A new sort of microscope can help researchers understand and ultimately develop the inner workings of quantum computing. Jigang Wang from Iowa State Univer­sity described his instrument that works in extreme scales of space, time and energy – billionths of a meter, quadrillionths of a second and trillions of electro­magnetic waves per second. He calls the instrument a “cryogenic magneto-terahertz ccanning near-field optical microscope (cm-SNOM). “It’s the first in the world.”

The instrument can focus down to about 20 nanometers, while operating below liquid-helium temperatures and in strong, Tesla magnetic fields. That’s small enough to get a read on the super­conducting properties of materials in these extreme environ­ments. Superconducting materials are being considered for quantum computing. Super­conducting quantum bits, or qubits, are the heart of the new technology. One strategy to control supercurrent flows in qubits is to use strong light wave pulses. “Super­conducting technology is a major focus for quantum computing,” Wang said. “So, we need to understand and characterize super­conductivity and how it’s controlled with light.”

And that’s what the cm-SNOM instrument is doing. Wang and his team of are taking the first ensemble average measure­ments of supercurrent flow in iron-based super­conductors at terahertz energy scales and the first cm-SNOM action to detect terahertz supercurrent tunneling in a high-temperature, copper-based, cuprate superconductor. “This is a new way to measure the response of super­conductivity under light wave pulses,” Wang said. “We’re using our tools to offer a new view of this quantum state at nanometer-length scales during terahertz cycles.”

Ilias Perakis from the University of Alabama at Birmingham has developed the theoretical understanding of light-controlled super­conductivity. He said, “By analyzing the new experimental datasets, we can develop advanced tomography methods for observing quantum entangled states in super­conductors controlled by light.” 

Now that those measurements are happening at the ensemble level, Wang is looking ahead to the next steps to measure super­current existence using the cm-SNOM at simultaneous nanometer and terahertz scales. His group is searching for ways to make the new instrument even more precise. Could measurements go to the precision of visualizing super­current tunneling at single Josephson junctions, the movement of electrons across a barrier separating two super­conductors? “We really need to measure down to that level to impact the optimi­zation of qubits for quantum computers,” he said. “That’s a big goal. And this is now only a small step in that direction. It’s one step at a time.” (Source: Iowa State U.)

Reference: L. Luo et al.: Quantum coherence tomography of light-controlled superconductivity, Nat. Phys. 19, 201 (2023); DOI: 10.1038/s41567-022-01827-1

Link: Ultrafast Quantum Materials Laboratory, Dept. of Physics and Astronomy, Iowa State University, Ames, USA