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arXiv:1502.03079 · New Journal of Physics

Observation of coherent population trapping in a V-type two-electron system

Alok K. Singh, Vasant Natarajan A

Published Mar 31, 2015• 4 citations
arXivHTMLPDF

Abstract

We observe coherent population trapping (CPT) in a two-electron atom— 174 Yb—using the transition. CPT is not possible for such a transition according to one-electron theory because the magnetic sublevels form a V-type system, but in a two-electron atom like Yb, the interaction of the electrons transforms the level structure into a Λ -type system, which allows the formation of a dark state and hence the observation of CPT. Since the two levels involved are degenerate, we use a magnetic field to lift the degeneracy. The single fluorescence dip then splits into five dips—the central unshifted one corresponds to coherent population oscillation, while the outer four are due to CPT. The linewidth of the CPT resonance is about 300 kHz and is limited by the natural linewidth of the excited state, which is to be expected because the excited state is involved in the formation of the dark state.

DOI
10.1088/1367-2630/17/3/033044

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We observe coherent population trapping (CPT) in a two-electron atom— 174 Yb—using the transition. CPT is not possible for such a transition according to one-electron theory because the magnetic sublevels form a V-type system, but in a two-electron atom like Yb, the interaction of the electrons transforms the level structure into a Λ -type system, which allows the formation of a dark state and hence the observation of CPT. Since the two levels involved are degenerate, we use a magnetic field to lift the degeneracy. The single fluorescence dip then splits into five dips—the central unshifted one corresponds to coherent population oscillation, while the outer four are due to CPT. The linewidth of the CPT resonance is about 300 kHz and is limited by the natural linewidth of the excited state, which is to be expected because the excited state is involved in the formation of the dark state.
Abstract We study coherent population oscillations in an odd isotope of the two-electron atom Yb. The experiments are done using magnetic sublevels of the F g = 5 / 2 → F e = 3 / 2 hyperfine transition in 173 Yb of the 1 S 0 → 3 P 1 intercombination line. The experiments are done both with and without an applied magnetic field. In the absence of an applied field, the complicated sublevel structure along with the saturated fluorescence effect causes the linewidth to be larger than the 190 kHz natural linewidth of the transition. In the presence of a field (of magnitude 330 mG), a well-defined quantization axis is present which results in the formation of two M-type systems. The total fluorescence is then limited by spin coherence among the ground sublevels. In addition, the pump beam gets detuned from resonance which results in a reduced scattering rate from the 3 P 1 state. Both of these effects result in a reduction of the linewidth to a subnatural value of about 100 kHz.
We report our study on coherent population trapping (CPT) in a $\ensuremath{\Lambda}$ configuration coupled by magnetic dipole interactions. The $\ensuremath{\Lambda}$ configuration is formed by bichromatic radio-frequency fields coupling a pair of Zeeman sublevels in one hyperfine level of an alkali-metal atom to another sublevel in the other hyperfine level. The Zeeman sublevel at the apex is coupled to a state in the nPnPnP manifold via an optical transition on one of the spectroscopic DDD lines to induce optical pumping to a dark state. The configuration is closed without leakage to other Zeeman sublevels. We use lithium atoms in an optical trap for our experimental study. The system allows independent control of the main parameters characterizing CPT, which include the upper-state decay rate and the decoherence rate as well as the Rabi frequencies. By turning off the applied fields, the system is frozen so that its quantum state can be measured precisely. We studied the line shapes and dynamics of the CPT system, and measured the phase relation of the dark superposition state to find excellent agreement with theory. The possible application of the scheme as a method to cool optically trapped atoms below the recoil limit in a manner analogous to velocity-selective coherent population trapping is discussed.
This work investigates the possibility of realizing coherent population trapping (CPT) in a two-electron three-level system with parallel spins. Making an analogy with solid-state physics, this study demonstrates that a V-type three-level system with two electrons can be described as a /spl Lambda/-type three-level system for one single hole. Cancellation of the probe field absorption is obtained when the hole is placed in the dark state. Alternatively, in terms of the two-electron V-type system, the CPT condition is realized when one electron is placed in the ground state and the other one in the bright combination of the excited states. In addition, when the electron-electron Coulomb repulsion is taken into account, it is shown that this technique offers the possibility of measuring, with subnatural resolution, some superposition-state matrix elements of the electron-electron correlation that due to their time dependent nature are inaccessible by standard measuring procedures.
Coherent population trapping (CPT) resonances on the degenerate two-level system of the (Fg=2→Fe=1) transition of the 87Rb D1 line were investigated in an uncoated vacuum cell by means of level-crossing type experiments-sweeping a magnetic field around zero value and using a single laser beam. The two-photon resonance manifests itself as fluorescence canceling when the system of levels is degenerate (at zero point of magnetic field). The influence of weak transverse magnetic fields on the amplitude and width of the resonance was investigated. The CPT resonances were observed with linear, circular and elliptical polarized laser light. Transverse magnetic field widens the resonances obtained by linear and circular polarization but splits the resonance created by slightly elliptically polarized laser light. The dependence of this splitting on the elliptical ratio and experimental geometry were investigated. Due to its narrow width and sensitivity to low magnetic fields (< 1 G) this resonance is very promising for small magnetic field measurement.
Phase-dependent coherent population trapping (CPT) was observed using a thermal Na atomic beam irradiated by four laser fields derived from two dye lasers. Investigations were made on Λ- and double Λ-systems created in the D 1 line, both with and without magnetic field that lifts degeneracy of the Zeeman sublevels. In all cases a sharp decrease of fluorescence intensity was observed for certain values of the relative phase of the light waves, indicating that the occurrence of CPT depends on this phase. The shape of the phase dependence as well as its behaviour with respect to laser intensity fits well theoretical predictions.
Summary form only given. We have performed a series of experiments on coherent population trapping (CPT) in the presence of strong magnetic flux densities up to 1 mT. CPT can be observed in /spl Lambda/-systems where two ground states are coupled to a common excited state by two near-resonant light fields. Effective formation of ground state coherences takes place when the frequency difference of the light fields precisely matches the ground state splitting. This process leads to reduced absorption in the medium and thus reduced fluorescence intensity ("dark resonance") at the optical resonance frequency.