Power/Performance Bits: Oct. 12

More stable quantum states; tiny, bright LED; ink for printed electronics.


More stable quantum states
Researchers at the University of Chicago found a way to make quantum systems retain coherency 10,000 times longer.

The fragile nature of quantum states remains a challenge for developing practical applications of quantum computing, as they can be easily disrupted by background noise coming from vibrations, temperature changes or stray electromagnetic fields.

Approaches to extending the length of coherency include physical isolation, which is unwieldy, and extremely pure materials, which is costly. Instead, the team looked to ways to reduce the impact of noise.

“With this approach, we don’t try to eliminate noise in the surroundings; instead, we “trick” the system into thinking it doesn’t experience the noise,” said Kevin Miao, a postdoctoral researcher at UChicago.

In tandem with the usual electromagnetic pulses used to control quantum systems, the team applied an additional continuous alternating magnetic field. By precisely tuning this field, the scientists could rapidly rotate the electron spins and allow the system to “tune out” the rest of the noise.

“To get a sense of the principle, it’s like sitting on a merry-go-round with people yelling all around you,” Miao explained. “When the ride is still, you can hear them perfectly, but if you’re rapidly spinning, the noise blurs into a background.”

This small change allowed the system to stay coherent up to 22 milliseconds, four orders of magnitude higher than without the modification. The system was able to almost completely tune out some forms of temperature fluctuations, physical vibrations, and electromagnetic noise.

“This approach creates a pathway to scalability,” said David Awschalom, professor of molecular engineering at UChicago, senior scientist at Argonne National Laboratory, and director of the Chicago Quantum Exchange. “It should make storing quantum information in electron spin practical. Extended storage times will enable more complex operations in quantum computers and allow quantum information transmitted from spin-based devices to travel longer distances in networks.”

The researchers used a solid-state silicon carbide quantum system, but believe the technique is applicable to other types, such as superconducting quantum bits and molecular quantum systems. Plus, it could make other types of systems plausible.

“There are a lot of candidates for quantum technology that were pushed aside because they couldn’t maintain quantum coherence for long periods of time,” Miao said. “Those could be re-evaluated now that we have this way to massively improve coherence.”

Tiny, bright LED
Scientists at the National Institute of Standards and Technology (NIST), University of Maryland, Rensselaer Polytechnic Institute, and the IBM Thomas J. Watson Research Center developed a new, tiny LED that is 100 to 1,000 times brighter compared to conventional submicron-sized LED designs.

“It’s a new architecture for making LEDs,” said NIST’s Babak Nikoobakht, who conceived the new design. “We use the same materials as in conventional LEDs. The difference in ours is their shape.”

The new design is able to overcome efficiency droop, which leads to a leveling off of LED brightness even if more electricity is supplied.

Unlike the flat, planar design used in conventional LEDs, the researchers built a light source out of 5um long, thin zinc oxide fins. “We saw an opportunity in fins, as I thought their elongated shape and large side facets might be able to receive more electrical current,” Nikoobakht said. “At first we just wanted to measure how much the new design could take. We started increasing the current and figured we’d drive it until it burned out, but it just kept getting brighter.”

The design produces light wavelengths on border between violet and ultraviolet. “A typical LED of less than a square micrometer in area shines with about 22 nanowatts of power, but this one can produce up to 20 microwatts,” Nikoobakht added. “It suggests the design can overcome efficiency droop in LEDs for making brighter light sources.”

As the team increased the current, the LED’s comparatively broad emission eventually narrowed to two wavelengths of intense violet color, becoming a laser. “Converting an LED into a laser takes a large effort. It usually requires coupling a LED to a resonance cavity that lets the light bounce around to make a laser,” Nikoobakht said. “It appears that the fin design can do the whole job on its own, without needing to add another cavity.”

“It’s got a lot of potential for being an important building block,” Nikoobakht said. “While this isn’t the smallest laser people have made, it’s a very bright one. The absence of efficiency droop could make it useful.”

Ink for printed electronics
Researchers from the University of Cambridge, Durham University, Beihang University, Nanjing Tech University, Imperial College London, Aalto University, and Chinese University of Hong Kong found a new ink formula to make printing electronics more accurate and reproducible.

The team tackled the ‘coffee ring effect,’ where liquid evaporates faster at the edges, leaving an accumulation of solid particles that result in a dark ring. Inks behave in the same way and can end up with irregular shapes particularly when printed on hard substrates like silicon wafers and plastics.

The researchers developed a mixture of isopropyl alcohol and 2-butanol that allows ink particles to distribute evenly across the droplet, generating shapes with uniform thickness and properties.

“The natural form of ink droplets is spherical – however, because of their composition, our ink droplets adopt pancake shapes,” said Tawfique Hasan from the Cambridge Graphene Centre of the University of Cambridge.

The team optimized over a dozen ink formulations containing different materials, from 2D materials like graphene, black phosphorus, and boron nitride to more complex heterostructures of multiple 2D materials and nanostructured materials.

While drying, the new ink droplets deform smoothly across the surface, spreading particles consistently. The new inks also avoid the use of polymers or surfactants, which inhibit the coffee ring effect but can hamper the electronic properties of graphene and other 2D materials.

To test the new ink’s reproducibility and scalability, the researchers printed 4500 nearly identical devices on a silicon wafer and plastic substrate. In particular, they printed gas sensors and photodetectors, both displaying very little variations in performance.

“Understanding this fundamental behavior of ink droplets has allowed us to find this ideal solution for inkjet printing all kinds of two-dimensional crystals,” said Guohua Hu, an assistant professor at the Chinese University of Hong Kong. “Our formulation can be easily scaled up to print new electronic devices on silicon wafers, or plastics, and even in spray painting and wearables, already matching or exceeding the manufacturability requirements for printed devices.”

The team expects industrial applications of the new inks soon, and said their printed sensors and photodetectors showed promising results in terms of sensitivity and consistency, even exceeding the usual industry requirements.

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