United States: Creation of a Quantum Computer with 6,100 Qubits

Quantum computers require a large number of qubits to tackle complex problems in physics, chemistry, and other fields. Unlike classical bits, qubits can exist in two states simultaneously, a phenomenon known as superposition. This unique feature of quantum physics enables quantum computers to carry out certain complex calculations more effectively than their classical counterparts, but it also means that qubits are delicate. To address this, researchers are building quantum computers with additional and redundant qubits to correct potential errors. This necessitates that robust quantum computers will require hundreds of thousands of qubits.

Today, in line with this vision, physicists at Caltech have created the largest qubit network ever assembled: 6,100 neutral atoms trapped in a grid by lasers. Previous networks of this type contained only a few hundred qubits.

This significant advancement comes amid a frantic race to scale up quantum computers. Several approaches are being developed, including those based on superconducting circuits, trapped ions, and neutral atoms, like in this new study.

"It's an exciting time for neutral atom quantum computing," says Manuel Endres, a physics professor at Caltech. "We now see a pathway towards large quantum computers with error correction. The building blocks are in place."

M. Endres is the lead researcher of the study published today in Nature. Three Caltech graduate students led the study: Hannah Manetsch, Gyohei Nomura, and Elie Bataille.

The team used optical tweezers, highly focused laser beams, to trap thousands of individual cesium atoms in a grid. To construct the atom network, the researchers split a laser beam into 12,000 tweezers, collectively holding 6,100 atoms in a vacuum chamber. "On the screen, we can actually see each qubit as a bright spot," notes Hannah Manetsch. "It's a striking image of large-scale quantum hardware."

A major achievement was demonstrating that this larger scale did not come at the expense of quality. Even with more than 6,000 qubits in a single network, the team maintained them in superposition for about 13 seconds, nearly 10 times longer than was possible in previous similar networks, while manipulating individual qubits with 99.98% precision. "It is often thought that scaling up, with more atoms, comes at the cost of precision, but our results show that we can achieve both," adds Mr. Nomura. "Qubits are not useful without quality. We now have both quantity and quality."

The team also demonstrated that they could move atoms several hundred micrometers across the network while maintaining superposition. The ability to move qubits is a key feature of neutral atom quantum computers that enables more effective error correction compared to traditional wired platforms like superconducting qubits.

Mr. Manetsch compares the task of moving individual atoms while keeping them in a state of superposition to balancing a glass of water while running. "Trying to hold onto an atom while moving is like trying not to spill the glass of water. Trying to keep the atom in a state of superposition is like being careful not to run too fast to avoid the water spilling," she explains.

The next major step in this field is implementing quantum error correction at the scale of thousands of physical qubits, and this work shows that neutral atoms are a serious candidate to achieve that. "Quantum computers will need to encode information in a way that tolerates errors so that we can actually perform useful calculations," specifies Mr. Bataille. "Unlike classical computers, qubits cannot simply be copied due to the so-called 'no-cloning' theorem, so error correction must rely on more subtle strategies."