World first quantum entanglement of single molecules

Physicists at Princeton University in the US have linked together individual molecules in quantum mechanical “entangled” states.

“This is a breakthrough in the world of molecules because of the fundamental importance of quantum entanglement,” says Lawrence Cheuk, an assistant professor of physics at Princeton University. “But it is also a breakthrough for practical applications because entangled molecules can be the building blocks for many future applications.”

“One of the motivations in doing quantum science is that in the practical world, it turns out that if you harness the laws of quantum mechanics, you can do a lot better in many areas,” says co-author of the paper published in Science, graduate student Connor Holland.

Quantum entanglement is when particles (or molecules) share a single physical state. This has the effect of “linking” the two particles. If something happens to one, it happens to the other – instantaneously. Theoretically, the effects of quantum entanglement should be seen instantly at any distance – even if the two entangled objects are on opposite ends of the universe.

The experiment could help the development of quantum computers, simulators and sensors. Such devices promise to perform functions faster, more accurately and with far more complexity than today’s computers and sensors.

Cheuk says that quantum entanglement is the key to “quantum advantage” – the (at this point) theoretical point at which quantum devices outperform modern “classical” computers.

Achieving quantum advantage has proven a challenge. Multiple technologies – including trapped ions, photons, superconducting circuits – have been tested to try and make quantum bits for a quantum device.

Molecules had before now defied controlled quantum entanglement. But they also have certain advantages over single atoms. Molecules can be arranged in more ways, giving them more “degrees of freedom.” This means they can interact in new ways.

“What this means, in practical terms, is that there are new ways of storing and processing quantum information,” says co-author Yukai Lu, also a graduate student. “For example, a molecule can vibrate and rotate in multiple modes. So, you can use two of these modes to encode a qubit. If the molecular species is polar, two molecules can interact even when spatially separated.”

Cheuk’s team was able to achieve entanglement by laser-cooling molecules to ultracold temperatures where quantum effects dominate. They then arranged the molecules in long lines and encoded the qubit into non-rotating and rotating states (analogous to 0 and 1 in classical computer bits).

Using microwaves, the individual molecules were made to interact, leading to entanglement.

Another research group led by researchers at Harvard University and MIT published a separate study in the same issue of Science with similar results.