Google says its quantum computer can reveal the structure of molecules

Google Quantum AI researchers used their Willow quantum computer to interpret data from nuclear magnetic resonance (NMR) spectroscopy, a mainstay of chemistry and biology research. This work places quantum computers on the verge of being able to usefully augment current molecular technologies.

The most rigorously proven uses of quantum computers are for decryption of cryptography, but current devices are too small and error-prone to run decryption algorithms. Another area where they could make progress is speeding up the procedures used to discover new drugs and materials. Such procedures are inherently quantum in nature and therefore correspond well to the capabilities of quantum computers. Hartmut Neven and his colleagues at Google Quantum AI demonstrated an example in which 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 Echoes and how it can be applied to NMR, used to determine the microscopic details of a molecule’s structure.

The idea at the heart of Quantum Echoes is similar to the butterfly effect – the phenomenon in which a small disturbance leads to big consequences in the larger system to which it belongs, like the flapping of a butterfly’s wings leading to a distant storm. The researchers used a quantum version in a system composed of 103 qubits within Willow.

In experiments, the researchers first applied a specific sequence of operations to their qubits, which changed the qubits’ quantum states in a controlled manner. Next, they chose a specific qubit to disrupt, which would act like a “quantum butterfly”, before applying the same sequence of operations as before but reversed in time, like rewinding a videotape. Finally, the team measured the quantum properties of the qubits, which they analyzed to obtain information about the entire system.

In the simplest sense, the NMR procedure used in laboratories also relies on tiny disturbances, this time by prodding real molecules with electromagnetic waves and then analyzing how the system reacts to determine the relative positions of the atoms, like a molecular ruler.. When qubit manipulations mimic this process, a mathematical analysis of the qubits could also result in details about the molecule’s structure. Tits quantum computing stage has a chance of allowing us to see between atoms further away from each other, » says team member Tom O’Brien. “We are building a longer molecular ruler.”

The team estimates that running a protocol similar to Quantum Echoes on a conventional supercomputer would take about 13,000 times longer. Their tests also showed that two different quantum computers could each run quantum echoes and produce the same results, which was not the case for some of the quantum algorithms the team championed in the past. O’Brien says this is possible in part because of rapid improvements in the quality of Willow’s hardware, such as decreasing the error rates of its qubits.

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

“The question of determining molecular structure is extremely important and relevant,” says Keith Fratus of HQS Quantum Simulations, a Germany-based company that develops quantum algorithms. He said linking an established technique like NMR to calculations performed on a quantum computer is an important step, but for now the technique’s usefulness would likely be limited to highly specialized studies in biology.

Dries Sels, of New York University, says the team’s experiment uses a larger quantum computer and takes into account more complex NMR protocols and molecules than those previously modeled on quantum computers, including by him and his colleagues. “Quantum simulation is often cited as one of the main potential use cases for quantum computers, but there are remarkably few examples of industrially interesting cases… I think inferring models on spectroscopic data, like NMR, could prove useful,” he says. “I don’t think we’re there yet, but work like this motivates us to continue studying the problem.”

O’Brien says the application of quantum echoes to NMR will become more useful as the team continues to improve the performance of its qubits. The fewer errors they make, the more they can be used simultaneously for the protocol, thus taking into account increasingly larger molecules.

In the meantime, the search for the best uses for quantum computers is certainly far from over. Running quantum echoes on Willow is extremely impressive experimentally, but the mathematical analysis it enables is unlikely to be widely used, says Curt von Keyserlingk of King’s College London. Until it can definitively surpass what NMR scientists have already been doing for decades, he says, its main appeal will be to physics theorists who focus on fundamental studies of quantum systems. And the protocol may not be completely future-proof – von Keyserlingk says he already has ideas for how conventional computing could compete with it.