Entanglement has a role to play in quantum criticality in strange metals, according to a new study from Vienna University of Technology (TU Wien) in Austria and Rice University in the USA. [Prochaska, l et al., Science (2020); DOI: 10.1126/science.aag1595]

"When we think about quantum entanglement, we think about small things," explains Rice's Qimiao Si. "We don't associate it with macroscopic objects. But at a quantum critical point, things are so collective that we have this chance to see the effects of entanglement, even in a metallic film that contains billions of billions of quantum mechanical objects."

The researchers have investigated a "strange metal" - compound of ytterbium, rhodium, and silicon - that can pass between two well-studied quantum phases through a critical transition. The work builds on more than two decades of work investigating strange metals and high-temperature superconductors that can undergo quantum phase changes. Understanding such systems could lead to new computing and communications technologies.

The team made ultrapure films of YbRh2Si2 on a germanium substrate and studied their quantum behavior at close to absolute zero. The material transitions from an ordered magnetic phase to a non-magnetic phase at this temperature. Terahertz spectroscopy on the films at 1.4 Kelvin reveals the material's optical conductivity as it is cooled through the transition point. "With strange metals, there is an unusual connection between electrical resistance and temperature," explains team member Silke Bühler-Paschen of TU Wien. "In contrast to simple metals such as copper or gold, this does not seem to be due to the thermal movement of the atoms, but to quantum fluctuations at the absolute zero temperature."

Si and Bühler-Paschen first discussed the potential of these experiments more than 15 years ago when they were exploring the means to test a new class of quantum critical point. The key factor in the current success is the quantum entanglement between spin and charge. At that time, there instrumentation was not available to carry out the experiments. Terahertz technology has changed all that in recent years.

"Conceptually, it was really a dream experiment," Si explains. "Probe the charge sector at the magnetic quantum critical point to see whether it's critical, whether it has dynamical scaling. If you don't see anything that's collective, that's scaling, the critical point has to belong to some textbook type of description. But, if you see something singular, which in fact we did, then it is very direct and new evidence for the quantum entanglement nature of quantum criticality."

Quantum entanglement will underpin the storage and processing of quantum information, so it is important to understand the fundamental science in the materials that might be used to build quantum devices. The same phenomenon is also thought to be the likely solution to developing high-temperature superconductors.