Google says its quantum computer can reveal the structure of molecules

Google Quantum AI researchers used their Quantum computer Willow to help interpret nuclear magnetic resonance (NMR) spectroscopy data, the foundation of chemical and biological research. This work puts quantum computers on the brink of being able to usefully complement conventional molecular technologies.

The most rigorously proven use of quantum computers is to break cryptography, but today's devices are too small and error-prone to run decryption algorithms. However, another area where they could make progress is in speeding up the procedures used to discover new drugs and materials. Such procedures are quantum in nature, so they are well suited to the capabilities of quantum computers. Hartmut Neven and his colleagues at Google Quantum AI demonstrated one example where a quantum computer's ability to “speak the same language as nature” could prove valuable.

The team's work focused on a computational protocol called Quantum Echo and how it could be applied to NMR, which is used to determine microscopic details of a molecule's structure.

The idea behind Quantum Echo is similar to the butterfly effect, a phenomenon in which a small disturbance causes large effects in the larger system to which it belongs, such as the flapping of a butterfly's wings leading to a distant storm. The researchers used a quantum version of this in the 103-qubit system at Willow.

In the experiments, the researchers first applied a specific sequence of operations to their qubits that changed the quantum states of the qubits in a controlled manner. They then selected one specific qubit to perturb, which would act as a “quantum butterfly,” and then applied the same sequence of operations as before, but in reverse order in time, such as rewinding a videotape. Finally, the team measured the quantum properties of the qubits, which they analyzed to gain information about the entire system.

In its simplest sense, the NMR procedure used in laboratories is also based on tiny perturbations: this time they nudge real molecules with electromagnetic waves and then analyze how the system reacts to determine the relative positions of atoms, like a molecular ruler.. When qubit manipulations mimic this process, the mathematical analysis of the qubits can also be translated into details of the structure of the molecule. This move into quantum computing gives us the chance to see atoms farther apart, team member says Tom O'Brien. “We are creating a longer molecular line.”

The team estimates that running a protocol like Quantum Echoes on a typical supercomputer would take about 13,000 times longer. Their tests also showed that two different quantum computers could each run a quantum echo and produce the same results, which was not the case with some of the quantum algorithms the team has become champion in the past. O'Brien says this is possible in part because the world situation is rapidly improving. Willow equipment qualityfor example, reducing qubit error rates.

But there are still improvements to be made. When the researchers used Willow and quantum echo on two organic molecules, they only used up to 15 qubits at a time, and the calculation result could still be compared with conventional, non-quantum methods. In other words, the team has yet to prove that Willow has a clear practical advantage over its classic counterparts. The demonstration of this particular application of Quantum Echoes is currently preliminary and has not undergone a formal peer review process.

“The issue of determining the molecular structure is extremely important and relevant,” says Keith Fratus at HQS Quantum Simulations, a German company developing quantum algorithms. He says linking an established technique such as NMR with calculations performed on a quantum computer is an important step, but for now the technique's usefulness is likely to be limited to highly specialized research in biology.

Dries Sels from New York University say the team's experiment uses a larger quantum computer and examines more complex NMR protocols and molecules than previously simulated on quantum computers, including by him and his colleagues. “Quantum simulation is often cited as one of the key promising use cases for quantum computers, but there are very few industrially interesting examples… I think model inference from spectroscopic data such as NMR could be useful,” he says. “I don’t think we’re there yet, but work like this provides motivation to continue studying the problem.”

O'Brien says Quantum Echoes' application to NMR will become more useful as the team continues to improve the performance of its qubits. The fewer mistakes they make, the more of them can be used in the protocol simultaneously, taking into account increasingly larger molecules.

Meanwhile, the search for the best use of quantum computers is certainly far from over. Running Quantum Echoes on Willow is extremely exciting from an experimental standpoint, but the mathematical analysis it enables is unlikely to be widely used, he says. Kurt von Keyserlingk at King's College London. He says that until it can definitively surpass what NMR scientists have already been doing for decades, its main appeal will be with theoretical physicists who focus on fundamental research on quantum systems. And the protocol may not be completely future von Keyserlingk says he already has ideas about how traditional computers can compete with him.