Reviewed by Danielle Ellis, B.Sc.Sep 4 2024
According to a study published in Physical Review Letters on August 30th, 2024, researchers at the Institute for Molecular Science discovered quantum entanglement between electronic and motional states in their ultrafast quantum simulator, which is caused by the repulsive force caused by the strong interaction between Rydberg atoms.
Atoms in the optical lattice, trapped at a distance of 0.5 micron, are excited to the Rydberg state by the ultrafast excitation technique. Interaction between close Rydberg atoms results in the repulsive force. Image Credit: Takafumi Tomita (Kenji Ohmori group)
They also suggest a novel quantum simulation approach that incorporates repulsive forces between particles.
Cold atoms trapped and constructed by optical traps have gained interest as a platform for quantum technology applications such as quantum computing, modeling, and sensing. Quantum entanglement, or the correlation between quantum states of corresponding atoms, is fundamental in quantum technology. Rydberg states, or giant electronic orbitals, are utilized to produce quantum entanglement on cold atom platforms.
The authors of this study thoroughly investigated the quantum state in the ultrafast quantum simulator, and they discovered that the quantum entanglement between electronic and motional states is created by the powerful repulsive force between atoms in the Rydberg state, along with the quantum entanglement between atoms' electronic states.
After being cooled to 100 nanokelvin via laser cooling, the 300,000 Rubidium atoms were placed into the optical trap to form an optical lattice with a 0.5 micron spacing. Subsequently, an ultrashort pulse laser light lasting just 10 picoseconds was used to produce the quantum superposition of the ground state with an electron in the 5s orbital and the Rydberg state with an electron in the gigantic 29s orbital.
Previous studies limited the distance between Rydberg atoms to around 5 microns because a Rydberg atom prevents adjacent atoms from being excited to the Rydberg state, a phenomenon known as Rydberg blockade. The scientists avoided this effect by using ultrafast stimulation with an ultrashort pulse laser light.
Observing the time development of the quantum superposition, the scientists discovered that quantum entanglement between electronic and motional states and entanglement among electronic states form in a few nanoseconds.
This can be explained by the repulsive force between atoms in the Rydberg state caused by the extremely strong interaction, which introduces the connection between “either the atom is in the Rydberg state or not” and “either the atom is moving or not.”
This phenomenon arises only when Rydberg atoms are close to the spread of the atomic wavefunction in the optical lattice (60 nanometers). The scientists’ unique ultrafast excitation approach, which allows for a distance of 0.5 microns, made its detection possible.
The researchers also presented a novel quantum simulation approach that incorporates repulsive forces between particles, such as electrons in materials. The repulsive force may be introduced by stimulating atoms in Rydberg states on a nanosecond scale with ultrafast pulse lasers.
By doing this repeatedly, the repulsive force between atoms trapped in the optical lattice can be arbitrarily regulated. This approach is anticipated to provide a novel quantum simulation using the motional states of particles with repulsive forces.
This study’s research group is also gaining attention for constructing an ultrafast cold-atom quantum computer that speeds up a two-qubit gate operation by two orders of magnitude compared to traditional cold-atom quantum computers.
The ultrafast cold-atom quantum computer uses Rydberg states to accomplish a two-qubit gate operation, and the influence of atomic motion during the interaction is one of the primary reasons for the operation’s fidelity to be reduced.
The study empirically disclosed the method by which quantum entanglement between electronic and motional states is formed, which represents significant progress in improving the fidelity of the two-qubit gate operation and creating socially usable quantum computers in the future.
Journal Reference:
Bharti, V., et. al. (2024) Strong Spin-Motion Coupling in the Ultrafast Dynamics of Rydberg Atoms. Physical Review Letters. doi.org/10.1103/PhysRevLett.133.093405