Quantum computers won’t be truly useful until they fix their mistakes
David Bonaldo / Alamy
Quantum computers are already here, but they make too many errors. This is arguably the biggest hurdle for the technology to become truly useful, but recent successes suggest that a solution may be on the horizon.
Errors occur even in traditional computers, but there are well-established techniques for fixing them. They rely on redundancy, where extra bits are used to detect when 0s are incorrectly converted to 1s or vice versa. However, in the quantum world, this is much more challenging.
The laws of quantum mechanics prevent information from being duplicated inside a quantum computer, so redundancy must be achieved by spreading information across groups of qubits – the building blocks of quantum computers – and using phenomena that only exist in quantum settings, such as when pairs of particles become linked via quantum entanglement. These qubit groups are called logical qubits and figuring out the optimal way to create and use them is important to determining how to eliminate errors.
The recent surge in progress has researchers optimistic. “This is a very exciting time in error correction. For the first time, theory and practice are really making contact,” says robert schoelkopf At Yale University.
The biggest obstacle to quantum error correction is that the number of qubits required to create a logical qubit is large, making it expensive and challenging to build an entire quantum computer. But xiyayu linpeng He and his team at the International Quantum Academy in China have recently demonstrated that this is not necessarily the case.
The researchers found that simply combining two superconducting qubits with a small resonator could create a larger qubit, which both made fewer errors and could automatically flag the error when it occurred. They then went a step further to show how three such qubits could be grouped together via quantum entanglement to build computational power without latent errors.
Skoelkopf’s team too recently featured How many of the operations required to program a quantum computer can be implemented with a single type of qubit and with exceptionally low error rates, with some errors occurring rarely once in a million qubit manipulations.
Even though many errors would be caught by such an approach, useful quantum computers would involve thousands of logical qubits, meaning some would still creep in. Ariane Vezvei He and his colleagues at the start-up Quantum Elements have been testing a way to add further error protection to logical qubits, like wearing a raincoat under an umbrella.
The main idea is to not allow any qubits to remain inactive for too long, as this would cause them to lose their special quantum properties and become corrupted. The team showed that giving inactive qubits an additional “kick” of electromagnetic radiation could create the most reliable entanglement yet between logical qubits.
The exact recipe for how to combine physical qubits into logical qubits actually matters for some of the most precise calculations David Munoz Ramo Quantum computing firm Quantinum and their colleagues discovered it while testing an algorithm that could determine the lowest possible energy in the hydrogen molecule. There, the precision required is so high that basic error-correction methods are not sufficient.
Such innovation in error-correction programs will be critical to the success or failure of quantum computers, say james wooton Start-up Moth at Quantum. “We’re still in the stage where researchers are learning how all the pieces of error correction fit together.” He says quantum computers still can’t work effectively without errors, but we’re starting to see its engineering foundations.
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