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Rydberg Quantum Simulators

Periodic Reporting for period 1 - RYSQ (Rydberg Quantum Simulators)

Reporting period: 2015-03-01 to 2016-08-31

The central objective of the RySQ project is to implement and exploit Quantum Simulators based on Rydberg atoms (called Rydberg Quantum Simulators, RQS), because their outstanding versatility allows us to address not just one but a whole family of quantum simulations, by exploiting different aspects of the same experimental and theoretical tools. Unique features of laser excited Rydberg atoms are their long-range van-der-Waals or dipolar interactions, which are simultaneously very large, and entirely controllable by external fields. They offer therefore many different “modes of operation”, with either single atom or collective variables, dissipative, monitored and coherent dynamics, short and long range interactions, qubits and multi-level systems. Therefore, RQS provide a powerful toolbox for designing many-body quantum systems for quantum simulation, and to study static and dynamical behaviors, effects of dissipation, transport phenomena, applied to exotic and elusive phases of matter, including frustrated phases, lattice gauge theories, and non-equilibrium dynamics.
A first step of the project is to study the influence of technical imperfections and environmental noise on the performance of RQS, and to develop criteria to quantify their advantages in performing various computational tasks. These studies are important for assessing and extending the potential of Rydberg atom systems to perform viable computations and simulations of different complex problems in fundamental and applied physics, chemistry, and engineering. The promise of a successful RQS can only be realised if the right technology is available for its implementation; such technologies must be identified, and developed early, so that they can contribute as quickly as possible to the main goal
In practice, the accurate theoretical understanding and detailed experimental control of interactions between the constituents of a many-body quantum system are vital to the operation of quantum simulation platforms. Ultimately, the goal is to provide the foundation for unique types of quantum simulation platforms involving strong and long-ranged interactions that can be controlled both in space and time. This includes geometric control over the atomic system, a feature that is unique to long-range interacting systems, where the mutual interaction strength depends on the separation between the atoms, and can be controlled by applying electric and magnetic fields.
Real world systems always involve dissipation, which can even be correlated for Rydberg systems. Besides fundamentally interesting aspects of dissipatively driven phase transitions, one important aspect of the studies is the quantum simulation of quantum magnetism, which underlies many technologically relevant phenomena such as high temperature superconductivity. Another issue to engineer optimal platforms for preparing quantum many-body states far from equilibrium and to study the transport of excitations in a controlled way. While equilibrium states of physical systems are well understood, the understanding of non-equilibrium phenomena, in particular involving transport of energy, poses a deep challenge to modern science. By direct imaging of the Rydberg atoms, one can monitor the migration of excitations in a time and spatially resolved manner and control their dynamics. RQS will thus shed new light on energy transport in a many-body system coupled to an environment, similar to, e.g. light-harvesting systems in photosynthesis, which, in turn, might enable to develop novel devices, e.g. for with enhancing light conversion efficiencies.
Finally, dephasing and decay are the intrinsic dissipative processes prevalent in any open quantum system and the dominant mechanisms for the loss of coherence and entanglement. In the context of RQS, various approaches can be implemented for realizing controlled dissipation such as dissipation on the single atom level as well as dephasing into collective states, and to
During the first period of the project, benchmarks for RQS have been elaborated, and extensive numerical simulations of a quantum search algorithm have been performed for several Rydberg atoms subject to realistic dissipation. Also, the quantum Zeno dynamics of a cavity interacting with Rydberg atoms has been explored, and new techniques have been demonstrated to control the state of Rydberg atoms, and to implement quantum-enabled metrology. This includes tools to characterize electric fields close to surfaces, coherent population transfer close to a superconducting coplanar waveguide, and the coupling of spatially separated Rydberg atoms via a superconducting cavity at finite temperature. These studies are important for the development of interfaces between Rydberg atoms and superconducting resonators, which are very versatile tools for quantum state manipulations.
For interfacing Rydberg atoms and optical photons, we have demonstrated interaction between photons converted to Rydberg excitations in spatially separated atomic ensembles. Other studies include the quantum theory of 1D or 2D interacting Rydberg polaritons in an optical resonator, and an experimental approach for combined measurements of the spatially resolved optical spectrum and the Rydberg excitation number in an ultracold gas of atoms. Very significant progress were also made for exploiting higher angular momenta of the Rydberg atoms for encoding and processing information, for using atoms with more optically active electrons, and also for exciting Rydberg levels of charged particles (ions) in a trap. It was also shown how to use arrays of atoms for creating entanglement and simulating spins in lattices. In addition, the very promising approaches of Rydberg-dressing for coherently controlling extended range interactions has been demonstrated on a many-body level for the first time in experiments, using a novel laser system for strong excitation of Rydberg states. This has been backed up by theory beyond simplified models, and elaborate experiments that probe two- and few-body processes to an unprecedented level of detail. Another experimental development was a novel microtrap-based platform for the study of quantum magnetism, and the demonstration of coherent long-range interactions in optical lattices, and of impurity triggered dissipative processes.
On the theoretical side, optimal control schemes have been developed to optimally steer RQS to a defined target state and analytical formulas have been found to describe emerging correlations between Rydberg excitations. Important progress has been made also in the development of numerical methods to simulate the non-equilibrium dynamics of strongly coupled systems. In a combined theoretical-experimental effort, it became possible to establish ultracold Rydberg atoms as a system to study the dynamics of a spin system under the influence of long-range forces beyond equilibrium. It could be demonstrated also that photonic devices such as, e.g. photon transistors, can be realized with Rydberg atoms. Different facets of energy transport in Rydberg systems have been explored, showing that RQS represent an appropriate platform to simulate energy transport under extreme conditions. Insights gained from theoretical approaches to describe RQS have fruitfully been applied to other materials, thus translating the underlying concepts towards material science.
Overall, we significantly improved our understanding of driven dissipative systems. As an example we demonstrated that the appearance of non-equilibrium phase transitions in purely dissipative systems, and started to understand the impact quantum fluctuations have on driven dissipative systems. On the experimental side, impressive steps in controlling decoherence by influencing the decay of Rydberg excitations by depumping to a fast decaying intermediate state have been achieved.
Generally speaking, the results described above represent significant advances beyond the present state of the art, bringing closer the prospects of realizing functional quantum simulators, which will facilitate development of new materials and devices and exploitation of quantum technologies for the benefit of the society. The immediate impact will be felt within in the consortium and the academic community, as new directions emerge from these steps. In particular, the devised novel schemes to implementing and controlling spin-like interactions and collisional interactions through Rydberg-dressing of atomic and molecular states provide highly promising approaches to quantum simulations that are explored in experiments within the RySQ consortium, and stimulate substantial experimental efforts around the world. In this regard, the demonstration of Rydberg-dressing in an atomic lattice constitutes a major milestone at the current frontier of quantum simulations of spin systems.
Also, the demonstration of fully controlled Ising type quantum magnetism observable on the level of a single elementary spin defines the border of research on artificial magnets. Furthermore, system sizes have been reached, which are beyond the reach of simulations on classical computers, readily enabling quantum simulations of tailored magnetic Hamiltonians. Bottom-up approaches starting with well-controlled systems consisting of few atoms and photons can be compared to top-down settings, in which the collective response of the system is investigated, e.g. by monitoring the collective spin of the system for Heisenberg-like spin Hamiltonians. Finally, the potential of driven non-equilibrium systems with controlled dissipation as a novel type of quantum simulator has been demonstrated, and illustrates the benefits due to the strong interactions between Rydberg states.