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AQuS Report Summary

Project ID: 640800
Funded under: H2020-EU.1.2.2.

Periodic Reporting for period 1 - AQuS (Analog quantum simulators for many-body dynamics)

Reporting period: 2015-01-01 to 2016-06-30

Summary of the context and overall objectives of the project

Quantum simulators promise to provide unprecedented insights into physical phenomena not accessible with classical computers and have the potential to enable radically new technologies. Analog dynamical quantum simulators constitute a most promising class of architectures to fulfil the ultimate promise to devise quantum machines outperforming classical computers. The AQuS project undertakes a two-fold approach: On the one hand, we devise versatile and practical platforms for dynamical simulators – making use of systems of ultra-cold atoms in optical lattices and the continuum, as well as cavity polaritons. We conduct an interdisciplinary research programme of certifying quantum devices and assess them in their computational capabilities, addressing largely unexplored key questions on the power of quantum simulators. On the other, we make use of those devices to probe important questions in fundamental and applied physics, ranging from technology-relevant problems, concerning transport processes or glassy dynamics, via long-standing challenges in the physics of non-equilibrium and thermalisation phenomena, through puzzles in notions of quantum turbulence, to questions in the study of quantum gravity.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The central objectives of the AQuS project are to lay the grounds for experimentally feasible analog quantum simulation, focusing on three promising platforms: Ultracold atoms in optical lattices and the continuum as well as cavity polaritons. During the first reporting period strong progress was made in all three experimental fields, complemented by extensive support and complementary developments on the theory side.

Both, experimental and theoretical efforts were focused, within a second line of objectives, on working out measures of certification of quantum simulators, aspects of complexity and robustness, as well as exploring the implications of universal properties. The key aim was to find ways how to build trust in quantum simulators. For this, specific analog quantum simulators, including low-dimensional cold gases in lattices and uniform traps, and honeycomb polariton lattices, were certified by comparing observables with available theoretical predictions, up to high-order correlation functions. This helped to build trust for the use of these simulators in extended geometries where no clear theoretical predictions are feasible. An example, where this could be explored are identical experiments on many-body localisation in single and coupled one-dimensional lattice systems.

Much progress was made on the implications of universality for quantum simulators. On the theory side, different avenues have been taken, approaching the scaling properties with analytical field theoretical methods, numerical simulations, and holographic calculations.

The third line of objectives aims at the use of the platforms built to quantum simulate intricate dynamical many-body phenomena. Experiment and theory developed methods to identify universal scaling properties in systems quenched suddenly close to critical configurations, studying in particular the properties of the self-similar dynamical evolution following a quench. Ongoing experimental efforts show first promising results pointing towards the identification on non-thermal fixed points and the corresponding universal scaling functions.

Prethermalisation to states well described by a General Gibbs ensemble were demonstrated in low-dimensional cold gas experiments and characterised by means of phase-correlation functions up to tenth order. Extending these setups, ground-state properties of a quantum sine-Gordon model could be simulated, and the relevant eigenmodes be identified by means of analysing the measured correlations.

Quenches in non-resonantly pumped polariton condensates as well as quenches in quantum fluids of light in propagating geometries were studied theoretically, the latter towards the development of simulators of analog gravity phenomena. Polariton systems, in a joint experiment-theory effort were used to realize edge states in the s-wave band as well as the much more complicated p-wave case.

On the basis of the cold-gas lattice platform a first successful realization of many-body localisation could be presented within a geometry of uncoupled one-dimensional chains. The system allowed to study the dimensional crossover to coupled chains and to identify the sources of breakdown of localised states. The same system could be used to rigorously prove information propagation over arbitrary distances. In a further simulation experiment, ballistic transport in a one-dimensional system of hardcore bosons could be demonstrated when melting the initial Mott insulator state, and scaling beyond the simple Kibble-Zurek prediction for slow ramps was observed.

Finally, extensive efforts were made towards the quantum simulation of Hawking radiation in analog gravity settings based on atomic as well as polaritonic systems. While a large part of the studies was done on the theory side, a configuration involving two counterpropagating polariton fluids could be realised in experiment, which could lead to the formation of successive acoustic horizons for the polariton fluid.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

The AQuS project so far has made considerable progress towards the expected impacts. The development of a variety of different platforms has helped towards the expected impact of establishing quantum simulators as a quantum computational procedure going beyond classical computational capabilities. A large part of the progress has been building the experimental tools and comparing specific applications with viable theoretical computations. The decisive steps achieved in this context concern the detailed understanding of the observations, down to the experimental noise level, which is necessary to build trust in the respective setups for use in more intricate configurations where no theoretical results are available. On the basis of the developed simulator platforms, a series of demanding challenges in fundamental physics could be addressed, including aspects of relaxation and thermalization, universal scaling dynamics, many-body localization, and edge states in polariton lattices. These experiments as well as the theoretical results gained in the AQuS context have a strong impact on the international scientific communities studying ultra cold gases, quantum fluids, statistical physics, as well as solid-state and high-energy nuclear physics. From the technological point of view, the experiments have a high potential impact in the development of integrated optical physics and concomitant technological development, in particular for ICT-related technologies. A further important socio-economic impact of the work within AQuS arises from the training of highly skilled researchers (master students, doctoral students, postdocs) who regularly move on to take up responsibility in the high-technology industry, in research and development as well as in management areas.

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