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Realization of a Quantum Integer-Spin Chain with Controllable Interactions

Illustration of energy level transitions in the ion chain. Image credit, Kelley/JQI 

PFC-supported researchers have used trapped atomic ions to construct a system that could potentially support a type of symmetry-protected quantum state. For this research they used a three-state system, called a qutrit, to demonstrate a proof-of-principle experiment for performing quantum simulations with integer spins. The result is the first demonstration of using multiple interacting qutrits for doing quantum information operations and simulation.

To date, most of the work in ion trap quantum simulation has focused on manipulating arrays of so-called spin-½ particles, where each ion spin has two available energy levels. Increasingly, there is interest in moving beyond spin-½ to simulations of higher order spin systems, which demand more computational power and may yield insight into difficult-to-calculate problems.

To engineer a spin-1 system, the researchers electromagnetically trap a linear crystal of atomic ytterbium (Yb) ions. With two ions, the team demonstrated the basic techniques necessary for quantum simulation: preparing initial states (placing the ions in certain internal states), observing the state of the system after some evolution, and verifying that the ions are entangled, here with 86% fidelity.

To prepare the system in certain initial states, the team first placed the system into its ground state, the lowest energy state in the presence of a large effective magnetic field. By adjusting the parameters (here laser amplitudes and frequencies) the team can open up and follow pathways between different energy levels. They found that for some states there is not a simple path without breaking a symmetry or going through a phase transition. For instance, when they added a third ion, they could no longer guide the system smoothly into its ground state, indicating the possible existence of a state with some additional symmetry protections. This experiment paves the way for future exploration of protected order in spin-1 systems and their use in quantum information applications.

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