Jigang Wang gave a quick idea of a new kind of microscope that could help researchers understand and ultimately improve the inner workings of quantum computing.
Wang, a professor of physics and astronomy at Iowa State University and also affiliated with the U.S. Department of Energy’s Ames National Laboratory, said the instrument produces electromagnetic waves at extreme scales of space, time, and energy—billionths of a meter, quadrillionth of a second, and trillions of electromagnetic waves per second.
Wang added to the control systems, the laser source, the mirror maze that creates an optical path for light pulses at trillions of revolutions per second, the superconducting magnet surrounding the sample area, the custom-built atomic force microscope, the bright yellow cryostat that lowers sample temperatures to liquid helium temperatures of about -450 degrees Fahrenheit.
Wang calls the instrument the Cryogenic Magneto-Terahertz Scanning Near Field Optical Microscope. (It’s cm-SNOM for short.) It’s located in the Ames National Laboratory’s Precision Instrument Facility, just northwest of the Iowa State campus.
It took five years and $2 million — $1.3 million from the Los Angeles WM Keck Foundation and $700,000 from the State of Iowa and Ames National Laboratory — to make the tool. He has been collecting data and contributing to experiments for less than a year.
“Nobody has it,” Wang said of the extreme scale nanoscope. “A first in the world.”
Working below liquid helium temperatures and in strong Tesla magnetic fields, it can focus down to about 20 nanometers, or 20 billionths of a meter. This is small enough to read the superconducting properties of materials in these extreme environments.
Superconductors are materials that conduct electricity – electrons – without resistance or heat, often at very cold temperatures. Superconducting materials have many uses, including medical applications such as MRI scans and magnetic racetracks for charged subatomic particles accelerating around accelerators such as the Large Hadron Collider.
Now superconducting materials are being considered for quantum computing, the next generation of computing power based on mechanics and energies at the atomic and subatomic scales of the quantum world. Superconducting quantum bits, or qubits, are the heart of the new technology. One strategy for controlling supercurrent flows in qubits is to use powerful pulses of light waves.
“Superconductivity technology is the main focus for quantum computing,” Wang said. “So we need to understand and characterize superconductivity and how it’s controlled by light.”
And that’s what the cm-SNOM device does. As described in a newly published research paper by the journal Nature Physics and a preprint article published on the arXiv website (see side bars), Wang and a research team are taking the first batch-averaging measurements of supercurrent flow in iron-based superconductors at terahertz. (trillions of waves per second) energy scale and the first cm-SNOM action to detect terahertz supercurrent tunneling in a high-temperature, copper-based, copper-rated superconductor.
“This is a new way to measure the response of superconductivity under light wave pulses,” Wang said. “We use our tools to offer new insight into this quantum state at nanometer-long scales during terahertz cycles.”
“By analyzing new experimental datasets, we can develop advanced tomography methods to observe quantum entangled states in light-controlled superconductors. “
The researchers’ paper reports that the interactions that “drive these supercurrents” are “still poorly understood, in part due to a lack of measurement.”
Now that these measurements are happening at the ensemble level, Wang is looking at the next steps to quantify supercurrent presence using cm-SNOM at simultaneous nanometer and terahertz scales. With support from the Center for Superconducting Quantum Materials and Systems, led by the U.S. Department of Energy’s Fermi National Accelerator Laboratory in Illinois, his group is looking for ways to make the new instrument even more precise. Can the measurements go to the precision of visualizing supercurrent tunneling at single Josephson junctions, which is the movement of electrons across a barrier separating the two superconductors?
“We really need to measure up to this level to affect the optimization of qubits for quantum computers,” he said. “This is a big goal. And this is now a small step in that direction. Step by step.”