New 'trick' fixes major flaw with lasers in neutral-atom quantum computers — inching us closer to more powerful systems

Researchers have created a new type of "quantum operation" that is dramatically more stable than previous methods. The achievement brings one hardware design, in particular — neutral‑atom qubits — a step closer to powering useful quantum computers.

Quantum computers use qubits that can exist in a state of 0, 1 or a superposition of both. Key to their processing power are "gates" capable of shuffling qubits between those states so they can run calculations in parallel. One critical type of gate is called a swap gate, which allows information to be routed through a machine by exchanging two qubits' states.

Many quantum systems rely on highly excited electronic states or collisions between atoms, as well as on the tunnel effect, in which particles slip through obstacles that would be impassable according to classical physics. However, swap gates that use those techniques (particularly the tunnel effect) are subject to how quickly lasers — which suspend neutrally charged atoms in place to form the qubits — can be turned on and how powerful they are.

This means that tiny fluctuations in the timing or strength of a laser could introduce errors and a lack of fidelity into the system, making a gate unreliable.

It feeds into the major bottleneck preventing scientists from scaling up quantum computing so they can be more powerful than the world's fastest supercomputers: qubits are highly susceptible to sustaining errors and breaking down during calculations. This rate is roughly 1 in 1,000 versus 1 in 1 trillion for conventional bits.

To resolve this issue, scientists at ETH Zurich devised a way to make qubits in neutral-atom quantum computers far more stable than ever before. They outlined their findings in a study published April 8 in the journal Nature.

Opening the gateway to more stable quantum computers

Rather than relying on conventional gates, the team used a subtler physical effect called a geometric phase. Unlike other methods for implementing quantum gates for neutral atoms or trapped particles, which depend on how fast and hard atoms are pushed, their swap gate exploits the path the atoms take through an artificial "crystal of light" built by intersecting laser beams (called an optical lattice).

Neutral‑atom platforms promise thousands of qubits in a single device. This setup uses tens of thousands of potassium atoms cooled to near absolute zero and held in place by laser light. Yann Hendrick Kiefer, a postdoctoral researcher at the ETH Zürich Institute for Quantum Electronics and first author of the study, told Live Science how this works.

"Laser light is nothing but monochromatic electromagnetic radiation," Kiefer said in an email. "If a neutral atom is placed inside this electric field a dipole moment is induced which leads to a force that enables us to hold atoms in place."

When two of those potassium atoms are brought close enough that their quantum waves overlap, their combined state changes in a way that depends only on the geometry of their motion, not on how quickly they move or how intense the lasers are. This makes the swap operation far less sensitive to experimental noise.

"Quantum mechanics is described by wave functions," Kiefer said. "Manipulation of this wavefunction generally introduces a phase on the wavefunction, which can be either of dynamical or geometric origin."

Dynamical quantum methods create this phase based on highly precise control over things like energy levels, timing, and laser strength, which means even tiny mistakes can cause errors. The geometric approach works differently: instead of depending on exact timing or force, it depends mainly on the overall path the system takes from start to finish. Because of that, it’s naturally less sensitive to outside disturbances or small imperfections, making these quantum operations more stable and reliable.

Building machines that will need far fewer qubits than we thought

Using this method, the research team achieved a very robust swap gate with a precision of better than 99.91%, operating in under a millisecond (one-thousandth of a second) across a system with a remarkable 17,000 qubit pairs. While some superconducting or trapped‑ion gates can be sub‑microsecond (one-millionth of a second), those systems typically run such gates on only a handful of qubit pairs at once.

The team also proved that they were capable of creating "half-swap" gates, which are critical for running real quantum algorithms. Half‑swap gates — a quantum operation that only swaps two qubits partway instead of completely — are vital because entanglement is the special ingredient in quantum computing. A full swap mostly just moves information around, but a half-swap can both partially exchange information, and create correlations between qubits that classical bits can't have. The scientists hope to eventually pair these robust swaps with a quantum gas microscope — which can image and target individual atom pairs — to build a more flexible, programmable quantum computing architecture.

That said, Kiefer admits a practical quantum computer is still way off. "Quantum computing on a practical scale still requires significant advancements," he said. "The most limiting factors are twofold: scale and fidelity."

However, Kiefer remains optimistic. He cited a recent study that explored how we could one day solve complex problems like Shor's algorithm with a system that uses as few as 10,000 qubits, rather than the millions we previously assumed we would need.

Shor's algorithm is a quantum recipe that can quickly crack certain kinds of modern encryption by finding the secret prime‑number ingredients of a big number faster than a classical computer can, and it remains a widely used benchmark in quantum computing research.

"There is a lot of work to be done before actually solving Shor's algorithm," Kiefer said, "but we are entering the phase in which the dream of quantum computing might actually be slowly converted into reality — exciting times!"

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