Petar Jurcevic

Quasiparticle engineering and entanglement propagation in a quantum many-body system

The key to explaining and controlling a range of quantum phenomena is to study how information propagates around many-body systems. Quantum dynamics can be described by particle-like carriers of information that emerge in the collective behaviour of the underlying system, the so-called quasiparticles. These elementary excitations are predicted to distribute quantum information in a fashion determined by the system’s interactions. Here we report quasiparticle dynamics observed in a quantum many-body system of trapped atomic ions. First, we observe the entanglement distributed by quasiparticles as they trace out light-cone-like wavefronts. Second, using the ability to tune the interaction range in our system, we observe information propagation in an experimental regime where the effective-light-cone picture does not apply. Our results will enable experimental studies of a range of quantum phenomena, including transport, thermalization, localization and entanglement growth, and represent a first step towards a new quantum-optic regime of engineered quasiparticles with tunable nonlinear interactions.

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.

Measurement-based quantum computation with trapped ions.

Measurement-based quantum computation represents a powerful and flexible framework for quantum information processing, based on the notion of entangled quantum states as computational resources. The most prominent application is the one-way quantum computer, with the cluster state as its universal resource. Here we demonstrate the principles of measurement-based quantum computation using deterministically generated cluster states, in a system of trapped calcium ions. First we implement a universal set of operations for quantum computing. Second we demonstrate a family of measurement-based quantum error correction codes and show their improved performance as the code length is increased. The methods presented can be directly scaled up to generate graph states of several tens of qubits.

Experimental generation of quantum discord via noisy processes.

B. P. Lanyon,1, 2 P. Jurcevic,1, 2 C. Hempel,1, 2 M. Gessner,3, 4 V. Vedral,5, 6, 7 R. Blatt,1, 2 and C. F. Roos1, 2 1Institut für Quantenoptik und Quanteninformation, Österreichische Akademie der Wissenschaften, Technikerstr. 21A, 6020 Innsbruck, Austria 2 Institut für Experimentalphysik, Universität Innsbruck, Technikerstr. 25, 6020 Innsbruck, Austria 3 Department of Physics, University of California, Berkeley, California 94720, USA 4 Physikalisches Institut, Universität Freiburg, Hermann-Herder-Strasse 3, D-79104 Freiburg, Germany 5 Centre for Quantum Technologies, National University of Singapore, 3 Science Drive 2, 117543, Singapore 6 Clarendon Laboratory, Department of Physics, University of Oxford, Parks Road, Oxford OX1 3PU, U.K. 7Department of Physics, National University of Singapore, 2 Science Drive 3, 117542, Singapore (Dated: May 9, 2019)

Experimental violation of multipartite Bell inequalities with trapped ions.

We report on the experimental violation of multipartite Bell inequalities by entangled states of trapped ions. First, we consider resource states for measurement-based quantum computation of between 3 and 7 ions and show that all strongly violate a Bell-type inequality for graph states, where the criterion for violation is a sufficiently high fidelity. Second, we analyze Greenberger-Horne-Zeilinger states of up to 14 ions generated in a previous experiment using stronger Mermin-Klyshko inequalities, and show that in this case the violation of local realism increases exponentially with system size. These experiments represent a violation of multipartite Bell-type inequalities of deterministically prepared entangled states. In addition, the detection loophole is closed.

Entanglement-enhanced detection of single-photon scattering events

A highly efficient method is demonstrated for detecting individual photons scattering from short-lived transitions in single trapped ions. An entangled state is used to amplify the tiny momentum kick an ion receives on scattering a photon. Cat-state spectroscopy has an 18-fold higher measurement sensitivity than the direct detection method.

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)

Trapped ions for simulating interacting spins

Strings of laser-cooled trapped ions can be precisely controlled and manipulated with coherent narrow-band laser light. The use of entangling laser-ion interactions opens up the prospect of simulating the physics of interacting spins. I will present experiments that we have carried out with small ion crystals [1] and discuss the prospects of doing experiments with long ion strings.