Power/Performance Bits: June 2

Neuromorphic memristor
Researchers at the University of Massachusetts Amherst used protein nanowires to create neuromorphic memristors capable of running at extremely low voltage. A challenge to neuromorphic computing is mimicking the low voltage at which the brain operates: it sends signals between neurons at around 80 millivolts.

Jun Yao, an electrical and computer engineering researcher at UMass Amherst, said, “This is the first time that a device can function at the same voltage level as the brain. People probably didn’t even dare to hope that we could create a device that is as power-efficient as the biological counterparts in a brain, but now we have realistic evidence of ultra-low power computing capabilities. It’s a concept breakthrough and we think it’s going to cause a lot of exploration in electronics that work in the biological voltage regime.”

The protein nanowires used by the team were harvested from Geobacter, a bacterium that grows conductive filaments. In addition to being potentially less expensive than silicon nanowires, they are more stable in water or bodily fluids, an important feature for biomedical applications.

The researchers experimented with a pulsing on-off pattern of positive-negative charge sent through a tiny metal thread in a memristor, which creates an electrical switch. They used a metal thread because protein nanowires facilitate metal reduction, changing metal ion reactivity and electron transfer properties. UMass Amherst microbiologist Derek Lovely noted that this microbial ability is not surprising, because wild bacterial nanowires breathe and chemically reduce metals to get their energy the way we breathe oxygen.

As the on-off pulses create changes in the metal filaments, new branching and connections are created, Yao explained, creating an effect similar to learning – new connections – in a real brain. “You can modulate the conductivity, or the plasticity of the nanowire-memristor synapse so it can emulate biological components for brain-inspired computing. Compared to a conventional computer, this device has a learning capability that is not software-based.”

The team plans to follow up this discovery with more research on mechanisms, and to “fully explore the chemistry, biology and electronics” of protein nanowires in memristors, Fu says, plus possible applications, which might include a device to monitor heart rate, for example. Yao adds, “This offers hope in the feasibility that one day this device can talk to actual neurons in biological systems.”

Hot qubits
Researchers at the University of New South Wales Sydney, Université de Sherbrooke, Aalto University, and Keio University propose a way to make quantum computers run at warmer temperatures. While still far from room temperature, it could drastically reduce the amount spent of refrigeration.

The proof-of-concept quantum processor unit cell uses a silicon chip and works at 1.5 Kelvin, significantly warmer than chip-based technology using superconducting qubits. “This is still very cold, but is a temperature that can be achieved using just a few thousand dollars’ worth of refrigeration, rather than the millions of dollars needed to cool chips to 0.1 Kelvin,” noted Professor Andrew Dzurak of UNSW Sydney. “While difficult to appreciate using our everyday concepts of temperature, this increase is extreme in the quantum world.”

The unit cell comprises two qubits confined in a pair of quantum dots embedded in silicon. Scaled up, the team says it could be manufactured using existing silicon fabs. It would also be easier to integrate with conventional silicon chips, which will be needed to control the quantum processor.

The qubit pairs are initialized and read using electrons tunneling between the two quantum dots, and electrically driven spin resonance is used to coherently control the qubits.

“Our new results open a path from experimental devices to affordable quantum computers for real world business and government applications,” said Dzurak.

They’re not the only team with recent success pushing quantum computing to warmer temperatures: researchers from Delft University of Technology in collaboration with QuTech and Intel used similar silicon technology to create qubits that can be controlled at 1.1 Kelvin.

The silicon-based qubits use standard production technology. “In order to work at a higher temperature, we had to make improvements at all stages of the experiment. We have created silicon qubits that can be isolated from unwanted interactions,” said Gertjan Eenink, a PhD student at Delft.

Luca Petit, a PhD student at Delft, noted, “Performing quantum calculations at 1.1 Kelvin depended on us reducing all possible sources of noise, and developing measurement procedures that are temperature-resistant. It was a fantastic moment when everything came together and we were able to perform quantum operations with two silicon qubits at this temperature for the first time.”

“We are now working towards a system that contains more and higher quality qubits,” said Menno Veldhorst of Delft. “Operating at 1.1 Kelvin has significant benefits, and we can now start thinking about integrating quantum hardware and classic hardware onto one single chip. In doing so, we will create the quantum integrated circuit.”