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arXiv:1509.01303 · Physical Review A

Large energy superpositions via Rydberg dressing

Mohammadsadegh Khazali, Hon Wai Lau, Adam Humeniuk, Christoph Simon

Published Aug 11, 2016• 33 citations
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Abstract

We propose to create superposition states of over 100 strontium atoms in a ground state or metastable optical clock state using the Kerr-type interaction due to Rydberg state dressing in an optical lattice. The two components of the superposition can differ by an order of 300 eV in energy, allowing tests of energy decoherence models with greatly improved sensitivity. We take into account the effects of higher-order nonlinearities, spatial inhomogeneity of the interaction, decay from the Rydberg state, collective many-body decoherence, atomic motion, molecular formation, and diminishing Rydberg level separation for increasing principal number.

DOI
10.1103/physreva.94.023408

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We propose to create superposition states of over 100 strontium atoms in a ground state or metastable optical clock state using the Kerr-type interaction due to Rydberg state dressing in an optical lattice. The two components of the superposition can differ by an order of 300 eV in energy, allowing tests of energy decoherence models with greatly improved sensitivity. We take into account the effects of higher-order nonlinearities, spatial inhomogeneity of the interaction, decay from the Rydberg state, collective many-body decoherence, atomic motion, molecular formation, and diminishing Rydberg level separation for increasing principal number.
This thesis describes Rydberg spectroscopy and dressing experiments in an ultracold strontium gas. The strontium atoms are cooled to sub-uK temperatures in a narrowline magneto-optical trap, where Rydberg atoms are created using a two-photon excitation scheme. This required the development of a high-power ultraviolet laser system at 319 nm. The laser has a large tuning range for access to triplet Rydberg states from principal quantum numbers of 35 to > 300. By performing Rydberg spectroscopy in a magneto-optical trap, we show that narrow spectra can be obtained where the line centre is determined to ~ 10 kHz. A frequency comb is then employed to make absolute frequency measurements of optical transitions to accuracies of < 2 MHz. Techniques are outlined for improving the accuracy further, showing that overall uncertainties on the order of 10 kHz can be obtained. The reliable measurement of Rydberg levels is important for studying the variation in quantum defect across Rydberg series, and hence improving the accuracy of atomic models. In this work we also develop a novel system in which to realise Rydberg dressing. By resonantly coupling the excited state of the cooling transition to a high lying Rydberg state, we create a Rydberg-dressed magneto-optical trap. We demonstrate that the atoms acquire Rydberg properties, while undergoing continuous cooling at ~ 1 uK. The lifetime of the trap is proved to be sufficiently long to observe interactions, however the interaction strength is currently limited by the non-uniform spatial profile of the dressing beam. Straightforward methods to overcome this limitation are presented. As such this work should lead to future experiments, whereby tuneable long-range interactions can be observed in the dynamics of the dressed magneto-optical trap.
Dipole-dipole interactions, the strongest, longest-range interactions possible between two neutral atoms, cannot be better manifested anywhere else than in a Rydberg atomic system. Rydberg atoms, having high principal quantum numbers n>>1 and dipole moments that scale as n^2, provide a powerful tool to examine dipole-dipole interactions. Therefore, we have studied the production and production rates of strontium Rydberg atoms created using two-photon excitation and have explored their properties in two distinct experiments. In the first experiment, very-high-n (n~300) Rydberg atoms are produced in a tightly collimated atomic beam allowing spectroscopic studies of their energy levels and their Stark effects. Simulations using a two-active-electron model, developed by our theoretical collaborators, allow detailed analysis of the results and are in remarkable agreement with the experimental results. The high density of Rydberg atoms achieved ~ 5*10^5 cm^(-3), in this experiment will allow studies of strongly interacting Rydberg-Rydberg systems. The second experiment, in which a cold strontium Rydberg gas is excited in a magneto-optic trap, features an imaging technique offering both spatial and temporal resolution. We use this technique to observe and study the evolution of an ultra-cold strontium Rydberg gas which reveals the importance of Rydberg-Rydberg interactions in the early stages of this evolution. Strongly interacting Rydberg gas provides an opportunity to realize a very strongly-correlated ultra-cold plasma.
This paper is a proposal for the generation of a many-body entangled state in atomic and mechanical systems. Here the detailed feasibility study shows that application of a strong Rydberg dressing interaction and a fast bifurcation scheme in a Bose-Einstein condensate of Rb atoms, results in the formation of large cat states. By detailed study of the decoherence effects using the quantum jump Monte Carlo approach and taking into account the obstacles like collective decoherence and level mixing, this proposal predicts the formation of a 700-atom cat state. Subsequent transfer of the generated superposition to far separated mechanical oscillators is proposed, using dipole coupling between Rydberg atoms and charged cantilevers.
We explore the prospects for confining alkaline-earth Rydberg atoms in an optical lattice via optical dressing of the secondary core–valence electron. Focussing on the particular case of strontium, we identify experimentally accessible magic wavelengths for simultaneous trapping of ground and Rydberg states. A detailed analysis of relevant loss mechanisms shows that the overall lifetime of such a system is limited only by the spontaneous decay of the Rydberg state, and is not significantly affected by photoionization or autoionization. The van der Waals C 6 coefficients for the Sr(5s n s 1 S 0 ) Rydberg series are calculated, and we find that the interactions are attractive. Finally we show that the combination of magic-wavelength lattices and attractive interactions could be exploited to generate many-body Greenberger–Horne–Zeilinger states.
Cat states are maximally entangled states with applications in metrology and fault-tolerant quantum computation. Experiments have revealed that Rydberg collective avalanche decoherence acts as a bottleneck for cat creation with Rydberg atoms. This process initiates after the blackbody-radiation-induced decay of Rydberg atoms and sets a strong limit on the cat creation time. These findings necessitate the exploration of new ideas to accelerate current Rydberg cat schemes. To enhance the interaction-to-loss ratio, this paper delves into cat-state formation in the strong-Rydberg-dressing regime, uncovering the emergence of cat states despite the presence of complex orders of nonlinearities. This unexplored regime demonstrates the potential for rapid cat-state formation, which is particularly beneficial for operation in typical two-dimensional lattices in Rydberg laboratories. In an extreme case, this paper demonstrates that second-order nonlinearity could be isolated under resonant Rydberg driving if a large number of atoms are accommodated inside the blockade volume. The resonant model significantly enhances the interaction-to-loss ratio while circumventing the adiabaticity condition, allowing fast switching of lasers. In addition, this paper presents a method for generating multicomponent cat states, which are superpositions of mmm coherent spin states ($|m\text{\ensuremath{-}}\text{CSS}\ensuremath{\rangle}$). The maximum value of mmm is determined by the number of atoms within the blockade radius, where m=Nm=\sqrt{N}m=N​. The states with larger mmm are more robust against the presence of multiple orders of nonlinearity in the strong-dressing Hamiltonian and are accessible in a much shorter time compared to traditional two-component cat states.
We show that optical spectroscopy of Rydberg states can provide accurate in situ thermometry at room temperature. Transitions from a metastable state to Rydberg states with principal quantum numbers of 25-30 have 200 times larger fractional frequency sensitivities to blackbody radiation than the strontium clock transition. We demonstrate that magic-wavelength lattices exist for both strontium and ytterbium transitions between the metastable and Rydberg states. Frequency measurements of Rydberg transitions with 10(-16) accuracy provide 10 mK resolution and yield a blackbody uncertainty for the clock transition of 10(-18).