ETH Zurich researchers reported a swap gate with 99.91% fidelity across 17,000 qubit pairs in less than a millisecond.
Researchers at ETH Zurich have demonstrated quantum logical operations with neutral atom qubits. The quantum gates are based on geometric phases and are designed to be robust against experimental noise.
Qubits for quantum computing can be built in different ways, including superconducting circuits, trapped ions and neutral atoms. Neutral atoms trapped with laser light offer some advantages, including lower sensitivity to certain disturbances and the potential to support systems with several thousand qubits in a single setup.
This is important for Control Engineering because advances in quantum computing benefit the design of automation, controls and instrumentation, as well as cybersecurity. For prior news from ETH Zurich, see how ETH Zurich researchers have developed a new control approach that enables a legged robot, called ANYmal, to move quickly and robustly over difficult terrain.
Neutral atoms also present technical challenges. In quantum computers, qubits exist in superposition states of the logic values 0 and 1. Performing calculations requires quantum logic operations, also known as quantum gates.
Until now, such quantum gates have mainly used highly excited electronic states (Rydberg atoms), collisions between atoms or the tunnel effect. The tunnel effect, in which particles pass through barriers that would not be crossed in classical physics, depends strongly on laser intensity. As a result, small imperfections or fluctuations can reduce quantum gate performance.
Reduced sensitivity to experimental noise
A team of researchers at ETH Zurich led by Tilman Esslinger, professor at the Institute for Quantum Electronics, has demonstrated a so-called swap gate, or quantum exchange, using a geometric phase with high fidelity. In this approach, the particle state changes according to the path taken rather than external disturbances which can make the system more robust against experimental noise.
The researchers also demonstrated that the gate can be applied to several thousands qubits simultaneously. The results, recently published in Nature, could support further development of quantum computers based on neutral atoms.
Quantum exchange with abstract phases
A swap gate exchanges the quantum states of two qubits. For example, if qubit A is initially in state 0 and qubit B is in state 1, after the swap gate is applied, qubit A is in state 1 and qubit B is in state 0. Swap gates are used to route quantum information within a large quantum computer.
“A few years ago, researchers managed to realize such gates using neutral atoms in their lowest energy state, albeit by exploiting dynamical phases due to tunneling and collisions,” says postdoc Yann Kiefer. Dynamical phases arise when particles move in space or interact with each other. These phases affect the oscillatory state of the particles’ quantum mechanical wave function, which in turn influences the probability of observing the particles in a particular quantum state.
Geometric phases, by contrast, are less intuitive. They can occur when the direction of an electron spin changes. After a360 degree rotation, the spin points in the same direction, but the phase of its wavefunction differs by 180 degrees.
Esslinger and his team used this principle to implement a swap gate. They trapped ultracold potassium atoms in optical lattices and adjusted the laser beams to bring pairs of atoms close enough for their wavefunctions to overlap. The atoms’ spin states served as qubits.
Swap gate operates across 17,000 qubit pairs
Because the potassium atoms used were fermions, which cannot occupy the same quantum state, the manipulation produceda geometric phase. “Unlike dynamical phases, this geometric phase is largely independent of the speed with which we manipulate the atoms, or how strongly the laser intensity fluctuates during the process,” explains Konrad Viebahn, junior group leader for the experiment. This produced a swap gate that exchanges the states of two qubits in less than a millisecond with 99.91 % fidelity, simultaneously across 17,000 qubit pairs.
“We can now make lots of swap gates with neutral atoms”, says Tilman Esslinger, “but of course we still need a few other ingredients to build a working quantum computer.” According to Esslinger, a next step is combining the swap gates with a quantum gas microscope. This could make it possible to resolve individual qubit pairs and address them selectively, allowing swap gates to be applied to selected qubits.
The researchers also demonstrated realize half-swap gates by introducing collisions between the atoms. These gates entangle the qubits, a requirement for many quantum algorithms.
References
Kiefer Y, Zhu Z, Fischer L, Jele S, Gächter M, Bisson G, Viebahn K, Esslinger T: Protected quantum gates using qubit doublons in dynamical optical lattices. Nature, April 8, 2026, DOI: external page10.1038/s41586-026-10285-1
Edited by Puja Mitra, WTWH Media, for Control Engineering, from a ETH Zurich news release.
