System Bits: Oct. 1

Jumping the gap in microchips
A quasi-particle that travels along the interface of a metal and dielectric material may be the solution to problems caused by shrinking electronic components, according to an international team of engineers.

“Microelectronic chips are ubiquitous today,” said Akhlesh Lakhtakia, Evan Pugh University Professor and Charles Godfrey Binder Professor of Engineering Science and Mechanics, Penn State University. “Delay time for signal propagation in metal-wire interconnects, electrical loss in metals leading to temperature rise, and cross-talk between neighboring interconnects arising from miniaturization and densification limits the speed of these chips.”

These electronic components are in our smartphones, tablets, computers, and security systems, and they are used in hospital equipment, military installations, and our transportation infrastructure.

Researchers have explored a variety of ways to solve the problem of connecting various miniaturized components in a world of ever shrinking circuits. While photonics, the use of light to transport information, is attractive because of its speed, this approach is problematic because the waveguides for light are bigger than current microelectronic circuits, which makes connections difficult.

A section of a circuit board showing microcircuits. Image credit: Antoine Bercovici

Surface plasmon-polariton (SPP) waves have been known for a long time, but the mathematics behind them requires solving complex equations. Lakhtakia and colleagues investigated the theoretical transport of information by using an SPP wave as a carrier wave and pulsing the wave in a way similar to the way Morse code electrically pulses through a telegraph line.

The researchers report in a recent issue of Scientific Reports that “The signal can travel long distances without significant loss of fidelity,” and that “signals can possibly be transferred by SPP waves over several tens of micrometers (of air) in microelectronic chips.”

They also note that calculations indicate that SPP waves can transfer information around a concave corner — a situation, along with air gaps, that is common in microcircuitry.

SPPs are a group phenomenon. These quasi-particles travel along the interface of a conducting metal and a dielectric — a non-conducting material that can support an electromagnetic field — and on a macroscopic level, appear as a wave.

According to Lakhtakia, SPPs are what give gold its particular shimmery shine. A surface effect, under certain conditions electrons in the metal and polarized charges in the dielectric material can act together and form an SPP wave. This wave, guided by the interface of the two materials can continue propagating even if the metal wire has a break or the metal dielectric interface terminates abruptly. The SPP wave can travel in air for a few 10s of micrometers or the equivalent of 600 transistors laid end-to-end in 14-nanometer technology chips.

SPP waves also only travel when in close proximity to the interface, so they do not produce crosstalk.

The problem with using SPP waves in designing circuits is that while researchers know experimentally that they exist, the theoretical underpinnings of the phenomenon were less defined. The Maxwell equations that govern SPP waves cover continuum of frequencies and are complicated.

“Instead of solving the Maxwell equations frequency by frequency, which is impractical and prone to debilitating computational errors, we took multiple snapshots of the electromagnetic fields,” said Lakhtakia.

These snapshots, strung together, become a movie that shows the propagation of the pulse-modulated SPP wave.

“We are studying tough problems,” said Lakhtakia. “We are studying problems that were unsolvable 10 years ago. Improved computational components changed our way of thinking about these problems, but we still need more memory.”

Also working on this project were Rajan Agrahari, graduate student in electronics engineering and Pradip K. Jain, professor of electronics engineering, both at the Indian Institute of Technology, Varanasi, India.

The Council of Scientific and Industrial Research, India, and the Charles Godfrey Binder Endowment at Penn State, supported this work.

Army funds quantum internet project
A U.S. Army research result brings the quantum internet a step closer. Such an internet could offer the military security, sensing and timekeeping capabilities not possible with traditional networking approaches.

The U.S. Army’s Combat Capability Development’s Army Research Laboratory’s Center for Distributed Quantum Information, funded and managed by the lab’s Army Research Office, saw researchers at the University of Innsbruck achieve a record for the transfer of quantum entanglement between matter and light – a distance of 50 kilometers using fiber-optic cables.

Entanglement is a correlation that can be created between quantum entities such as qubits. When two qubits are entangled and a measurement is made on one, it will affect the outcome of a measurement made on the other, even if that second qubit is physically far away.

“This [50 kilometers] is two orders of magnitude further than was previously possible and is a practical distance to start building inter-city quantum networks,” said Dr. Ben Lanyon, experimental physicist at University of Innsbruck and the principal investigator for the project, whose findings are published in the Nature journal Quantum Information.

Intercity quantum networks would be composed of distant network nodes of physical qubits, which are, despite the large physical separation, nevertheless entangled. This distribution of entanglement is essential for establishing a quantum internet, researchers said.

“The demonstration is a major step forward for achieving large-scale distributed entanglement,” said Dr. Sara Gamble, co-manager of the Army program supporting the research. “The quality of the entanglement after traveling through fiber is also high enough at the other end to meet some of the requirements for some of the most difficult quantum networking applications.”

The research team started the experiment with a calcium atom trapped in an ion trap. Using laser beams, the researchers wrote a quantum state onto the ion and simultaneously excited it to emit a photon in which quantum information is stored. As a result, the quantum states of the atom and the light particle were entangled.

The challenge is to transmit the photon over fiber-optic cables.

“The photon emitted by the calcium ion has a wavelength of 854 nanometers and is quickly absorbed by the optical fiber,” Lanyon said.

His team therefore initially sent the light particle through a nonlinear crystal illuminated by a strong laser. The photon wavelength was converted to the optimal value for long-distance travel — the current telecommunications standard wavelength of 1,550 nanometers.

The researchers then sent this photon through the 50-kilometer-long optical-fiber line. Their measurements show that atom and light particles were still entangled even after the wavelength conversion and the distance traveled.

“The choice to use calcium means these results also provide a direct path to realizing an entangled network of atomic clocks over a large physical distance, since calcium can be co-trapped with a high-quality ‘clock’ qubit. Large-scale entangled clock networks are of great interest to the Army for precision position, navigation, and timing applications,” said Dr. Fredrik Fatemi, an Army researcher who also co-manages the program.

Aluminum battery design could be more sustainable
A new concept for an aluminum battery has twice the energy density as previous versions, is made of abundant materials, and could lead to reduced production costs and environmental impact. The idea has potential for large scale applications, including storage of solar and wind energy. Researchers from Chalmers University of Technology, Sweden, and the National Institute of Chemistry, Slovenia, are behind the idea.

Using aluminum battery technology could offer several advantages, including a high theoretical energy density, and the fact that there already exists an established industry for its manufacturing and recycling. Compared with today’s lithium-ion batteries, the researchers’ new concept could result in markedly lower production costs.

“The material costs and environmental impacts that we envisage from our new concept are much lower than what we see today, making them feasible for large scale usage, such as solar cell parks, or storage of wind energy, for example,” says Patrik Johansson, Professor at the Department of Physics at Chalmers. “Additionally, our new battery concept has twice the energy density compared with the aluminum batteries that are state of the art today.”

Previous designs for aluminum batteries have used the aluminum as the anode (the negative electrode) – and graphite as the cathode (the positive electrode). But graphite provides too low an energy content to create battery cells with enough performance to be useful.

But in the new concept, presented by Patrik Johansson and Chalmers, together with a research group in Ljubljana led by Robert Dominko, the graphite has been replaced by an organic, nanostructured cathode, made of the carbon-based molecule anthraquinone.

The anthraquinone cathode has been extensively developed by Jan Bitenc, previously a guest researcher at Chalmers from the group at the National Institute of Chemistry in Slovenia.
The advantage of this organic molecule in the cathode material is that it enables storage of positive charge-carriers from the electrolyte, the solution in which ions move between the electrodes, which make possible higher energy density in the battery.

“Because the new cathode material makes it possible to use a more appropriate charge-carrier, the batteries can make better usage of aluminum’s potential. Now, we are continuing the work by looking for an even better electrolyte. The current version contains chlorine – we want to get rid of that,” says Chalmers researcher Niklas Lindahl, who studies the internal mechanisms which govern energy storage.

So far, there are no commercially available aluminum batteries, and even in the research world, they are relatively new. The question is if aluminum batteries could eventually replace lithium-ion batteries.

“Of course, we hope that they can. But above all, they can be complementary, ensuring that lithium-ion batteries are only used where strictly necessary. So far, aluminum batteries are only half as energy dense as lithium-ion batteries, but our long-term goal is to achieve the same energy density. There remains work to do with the electrolyte, and with developing better charging mechanisms, but aluminum is in principle a significantly better charge carrier than lithium, since it is multivalent – which means every ion ‘compensates’ for several electrons. Furthermore, the batteries have the potential to be significantly less environmentally harmful,” says Patrik Johansson.

Read the article, “Concept and electrochemical mechanism of an Al metal anode – organic cathode battery,” published in the journal Energy Storage Materials.