System Bits: April 12

Nanotubes line up; stabilizing quantum bits; quantum cryptography.

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Highly aligned, wafer-scale films
Rice University researchers, with support from Los Alamos National Laboratory, have created inch-wide, flexible, wafer-scale films of highly aligned and closely packed carbon nanotubes with the help of a simple filtration process.

The chirality-enriched single-walled carbon nanotubes assemble themselves by the millions into long rows that are aligned better than once thought possible, offering possibilities for making flexible electronic and photonic (light-manipulating) devices. Think of a bendable computer chip, rather than a brittle silicon one, and the potential becomes clear, they said.

A carbon nanotube is a cylinder of graphene, with its atoms arranged in hexagons. How the hexagons are turned sets the tube’s chirality, and that determines its electronic properties. Some are semiconducting like silicon, and others are metallic conductors.

Rice University researchers have discovered a simple method to make films of highly aligned carbon nanotubes. The films can be separated from their backgrounds and show potential for use in electronic and photonic applications. (Source: Rice University)

Rice University researchers have discovered a simple method to make films of highly aligned carbon nanotubes. The films can be separated from their backgrounds and show potential for use in electronic and photonic applications. (Source: Rice University)

A film of perfectly aligned, single-chirality nanotubes would have specific electronic properties, and controlling the chirality would allow for tunable films, but nanotubes grow in batches of random types. For now, the Rice researchers use a simple process developed at the National Institute of Standards and Technology to separate nanotubes by chirality. While not perfect, it was good enough to let the researchers make enriched films with nanotubes of different types and diameters and then make terahertz polarizers and electronic transistors.

The discovery is a step forward in a long quest for aligned structures, and at the same time, the researchers found that their completed films could be patterned with standard lithography techniques, which is another plus for manufacturers.

Making quantum computing more practical
According to MIT researchers, while quantum computers are largely hypothetical devices that could perform some calculations much more rapidly than conventional computers can, they exploit a property called superposition, which describes a quantum particle’s counterintuitive ability to, in some sense, inhabit more than one physical state at the same time. And this superposition is fragile; finding ways to preserve it is one of the chief obstacles to developing large, general-purpose quantum computers. Now however, the team has described a new approach to preserving superposition in a class of quantum devices built from synthetic diamonds, which could ultimately prove an important step toward reliable quantum computers.

They reminded that in most engineering fields, the best way to maintain the stability of a physical system is feedback control: a measurement is made, such as the current trajectory of an airplane, or the temperature of an engine, and on that basis a control signal is produced that nudges the system back toward its desired state. But the problem with using this technique to stabilize a quantum system is that measurement destroys superposition, so quantum-computing researchers have traditionally had to do without feedback.

As a result, open-loop control is typically used. It is decided a priori how the system will be controlled and then the controller is applied. Then the team hopes for the best, and that they knew enough about the system that the control applied will do what they thought it should. Feedback should be more robust, because it allows for adaptation to what’s going wrong.

Further, the researchers have described a feedback-control system for maintaining quantum superposition that requires no measurement, and instead of having a classical controller to implement the feedback, a quantum controller is used. And because the controller is quantum, a measurement is not needed to know what’s going on.

Super fast, super secure quantum cryptography
University of Cambridge researchers along with Toshiba Research Europe have developed a new method to overcome one of the main issues in implementing a quantum cryptography system, raising the prospect of a useable ‘unbreakable’ method for sending sensitive information hidden inside particles of light.

By ‘seeding’ one laser beam inside another, the team demonstrated that it is possible to distribute encryption keys at rates between two and six orders of magnitude higher than earlier attempts at a real-world quantum cryptography system.

Depiction of indistinguishable photons leaving through the same output port of a beam splitter (Source: University of Cambridge)

Depiction of indistinguishable photons leaving through the same output port of a beam splitter (Source: University of Cambridge)

It is well accepted that encryption is a vital part of modern life, as it enables sensitive information to be shared securely. In conventional cryptography, the sender and receiver of a particular piece of information decide the encryption code, or key, up front, so that only those with the key can decrypt the information. But as computers get faster and more powerful, encryption codes get easier to break.

Quantum cryptography promises ‘unbreakable’ security by hiding information in particles of light, or photons, emitted from lasers, whereby quantum mechanics are used to randomly generate a key via a new quantum cryptography protocol known as measurement-device-independent quantum key distribution (MDI-QKD).