We investigate the transient coherent transmission of light through an optically thick cold strontium gas. We observe a coherent superflash just after an abrupt probe extinction, with peak intensity more than three times the incident one. We show that this coherent superflash is a direct signature of the cooperative forward emission of the atoms. By engineering fast transient phenomena on the incident field, we give a clear and simple picture of the physical mechanisms at play.PACS numbers: 42.50. Md, 42.25.Dd For many decades, coherent transient phenomena have been used to characterize decays and dephasing in resonantly driven two-level systems [1,2]. A rich variety of systems, with their own particularities, ranging from NMR [3,4] to electromagnetic resonances in atoms [5][6][7][8], molecules [9][10][11][12] and nuclei [13,14], have been used. A simple situation arises when an electromagnetic wave is sent through a sample composed of atomic (or molecular) scatterers. The abrupt switch off of a monochromatic quasiresonant excitation leads to free induction decay in the forward direction [9]. Temporal shapes and characteristic decay times of free induction decay depend on quantities such as laser frequency detuning [5], optical thickness [8,15], and on the presence of inhomogeneous broadening [9] and nonlinearities [16]. For an optically thick medium, since the incoming light is almost completely depleted by scattering in the stationary regime, the free induction decay signal takes the form of a coherent flash of light [8]. Its duration is reduced with respect to the single scatterer lifetime by a factor of the order of the optical thickness [8]. Consequently, its experimental observation, using standard optical transitions (lifetime in the nanosecond range), is rather challenging [17]. In this Letter, we solve this issue by performing free induction decay on the intercombination line of a cold strontium atomic gas. We gain physical insight into coherent transmission, and observe a coherent superflash of light, i.e., a transmitted intensity larger than the incident one [see Fig. 1(c)]. The superflash is due to strong phase rotation and large amplitude of the forward scattered field which are directly measured in our experiment.Related effects have been observed in Mössbauer spectroscopy experiments, where a temporal phase change in the γ radiation can lead to transient oscillations of the intensity transmitted through a sample [13,14]. These oscillations are rather small, typically of the order of 1%. This is because the γ emitter used has a short coherence time. Note that no superflash was ever observed. In a refined "γ echo" experiment, a coincidence detection made it possible to shift the phase of the emitter at a specific time during its exponential decay, leading to a revival of the forward transmitted intensity [18]. Laser spectroscopy is, however, a much easier and flexible tool. First, the temporal or spectral properties of the source can be tuned almost at will, and second, a dilute cold atomic gas c...
We describe an experimental apparatus capable of achieving a high loading rate of strontium atoms in a magneto-optical trap operating in a high vacuum environment. A key innovation of this setup is a two dimensional magneto-optical trap deflector located after a Zeeman slower. We find a loading rate of 6 × 10 9 s −1 whereas the lifetime of the magnetically trapped atoms in the 3 P2 state is 54 s.
PACS. 32.10.Fn -Fine and hyperfine structure. PACS. 42.50.Gy -Effects of atomic coherence on propagation, absorption, and amplification of light. PACS. 42.50.-p -Quantum optics.Abstract. -We use the phenomenon of electromagnetically-induced transparency in a threelevel atomic system for hyperfine spectroscopy of upper states that are not directly coupled to the ground state. The three levels form a ladder system: the probe laser couples the ground state to the lower excited state, while the control laser couples the two upper states. As the frequency of the control laser is scanned, the probe absorption shows transparency peaks whenever the control laser is resonant with a hyperfine level of the upper state. As an illustration of the technique, we measure hyperfine structure in the 7S 1/2 states of 85 Rb and 87 Rb, and obtain an improvement of more than an order of magnitude over previous values.The use of coherent-control techniques in three-level systems is now an important tool for modifying the absorption properties of a weak probe laser [1,2,3]. For example, in the phenomenon of electromagnetically induced transparency (EIT), an initially absorbing medium is made transparent to a probe beam when a strong control laser is switched on [4,5]. EIT techniques have several practical applications in probe amplification [6], lasing without inversion [7], and suppression of spontaneous emission [3,8,9,10]. Experimental observations of EIT have been mainly done using alkali atoms (such as Rb and Cs), where the transitions have strong oscillator strengths and can be accessed with low-cost tunable diode lasers.In this paper, we use the phenomenon of EIT in a novel application, namely high-resolution spectroscopy of hyperfine structure in excited states. The experiments are done in a ladder system, where the control laser drives the upper transition and the probe laser measures absorption on the lower transition. In normal EIT experiments, the frequency of the probe laser is scanned while the frequency of the control laser is kept fixed. By contrast, in our technique, it is the frequency of the control laser that is scanned while the probe laser remains locked on resonance. The probe-absorption signal then shows transparency peaks every time the control laser comes into resonance with a hyperfine level of the excited state.Measurement of hyperfine structure in excited states is important because these states are used in diverse experiments ranging from atomic signatures of parity non-conservation (PNC)
Topology, geometry, and gauge fields play key roles in quantum physics as exemplified by fundamental phenomena such as the Aharonov–Bohm effect, the integer quantum Hall effect, the spin Hall, and topological insulators. The concept of topological protection has also become a salient ingredient in many schemes for quantum information processing and fault-tolerant quantum computation. The physical properties of such systems crucially depend on the symmetry group of the underlying holonomy. Here, we study a laser-cooled gas of strontium atoms coupled to laser fields through a four-level resonant tripod scheme. By cycling the relative phases of the tripod beams, we realize non-Abelian SU(2) geometrical transformations acting on the dark states of the system and demonstrate their non-Abelian character. We also reveal how the gauge field imprinted on the atoms impact their internal state dynamics. It leads to a thermometry method based on the interferometric displacement of atoms in the tripod beams.
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