Physicists ‘entangle’ individual molecules for the first time, accelerating the possibilities of quantum computing

For the first time, a team of Princeton physicists has succeeded in linking individual molecules into special states that are mechanically “quantum entangled.” In these bizarre states, molecules remain correlated with each other – and can interact simultaneously – even if they are miles apart, or even if they occupy opposite ends of the universe. This research was recently published in the journal Science.

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

These include, for example, quantum computers capable of solving certain problems much faster than conventional computers, quantum simulators capable of modeling complex materials whose behaviors are difficult to model, and quantum sensors capable of measuring faster than their traditional counterparts.

“One of the motivations for doing quantum science is that in the practical world, it turns out that if you exploit the laws of quantum mechanics, you can do much better in many areas,” Connor said Holland, a graduate student in physics. department and co-author of the work.

The ability of quantum devices to outperform classical devices is known as “quantum advantage.” And at the heart of quantum advantage are the principles of superposition and quantum entanglement. While a classical computer bit can take the value 0 or 1, quantum bits, called qubits, can be in a superposition of 0 and 1 simultaneously.

The latter concept, entanglement, is a major cornerstone of quantum mechanics and occurs when two particles become inextricably linked to each other, such that this bond persists, even if a particle is years away. light of the other. This is the phenomenon that Albert Einstein, who initially doubted its validity, described as “frightening action at a distance”.

Since then, physicists have demonstrated that entanglement is actually an accurate description of the physical world and how reality is structured.

“Quantum entanglement is a fundamental concept,” Cheuk said, “but it is also the key ingredient that confers quantum advantage.”

But creating a quantum advantage and achieving controllable quantum entanglement remains a challenge, not least because engineers and scientists still don’t know which physical platform is best for creating qubits.

Over the past few decades, many different technologies, such as trapped ions, photons, and superconducting circuits, to name a few, have been explored as candidates for quantum computers and devices. The optimal quantum system or qubit platform could very well depend on the specific application.

However, until this experiment, molecules had long defied controllable quantum entanglement. But Cheuk and his colleagues found a way, through careful laboratory manipulation, to control individual molecules and bring them into these nested quantum states.

They also believed that molecules had certain advantages, over atoms, for example, that made them particularly suitable for certain applications in quantum information processing and quantum simulation of complex materials. Compared to atoms, for example, molecules have more quantum degrees of freedom and can interact in new ways.

“What this means, in practical terms, is that there are new ways to store and process quantum information,” said Yukai Lu, a graduate student in electrical and computer engineering and co-author of the paper. article. “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 they are spatially separated.”

However, the molecules prove notoriously difficult to control in the laboratory due to their complexity. The degrees of freedom that make them attractive also make them difficult to control or corral in the laboratory.

Cheuk and his team overcame many of these challenges through carefully considered experimentation. They first chose a molecular species that was both polar and could be cooled by laser. They then laser-cooled the molecules to ultracold temperatures, where quantum mechanics takes center stage.

The individual molecules were then captured by a complex system of tightly focused laser beams, called “optical tweezers.” By designing the positions of the tweezers, they were able to create large arrays of single molecules and position them individually in any desired one-dimensional configuration. For example, they created lone pairs of molecules and chains of molecules without defects.

Next, they encoded a qubit in a non-rotating and rotating state of the molecule. They were able to show that this molecular qubit remained coherent; that is, he remembered his superposition. In short, the researchers demonstrated the ability to create well-controlled and coherent qubits from individually controlled molecules.

To entangle the molecules, they had to interact. Using a series of microwave pulses, they were able to make individual molecules interact with each other in a coherent manner.

By allowing the interaction to take place for a specific duration, they were able to implement a two-qubit gate that would entangle two molecules. This is important because such a two-qubit entangled gate is a building block for both universal digital quantum computing and the simulation of complex materials.

The potential of this research to study different areas of quantum science is significant, given the innovative features offered by this new molecular tweezer array platform. The Princeton team is particularly interested in exploring the physics of many interacting molecules, which can be used to simulate many-body quantum systems in which interesting emergent behaviors, such as new forms of magnetism, may appear.

“Using molecules for quantum science represents a new frontier, and our demonstration of on-demand entanglement is a key step in demonstrating that molecules can be used as a viable platform for quantum science,” said Cheuk.

In a separate article published in the same issue of Sciencean independent research group led by John Doyle and Kang-Kuen Ni of Harvard University and Wolfgang Ketterle of the Massachusetts Institute of Technology obtained similar results.

“The fact that they obtained the same results confirms the reliability of our results,” Cheuk said. “They also show that molecular tweezer arrays are becoming an exciting new platform for quantum science.”