Unlike standard qubits, logical qubits can better carry out computations unmarred by errors, making the new device a potentially important step towards practical quantum computing. A quantum computer calculation depends on the number of qubits it contains.
Recently, IBM and California-based Atom Computing unveiled devices with more than 1000 qubits, nearly tripling the size of previously largest quantum computers. However, these devices have not led to an immediate and dramatic increase in computing capability because larger quantum computers often make more errors.
To make a quantum computer that can correct its errors, researchers from the quantum computing start-up QuEra in Boston and several academics focused instead on increasing its number of logical qubits, which are groups of qubits connected through quantum entanglement.
In conventional computers, error correction relies on keeping multiple redundant copies of information. Still, quantum information is fundamentally different and cannot be copied -- so researchers use entanglement to spread it across several qubits, which achieves a similar redundancy.
Harvard University’s Dolev Bluvstein said that the researchers started with several thousand rubidium atoms in an airless container when making their quantum computer. They used 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 precisely by hitting them with lasers. Accordingly, 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, 7 qubits at a time -- to make a logical qubit.
By doing this, the researchers could make as many as 48 logical qubits simultaneously. This is more than 10 times the number of logical qubits that have ever been created before. The new quantum computer greatly benefits from being made of atoms that are controlled by light because this kind of control is very efficient.
QuEra's computer makes its qubits interact and exchange information by moving them closer to each other inside the computer 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 in quantum computers using physical qubits.