Christine Maier

Direct observation of dynamical quantum phase transitions in an interacting many-body system

The theory of phase transitions represents a central concept for the characterization of equilibrium matter. In this work we study experimentally an extension of this theory to the nonequilibrium dynamical regime termed dynamical quantum phase transitions (DQPTs). We investigate and measure DQPTs in a string of ions simulating interacting transverse-field Ising models. During the nonequilibrium dynamics induced by a quantum quench we show for strings of up to 10 ions the direct detection of DQPTs by revealing nonanalytic behavior in time. Moreover, we provide a link between DQPTs and the dynamics of other quantities such as the magnetization, and we establish a connection between DQPTs and entanglement production.

Spectroscopy of Interacting Quasiparticles in Trapped Ions.

The static and dynamic properties of many-body quantum systems are often well described by collective excitations, known as quasiparticles. Engineered quantum systems offer the opportunity to study such emergent phenomena in a precisely controlled and otherwise inaccessible way. We present a spectroscopic technique to study artificial quantum matter and use it for characterizing quasiparticles in a many-body system of trapped atomic ions. Our approach is to excite combinations of the systems fundamental quasiparticle eigenmodes, given by delocalized spin waves. By observing the dynamical response to superpositions of such eigenmodes, we extract the system dispersion relation, magnetic order, and even detect signatures of quasiparticle interactions. Our technique is not limited to trapped ions, and it is suitable for verifying quantum simulators by tuning them into regimes where the collective excitations have a simple form.

Efficient tomography of a quantum many-body system

Traditionally quantum state tomography is used to characterize a quantum state, but it becomes exponentially hard with the system size. An alternative technique, matrix product state tomography, is shown to work well in practical situations. Quantum state tomography is the standard technique for estimating the quantum state of small systems1. But its application to larger systems soon becomes impractical as the required resources scale exponentially with the size. Therefore, considerable effort is dedicated to the development of new characterization tools for quantum many-body states2,3,4,5,6,7,8,9,10,11. Here we demonstrate matrix product state tomography2, which is theoretically proven to allow for the efficient and accurate estimation of a broad class of quantum states. We use this technique to reconstruct the dynamical state of a trapped-ion quantum simulator comprising up to 14 entangled and individually controlled spins: a size far beyond the practical limits of quantum state tomography. Our results reveal the dynamical growth of entanglement and describe its complexity as correlations spread out during a quench: a necessary condition for future demonstrations of better-than-classical performance. Matrix product state tomography should therefore find widespread use in the study of large quantum many-body systems and the benchmarking and verification of quantum simulators and computers.

Electromagnetically-induced-transparency ground-state cooling of long ion strings

Regina Lechner, 2 Christine Maier, 2 Cornelius Hempel∗,1, 2 Petar Jurcevic, 2 Ben P. Lanyon, 2 Thomas Monz, Michael Brownnutt†,2 Rainer Blatt, 2 and Christian F. Roos‡1, 2 Institut für Quantenoptik und Quanteninformation, Österreichische Akademie der Wissenschaften, Technikerstraße 21a, 6020 Innsbruck, Austria Institut für Experimentalphysik, Universität Innsbruck, Technikerstraße 25, 6020 Innsbruck, Austria (Dated: April 5, 2016)

Coherent rydberg excitation of a single trapped ion

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.

Rydberg excitation of trapped strontium ions(Conference Presentation)

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

Two — Photon Rydberg excitation of trapped strontium ions

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].