Quantum computer sets record on path towards error-free calculations

Another quantum computing record has been broken. A team has built a quantum computer with the largest ever number of so-called logical qubits, or quantum bits. Unlike standard qubits, logical qubits are better able to carry out computations unmarred by errors, making the new device a potentially important step towards practical quantum computing.

The complexity of a calculation a quantum computer can complete depends on the number of qubits it contains. Recently, IBM and California-based Atom Computing unveiled devices with more than 1000 qubits, nearly triple that of the previous largest quantum computers. But the existence of these devices hasn’t led to an immediate and dramatic increase in computing capability, because larger quantum computers often also make more errors.

To make a quantum computer that can correct its errors, Dolev Bluvstein at Harvard University and his colleagues, including some from the quantum computing start-up QuEra in Boston, focused instead on increasing its number of logical qubits – groups of qubits that are connected to each other through quantum entanglement.

In conventional computers, error-correction relies on keeping multiple redundant copies of information. But quantum information is fundamentally different and cannot be copied – so researchers use entanglement to spread it across several qubits, which achieves a similar redundancy, says Bluvstein.

To make their quantum computer, the researchers started with several thousand rubidium atoms in an airless container. They then used forces from lasers and magnets to cool the atoms to temperatures close to absolute zero where their quantum properties are most prominent. Under these conditions, they could control the atoms’ quantum states very precisely by again hitting them with lasers. They first created 280 qubits from the atoms and then went a step further by using another laser pulse to entangle groups of those – for instance, seven qubits at a time – to make a logical qubit. By doing this, the researchers were able to make as many as 48 logical qubits at one time. This is more than 10 times the number of logical qubits that have ever been created before.

“It’s a big deal to have that many logical qubits. A very remarkable result for any quantum computing platform,” says Mark Saffman at the University of Wisconsin-Madison. He says the new quantum computer greatly benefits from being made of atoms that are controlled by light because this kind of control is very efficient.

The computer makes its qubits interact and exchange information by moving them closer to each other with optical “tweezers” made of laser beams. In contrast, chip-based quantum computers, like those made by IBM and Google, must use multiple wires to control each qubit.

Bluvstein and his colleagues implemented several computer operations, codes and algorithms on the new computer to test the logical qubits’ performance. He says that though these tests were more preliminary than the calculations that quantum computers will eventually perform, the team already found that using logical qubits led to fewer errors than what is seen in quantum computers using physical qubits.

Researchers typically estimate that fully fault-tolerant, or error-free, quantum computers will require thousands of logical qubits, but Jeff Thompson at Princeton University says it is exciting to see that some ideas about fault tolerance have already been explored in the new experiments. He says this is a definite step forward, and more progress is bound to follow as atom-based computers have been improving rapidly. “We are witnessing some of the [atom-based computer’s] defining moments,” says Thompson.