Quantum Simulations with Trapped Ions

Our knowledge of the behavior of quantum many-body systems is currently limited to those that happen to be theoretically or numerically tractable and those that can be studied in detail experimentally. The world, however, exhibits many systems that are too complex for a complete theoretical description, but too large for an efficient numerical simulation on a classical computer or microscopic experimental probing. Such poorly-understood systems include exotic forms of matter such as spin-liquids and some high-temperature superconductors. We would like to understand the behavior of quantum many-body systems better in order to help us to design useful new materials, understand observed but unexplained behavior, and investigate the borderline between quantum mechanical and thermodynamic behavior.

Recently, a new approach called quantum simulation has begun to be applied that is in many ways an ideal intermediate between in situ experiment and numerical simulation. In quantum simulation, we can use a well-controlled, isolated, laboratory quantum system to model the quantum many-body phenomenon of interest. The quantum simulator, then, utilizes the enormous, classically intractable Hilbert space we allow it to fill and, for instance, either does or does not exhibit the superconductivity we were not sure was associated with the Hamiltonian imposed on the simulator. We are using trapped atomic ions as a platform for quantum simulations of a class of many-body quantum systems called lattice spin models. Each ion simulates the quantum-mechanical spin-1/2 degree of freedom of an electron, and by using appropriately tuned laser fields, we can cause these simulated spins to interact in a manner that is governed by the rules we impose on the system. By looking at the resulting spectra, dynamics, phase transitions, and states, we hope to learn about systems that have to this point been inaccessible to us.

Cold Polar Molecules

The ability to laser cool gas-phase samples of a small subset of elements found in the periodic table has established ultracold atoms as premier systems for precision measurement and control. Select species have been cooled to quantum degeneracy and are finding use as elements in quantum information processors, quantum simulators, and optical atomic clocks. These atoms, however, are extremely symmetric and simple objects when compared to molecules. Even a simple diatomic molecule made from two different elements offers a vast array of new control possibilities if we can produce samples that are as cold and dense as is routinely achieved for atomic samples. It turns out that this is a much more demanding task than it was for atoms, due essentially to the addition of the vibrational degree of freedom that molecules possess.

We are pursuing ways to produce cold samples of polar molecules in the lab. Our focus is on simplicity of both concept and apparatus, and we use a host of different tools to help us, including parts of the electromagnetic spectrum ranging from UV lasers to microwaves to quasi-DC electric fields. As with most atomic physics labs, we also utilize lasers for many tasks, including Q-switched, mode-locked, and continuous-wave lasers. Our plan is to use these tools to decelerate, cool, and trap dense samples of cold molecules for quantum simulation and cold, controlled chemistry.