Power/Performance Bits: Jan. 5

Quiet qubits
Researchers at the University of New South Wales Sydney recorded the lowest noise levels yet for a semiconductor qubit. Charge noise caused by material imperfections interferes with the information encoded on qubits, reducing accuracy.

“The level of charge noise in semiconductor qubits has been a critical obstacle to achieving the accuracy levels we need for large-scale error-corrected quantum computers,” said Ludwik Kranz, a PhD student at UNSW’s Centre for Quantum Computation and Communication Technology (CQC2T) working with the Centre’s spin off company Silicon Quantum Computing (SQC).

“Our research has demonstrated that we can reduce charge noise to a significantly low level, minimizing the impact it has on our qubits,” says Kranz. “By optimizing the fabrication process of the silicon chip, we achieved a noise level 10 times lower than previously recorded. This is the lowest recorded charge noise of any semiconductor qubit.”

The team found that the presence of defects either within the silicon chip or at the interface to the surface were significant contributors to the charge noise. “This was a surprise, as we have spent a lot of time optimizing the quality of our silicon chip but this showed that even a few impurities nearby can affect the noise,” said Kranz.

To reduce noise, the researchers further reduced impurities in the silicon chip and positioned the atoms away from the surface and interfaces where most of the noise originates. “Our results continue to show that silicon is a terrific material to host qubits. With our ability to engineer every aspect of the qubit environment, we are systematically proving that atom qubits in silicon are reproducible, fast and stable,” said Prof. Michelle Simmons, Director CQC2T.

Another aspect impacting noise is the length of computation. “From the noise spectrum we measured, we know that the longer the computation — the more noise affects our system,” said Dr Sam Gorman of CQC2T. “This has major implications for the design of future devices, with quantum operations needing to be completed in exceptionally short time frames so that the charge noise doesn’t become worse over time, adding errors to the computation.”

In tests, the team was able to read out qubits in 1 microsecond. “This research combined with our lowest charge noise results shows that it is possible to achieve a 99.99% fidelity in atom qubits in silicon,” said Prof. Simmons, who is also the founder of SQC. “Our team is now working towards delivering all of these key results on a single device — fast, stable, high fidelity and with long coherence times — moving a major step closer to a full-scale quantum processor in silicon.”

SQC’s goal is to demonstrate the capability required to reliably produce a 10-qubit prototype quantum integrated processor by 2023.

Printing sensors on skin
Researchers from the Harbin Institute of Technology, Beijing Institute of Technology, and Pennsylvania State University created wearable sensors that can be directly printed on human skin without heat.

A challenge in printing electronics directly on skin is the bonding process for the sensor’s metallic components: this sintering process usually requires temperatures of around 572 degrees Fahrenheit (300 degrees Celsius) to bond the sensor’s silver nanoparticles together.

“The skin surface cannot withstand such a high temperature, obviously,” said Huanyu “Larry” Cheng, a professor in the Penn State Department of Engineering Science and Mechanics. “To get around this limitation, we proposed a sintering aid layer — something that would not hurt the skin and could help the material sinter together at a lower temperature.”

The team was able to lower the temperature to about 212 F (100 C) with the aid of an additional nanoparticle. “That can be used to print sensors on clothing and paper, which is useful, but it’s still higher than we can stand at skin temperature,” Cheng said, who noted that about 104 F (40 C) could still burn skin tissue. “We changed the formula of the aid layer, changed the printing material and found that we could sinter at room temperature.”

With a novel layer to help the metallic components of the sensor bond, an international team of researchers printed sensors directly on human skin. (Credit: Ling Zhang, Penn State/Cheng Lab and Harbin Institute of Technology)

The room temperature sintering aid layer consists of polyvinyl alcohol paste (used in peelable face masks) and calcium carbonate. The layer reduces printing surface roughness and allows for an ultrathin layer of metal patterns that can bend and fold while maintaining electromechanical capabilities. When the sensor is printed, the researchers use an air blower, such as a hair dryer set on cool, to remove the water that is used as a solvent in the ink.  “The outcome is profound,” Cheng said. “We don’t need to rely on heat to sinter.”

According to Cheng, the sensors are capable of precisely and continuously capturing temperature, humidity, blood oxygen levels and heart performance signals. The researchers also linked the on-body sensors into a network with wireless transmission capabilities to monitor the combination of signals as they progress.

The sensor can remain in tepid water for a few days or be removed with hot water. “It could be recycled, since removal doesn’t damage the device,” Cheng said. “And, importantly, removal doesn’t damage the skin, either. That’s especially important for people with sensitive skin, like the elderly and babies. The device can be useful without being an extra burden to the person using it or to the environment.”

The researchers plan to alter the technology to target specific applications as needed.