A Cold Ensemble of 91 Ions express
A tiny crystal of charged atoms, trapped in a trap, trembles with heat. Each ion oscillates like a ball on a spring, and these vibrations hinder using the crystal for precise quantum operations. Usually, excess energy is removed with a laser — the beam hits the ions and slows their motion, like a stream of air damping a pendulum.
But when the crystal contains dozens of ions, an unexpected effect emerges. At a certain moment, the laser not only slows the jitter but makes all ions exchange energy with their internal "little magnets"—the spins. This exchange happens so cooperatively that the vibrations of the entire crystal die down all at once. In an experiment with 91 ions, after a single such pulse, the residual heat turned out to be 8000 times smaller than for a single ion. The more ions, the cleaner the result — the dependence goes as one divided by the square of the number of particles.
By repeating the pulses, the physicists brought the crystal to a state where motion almost vanished. Moreover, the behavior of the large ensemble ceased to depend on how exactly the ions jiggled initially — the collective rhythm dominates everything. Such a "dance" can be used as an ultrasensitive sensor: by measuring the spins after the pulse, scientists reconstruct the vibrational pattern without disturbing the ion crystal itself.
Original abstract
as written by the authors arXiv:2606.21374
Low-entropy motional states of ion Coulomb crystals are an essential prerequisite for a plethora of applications and are typically prepared by laser cooling. As larger crystals are operated in the quantum regime, it remains unclear, and has recently become debated, whether increasing the ion number can be beneficial for cooling. Here, we investigate theoretically and experimentally many-ion sideband cooling and the role of collective effects in different spin-motion coupling regimes. For weak coupling, the many-body effects are insignificant. In the strong-coupling regime, however, the spin and motional subsystems undergo a coherent state swap, enabling cooling by a suitably timed laser pulse. Using planar Coulomb crystals with up to 91 ions, we demonstrate that the residual mean phonon occupation after one such pulse scales as $$1/N^2$$ with the number of ions. By iterating the pulses, we measure mean phonon occupations $$<2\cdot10^{-4}$$. For large crystals in the coherent regime, we further show that the spin-motion dynamics becomes largely independent of the initial phonon statistics. Through spin measurements, the state-swap mechanism can be utilized to probe the phonon distribution in the mode.
The vibrational energy after cooling was so small that if it were converted into the height a grain of sand would rise, it would lift only a few atomic layers.