Power/Performance Bits: May 26

Warmer quantum computing
Researchers at the University of New South Wales Sydney, Université de Sherbrooke, Aalto University, and Keio University developed a proof-of-concept quantum processor unit cell on a silicon chip that works at 1.5 Kelvin – 15 times warmer than current chip-based technology that uses 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,” said Andrew Dzurak, a professor at UNSW Sydney. “While difficult to appreciate using our everyday concepts of temperature, this increase is extreme in the quantum world.”

After the team first announced their experimental results via an academic pre-print archive, a group in the Netherlands led by a former post-doctoral researcher in Dzurak’s group, Menno Veldhorst, announced a similar result using the same silicon technology, confirming the ‘hot qubit’ behavior.

The unit cell developed by the UNSW team comprises two qubits confined in a pair of quantum dots embedded in silicon. The new method initializes and reads qubit pairs using electrons tunneling between two quantum dots.

The result, scaled up, could be manufactured using existing silicon chip factories, and would operate without the need for multi-million-dollar cooling. It would also be easier to integrate with conventional silicon chips, which will be needed to control the quantum processor.

A quantum computer capable of performing complex tasks requires millions of qubit pairs, which is likely to be at least a decade away. “Every qubit pair added to the system increases the total heat generated,” said Dzurak, “and added heat leads to errors. That’s primarily why current designs need to be kept so close to absolute zero.”

Efficient optical
Researchers at Peking University, University of Pennsylvania, and Massachusetts Institute of Technology (MIT) developed a new design for optical devices that radiate light only in a single direction, reducing energy loss.

In optical fibers, light tends to flow in one direction. On-chip couplers are used to connect fibers to chips, where light signals are generated, amplified, or detected. While most light going through the coupler continues through to the fiber, some of the light travels in the opposite direction, leaking out.

A large part of energy consumption in data traffic is due to this radiation loss. Total data center energy consumption is two percent of the global electricity demand, and demand increases every year.

Studies have suggested that there is a 25% loss at each interface between optical fibers and chips, which compounds quickly in a data center.

“You may need to pass five nodes (10 interfaces) to communicate with another server in a typical medium-sized data center, leading to a total loss of 95 percent if you use existing devices,” said Jicheng Jin, University of Pennsylvania doctoral student. “In fact, extra energy and elements are needed to amplify and relay the signal again and again, which introduces noise, lowers signal-to-noise ratio, and, ultimately, reduces communication bandwidth.”

To address the problem, the team looked at breaking the left-right symmetry in their device and found that doing so could reduce the loss to zero.

To better understand this phenomenon, the team developed a theory based on topological charges. Topological charges forbid radiation in a specific direction. For a coupler with both up-down and left-right symmetries, there is one charge on each side, forbidding the radiation in the vertical direction.

“Imagine it as two-part glue,” said Bo Zhen, assistant professor, department of physics and astronomy at University of Pennsylvania. “By breaking the left-right symmetry, the topological charge is split into two half charges – the two-part glue is separated so each part can flow. By breaking the up-down symmetry, each part flows differently on the top and the bottom, so the two-part glue combines only on the bottom, eliminating radiation in that direction. It’s like a leaky pipe has been fixed with a topological two-part glue.”

The team eventually settled on a design with a series of slanted bars, which break left-right and up-down symmetries at the same time. To fabricate such structures, they developed a novel etching method: silicon chips were placed on a wedge-like substrate, allowing etching to occur at a slanted angle. In comparison, standard etchers can only create vertical side walls. As a future step, the team hopes to further develop this etching technique to be compatible with existing foundry processes and also to come up with an even simpler design for etching.

“These exciting results have the potential to spur new research investments for Army systems,” said Dr. Michael Gerhold, program manager, optoelectronics, Army Research Office (which funded the research). “Not only do the coupling efficiency advances have potential to improve data communications for commercial data centers, but the results carry huge impact for photonic systems where much lower intensity signals can be used for the same precision computation, making battery powered photonic computers possible.”

The researchers expect applications both in helping light travel more efficiently at short distances, such as between an optical fiber cable and a chip in a server, and over longer distances, such as long-range Lidar systems.