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Single strontium Rydberg ion confined in a Paul trap

Gerard T. Higgins, Weibin Li, Fabian Pokorny, Chi Zhang, Florian Kress, Maier, Christiane, Johannes Haag, Quentin Bodart, Igor Lesanovsky, Markus Hennrich

Published May 2, 2017• 41 citations
OpenAlex

Abstract

Trapped Rydberg ions are a promising new system for quantum information processing. They have the potential to join the precise quantum operations of trapped ions and the strong, long-range interactions between Rydberg atoms. Combining the two systems is not at all straightforward. Rydberg atoms are severely affected by electric fields which may cause Stark shifts and field ionization, while electric fields are used to trap ions. Thus, a thorough understanding of the physical properties of Rydberg ions due to the trapping electric fields is essential for future applications. Here we report the observation of two fundamental trap effects. First, we investigate the interaction of the Rydberg electron with the trapping electric quadrupole fields which leads to Floquet sidebands in the excitation spectra. Second, we report on the modified trapping potential in the Rydberg state compared to the ground state which results from the strong polarizability of the Rydberg ion. By controlling both effects we observe resonance lines close to their natural linewidth demonstrating an unprecedented level of control of this novel quantum platform.

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Control of Ion Motion Using Rydberg Excitation

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Trapped Rydberg ions are a novel approach to quantum information processing [1], [2]. Qubit rotations in the ion's lower lying electronic states are combined with entanglement operations that take advantage of strong Rydberg interactions [3]. In our experimental setup we excite trapped strontium ions from the metastable 4D state to the Rydberg manifold using a two photon excitation process. Certain properties of the ion become more prominent for highly excited states. One example is the polarizabilty which reacts to the surrounding electric field of the trapping electrodes. While effects due to the polarizability are negligible for lower lying states, they become more prominent for highly-excited states. This change leads to an altered trapping potential for the high lying Rydberg states, as shown in Fig. 1 [4]. For previous experiments those shifts have always been compensated for to perform, for example, sub-microsecond entangling gates between trapped ions [5]. However, this trapping field displacement can also be used to coherently control the ions' motion.

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Trapped Rydberg ions are a promising new system for quantum information processing. They have the potential to join the precise quantum operations of trapped ions and the strong, long-range interactions between Rydberg atoms. Combining the two systems is not at all straightforward. Rydberg atoms are severely affected by electric fields which may cause Stark shifts and field ionization, while electric fields are used to trap ions. Thus, a thorough understanding of the physical properties of Rydberg ions due to the trapping electric fields is essential for future applications. Here we report the observation of two fundamental trap effects. First, we investigate the interaction of the Rydberg electron with the trapping electric quadrupole fields which leads to Floquet sidebands in the excitation spectra. Second, we report on the modified trapping potential in the Rydberg state compared to the ground state which results from the strong polarizability of the Rydberg ion. By controlling both effects we observe resonance lines close to their natural linewidth demonstrating an unprecedented level of control of this novel quantum platform.
Trapped Rydberg ions are a promising new system for quantum information processing. They have the potential to join the precise quantum operations of trapped ions and the strong, long-range interactions between Rydberg atoms. Technically, the ion trap will need to stay active while exciting the ions into the Rydberg state, else the strong Coulomb repulsion will quickly push the ions apart. Thus, a thorough understanding of the trap effects on Rydberg ions is essential for future applications. Here we report the observation of two fundamental trap effects. First, we investigate the interaction of the Rydberg electron with the quadrupolar electric trapping field. This effect leads to Floquet sidebands in the spectroscopy of Rydberg D-states whereas Rydberg S-states are unaffected due to their symmetry. Second, we report on the modified trapping potential in the Rydberg state compared to the ground state which results from the strong polarizability of the Rydberg ion. We observe the resultant energy shifts as a line broadening which can be suppressed by cooling the ion to the motional ground state in the directions orthogonal to the excitation laser.
Summary form only given. Trapped Rydberg ions are a promising new approach to quantum information processing [1, 2]. This idea combines qubit encoding in the ions' ground states with entanglement operations via the Rydberg interaction. It promises to speed up entanglement operations and make them available in larger ion crystals.In this presentation, we will report on our experimental results with trapped strontium Rydberg ions. In particular, we have recently mapped the population from ground to the 42S1/2Rydberg state and back using stimulated Raman adiabatic passage (STIRAP) [3] - to our knowledge the first observed coherently manipulated Rydberg excitation of an ion. Moreover, we will present our observation of fundamental trap effects due to the interaction of the Rydberg ion with the electric trapping field [4]. A solid understanding of these effects is essential for future applications in quantum information processing.In the near future, we plan to excite the ions to even higher Rydberg states and to apply microwave radiation to induce large oscillating dipole moments. In this way, ions should be able to interact at a distance as required for quantum gates via the Rydberg interaction.
We observe an atomic ion's response to an electric quadrupole field to second and higher orders; this arises from the ion's electric quadrupole polarizability and hyperpolarizabilities.We probe a single 88 Sr + ion which is confined in the electric fields of a Paul trap and excited to Rydberg states.The quadrupolar trapping fields cause atomic energy level shifts and give rise to spectral sidebands.The effects on Rydberg S 1/2 states are described well by second-order perturbation calculations.The stronger effects on Rydberg P 1/2 states are captured by full diagonalization of the Hamiltonian.
Trapped Rydberg ions are a novel approach for quantum information processing [1,2]. This idea joins the advanced quantum control of trapped ions with the strong dipolar interaction between Rydberg atoms. For trapped ions this method promises to speed up entangling interactions [3] and to enable such operations in larger ion crystals [4]. We report on the first experimental realization of trapped strontium Rydberg ions. A single ion was confined in a linear Paul trap and excited to Rydberg states (25S to 37S) using a two-photon excitation with 243nm and 308nm laser light. The transitions we observed are narrow and the excitation can be performed repeatedly which indicates that the Rydberg ions are stable in the ion trap. Similar results have been recently reported on a single photon Rydberg excitation of trapped calcium ions [5]. The tunability of the 304-309nm laser should enable us to excite our strontium ions to even higher Rydberg levels. Such highly excited levels are required to achieve a strong interaction between neighboring Rydberg ions in the trap as will be required for quantum gates using the Rydberg interaction. References [1] M. Müller, L. Liang, I. Lesanovsky, P. Zoller, New J. Phys. 10, 093009 (2008). [2] F. Schmidt-Kaler, et al., New J. Phys. 13, 075014 (2011). [3] W. Li, I. Lesanovsky, Appl. Phys. B 114, 37-44 (2014). [4] W. Li, A.W. Glaetzle, R. Nath, I. Lesanovsky, Phys. Rev. A 87, 052304 (2013). [5] T. Feldker, et al., arXiv:1506.05958
Usually the influence of the quadratic Stark effect on an ion's trapping potential is minuscule and only needs to be considered in atomic clock experiments. In this work we excite a trapped ion to a Rydberg state with polarizability ∼8 orders of magnitude higher than a low-lying electronic state; we find that the highly polarizable ion experiences a vastly different trapping potential owing to the Stark effect. We observe changes in trap stiffness, equilibrium position, and minimum potential, which can be tuned using the trapping electric fields. These effects lie at the heart of several proposed studies, including a high-fidelity submicrosecond entangling operation; in addition we demonstrate these effects may be used to minimize ion micromotion. Mitigation of Stark effects is important for coherent control of Rydberg ions; we illustrate this by carrying out the first Rabi oscillations between a low-lying electronic state and a Rydberg state of an ion.
Trapped Rydberg ions are a novel approach to quantum information processing [1], [2]. Qubit rotations in the ion's lower lying electronic states are combined with entanglement operations that take advantage of strong Rydberg interactions [3]. In our experimental setup we excite trapped strontium ions from the metastable 4D state to the Rydberg manifold using a two photon excitation process. Certain properties of the ion become more prominent for highly excited states. One example is the polarizabilty which reacts to the surrounding electric field of the trapping electrodes. While effects due to the polarizability are negligible for lower lying states, they become more prominent for highly-excited states. This change leads to an altered trapping potential for the high lying Rydberg states, as shown in Fig. 1 [4]. For previous experiments those shifts have always been compensated for to perform, for example, sub-microsecond entangling gates between trapped ions [5]. However, this trapping field displacement can also be used to coherently control the ions' motion.