An artist’s concept of a quantum computer
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A quantum bit inspired by Schrödinger’s cat has managed to resist making errors for an unusually long time in a quantum computing experiment. This may make it a promising building block for more reliable quantum computers in the future.
Researchers have long believed that quantum computers can solve problems that are impossible for conventional computers, but there have been very few demonstrations of such capability so far. This is because quantum computers tend to make errors as they compute, but building a quantum computer powerful enough to correct its own errors is technically difficult.
Zaki Leghtas at the École Normale Supérieure in France and his colleagues, in collaboration with the quantum computing start-up Alice & Bob, have now created a quantum bit, or qubit, that avoids making a particularly common type of error for the unprecedentedly long time of 10 seconds.
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They made their qubit by trapping light in a small hole on a chip filled with tiny circuits made from perfectly conducting – or “superconducting” – wires. The light could oscillate back and forth in two different ways inside the hole. But instead of forcing it to oscillate one way only, the team made it do both – creating a quantum superposition similar to the one involving the cat in Erwin Schrödinger’s famous thought experiment. This type of qubit is, accordingly, called a “cat qubit”.
Leghtas says that for more than 10 years, physicists have theorised that cat qubits should be particularly unlikely to make so-called bit-flip errors, which are equivalent to the digital 0s in a conventional computer spontaneously becoming 1s, or vice versa. But demonstrating that cat qubits in the lab are so resistant to bit-flips is not straightforward.
For several years, he says he and his colleagues were detecting bit-flip errors in their cat qubit every few milliseconds. Recently, however, they realised that many of these errors were actually induced by the way they were measuring the cat qubit’s states. Redesigning that process led them to a major technical leap: their cat qubit can now function for 10 seconds without bit-flipping, which is 10,000 times longer than in any past experiment.
The researchers have only built one cat qubit with this property so far, but building more of them could be a step towards reliably useful quantum computers. This is because a computer built with the cat qubits could devote more of them to computation, rather than reserving just a few for computation and using the others to correct bit-flip errors in the computational qubits. Leghtas says that using these cat qubits could cut the number of qubits needed for error-correction by about 10 times compared with other qubit designs involving superconducting circuits.
Christian Andersen at the Delft University of Technology in the Netherlands says that while 10 seconds in between bit-flips is a very long time for a qubit, it is not the only qubit property that matters. There is a trade-off between making the cat qubit more resilient to bit-flip errors and having it inadvertently become more prone to other kinds of errors. Future studies will have to find the most practical way to deal with that, he says.
“This is really cool, it’s nice progress, but there are also many challenges,” says Andersen.
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