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System Bits: Sept. 25

Schottky diodes get better equations; quantum computers; new qubit measurements.


Schottky diodes: One 2D material equation to rule them all
Specifying the right materials for the heterostructure of 2D Schottky diodes—which consist of a metal touching a semiconductor—means designers have to wade through sometimes conflicting theoretical models to select materials. “It is not uncommon to see a model, whose underlying physics fundamentally contradicts with the physical properties of 2D materials, being deployed to analyze a 2D material Schottky diode,” explained a news article from Singapore University of Technology and Design (SUTD).

Researchers from SUTD have now discovered a single equation that may help clear up some of the model confusion to make 2D Schottky diode design more straightforward. The researchers say equation is widely applicable to broad classes of 2D systems, including semiconductor quantum well, graphene, silicene, germanene, stanene, transition metal dichalcogenides and the thin-films of topological solids.

By finding similarities the carrier transport—the electrical current—flowing across 2D material Schottky diodes, the researchers found a universal scaling law that describes different variants of 2D-material-based Schottky diodes. They discovered that the reversed saturation current scales universally with temperature, and how it relates to vertical versus lateral heterostructures. The Schottky barrier height can then be calculated.

“The universal scaling laws signal the breakdown of β=2 scaling in the classic diode equation widely used over the past sixty years,” concluded SUTD researchers in their article published in Physical Review Letters. “Our model resolves some of the conflicting results from prior works and is in agreement with recent experiments.”

Read more in the Singapore University of Technology and Design: Research News. Read SUTD’s article published in Physical Review Letters, Universal Scaling Laws in Schottky Heterostructures Based on Two-Dimensional Materials.

“Maxwell’s demon” helps researchers move toward quantum computers
Researchers at Penn State say they have reduced the entropy in a super-cooled lattice of atoms—which puts a step closer to the quantum computer. Using a laser to trap the atoms, researchers were able to manipulate them into organized blocks. The uncharged atoms in the system could be used as ‘quantum bits’ or ‘qubits’ someday to run calculations and encode data under the quantum mechanical phenomena that allow them to be in multiple states simultaneously.

The researchers can move the atoms into 125 positions arranged as a 5 by 5 by 5 cube. “Organizing the atoms into a packed 3D grid allows us to fit a lot of atoms into a small area and makes computation easier and more efficient,” said Penn State physics professor David Weiss, the leader of the research team.

In describing the discovery, researchers invoked a thought experiment from the 1870s called Maxwell’s Demon. The demon is supposed to stop entropy, which according to the second law of thermodynamics always—usually—increases in a system. Entropy can also be in an equilibrium state or can be negative only in certain cases when something acts upon it. The second law of thermodynamics makes perpetual motion impossible, but the demon thought experiment imagined the demon created some kind of perpetual motion. The researchers say they may be playing the demon’s part. To understand the analogy, read more here.

New way to measure quantum bits focuses on microwave photons
The way researchers currently measure qubits—or quantum bits—is not going to scale up to the millions of qubits that quantum processors will eventually produce. Researchers from the Plourde Group at Syracuse University, working with collaborators at the University of Wisconsin (UW)-Madison, have published an article detailing a new way to measure qubits that may be able to handle that work load.

To measure qubits, researchers now use low-noise cryogenic amplifiers and a lot of room-temperature microwave hardware. The 50 qubits that Google and IBM squeezed out of their quantum processors is nowhere near the at least several hundreds needed to do useful work—and they had to cool the superconducting microwave circuits to near absolute zero.

“We focus on detecting microwave photons,” said Britton Plourde, professor of physics in Syracuse University and editor in chief of IEEE Transactions on Applied Superconductivity, in a news release. “Our approach replaces the need for a cryogenic amplifier and could be extended, in a straightforward way, toward eliminating much of the required room-temperature hardware, as well.” The Plourde Group researchers use a microwave photo counter that has “raw single-shot measurement fidelity of 92%.”

Unlike digital bits, which have only one state at time, qubits can exist in two states at once. The hope is quantum computing can someday solve problems that even supercomputers can’t solve.

Researchers published their findings in Science magazine. Read more here.