Ultrafast Rydberg Experiments with Ultracold Atoms in Optical Tweezers

Rydberg atoms, with their giant electronic orbitals, exhibit dipole-dipole interaction reaching the GHz range at a distance of a micron (C₃ ~ GHz.μm³), making them a prominent contender for realizing ultrafast quantum operations. However, such strong interactions between single atoms have never been harnessed because of the stringent requirements on the fluctuation of the atom positions and the necessary excitation strength. Here, we introduce novel techniques to enter this regime and explore it with two strongly-interacting single atoms [1].

First, we trap ⁸⁷Rb atoms in holographic tweezers focused with a high-NA lens (0.75), allowing to bring two atoms at distance as close as 1.2 µm. At such close distance, thermal motion of the atoms within the tweezers becomes a major source of noise for the dipole-dipole coupling. We thus apply Raman-sideband cooling to suppress thermal fluctuations (~100 nm) and bring the atom to the motional ground-sate of the tweezers. There, the atoms position is only affected by quantum fluctuation (~ 25 nm) which allows to unlock coherent ultrastrong interaction. We further reduce quantum fluctuation by creating squeezed states of motion [2,3].

Then, we prepare atoms in the Rydberg state using ultrashort, picosecond, laser pulses. In contrast to the standard approach based on cw-laser excitation, this allows to overcome the Rydberg blockade for our strongly interacting Rydberg orbitals. Following excitation, atoms experience the dipole-dipole interaction, which, for our particular choice of Rydberg state, gives rise to an energy exchange between the two atoms. We observe this coherent dynamics occurring on the nano-second timescale. After a full exchange, the atoms are back in their initial orbitals with a π-phase shift. We measured this phase shift by probing the superposition of a ground and Rydberg orbital by Ramsey interferometry with attosecond precision. This phase shift is the key to the realization of an ultrafast two-qubit C-Z gate.

These demonstrations open the path for “ultrafast Rydberg experiments”, such as the realization of a nanosecond CZ gate operating at the speed-limit set by the strong dipole-dipole interaction between Rydberg atoms.