Sound waves, light waves and other types of waves, generally spread freely through space and over time. In 1958, physicist Philip W. Anderson first described a phenomenon via which irregularities or other sources of disorder in materials would prevent waves from propagating freely, which is now known as Anderson localization.

In quantum systems, one can observe quantum states that are spread throughout a system (i.e., extended), confined to a small region (i.e., localized) or somewhere between the two (i.e., critical). Critical quantum states have so far proved to be very difficult to identify and study using Anderson’s localization theory.

Researchers at the International Quantum Academy and Southern University of Science and Technology in China recently set out to further explore critical quantum states in a quantum processor based on superconducting qubits.

Magnetic fields are generally known to destroy superconductivity in a material. However, in exceptional cases, they can lead to what is known as “re-entrant superconductivity”—where superconductivity disappears as expected, but then unexpectedly returns when the magnetic field is increased further.

This behavior is sometimes seen in bulk, three-dimensional materials, but now, in a study published in Science Advances, a team led by the RIKEN Center for Emergent Matter Science (CEMS) in Japan has seen the phenomenon in a very thin conducting layer at the boundary between two insulating oxide materials. Because oxide interfaces can be precisely engineered and controlled, the discovery provides a new platform for investigating unconventional forms of superconductivity and the quantum mechanisms that allow it to survive under unusual conditions.