Quantum computers won’t be truly useful until they can correct their errors
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Quantum computers are already here, but they make far too many mistakes. This is arguably the biggest obstacle to the technology becoming truly useful, but recent breakthroughs suggest a solution may be on the horizon.
Errors also creep into traditional computers, but there are well-established techniques for correcting them. They rely on redundancy, where extra bits are used to detect when 0s mistakenly switch to 1s or vice versa. In the quantum world, however, it is much more challenging.
The laws of quantum mechanics prohibit 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 exploiting phenomena that only exist in quantum settings, such as when pairs of particles are linked via quantum entanglements. These groups of qubits are called logical qubits, and figuring out the optimal way to build and use them is critical to determining how best to eliminate errors.
A recent ongoing increase has scientists optimistic. “It’s a very exciting time in error correction. For the first time, theory and practice are really in contact,” says Robert Schoelkopf of Yale University.
One of the stumbling blocks for quantum error correction has been that the number of qubits needed to make a logical qubit tends to be large, making the full quantum computer expensive and challenging to build. But Xiayu Linpeng at the International Quantum Academy in China and his team have recently shown that this need not be the case.
The researchers found that just two superconducting qubits can be combined with a small resonator to create a larger qubit that both makes fewer errors and can automatically flag an error when it occurs. They then went a step further to show how three such qubits can be grouped together through quantum entanglement to build up computational power without hidden errors.
Schoelkopf’s team has also recently demonstrated how multiple operations necessary for quantum computing applications can be implemented with the same type of qubit and with exceptionally low error rates, with some errors occurring as rarely as once in a million qubit manipulations.
Although approaches like this will catch many errors, useful quantum computers will need to contain thousands of logical qubits, meaning some will still sneak in. So Arian Vezvaee at startup Quantum Elements and his colleagues have been testing a way to add additional error protection to logic qubits, like wearing a raincoat under an umbrella.
The key idea is not to let any qubits sit idle for too long, as this causes them to lose their special quantum properties and become destroyed. The team showed that giving idle qubits extra “kicks” of electromagnetic radiation can create the most reliable entanglement between logical qubits to date.
The exact recipe of how to combine physical qubits with logical ones really matters for some of the most precise calculations, as David Muñoz Ramo of the quantum computing firm Quantinuum and his colleagues found when they investigated an algorithm that determines the lowest possible energy a hydrogen molecule can have. There, the precision required is so high that basic error correction methods are not enough.
Such innovation in error-correcting programs will be critical to the success or failure of quantum computers, says James Wootton at the start-up Moth Quantum. “We’re still in a phase where scientists are learning how all the pieces of error correction fit together.” Quantum computers cannot yet work efficiently without errors, but we are beginning to see the technical basis for this, he says.
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