Quantum pendulum learns to hop over a barrier express
A laser beam holds a single ion in a vacuum, and that ion behaves like a ball trapped between two cups. The cups aren't pits in the ground but regions where the ion energetically prefers to be. In the classical world, a ball would roll into one cup and stay there. In the quantum world, the ion is smeared across both cups at once and occasionally slips through the barrier between them. Physicists in David Wineland's group learned to change the depth of the cups and the height of the barrier simply by tweaking the laser and electric field parameters.
The experiment is like tuning a musical instrument. The trapped ion oscillates almost perfectly harmonically, like a string producing a pure tone. But real molecules and materials behave more complexly: their vibrations are anharmonic, like the sound of an out-of-tune piano. To reproduce such anharmonicity, the researchers added special signals to the trap. With these, they shaped a double well of the desired form — symmetric or tilted. In a symmetric well, the ion tunnels quickly from one cup to the other. As soon as the wells were made slightly different in depth, tunneling nearly vanished — the particle got stuck in the deeper cup.
Such a tunable quantum system is a step toward a universal simulator. It can model the motion of atoms in a molecule, energy transfer in proteins, or phase transitions in solids. All of this boils down to how a quantum particle hops between states, and the shape of the well determines how easily it does so. Now that shape can be changed with a twist of a knob, without rebuilding the whole setup.
Original abstract
as written by the authors arXiv:2603.04744
Continuous-variable-discrete-variable (CV-DV) quantum simulators offer a natural route to simulating bosonic dynamics relevant to many branches of physics and chemistry. However, programmable simulation of arbitrary dynamics is an outstanding challenge. In particular, simulating anharmonic dynamics, which is ubiquitous across the physical sciences, is challenging due to the highly harmonic nature of oscillators used in CV-DV simulators. Here, we experimentally demonstrate programmable CV-DV quantum simulation of anharmonic dynamics in a range of double-well potentials, implemented in a trapped-ion system. We synthesise the time-evolution operators using a bosonic-quantum-signal-processing subroutine, which allows the potential to be tuned between experiments by controlling classical experimental parameters. We observe coherent dynamics in various double-well potentials, where a wavepacket tunnels through the potential barrier, and we suppress this effect by programmatically introducing asymmetry.
The trapped ion is laser-cooled to thousandths of a kelvin — colder than outer space. At that temperature, its motion follows quantum laws, not classical mechanics.
In the series 'Dark', quantum states of particles link different times. Here, an ion in two wells is a prototype of a quantum bit, which can be both 0 and 1 at the same time.