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arXiv:1408.1551 · Journal of Physics B Atomic Molecular and Optical Physics

Subluminal light propagation through an ultracold atom–molecule coupled resonant medium

Sandeep Sharma, Bimalendu Deb, Tarak Nath Dey

Published Jul 21, 2015• 2 citations
arXivHTMLPDF

Abstract

Abstract We theoretically investigate light propagation through a Feshbach-resonant atom–molecule coupled system. We show that when a diatomic molecular state in an electronically ground-state potential is coupled to the continuum of Feshbach-resonant atom–atom scattering states via an excited molecular state with a control (strong) and a probe (weak) laser field, the dispersive behaviour of the medium is significantly influenced by the Feshbach resonance. In particular, a Feshbach resonance helps to make the dispersion characteristics quite sharp, leading to the subluminal propagation of the probe. Our model calculations with realistic parameters show that it is possible to reduce the group velocity to 50 m s −1 with nearly 80% transmission. Furthermore, we demonstrate that it is possible to achieve significant enhancement of the delay-bandwidth product (DBP) of the probe by narrowing the width of the resonance, opening up new prospects for using a resonant atom–molecule coupled medium for efficient slow-light production and applications.

DOI
10.1088/0953-4075/48/17/175502

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Abstract We theoretically investigate light propagation through a Feshbach-resonant atom–molecule coupled system. We show that when a diatomic molecular state in an electronically ground-state potential is coupled to the continuum of Feshbach-resonant atom–atom scattering states via an excited molecular state with a control (strong) and a probe (weak) laser field, the dispersive behaviour of the medium is significantly influenced by the Feshbach resonance. In particular, a Feshbach resonance helps to make the dispersion characteristics quite sharp, leading to the subluminal propagation of the probe. Our model calculations with realistic parameters show that it is possible to reduce the group velocity to 50 m s −1 with nearly 80% transmission. Furthermore, we demonstrate that it is possible to achieve significant enhancement of the delay-bandwidth product (DBP) of the probe by narrowing the width of the resonance, opening up new prospects for using a resonant atom–molecule coupled medium for efficient slow-light production and applications.
We report on a proposal to reduce the group velocity of a small ``laser'' Bose-Einstein condensate upon passing through a larger ``medium'' condensate of the same isotopic species in analogy to slowing of light passing through dispersive media. We make use of ultracold collisions near a magnetic Feshbach resonance, which gives rise to a sharp variation in scattering length with collision energy and thereby changes the group velocity. A generalized Gross-Pitaevskii equation is used to derive an expression for the group velocity in a homogeneous medium as well as for the spatial delay $\ensuremath{\delta}$, through a finite-sized medium BEC with a Thomas-Fermi density distribution. We predict a maximum reduction of the group velocity by a half, which physically corresponds to freely propagating Feshbach molecules. For typical narrow Feshbach resonances and a medium with number density 1015{10}^{15}1015 cm${}^{\ensuremath{-}3}$, up to 8585%85 of the lower bound can be achieved, making the effect experimentally observable. We also derive constraints on the experimental realization of our proposal.
This thesis reports on the first observation of optical tuning of interactions in an ultracold atomic gas. To control the elastic interactions, we use optically induced scattering resonances. They are realized by coupling pairs of atoms to a bound molecular state with the help of laser light. In comparison to the well-known magnetic Feshbach resonances, these optical Feshbach resonances allow a very fast switching of interactions in a spatially resolved way. This is because laser light can be switched on and off much faster than magnetic fields and because the light intensity can be spatially structured. Our experiments are performed with a Bose-Einstein condensate (BEC) of Rb atoms, which we produce in our new BEC setup. This apparatus has been constructed in the framework of this thesis and offers excellent optical access to the ultracold atom cloud. To investigate the tunability of atomic interactions with single-frequency laser light, we optically couple pairs of free atoms to an electronically excited molecular state. The coupling is achieved with a laser that is tuned close to the respective free-bound photoassociation transition. By changing the laser detuning we are able to adjust the atomic scattering length to any value between 10 a0 and 190 a0, where a0 denotes the Bohr radius. The atomic scattering length is a single quantity that fully describes ultracold elastic interactions. Since laser induced atom losses restrict the duration of our experiments, Bragg spectroscopy is employed as a fast method to measure the scattering length. We also demonstrate that optical Feshbach resonances can be realized with a two-color Raman coupling scheme. To implement this scheme, a second laser is added that couples the excited molecular state to a weakly bound molecular level in a ground state potential. Tuning the second laser results in a modification of the atomic scattering length similar to the one observed with a single laser. In addition, this scheme allows to control the width of the resonance via the detuning of the first laser. For the one-color and two-color coupling schemes, we investigate how the elastic scattering properties depend on the different available parameters, i.e. the laser detunings and intensities. Our experimental findings can successfully be described with an available theoretical model, which treats ultracold collisions in the presence of laser light. Optical Feshbach resonances have the possibility of becoming a valuable tool for fast switching of interactions. This is of special interest in quantum computation schemes with neutral atoms, where interactions between different atomic qubits need to be controlled.
It is crucial to control the collision between ultracold atoms by applying external fields. We developed a theoretical model for investigating the s-wave scattering of ultracold atoms controlled by the magnetic field and laser field. The calculation is performed by using the close-coupling method and mapped Fourier grid method. Due to the interference between the photoassociation and bound-to-bound transitions, the bound state in the continuum, which is a resonance with a vanishing width, occurs at the magnetic field position near the magnetic Feshbach resonance. The widths of resonances in the neighborhood of the bound state in the continuum are narrow. Changing the laser intensity can shift the magnetic field position where the bound state in the continuum occurs through modifying the ground molecular state to induce wide resonances at desired magnetic field positions. By increasing the resonance width, the tunability of the real part of the scattering length at resonances can be significantly improved. Changing the laser intensity can also adjust the coupling between the ground and excited molecular states. When the coupling between the ground and excited molecular states approaches zero, a resonance is induced, and the photoassociation and bound-to-bound transitions are both significantly suppressed at this resonance. Therefore, the atomic loss peak due to spontaneous emission does not appear at this resonance. The magnetic field position of this resonance is stable against the change in laser frequency.
We develop a theoretical description of photoassociative stimulated Raman adiabatic passage (STIRAP) near a Feshbach resonance in a thermal atomic gas. We show that it is possible to use low-intensity laser pulses to directly excite pairs of atoms in the continuum near a Feshbach resonance and to transfer most of the atomic cloud to the lowest rovibrational level of the molecular ground state. For a broad resonance, commonly found in several mixtures of alkali atoms, our model predicts a transfer efficiency up to 97% for a given atom pair, and up to 70% when averaged over an atomic ensemble. The laser intensities and pulse durations needed for such an optimal transfer, 10 2 –10 3 W cm −2 and several microseconds, are easily achievable experimentally. A single pair of STIRAP pulses converts an estimated fraction f ∼10 −6 –10 −4 of atoms in an atomic ensemble, leading to the production of 10–1000 molecules in a large sample of 10 7 atoms. A total of ∼10 4 –10 6 pulse pairs are thus required to transfer most atoms into molecules. Such an efficiency compares with or surpasses currently available techniques for creating stable diatomic molecules, and the versatility of this approach simplifies its potential use for many molecular species.
Abstract The propagation of a weak probe field through a four-level inverted-Y type atomic system driven by two additional coherent fields, namely the control field and the repump field, is studied. The simulated probe absorption shows that electromagnetically induced transparency and the line shape of the transparency window under a Doppler-broadened condition are dependent on the residual Doppler averaging due to the wavelength mismatch between the control and probe fields. For 87 Rb the probe propagation changes from subluminal on-resonance under a Doppler-free condition to superluminal on-resonance under a Doppler-broadened condition. For both 87 Rb and 133 Cs, the narrow subluminal window occurring in the group index profile in a Doppler-free environment offers an excellent delay bandwidth for probe field propagation. Under this condition, the order of the delay in probe field propagation can be as long as a few hundred µ s and seems quite promising in the field of optical delay generation and light storage. The analysis shows that the residual Doppler averaging has a greater impact on the switching window under the variation of the Rabi frequency of the control field as compared to that of the repump field.
We present a theoretical analysis of the optical tuning of the scattering length in a Bose-Einstein condensate in which ultracold atoms in the ground state are exposed to laser light which is off-resonant with the transition frequency to an open-channel excited state. The scattering length can be altered by changing the intensity and frequency of the laser. Experimentally relevant systems are studied in detail, with particular reference to the possibility of Feshbach molecule creation in these systems.