Power-Performance Bits: Nov. 19

Different Species of Carbon Nanotubes
We all know that humans can be either left or right handed, but what about carbon nanotubes?

Apparently, single-walled carbon nanotubes come in a plethora of different “species,” each with its own structure and unique combination of electronic and optical properties. Researchers with the U.S. Department of Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) and the University of California (UC) Berkeley have developed a technique that can be used to identify the structure of an individual carbon nanotube and characterize its electronic and optical properties in a functional device.
http://newscenter.lbl.gov/news-releases/2013/11/12/taking-a-new-look-at-carbon-nanotubes-berkeley-researchers-develop-new-technique-for-imaging-individual-carbon-nanotubes/

“Using a novel high-contrast polarization-based optical microscopy set-up, we’ve demonstrated video-rate imaging and in-situ spectroscopy of individual carbon nanotubes on various substrates and in functional devices,” says Feng Wang, a condensed matter physicist with Berkeley Lab’s Materials Sciences Division. “For the first time, we can take images and spectra of individual nanotubes in a general environment, including on substrates or in functional devices, which should be a great tool for advancing nanotube technology.”

In this display showing optical imaging and spectroscopy of an individual nanotube on substrates and in devices, (a–c) are schematics of a nanotube on a fused-silica substrate, in a field-effect transistor device with two gold electrodes, and under an alumina dielectric layer; (d–f) are SEM images and (g-i) are direct optical images of these individual nanotubes. Courtesy of the researchers.

A single-walled carbon nanotube can be metallic or semiconducting depending on its exact structure. Semiconducting nanotubes can have very different electronic bandgaps, resulting in wildly different electronic or optical properties.

“To fully understand field-effect devices or optoelectronic devices made from single-walled carbon nanotubes, it is critical to know what species of carbon nanotube is in the device,” Wang says. “In the past, such information could not be obtained and researchers had to guess as to what was going on.”

The physical structure and electronic properties of each individual species of single-walled carbon nanotubes are governed by chirality, meaning their structure has a distinct left/right orientation or “handedness,” which cannot be superimposed on a mirror image. As a result, achieving chirality-controlled growth of carbon nanotubes and understanding the physics behind chirality-dependent devices are two of the biggest challenges in nanotube research.

Progress Toward Quantum Computing
In conventional computers, data is stored as a string of 1s and 0s. In the experiment conducted by Stephanie Simmons of Oxford University and Mike Thewalt of Simon Fraser University, Canada, quantum bits of information, ‘qubits’, were put into a ‘superposition’ state in which they can be both 1s and 0 at the same time — enabling them to perform multiple calculations simultaneously. What’s more, they showed that a normally fragile quantum state could survive at room temperature for a world record 39 minutes, overcoming a key barrier in building ultrafast quantum computers.

In the experiment, the team raised the temperature of a system, in which information is encoded in the nuclei of phosphorus atoms in silicon, from -269°C to 25°C and demonstrated that the superposition states survived at this balmy temperature for 39 minutes. Outside of silicon the previous record for such a state’s survival at room temperature was about two seconds. The team even found they could manipulate the qubits as the temperature of the system rose, and that they were robust enough for this information to survive being ‘refrozen’.

“Thirty-nine minutes may not seem very long, but as it only takes one-hundred-thousandth of a second to flip the nuclear spin of a phosphorus ion – the type of operation used to run quantum calculations – in theory over 2 million operations could be applied in the time it takes for the superposition to naturally decay by 1%. Having such robust, as well as long-lived, qubits could prove very helpful for anyone trying to build a quantum computer,” said Simmons.

The team began with a sliver of silicon doped with small amounts of other elements, including phosphorus. Quantum information was encoded in the nuclei of the phosphorus atoms: each nucleus has an intrinsic quantum property called ‘spin’, which acts like a tiny bar magnet when placed in a magnetic field. Spins can be manipulated to point up (0), down (1), or any angle in between, representing a superposition of the two other states.

The team prepared their sample at just 4°C above absolute zero (-269°C) and placed it in a magnetic field. Additional magnetic field pulses were used to tilt the direction of the nuclear spin and create the superposition states. When the sample was held at this cryogenic temperature, the nuclear spins of about 37% of the ions – a typical benchmark to measure quantum coherence – remained in their superposition state for three hours. The same fraction survived for 39 minutes when the temperature of the system was raised to 25°C.

‘These lifetimes are at least 10 times longer than those measured in previous experiments,’ said Stephanie Simmons. ‘We’ve managed to identify a system that seems to have basically no noise. They’re high-performance qubits.’

There is still some work ahead before the team can carry out large-scale quantum computations. The nuclear spins of the 10 billion or so phosphorus ions used in this experiment were all placed in the same quantum state. To run calculations, however, physicists will need to place different qubits in different states. “To have them controllably talking to one another – that would address the last big remaining challenge,” said Simmons.