Quantum Simulations in Arbitrary Ion Lattices (Q-SAIL)

In recent years proof-of-principle experiments have shown that trapped ions provide a viable system for quantum simulations. By increasing the versatility of the trapping potential beyond one-dimensional harmonic wells, one can realize ion lattices which would open up new possibilities for simulations. By this means simulations could be accomplished which are otherwise infeasible, even on current universal quantum simulators. Here we present a research plan to demonstrate simulations in traps with arbitrary potentials; firstly using higher-order anharmonic terms, and secondly using higher dimensionality. In the first instance we propose to use standard segmented traps in a new mode of operation: anharmonic, rather than harmonic, confinement. In this system we will develop the control and cooling techniques which will be required in more advanced traps. We will then develop novel traps to create periodic potentials, for a highly versatile, scalable, and robust simulator architecture. In these traps we will demonstrate the basic units of a quantum simulation in higher-order potentials. In parallel with this work we will develop traps for two-dimensional arrays of ions, facilitating qualitatively new ways to implement quantum simulations. Having previously demonstrated the basic principles in a millimeter-sized system, we will reduce the trap dimensions by an order of magnitude, allowing coherent quantum interactions to be performed. We will develop the attendant electronics and optics to allow scalability of the system to many ions, and we will demonstrate the basic elements of a quantum simulation in two dimensions. The theory of simulations in arbitrary lattices is currently a rapidly developing field. It is envisioned that both the one- and two-dimensional systems from this proposal will be used for simulations of lattice gauge theories and vacuum decay. Beyond this we will work closely with theorists to investigate new possibilities generated by our novel architectures. The funding applied for in this application is to provide salaries for two Ph.D. students to carry out the research, and to cover the running costs of the experiments.

The Q-SAIL project was aimed at the development of new ion trapping configurations for quantum simulation. Until now, quantum simulation experiments in this field have relied on a single linear string of ions. In order to drastically extend quantum simulation capabilities, it is necessary to move beyond this simple but limited configuration. One extension consists in holding each ion of the string in a separated, controllable potential well, which allows to effectively switch on and off the interaction between individual ions. Another possibility is to trap the ions in a two-dimensional array, with which solid state systems can be simulated much more efficiently. Both of these new 1D and 2D ion configurations require the use of microfabricated traps with specially designed electrode patterns. As a starting point for the quantum manipulation of large 1D ion chains, we demonstrated the simultaneous laser cooling of up to 18 ions, freezing all their possible radial movements in their lowest quantum state. In parallel, we developed microfabricated ion traps which were produced by our collaborators at the Fachhochschule Vorarlberg (FHV). In a 1D trap fabricated on a high-purity silicon substrate and operated at T ~ 10 K, we could keep an ion in its motional quantum ground state for more than a second with a high probability, a crucial requirement for quantum simulation. Despite this excellent result, silicon-based traps could not be used further in the project as they showed a sensitivity to stray laser light, leading to drifting trapping potentials. We then successfully tested an alternative microfabrication technique using YBCO, a ceramic material that becomes superconducting when cooled below a temperature Tc ~ 85 K. With specially designed YBCO traps, we could detect for the first time the normal to superconducting transition using a trapped ion, and validated the suitability of such traps for 1D configurations. For the development of 2D ion configurations, we investigated an array of 16 individual trapping wells arranged in a 4x4 square lattice. The fabrication of these traps proved very challenging, the complex electrode layout demanding the deposition of two metal layers separated by a third, electrically insulating layer. We had to improve the fabrication process multiple times before chips were produced that could hold the required trapping voltages. These 2D traps, already electrically tested, will be implemented in the ion trapping experiment in 2017. In a complementary setup we investigated the possibility to control and suppress static charges in ion traps. Such charges are known to generate uncontrolled stray electric fields in microfabricated ion traps, which can cause drifts in the ion-ion interaction strength that is essential to quantum simulation schemes. By directing photoelectrons with well-defined energy to an insulating surface, we were able to reliably bring the surface to positive and negative charge states, and to discharge it. Beyond ion trapping experiments, this charge mitigation technique is of interest for precision experiments operating in a high vacuum, where unwanted static charges can affect the measurements.