Highlights

Petar Jurcevic et al., PRL 119, 080501

Today, the equilibrium properties of quantum matter are theoretically described with great success. Yet, in recent years pioneering experiments have created quantum states beyond this equilibrium paradigm. Understanding properties of such nonequilibrium quantum states on a general level provides a significant challenge. Jurcevic at el. have now experimentally observed for the first time a phenomenon - a dynamical quantum phase transition - which promises to provide a general framework to identify and predict such general principles of quantum dynamics.

See also the Physics Viewpoint: Quantum Phase Transitions Go Dynamical

for his seminal contributions to nonlinear physics

for his outstanding contribution in the area of statistical physics and stochastic thermodynamics.

H.A.M. Leymann et al., Phys. Rev. X 7, 021045 (2017)

We investigate the switching of the coherent emission mode of a bimodal microcavity device, occurring when the pump power is varied. We compare experimental data to theoretical results and identify the underlying mechanism based on the competition between the effective gain, on the one hand, and the intermode kinetics, on the other. When the pumping is ramped up, above a threshold, the mode with the largest effective gain starts to emit coherent light, corresponding to lasing. In contrast, in the limit of strong pumping, it is the intermode kinetics that determines which mode acquires a large occupation and shows coherent emission. We point out that this latter mechanism is akin to the equilibrium Bose-Einstein condensation of massive bosons. Thus, the mode switching in our microcavity device can be viewed as a minimal instance of Bose-Einstein condensation of photons. Moreover, we show that the switching from one cavity mode to the other always occurs via an intermediate phase where both modes are emitting coherent light and that it is associated with both superthermal intensity fluctuations and strong anticorrelations between both modes.

Arnab Banerjee et al. Science 356, 1055 (2017)

The Kitaev quantum spin liquid (KQSL) is an exotic emergent state of matter exhibiting Majorana fermion and gauge flux excitations. The magnetic insulator ?-RuCl3 is thought to realize a proximate KQSL. We used neutron scattering on single crystals of ?-RuCl3 to reconstruct dynamical correlations in energy-momentum space. We discovered highly unusual signals, including a column of scattering over a large energy interval around the Brillouin zone center, which is very stable with temperature. This finding is consistent with scattering from the Majorana excitations of a KQSL. Other, more delicate experimental features can be transparently associated with perturbations to an ideal model. Our results encourage further study of this prototypical material and may open a window into investigating emergent magnetic Majorana fermions in correlated materials.

See also: A. Banerjee et al. Nature Materials, 15, 733 (2016).

R. Moessner and S. L. Sondhi, Nature Physics 13, 424 (2017)

Equilibrium thermodynamics is characterized by two fundamental ideas: thermalization—that systems approach a late time thermal state; and phase structure—that thermal states exhibit singular changes as various parameters characterizing the system are changed. We summarize recent progress that has established generalizations of these ideas to periodically driven, or Floquet, closed quantum systems. This has resulted in the discovery of entirely new phases which exist only out of equilibrium, such as the ?-spin glass/Floquet time crystal.

This is a progress article. For related original work see: Phase Structure of Driven Quantum Systems, Khemani et al. Phys. Rev. Lett. 116, 250401 (2016).

Shai R Joseph et al. eLife 2017;6:e23326

Upon fertilization, the genome of animal embryos remains transcriptionally inactive until the maternal-to-zygotic transition. At this time, the embryo takes control of its development and transcription begins. How the onset of zygotic transcription is regulated remains unclear. Here, we show that a dynamic competition for DNA binding between nucleosome-forming histones and transcription factors regulates zebrafish genome activation. Taking a quantitative approach, we found that the concentration of non-DNA-bound core histones sets the time for the onset of transcription. The reduction in nuclear histone concentration that coincides with genome activation does not affect nucleosome density on DNA, but allows transcription factors to compete successfully for DNA binding. In agreement with this, transcription factor binding is sensitive to histone levels and the concentration of transcription factors also affects the time of transcription. Our results demonstrate that the relative levels of histones and transcription factors regulate the onset of transcription in the embryo.

André Eckardt, Rev. Mod. Phys. 89, 011004 (2017)

Time-periodic forcing in the form of coherent radiation is a standard tool for the coherent manipulation of small quantum systems like single atoms. In the last years, periodic driving has more and more also been considered as a means for the coherent control of many-body systems. In particular, experiments with ultracold quantum gases in optical lattices subjected to periodic driving in the lower kilohertz regime have attracted much attention. Milestones include the observation of dynamic localization, the dynamic control of the quantum phase transition between a bosonic superfluid and a Mott insulator, as well as the dynamic creation of strong artificial magnetic fields and topological band structures. This Colloquium reviews these recent experiments and their theoretical description. Moreover, fundamental properties of periodically driven many-body systems are discussed within the framework of Floquet theory, including heating, relaxation dynamics, anomalous topological edge states, and the response to slow parameter variations.

We are glad to announce the arrival of Dr. Francesco Piazza, who heads the research group 'Strongly Correlated Light-Matter Systems'. The group will investigate the many-body properties of peculiar quantum “plasmas” of photons and atoms.

D. Zwicker, R. Seyboldt, C. A. Weber, A. A. Hyman and F. Jülicher, Nature Physics (2016)

We show that liquid droplets that are driven away from thermodynamic equilibrium by chemical reactions can undergo cycles of growth and division reminiscent of living cells. We propose such active droplets as simple models for prebiotic protocells. Our work shows that protocells could have been able to propagate and divide without having established membranes.

See also coverage in Chemistry World and two articles in Quanta Magazine: article 1 and article 2.