Observation of dynamical quantum phase transition

Direct observation of dynamical quantum phase transitions in an interacting many-body system.
P. Jurcevic, H. Shen, P. Hauke, C. Maier, T. Brydges, C. Hempel, B. P. Lanyon, M. Heyl, R. Blatt, C. F. Roos
Phys. Rev. Lett. 119, 080501 (2017)
arXiv version: PDF

Recent years have seen a surge of interest in out-of-equilibrium quantum many-body phenomena across the physical sciences. This surge has been fuelled by rapid experimental advances, which now make it possible to study exotic phenomena ranging from many-body localization to particle-antiparticle creation in the Schwinger model and light-induced superconductivity. However, further progress is hampered by the fact that, as of yet, no unifying theoretical framework exists to understand such dynamical phenomena on a general level. The recently proposed theory of dynamical quantum phase transitions (DQPTs) [1] provides a promising concept towards achieving this major goal.

In this paper, we present the first experimental in-depth study of DQPTs. This is enabled by our ability to manipulate and measure individual particles in our experimental setup, a chain of trapped ions realizing a quantum many-body system with magnetic Ising interactions. In our experiment, we investigate the dynamics of a long-range transverse-field Ising interaction by encoding spin-1/2 particles in two electronic levels of an ion and engineering the spin-spin interaction by suitable laser pulses that create a state-dependent interaction. We prepare strings of up to 10 ions such that they are initially aligned with the spin-spin interaction part of the Hamiltonian before suddenly switching on the transverse field. We observe the resulting dynamics by measuring the particles time-dependent magnetization as well as multi-spin correlation functions. In this way, we directly observe the defining characteristic of DQPTs,i .e. nonanalyticities in the time-dynamics of a quantum many-body system, by projecting the state of the system onto the two ground states of the Ising Hamiltonian and measuring the resulting populations as a function of time. Moreover, we also investigate the DQPTs for a variety of system parameters and show how the dynamics of observables such as the magnetization is controlled by DQPTs. Finally, we also establish a link between DQPTs and entanglement growth by measuring half-chain entropies and spin squeezing in a chain of six particles.

There is also viewpoint published in Physics discussing our work: Quantum Phase Transitions Go Dynamical

Literature

[1] M. Heyl, A. Polkovnikov, and S. Kehrein, Phys. Rev. Lett. 110, 135704 (2013)

Financial support

This research was supported by the Austrian Academy of Science, the University of Innsbruck, the Austrian Science Fund FWF, the European Research Council (ERC) Synergy Grant No. UQUAM, the Deutsche Akademie der Naturforscher Leopoldina under Grant No. LPDR 2015-01, the Deutsche Forschungsgemeinschaft via the Gottfried Wilhelm Leibniz Prize program, and the Institut für Quanteninformation GmbH.

CR, September 2017