Princeton researchers have succeeded in forcing molecules into quantum entanglement for the very first time.
Individual molecules have been forced into special states of quantum entanglement where they can remain correlated with each other, even if they occupy opposite ends of the Universe.
“This is a 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 the senior author of the paper.
“But it is also a breakthrough for practical applications because entangled molecules can be the building blocks for many future applications.”
The research, ‘On-Demand Entanglement of Molecules in a Reconfigurable Optical Tweezer Array,’ was recently published in the journal Science.
Applications of entangled molecules
Applications of molecules that have gone through quantum entanglement include quantum computers that can solve certain problems faster than conventional computers.
The molecules can also be used for quantum simulators that can model complex materials whose behaviours are difficult to model, and quantum sensors that can measure faster than their traditional counterparts.
Connor Holland, a graduate student in the physics department and a co-author of the work, said: “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.”
What is quantum entanglement?
The quantum advantage is the ability of quantum devices to outperform classical ones. At the core of quantum advantage are the principles of superposition and quantum entanglement.
A classical computer can assume the value of either 0 or 1, whilst qubits can be in a superposition of 0 and 1.
Quantum entanglement is a major cornerstone of quantum mechanics and occurs when two particles become so linked that it persists even if one particle is lightyears away from the other.
Entanglement is an accurate description of the physical world and how reality is structured.
“Quantum entanglement is a fundamental concept,” said Cheuk, “but it is also the key ingredient that bestows quantum advantage.”
Achieving controllable quantum entanglement remains a challenge
Building quantum advantage and achieving controllable quantum entanglement is challenging as scientists are unclear as to which physical platform is best for creating qubits.
Previously, many different technologies have been explored as candidates for quantum computers and devices. The optimal quantum system could depend on the specific application.
However, molecules have long defied controllable quantum entanglement until now.
Advantages of molecules compared to atoms
The Princeton University team manipulated individual molecules to control and coax them into interlocking quantum states. They believe that molecules have advantages over atoms that make them better suited for certain applications in quantum information processing and simulation of complex materials.
Compared to atoms, 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 of storing and processing quantum information,” said Yukai Lu, a graduate student in electrical and computer engineering and a co-author of the paper.
“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.”
However, despite their advantages, molecules are hard to control in the laboratory because they are complex. Their attractive degrees of freedom also make them hard to control in laboratory settings.
Steps taken for molecular quantum entanglement
First, the team picked a molecular species that is both polar and can be cooled with lasers. The molecules were cooled to ultracold temperatures where quantum mechanics can occur. Individual molecules were then picked up by a complex system of focused laser beams called optical tweezers.
Through the engineering of these tweezers, the team created large arrays of single molecules to position them in a one-dimensional configuration.
They then encoded a qubit into a non-rotating and rotating state of the molecule. This molecular qubit was shown to remain coherent – remembering its superposition. Thus, the team revealed that they could create well-controlled and coherent qubits out of individually controlled molecules.
To enable molecular quantum entanglement, the team ensured that the molecules could interact using a series of microwave pulses. By allowing this interaction for a precise amount of time, the team could implement a two-qubit gate that entangled two molecules. This is important because such an entangling two-qubit gate is a building block for universal quantum computing and the simulation of complex materials.
Potential for new breakthroughs in quantum science
The research will help to investigate different areas of quantum science. The team is particularly interested in exploring the physics of interacting molecules which can be used to simulate quantum many-body systems where interesting emergent behaviour like new forms of magnetism can appear.
Cheuk said: “Using molecules for quantum science is 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.”
Confirmation of results
In a separate article published in the journal Science, an independent research group reported the achievement of similar results.
Cheuk concluded: “The fact that they got the same results verify the reliability of our results.
“They also show that molecular tweezer arrays are becoming an exciting new platform for quantum science.”
