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The electro-optic effect, where the refractive index of a medium is modified by an electric field, is of central importance in non-linear optics, laser technology, quantum optics and optical communications. In general, electro-optic coefficients are very weak and a medium with a giant electro-optic coefficient would have profound implications for non-linear optics, especially at the single photon level, enabling single photon entanglement and switching. Here we propose and demonstrate a giant electro-optic effect based on polarizable dark states. We demonstrate phase modulation of the light field in the dark state medium and measure an electro-optic coefficient that is more than 12 orders of magnitude larger than in other gases. This enormous Kerr non-linearity also creates the potential for precision electrometry and photon entanglement.PACS numbers: 32.80.Rm, 42.50.Gy In 1875 Kerr showed that the refractive index (n r ) of a medium can be changed by applying an electric field [1] according to ∆n r = λ 0 B 0 E 2 0 , where λ 0 is the wavelength of the light field, E 0 is the applied electric field and B 0 is the electro-optic Kerr coefficient. Subsequently, the Kerr effect, or quadratic electro-optic effect, and the related linear electro-optic effect have become widely used in photonic devices such as electro-optic modulators (EOMs) [2,3]. The ac Kerr effect where the electric field is produced by another light beam is the basis of Kerr lens mode-locking [4], and has led to the development of femto and attosecond pulses [5]. Outside these successes, the wider applicability of the Kerr effect is limited by the fact that, in general, the Kerr non-linearity is very small. A larger non-linearity occurs close to a resonance, but at the expense of higher absorption of the signal light. A way around this problem is to use electromagnetically induced transparency (EIT) [6,7,8] where an additional light field, the coupling beam, renders a medium transparent on resonance. Enhanced ac Kerr non-linearities were predicted [9], and have been studied in experiments on Bose Einstein condensates [10] and cold atoms [11]. However, such an EIT medium produces insufficient non-linearity to implement single photon non-linear optics [8]. In addition, the potential to implement all-optical quantum computation using the ac Kerr effect [12] is limited by pulse distortion effects [13], so a new Kerr mechanism based on interactions [14] is desirable.In this paper, we demonstrate a giant dc electro-optic effect in an EIT medium by coupling to a highly excited Rydberg state which has a large polarizability. This renders the transmission through the medium highly sensitive to electric fields produced either externally or internally due to interparticle interactions. The Rydberg states have a polarizability that scales as the principal quantum number, n 7 , and the interactions between Rydberg atoms scale with an even higher power (n 11 for van der Waals interactions) [15]. These strong interac-tions lead to strongly correlated quantum st...

We demonstrate laser frequency stabilization to excited state transitions using cascade electromagnetically induced transparency. Using a room temperature Rb vapor cell as a reference, we stabilize a first diode laser to the D2 transition and a second laser to a transition from the intermediate 5P 3/2 state to a highly excited state with principal quantum number n = 19 − 70. A combined laser linewidth of 280 ± 50 kHz over a 100 µs time period is achieved. This method may be applied generally to any cascade system and allows laser stabilization to an atomic reference in the absence of a direct absorption signal.

We study electromagnetically induced transparency (EIT) of a weakly interacting cold Rydberg gas. We show that the onset of interactions is manifest as a depopulation of the Rydberg state and numerically model this effect by adding a density-dependent non-linear term to the optical Bloch equations. In the limit of a weak probe where the depopulation effect is negligible, we observe no evidence of interaction induced decoherence and obtain a narrow Rydberg dark resonance with a linewidth of <600 kHz, limited by the Rabi frequency of the coupling beam.PACS numbers: 03.67. Lx, 32.80.Rm, 42.50.Gy Ensembles of Rydberg atoms display fascinating manybody behavior due to their strong interactions [1]. These interactions lead to interesting cooperative effects such as superradiance [2,3,4] and dipole blockade [5,6,7,8], which may provide the basis for applications such as single-photon sources [9] and quantum gates [10,11,12]. For quantum information applications one is interested in the coherent evolution of the ensemble. Coherent excitation of Rydberg states has been achieved using adiabatic passage [13,14]. Rabi oscillations between ground and Rydberg states with dipole-dipole interactions have been observed [15,16]. Also, the coherence of a Rydberg ensemble has been measured directly using a spin echo technique [17]. In most experiments on ultracold Rydberg gases, the Rydberg atoms are detected indirectly following field ionization and subsequent detection of electrons (or ions) using a micro channel plate (MCP). However, recently we demonstrated non-destructive optical detection of Rydberg states in room temperature Rb vapor [18,19] using EIT [20,21]. The same technique was subsequently used to detect Rydberg states in a Sr atomic beam [22]. Rydberg EIT has number of potential applications, for example, a Rydberg EIT medium displays a dc electro-optic effect many orders of magnitude larger than other systems [23], and Rydberg EIT enables direct measurement of the coherence of the Rydberg ensemble.In this work we demonstrate EIT involving Rydberg states in an ultra-cold atomic sample. We show that interactions between Rydberg atoms lead to a rapid depopulation of the Rydberg state, and that the EIT spectra are extremely sensitive to small changes in the interaction strength. For example, changing the principal quantum number of the Rydberg state from n to n + 1 produces a significant change in the EIT spectrum. By using a weak probe, where the probability of populating the Rydberg state is low, we can eliminate this depopulation effect and obtain narrow Rydberg dark resonances with a linewidth of < 600 kHz, limited by the Rabi frequency of the coupling beam.The experimental setup and simplified level scheme are shown in figure 1 (a) and (b) respectively. A probe and coupling beam are combined using dichroic mirrors and counter-propagate through a cloud of laser-cooled 85 Rb atoms. The polarization of the beams are chosen to max- imize transition strengths (σ + -σ + ). The probe beam is derived from a diode laser a...

Optical detection of Rydberg states using electromagnetically induced transparency (EIT) enables continuous measurement of electric fields in a confined geometry. In this paper, we demonstrate the formation of rf-dressed EIT resonances in a thermal Rb vapour and show that such states exhibit enhanced sensitivity to dc electric fields compared to their bare counterparts. Fitting the corresponding EIT profile enables precise measurements of the dc field independent of laser frequency fluctuations. Our results indicate that space charges within the enclosed cell reduce electric field inhomogeneities within the interaction region.