Researchers at ETH Zurich have taken us a step closer to quantum supercomputers after achieving a major breakthrough with neutral-atom qubits, making them more stable during operation than ever before. To do so, the research team also had to develop a new type of quantum operation, which advances quantum computing. 

Quantum computers are considered the next frontier of computing, allowing computations at speeds that can’t be achieved by conventional silicon-based computers. Central to this capability of quantum computers are quantum bits or qubits that can exist in states of 0, 1 or a combination of both, known as a superposition. 

Quantum computers also use computing gates that allow qubits to be shuffled between these states and to run computations in parallel. One such gate critical to quantum operation is the swap gate, which allows two qubits to exchange their quantum states. 

What makes gates unreliable? 

To operate, swap gates rely on the tunnel effect, where particles can slip through obstacles in ways classical physics cannot comprehend, while quantum computers also use highly excited electronic states of atoms. All this depends on the strength and tunability of lasers, which suspend the atoms that make up qubits. 

Any fluctuations in the timing or strength of the lasers introduce errors into the system, making these gates unreliable. While errors with conventional bits are often seen as one in a trillion, they are more common with qubits, where the rate is one in a thousand. 

To overcome this, researchers at ETH Zurich used a subtler effect called the geometric phase, which exploits the path taken by atoms through an artificial ‘crystal of light’ built from intersecting laser beams. 

“Laser light is nothing but monochromatic electromagnetic radiation,” Yann Hendrick Kiefer, postdoctoral researcher at the Institute for Quantum Electronics at ETH Zurich told a major science outlet. “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.”

Geometry-based quantum swap

When two potassium atoms held in such a setup are brought close enough that their quantum waves overlap, their combined state changes, depending on their relative motion. Since the output is no longer dependent on how quickly the atoms move or how powerful the lasers are, but instead of the overall path that the system takes, it makes it less susceptible to outside disturbance and hence more stable. 

The superiority of the approach was demonstrated by the research team in a system with 17,000 qubit pairs. The researchers achieved 99.1 percent precision with their swap gates, which operated within a millisecond (a thousandth of a second). 

The researchers were also successful in creating ‘half-swap’ gates that only partially swap qubits, rather than completely, and are crucial for running real quantum algorithms. While a full swap moves information around, a half-swap can not only exchange information but also create correlations between qubits, something classical bits cannot do. 

Kiefer is optimistic that their approach could help solve problems like Shor’algorithm with just 10,000 qubits rather than millions that researchers had previously hypothesized. While such a quantum supercomputer may not be deployed anytime soon, the research brings us a step closer to that reality. 

The research findings were published in Nature.