A strange kind of quantumness may be key to quantum computers’ success

What is it about quantum computers that makes them more powerful than conventional machines? A new experiment suggests that the key ingredient may be the property of “quantum contextuality.”

Quantum computers are fundamentally different from all other computers because they use unique quantum phenomena not found in traditional electronics. For example, their building blocks, called qubits, are typically placed in superposition states – they seem to acquire two properties at once, usually mutually exclusive – or are connected by an inextricable connection quantum entanglement.

Now researchers from Google Quantum AI have used their Quantum computer Willow conduct several demonstrations showing that the property of quantum contextuality also plays a significant role.

Quantum contextuality reflects the strangeness of measuring the properties of quantum objects. While the color of a pen, say, is not affected by whether you measure it before or after measuring the length of the pen, for a quantum object the measurement results cannot be treated as pre-existing properties independent of all other measurements.

This contextuality has previously been studied in specialized experiments with quantum lightand in 2018 a group of researchers mathematically proven that it can also be used in a quantum computing algorithm.

Notably, this algorithm will allow a quantum computer to find a mathematical formula hidden inside a larger mathematical object in a fixed number of steps, no matter how big the object is. In other words, quantum contextuality allows you to find a needle in a haystack, regardless of its size.

The Google experiment implemented this algorithm by increasing the number of qubits, from a few to 105, which is equivalent to increasing the number of haystacks. Because Willow has more noise, it has fewer errors than ideal theoretical quantum computer for which the algorithm was written, the number of steps increased with the number of qubits. However, Willow still used fewer steps than the researchers estimate a traditional computer would require.

Thus, quantum contextuality appears to lead to quantum advantage—a case where a quantum computer uses its quantumness to surpass the performance of classical devices. In addition, the team implemented several other quantum computing protocols based on quantum contextuality and found that their effect was stronger than in previous studies.

“When I first heard about it, I said it can't be true. It's just amazing,” he says. Adam Cabello at the University of Seville in Spain.

“These results clearly demonstrate how modern quantum computers are pushing the boundaries of experimental quantum physics,” says Veer Bulchandani at Rice University in Texas. He says that any quantum computer that claims to gain useful quantum benefits must be able to perform these tasks, which is an indicator of its quantum nature.

However, the demonstration is not yet proof of a quantum advantage that could be exploited practical use because the 2018 algorithm rigorously proved the superiority of a quantum computer over classical computers only for more qubits than Willow's, and by using qubits that are less error-prone, says Daniel Deal at the University of Southern California. The next step might be to connect the new work with quantum error correction algorithms, he says.

In addition to offering a new benchmark for quantum computers, this experiment also highlights the importance of the most fundamental aspects of quantum physics. Cabello says researchers still lack a comprehensive theory of what exactly causes quantum advantage, but unlike the entanglement that often has to be created, contextuality is in some sense built into quantum objects. Quantum computers like Willow are now good enough to keep us busy weirdness of quantum physics very seriously, he says.