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arXiv:1605.06603

Study of Rydberg blockade mediated optical non-linearity in thermal vapor using optical heterodyne detection technique

Arup Bhowmick, Dushmanta Kara, Ashok Kumar Mohapatra

Published May 21, 2016• 0 citations
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Abstract

We demonstrate the phenomenon of blockade in two-photon excitations to the Rydberg state in thermal vapor. A technique based on optical heterodyne is used to measure the dispersion of a probe beam far off resonant to the D2 line of rubidium in the presence of a strong laser beam that couples to the Rydberg state via two-photon resonance. Density dependent suppression of the dispersion peak is observed while coupling to the Rydberg state with principal quantum number, n = 60. The experimental observation is explained using the phenomenon of Rydberg blockade. The blockade radius is measured to be about 2.2 μm which is consistent with the scaling due to the Doppler width of 2-photon resonance in thermal vapor. Our result promises the realization of single photon source and strong single photon non-linearity based on Rydberg blockade in thermal vapor.

DOI
10.48550/arxiv.1605.06603

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Acta Physica Sinica•Jan 1, 2023•0 citations
We demonstrate the phenomenon of blockade in two-photon excitations to the Rydberg state in thermal vapor. A technique based on optical heterodyne is used to measure the dispersion of a probe beam far off resonant to the D2 line of rubidium in the presence of a strong laser beam that couples to the Rydberg state via two-photon resonance. Density dependent suppression of the dispersion peak is observed while coupling to the Rydberg state with principal quantum number, n = 60. The experimental observation is explained using the phenomenon of Rydberg blockade. The blockade radius is measured to be about 2.2 μm which is consistent with the scaling due to the Doppler width of 2-photon resonance in thermal vapor. Our result promises the realization of single photon source and strong single photon non-linearity based on Rydberg blockade in thermal vapor.
We present the experimental demonstration of Rydberg blockade in thermal atomic vapor where the atoms are not necessarily be frozen. We show that not all the interacting atoms but only the atoms with same velocity collectively participate in the blockade process. Using this observation, we formulated a suitable model based on super atom picture to study blockade interaction in thermal vapor. We performed an experiment to measure Rydberg population in rubidium thermal vapor using optical heterodyne detection technique and density dependent suppression of Rydberg population is observed in suitable experimental parameter regime. Further analysis of the experimental data using the model verifies the scaling law for van der Waals interaction strength (C6)(C_{6})(C6​) with principal quantum number of the Rydberg state with 11%11\%11% error. Our result suggests multi-photon excitation in thermal vapor with suitable laser configuration to probe Rydberg blockade interaction based optical nonlinearity and many body effects.
We present the experimental demonstration of interaction induced enhancement in Rydberg excitation or Rydberg anti-blockade in thermal atomic vapor. We have used optical heterodyne detection technique to measure Rydberg population due to two-photon excitation to the Rydberg state. The anti-blockade peak which doesn't satisfy the two-photon resonant condition is observed along with the usual two-photon resonant peak which can't be explained using the model with non-interacting three-level atomic system. A model involving two interacting atoms is formulated for thermal atomic vapor using the dressed states of three-level atomic system to explain the experimental observations. A non-linear dependence of vapor density is observed for the anti-blockade peak which also increases with increase in principal quantum number of the Rydberg state. A good agreement is found between the experimental observations and the proposed interacting model. Our result implies possible applications towards quantum logic gates using Rydberg anti-blockade in thermal atomic vapor.
We discuss the optical-heterodyne-detection technique to study the absorption and dispersion of a probe beam propagating through a medium with a narrow resonance. The technique has been demonstrated for Rydberg electromagnetically induced transparency in rubidium thermal vapor and the optical nonlinearity of a probe beam with variable intensity has been studied. A quantitative comparison of the experimental result with a suitable theoretical model is presented. The limitations and the working regime of the technique are discussed.
We present a theoretical model of a four-photon excitation process to the Rydberg state in thermal atomic vapor where the motion-induced dephasing in the system is eliminated. This is achieved by arranging the four laser beams in a suitable geometry such that the residual wave vector is reduced to zero. The method of adiabatic elimination has been used to reduce the complex five-level system to an effective three-level system to study electromagnetically induced transparency (EIT) where the transition from ground state to second excited state can be considered as the effective probe and second excited state to the Rydberg state as the effective coupling transition. The effect of the blockade phenomenon is observed in the strong interaction regime, where the two atoms are considered to be moving with independent velocities and the system is Doppler averaged using Monte Carlo simulation technique. Also, the dephasing mechanisms in the system are investigated in detail. Though the system is not frozen during the excitation process, a strong blockade effect is still observed similar to the cold atom system. We conclude the paper with a proposal for experimentally investigating the four-photon excitation process to the Rydberg state in thermal rubidium vapor.
The realization of nonlinear optical effects at the single photon level would be a significant step forward for quantum information processing and communications. In particular, a strong dispersive and non-dissipative nonlinearity could enable the implementation of a two-photon phase gate. One possible strategy to reach such huge nonlinearities is to convert photons into strongly interacting particles, like collective excitations involving Rydberg atoms. Interactions between Rydbergs in fact lead to a “blockade” phenomenon, where each Rydberg atom blocks the excitation of its neighbors, which can result in strong nonlinearities. Here, we use an ensemble of cold Rydberg atoms inside an optical cavity to create large dispersive nonlinearities on a weak probe beam far detuned from a D2-line transition in87Rb atoms [1]. The used three-level ladder scheme, with a second control field detuned from resonance on the upper transition towards a Rydberg level, is shown in Fig.1a). A simple explanation of the nonlinear effect is the following: if a very weak probe beam is injected into the cavity in the presence of the blue light on the two-photon transition, it experiences the single-atom three-level dispersion described by the real part of the three-level susceptibility χ-3level. This corresponds to a certain shift of the transmitted cavity peak (Fig. 1b)).
Single photon source is a non-classical light field with anti-bunching effect, which has a potential applications in the research of fundamental physics problems, quantum precision measurement, quantum communication, quantum computing, etc. The strong interaction between highly excited Rydberg atoms presents an excitation blockade effect. In a dense Rydberg atomic ensemble, the excitation of more than one Rydberg atom within a blockade volume is suppressed, where the interactions of Rydberg atoms shift the atomic states out of resonance with an excitation laser. We consider here the generation of single photon source by using a four-wave mixing scheme in a room-temperature atomic vapor cell. In a homemade micrometer-sized atomic vapor cell, one-dimensional size is smaller than the radius of Rydberg blockade and the other two-dimensional size is limited by the size of focused laser beam. The blockade radius is on the order of a few micrometers, depending on the Rydberg atom states. An excitation blockade effect can be used to realize single photon source in thermal cesium vapor microcells. The micron cesium-cell is used to spatially localize atomic groups, which results in the atomic decoherence time on the order of microseconds or even nanoseconds. This requires a high-power pulsed laser to prepare the Rydberg atomic state at a nanosecond scale. Four-photon excitation schemes with narrow linewidth lasers are also used experimentally. The cesium-Rydberg state can usually be excited by the lasers with optical wavelengths 852 and 509 nm, respectively. The laser system is well-stabilized so that the detuning is small compared with the spontaneous linewidth of Rydberg state, while the laser power and temporal mode need to be specified for ns-time coherence in thermal cesium vapor microcells. The 852 nm laser can be achieved by modulating the continuous laser beam with the help of an electro-optic intensity modulator (EOIM). While this remains a technical challenge for 509 nm laser with ns-laser pulse. There is no EOIM to generate the ns-laser pulse with high power. We demonstrate a novel generation method of 509 nm laser system. In our experiments, a 1018 nm fiber laser is used to produce a continuous laser with a typical linewidth of ～8 kHz and power of 10 mW. The nanosecond pulse is generated with the help of an electro-optic intensity modulator (EOIM) by modifying the continuous laser beam. The peak power of modulated optical pulse is amplified to 4600 W by using a homemade fiber amplifier. The output beam of 1018 nm is then injected into a periodically poled lithium-niobate (PPLN) to generate the second harmonics laser of 509 nm. The typical peak power of 509 nm reaches 173 W by optimizing PPLN phase matching parameters. The pulse repetition frequency of the 509 nm laser can be continuously tuned in a range of 300 kHz–100 MHz, and the pulse width can be continuously tuned in a range of 1–100 ns. Peak power fluctuation of the pulses is about 1.3%. The power 509 nm laser with optimized pulse parameters can be used to excite the cesium atom with GHz bandwidth. Meanwhile the two seed source lasers is well established experimentally, which allows alternating pulses with a different wavelength. This is an essential capability for realizing a single photon source through four-wave mixing.