Quantum entanglement between electronic and motional states in cold-atom quantum simulator

Quantum entanglement between electronic and motional states in cold-atom quantum simulator
by Riko Seibo
Tokyo, Japan (SPX) Sep 05, 2024
Researchers at the Institute for Molecular Science have demonstrated quantum entanglement between electronic and motional states within an "ultrafast quantum simulator" powered by strong interactions among Rydberg atoms. Their work, which includes the introduction of a new quantum simulation method involving repulsive forces between particles, was published on August 30th in Physical Review Letters.

Cold atoms, confined and manipulated using optical traps, are gaining prominence as a platform for advancing "quantum technology," including quantum computing, quantum simulation, and quantum sensing. A key element in these technologies is "quantum entanglement," the correlation between quantum states of individual atoms. In this study, researchers employed "Rydberg states," characterized by giant electronic orbitals, to generate quantum entanglement in cold-atom systems. The study reveals that quantum entanglement not only occurs between electronic states of atoms but also between electronic and motional states, driven by strong repulsive forces between Rydberg atoms.

The researchers cooled 300,000 rubidium atoms to 100 nanokelvin using laser cooling and then trapped them in an optical lattice with a spacing of 0.5 microns. They generated a "quantum superposition" between the ground state (5s orbital) and the Rydberg state (29s orbital) using an ultrashort laser pulse lasting just 10 picoseconds.

In prior research, the proximity of Rydberg atoms was typically limited to about 5 microns due to the "Rydberg blockade," where an excited Rydberg atom prevents nearby atoms from being excited to the same state. The authors circumvented this limitation by utilizing ultrafast excitation with an ultrashort laser pulse.

Through time-evolution observations of the quantum superposition, the researchers discovered that quantum entanglement between electronic and motional states forms within just a few nanoseconds, in addition to the previously known entanglement among electronic states. This phenomenon results from the repulsive force between Rydberg atoms, which correlates the atom's state ("in the Rydberg state or not") with its motion ("moving or not"). This effect is observable only when Rydberg atoms are positioned within 0.5 microns, comparable to the atomic wavefunction's spread in the optical lattice (60 nanometers). The unique ultrafast excitation method enabled this observation.

The researchers also introduced a new quantum simulation method incorporating the repulsive force between particles, such as electrons in materials. This repulsive force can be induced by briefly exciting atoms into Rydberg states on a nanosecond scale using ultrafast pulse lasers. By rapidly repeating this process, the repulsive force between atoms in the optical lattice can be precisely controlled. This method could lead to quantum simulations that include the motional states of particles experiencing repulsive forces.

Additionally, the research group is drawing attention for developing an "ultrafast cold-atom quantum computer," which significantly accelerates two-qubit gate operations compared to conventional cold-atom quantum computers. The paper's findings, revealing the generation of quantum entanglement between electronic and motional states, represent a significant step toward improving the fidelity of these gate operations, contributing to the development of practical quantum computers in the future.

Research Report:Strong spin-motion coupling in the ultrafast dynamics of Rydberg atoms