Quantum entanglement waves measured for the first time

The approach can not only help understand complex systems but also serve as a platform for designing newer quantum materials.
Ameya Paleja
Artistic illustration depicts magnetic excitations of cobalt-phthalocyanine molecules, where entangled electrons propagate into triplons.
Artistic illustration depicts magnetic excitations of cobalt-phthalocyanine molecules, where entangled electrons propagate into triplons

Jose Lado/ Aalto University 

A collaborative effort from researchers at the Aalto University and the University of Jyväskylä in Finland created an artificial quantum material with unusual magnetic properties. Made using cobalt-phthalocyanine molecules, this material was used to measure quantum entanglement waves for the first time using real-space measurements, an organizational press release said.

At a microscopic level, quantum materials see a lot of electron interactions, which also determine their nature, such as high-temperature superconductivity or complex magnetic states. Quantum interactions also give rise to new electronic states.

In the case of two electrons, there are two entangled states known as singlet and triplet states. The energy supplied to electrons in the singlet state sends them to the triplet state. At times, however, the excitement moves through the material in the form of an entanglement wave, also known as a triplon.

Detecting the tricky triplons

Triplons do not occur in conventionally known magnetic materials. Moreover, physicists carry out experiments on macroscopic materials where measurements are expressed as an average across the entirety of the sample. This makes the detection of triplons tricky.

"These materials are very complex. They give you very exciting physics, but the most exotic ones are also challenging to find and study," said Peter Liljeroth, a professor of physics at Aalto University. "So, we are trying a different approach here by building an artificial material using individual components."

To overcome this issue, a collaborative research effort between Finnish organizations worked on developing a designer quantum material. This allows researchers to introduce properties and characteristics into the material that would otherwise not be seen in naturally occurring materials.

The collaboration led to the development of an artificial quantum material using cobalt-phthalocyanine molecules. These molecules contain two frontier electrons. The researchers bundled these molecules together and packed them in tight spaces, forcing them to interact with each other. The researchers were able to see the joint physics of both electrons.

What did researchers observe?

The researchers were successful in measuring the singlet-triplet excitations traversing through the building blocks of their artificial material. This was first observed in molecules of the artificial material and then in larger structures such as molecular chains and islands.

"Using very simple molecular building blocks, we are able to engineer and probe this complex quantum magnet in a way that has never been done before, revealing phenomena not found in its independent parts," said Robert Drost, a research fellow at the Aalto University. "While magnetic excitations in isolated atoms have long been observed using scanning tunneling spectroscopy, it has never been accomplished with propagating triplons."

By using simpler building blocks and moving toward more complex systems, the researchers are hopeful of understanding emergent behavior in quantum materials, the press release said.

Buoyed by their success, the team now plans to use the approach to create new materials with more complex building blocks. This can help them study more exotic magnetic excitations in quantum materials. This approach is expected to not only help understand complex physics but also serve as a platform for designing new quantum materials.

The research findings were published in the journal Physical Review Letters.


Quantum magnets provide a powerful platform to explore complex quantum many-body phenomena. One example is triplon excitations, exotic many-body modes emerging from propagating singlet-triplet transitions. We engineer a minimal quantum magnet from organic molecules and demonstrate the emergence of dispersive triplon modes in one- and two-dimensional assemblies probed with scanning tunneling microscopy and spectroscopy. Our results provide the first demonstration of dispersive triplon excitations from a real-space measurement.

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