A trapped ion quantum computer usually uses the motional modes of the ion crystal to exchange quantum information and to generate entanglement. As the number of ions in a trap is increased, the motional modes become progressively more complicated and their energy spacing is reduced. This makes the entangling operation increasingly difficult. An alternative method for a trapped-ion quantum computer may be realised by exciting some of the ions to Rydberg states. The large polarizability of Rydberg ions modifies the trapping potential thus allows tailoring the motional modes for entanglement operations [1]. Additionally entangling gates may be realised using the dipole interaction between Rydberg ions, without using motional modes at all [2,3].
A trapped ion quantum computer usually uses the motional modes of the ion crystal to exchange quantum information and to generate entanglement. As the number of ions in a trap is increased, the motional modes become progressively more complicated and their energy spacing is reduced. This makes the entangling operation increasingly difficult. An alternative method for a trapped-ion quantum computer may be realised by exciting some of the ions to Rydberg states. The large polarizability of Rydberg ions modifies the trapping potential thus allows tailoring the motional modes for entanglement operations [1]. Additionally entangling gates may be realised using the dipole interaction between Rydberg ions, without using motional modes at all [2,3].
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.
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.
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
Combining the strong, long-range interactions of cold Rydberg atoms with the controllability of trapped ions, trapped Rydberg ions provide a promising platform for scalable quantum information processing. As demonstrated in a breakthrough experiment [1], microwave dressing of Rydberg states induces permanent rotating dipole moments leading to strong interactions between highly excited ions. Due to the separation of timescales, the fast electronic dynamics of Rydberg ions decouple from the slower motional modes of the linear Coulomb crystal, which typically mediate entangling gates in ground-state ion systems. Therefore, Rydberg ions enable significantly faster gate operations. We demonstrate how the unique properties of trapped Rydberg ions can be exploited to realize fast and high-fidelity entangling gates, along with the associated challenges and strategies to overcome them. Different types of gate protocols for two- and three-qubit entangling gates with trapped Rydberg ions are presented and sources of infidelity are analyzed.
Trapped Rydberg ions are a promising new platform for quantum computation and simulation. They combine the exquisite control of trapped ion systems with the strong dipole-dipole interactions available in Rydberg atom systems. In order to realize strong interactions between Rydberg ions, microwave dressing is required to induce strong oscillating dipoles. Additionally, microwave dressing of Rydberg states with polarizabilities of opposite sign allows for the realization of Rydberg states with vanishing polarizability. Such states are insensitive to external electric fields and can be trapped independently of their state. Vanishing-polarizability Rydberg states allow for implementing Rydberg entangling gates in large ion crystals.
We present a method for measuring the polarizability of Rydberg ions confined in the harmonic potential of a Paul trap. For a highly excited electronic state, the coupling between the electronic wave function and the trapping field modifies the excitation probability depending on the motional state of the ion. This interaction strongly depends on the polarizability of the excited state and manifests itself in the state-dependent secular frequencies of the ion. We initialize a single trapped 40Ca+ ion from the motional ground state into coherent states with ∣α∣ up to 12 using electric voltages on the trap segments. The internal state, firstly initialised in the long-lived 3D5/2 state, is excited to a Rydberg S1/2-state via the 5P3/2 state in a two-photon process. We probe the depletion of the 3D5/2 state owing to the Rydberg excitation followed by a decay into the internal ground 4S1/2 state. By analysing the obtained spectra we extract the polarizability of Rydberg states which agree with numerical calculations. The method is easy-to-implement and applicable to different Rydberg states regardless of their principal or angular quantum numbers. An accurate value of the state polarizability is needed for quantum gate operations with Rydberg ion crystals.