scheme in our system.

Effect of Depolarizing and Quenching Collisions on the Contrast of Coherent Population Trapping Resonance

Effect of Depolarizing and Quenching Collisions on the Contrast of Coherent Population Trapping Resonance

Coherent Population Trapping Resonances under Multifrequency Optical Pumping

Coherent Population Trapping Resonances under Multifrequency Optical Pumping

Field Shift of Coherent Population Trapping Resonance Taking into Account Spatial Inhomogeneity of the Light Beam

Field Shift of Coherent Population Trapping Resonance Taking into Account Spatial Inhomogeneity of the Light Beam

Coherent Population Trapping Resonance Parameters and Field Shifts during the Detection of Signals from Different Cross-Sectional Areas of the Interaction of Laser Radiation with Atoms

Coherent Population Trapping Resonance Parameters and Field Shifts during the Detection of Signals from Different Cross-Sectional Areas of the Interaction of Laser Radiation with Atoms

Active Suppression of the Light Shift in an Atomic Clock Based on Coherent Population Trapping in 87Rb Vapor Using the Phase Jump Technique

The active suppression of light shift of the reference coherent population trapping (CPT) resonance in 87Rb vapor to improve the long-term stability of compact atomic clocks has been demonstrated experimentally. The approach is based on the phase jump technique used to synthesize an error signal, which is proportional to the light shift of the CPT resonance, from an optical transmission signal. The use of this error signal in an additional feedback loop allows one to stabilize the amplitude of a microwave signal for the phase modulation of laser radiation near a value at which the light shift of the CPT resonance is absent. This technique has made it possible to reduce the long-term instability of CPT atomic clocks in our experiments by a factor of 15 at an integration time of 10 000–20 000 s.

Spectrum and Light Shift of the Coherent Population Trapping Resonance in Cells with Antirelaxation Wall Coating in Specular and Diffuse Reflection Models

Spectrum and Light Shift of the Coherent Population Trapping Resonance in Cells with Antirelaxation Wall Coating in Specular and Diffuse Reflection Models

Frequency shift compensation for single and dual laser beam pass sensors of a coherent population trapping resonance based coupled dark state magnetometer

The Coupled Dark State Magnetometer is an atomic magnetometer designed for magnetic field measurements in space. Its measurement principle is based on the excitation of coupled dark state resonances prepared via the coherent population trapping effect. The coupled dark state resonances enable the compensation of disturbing frequency shifts which degrade the measurement accuracy. Results with a sensor design based on a single laser beam pass through the sensor unit reveal a reduced ability to compensate these disturbing effects compared to theoretical predictions. Therefore, a new sensor design is presented which enables a dual laser beam pass through the sensor unit. For almost all angles between the external magnetic field and the light propagation direction, the dual pass sensor design enables a better compensation of disturbing frequency shifts than the single pass sensor design - up to a factor of nine.

Motion-selective coherent population trapping by Raman sideband cooling along two paths in a Λ configuration

We report our experiment on sideband cooling with two Raman transitions in a $\Lambda$ configuration that allows selective coherent population trapping (CPT) of the motional ground state. The cooling method is applied to $^{87}$Rb atoms in a circularly-polarized one-dimensional optical lattice. Owing to the vector polarizability, the vibration frequency of a trapped atom depends on its Zeeman quantum number, and CPT resonance for a pair of bound states in the $\Lambda$ configuration depends on their vibrational quantum numbers. We call this scheme motion-selective coherent population trapping (MSCPT) and it is a trapped-atom analogue to the velocity-selective CPT developed for free He atoms. We observe a pronounced dip in temperature near a detuning for the Raman beams to satisfy the CPT resonance condition for the motional ground state. Although the lowest temperature we obtain is ten times the recoil limit owing to the large Lamb-Dicke parameter of 2.3 in our apparatus, the experiment demonstrates that MSCPT enhances the effectiveness of Raman sideband cooling and enlarges the range of its application. Discussions on design parameters optimized for MSCPT on $^{87}$Rb atoms and opportunities provided by diatomic polar molecules, whose Stark shift shows strong dependence on the rotational quantum number, are included.

Parametric Study of CPT Resonance in Rubidium Vapor Cell for Application in Atomic Clock

The performance of Coherent Population Trapping (CPT) based atomic clocks primarily depends on the characteristics of CPT resonance. We have performed experiments to study and optimize the characteristics of CPT resonance in 87Rb atoms by measuring its contrast and full-width-at-half maximum (FWHM) as function of laser excitation and temperature of atomic vapor cells with different dimensions. A four-level atomic model is used to simulate CPT resonance characteristics along the length of atomic vapor cell. The model incorporates scaling law to understand collision dynamics in cells with different radius for a range of laser excitation intensities and the results are compared with experimental data. The quality figure, calculated from the measured values of FWHM and contrast, decreases with increase in laser intensity and improves in cells with higher dimension (radius). The optimum temperature corresponding to maximum quality figure varies with laser excitation intensity as well as cell dimension. The underlying collision dynamics and density effects that are responsible for the observed resonance characteristics are discussed.

Inhomogeneous light shifts of coherent population trapping resonances

Coherent population trapping (CPT) in atomic vapors using all-optical interrogation has enabled the miniaturization of microwave atomic clocks. Light shifts induced by the CPT driving fields can impact the spectral profile of CPT resonances and are a common limit to the long-term stability of CPT clocks. Nonlinear light shifts have been observed in several CPT systems and have not been explored in detail. In this Letter, we demonstrate that nonlinear light shifts in CPT clocks can arise from spatially inhomogeneous CPT driving fields. We measure this effect using Gaussian laser beams in a buffer gas cell and show strong agreement with a four-level model describing the CPT Λ-system with a noninteracting “trap” state. We estimate the effect of this nonlinearity on recently developed light shift mitigation techniques and suggest improvements to existing techniques.

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.

Magnetic-field-insensitive coherent-population-trapping resonances excited by bichromatic linearly polarized fields on the D1 line of Cs133

We have experimentally demonstrated that magnetic-field-insensitive coherent-population-trapping (CPT) resonances are generated between the ground hyperfine levels on the ${D}_{1}$ line of $^{133}\mathrm{Cs}$ using a two-photon $\mathrm{\ensuremath{\Lambda}}$ scheme excited by lin||lin polarizations. The frequency shift of the CPT resonance is 0.04 Hz for the deviation of $1\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{T}$ at a ``magic'' magnetic field of $139\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{T}$ and is 50 times smaller than that of the conventional clock transition at a typical bias magnetic field for the clock operation. The amplitude of the CPT spectrum excited by lin||lin polarizations is enhanced as the excitation intensity increases and is well explained by the three-level model without trap levels. Thus, the CPT resonance on the ${D}_{1}$ line of $^{133}\mathrm{Cs}$ excited by a simple lin||lin scheme will be one of the best candidates for frequency reference of miniature atomic clocks.

A vertical cavity surface emitting laser for CPT atomic clock

A polarization-stabilized fundamental mode vertical cavity surface emitting laser (VCSEL) was developed for a coherent population trapping (CPT) Cs atomic clock. The VCSEL achieved a single stabilized polarization state through a directly-etched surface grating structure on the surface. The device polarization characteristics of different surface grating parameters were analysed using the finite-difference time-domain (FDTD) method. For the fundamental transverse mode condition, the lasing wavelength corresponded to the Cs atom’s D1 line. Owing to the polarization-dependent reflectivity of surface gratings, the VCSEL’s orthogonal polarization suppression ratio (OPSR) exceeded 20 dB. Since the surface grating eliminated the degeneracy of the two orthogonal linear polarization fundamental modes, a linewidth of approximately 53 MHz for the VCSEL could be obtained using the delayed self-homodyne method. A 7.6 GHz modulation bandwidth satisfied the atomic clock’s bandwidth requirement. We introduced the directly-etched surface grating VCSEL to the Cs atomic clock, and measured the CPT resonance signal.

CPT resonances under multifrequency pumping

The possibility of detecting coherent population trapping (CPT) resonances with combined microwave and HF modulation of the injection current of a diode laser has been demonstrated for the first time. A series of resonances is observed, separated from each other by half the RF modulation frequency.

Line shape of the coherent population trapping resonance in the case of a Gaussian spatial profile of a light beam

For coherent population trapping (CPT) resonance excited by a bichromatic field in a closed Λ-system, we have obtained an analytical expression for the line shape in the case of a Gaussian profile of the radiation intensity distribution, taking into account the light shift of the reference transition frequency. Due to the spatial inhomogeneity of the light shift, the line shape is asymmetric. The dependence of the shift of the resonance peak position on the light intenisty is investigated.

Line shape and light shift of coherent population trapping resonance under Ramsey interrogation in ‘hot’ atoms in an optically dense medium

A theory is developed for the effect of coherent population trapping (CPT) in ‘hot’ atoms under pulsed pumping in an optically dense medium with a buffer gas. The case is considered when the excited level is not degenerate. Based on an analysis of the shape of Ramsey resonances, we show that with increasing optical density of the medium, the dependence of the light shifts of the CPT resonance becomes more and more nontrivial. The dependence of the light shifts of the CPT resonance on the magnitude of the hyperfine splitting of the excited level, concentration of active atoms, temperature, and duration of the dark pause is constructed.