A Quantum Simulator with Circular States
Physicists have used arrays of Rydberg atoms—atoms whose outermost electron is excited to an extremely high energy level—to simulate quantum phenomena such as electron spins in a magnet. But Rydberg states come with an inherent compromise: They can have long coherence times, making it possible to execute lengthy quantum algorithms, or they can have atomic transitions that are controllable using convenient optical wavelengths; they cannot have both. Now Yohann Machu at Kastler Brossel Laboratory in France and colleagues have demonstrated a quantum simulator that combines atoms in two kinds of Rydberg states to achieve both long coherence times and optical addressability [1].
The two kinds of Rydberg states are distinguished by their angular momentum. In a Rydberg atom with the maximum angular momentum, the outermost electron has a circular orbit far from the atom’s nucleus. These so-called circular Rydberg atoms have long coherence times but require both optical and radio-frequency signals to prepare. A Rydberg atom with less angular momentum can be excited optically, but the outermost electron’s elliptical orbit takes it closer to the nucleus, increasing the likelihood that it relaxes to a lower energy state.
Machu and colleagues placed eight rubidium atoms in an array of single-atom optical traps. Four circular Rydberg atoms served as “data” qubits, which encoded the quantum information. Four noncircular Rydberg atoms served as optically controllable “ancilla” qubits, which the researchers used to influence the data qubits via dipole–dipole interactions. They also used measurements of the ancilla qubits to infer the data qubits’ states via an operation known as a quantum nondemolition measurement. Reading a state without destroying it allows qubits to be measured before the simulation is complete, giving a more detailed understanding of its evolution.
–Sophia Chen
Sophia Chen is a freelance science writer based in Columbus, Ohio.
References
- Y. Machu et al., “Nondestructive optical readout and manipulation of circular Rydberg atoms,” Phys. Rev. X 16, 021040 (2026).