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An ion quantum computer is like a chandelier with a dozen pendants: pull one, and they all start swinging. Each ion is a qubit, but when there are many, their collective vibrations crowd the stage. Any operation on one pair disturbs the neighbors, and the computation goes awry.

The authors found a way to push ions from the side, not from the front. A force applied transversely changes the wave's phase but doesn't set neighboring ions swinging. It's like pressing a guitar string from the side instead of plucking it: the sound changes, but the adjacent strings don't vibrate. In an experiment with 12 ions in a single trap, the entanglement error stayed below 0.5%.

This trick works with any qubit encoding scheme. The key is that the qubit states respond differently to light. Then the transverse force entangles only the chosen pair, leaving the other ions undisturbed.

Now the ion chandelier can swing in more complex but more controlled ways. Each ion is like an individual musician in an orchestra, and the conductor has learned to give cues without throwing off the rest.

Original abstract

as written by the authors arXiv:2603.07548

Spectral crowding of collective motional modes limits the fidelity of entangling interactions in trapped-ion quantum processors by inducing off-resonant coupling to spectator modes. We introduce a geometric-phase entangling interaction driven by a transverse, time-dependent structured-light force. By applying the force in a plane orthogonal to the optical propagation direction, we reduce the effects of spectral crowding while preserving single-ion addressing. The scheme is compatible with arbitrary qubit encodings, provided that the qubit states experience a differential AC Stark shift. We experimentally realise high-fidelity two-qubit gates with error rates below $$5\times10^{-3}$$ in ion crystals containing up to 12 ions confined within a single potential well. These results establish gradient-field light-shift gates as a scalable approach to high-fidelity entangling generation in spectrally crowded trapped-ion systems.