Scientists in Switzerland have come closer to building stable quantum computers after developing a swap gate made of neutral atoms that can deliver 99.9 percent accuracy while operating across 17,000 qubits.
Led by Tilman Esslinger, PhD, a professor at the Institute of Quantum Electronics at ETH Zurich, the research team turned to geometric phases, an approach that makes quantum logic operations more robust against experimental noise.
Using this method, the scientists realized a high-quality swap gate, or a quantum exchange. This is a fundamental quantum operation that exchanges the states of two qubits.
Swap gates typically rely on interactions like tunneling or collisions that are highly sensitive to imperfections. In contrast, this novel method depends on the path a quantum system takes rather than on unstable external factors.
The researchers also proved that the gate can be applied to several thousands of qubits at the same time.
A stable quantum step
Quantum computers rely on qubits, the fundamental, quantum-mechanical units of information. Unlike classical bits that are either 0 or 1, they can exist in multiple states at once. But controlling qubits precisely is notoriously difficult.
Even tiny fluctuations in temperature, laser intensity, or even environmental noise can disrupt them. To address the challenge, the team designed a swap gate based purely on geometric phases, that exchanges the quantum state of two qubits.
This means that if qubit A is in state 0 and qubit B is in state 1, after the execution of the swap gate, qubit A will be in state 1 and qubit B in state 0. These gates help route quantum information.
Hence, the outcome is set by the system’s path, not external fluctuations, which makes the gate more resistant to noise. To achieve this, the researchers trapped extremely cold potassium atoms in an optical lattice, a grid-like structure formed by laser light.
These atoms act as qubits, with their spin states encoding quantum information. By carefully manipulating the laser configuration, the team brought pairs of atoms close enough for their quantum wavefunctions to overlap in space. This way, they triggered the geometric phase that enables the swap operation.
Promising results
Because the potassium atoms are fermions and can not share the same quantum state, the manipulation produced a 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,” Konrad Viebahn, PhD, the experiment’s junior group leader, noted.
As a result, the new swap gate achieved a precision of 99.91 percent. It was also able to operate simultaneously across 17,000 qubit pairs.
“We can now make lots of swap gates with neutral atoms,” Esslinger concluded in a press statement. “But of course we still need a few other ingredients to build a working quantum computer.” He added the next step involves pairing swap gates with a quantum gas microscope to visualize and selectively control qubit pairs.
Meanwhile, by introducing atomic collisions, the researchers achieved “half”-swap gates, that cause the qubits to become quantum mechanically entangled. This is a prerequisite for executing quantum algorithms.
The study has been published in the journal Nature.