System Bits Nov 26

Scaling The Quantum Slopes
Like any task, there are easy and hard ways to control atoms and molecules as quantum systems, which are driven by tailored radiation fields. More efficient methods for manipulating quantum systems could help scientists realize the next generation of technology by harnessing atoms and molecules to create small but incredibly powerful devices, such as molecular electronics or quantum computers.

Of course, controlling quantum systems is as painstaking as it sounds, and requires scientists to discover the ideal radiation field that leads to the desired response from the system. Scientists know that reaching that state of quantum nirvana can be a long and expensive slog, but Princeton University researchers have found that the process might be more straightforward than previously thought.

The researchers report that quantum-control “landscapes” — the path of a system’s response from the initial field to the final desired field — appears to be unexpectedly simple. Although still a mountain of a task, finding a good control radiation field turns out to be very much like climbing a mountain, and scientists need only choose the right path. Like a hiker, a scientist can take a difficult, twisting path that requires frequent stops to evaluate which step to take next. Or, as the Princeton researchers show, they can opt for a straighter trail that cuts directly to the summit.

(Image courtesy of Arun Nanduri)

The researchers observe that these fast tracks toward the desired control field actually exist, and are scattered all over the landscape. They provide an algorithm that scientists can use to identify the starting point of the straight path to their desired quantum field.

The existence of nearly straight paths to reach the best quantum control was surprising because the landscapes were assumed to be serpentine, explained Arun Nanduri, who is working in the laboratory of Herschel Rabitz, Princeton’s Charles Phelps Smyth ’16 *17 Professor of Chemistry.

“We found that not only can you always climb to the top, but you can climb along a simple path to the top,” Nanduri said. “If we could consistently identify where these paths are located, a scientist could efficiently climb the landscape. Looking around for the next good step along an unknown path takes great effort. However, starting along a straight path requires you to look around once, and you can keep walking forward with your eyes closed, as it were.”

Following a straighter path could be a far more efficient way of achieving control of atoms and molecules for a host of applications, including manipulating chemical reactions and operating quantum computers, Nanduri said. The source of much scientific excitement, quantum computers would use “qubits” that can be entangled to potentially give them enormous storage and computational capacities far beyond the capabilities of today’s digital computers.

“We don’t know if our discovery will directly lead to futuristic quantum devices, but this finding should spur renewed research,” Nanduri said. “If straight paths to good quantum control solutions can be routinely found, it would be remarkable.”

If It Sounds Interesting It Could Be Phonons
The phonon, like the photon or electron, is a physical particle that travels like waves, representing mechanical vibration. Phonons transmit everyday sound and heat. Recent progress in phononics has led to the development of new ideas and devices that are using phononic properties to control sound and heat.

One application that has scientists buzzing is the possibility of controlling sound waves by designing and fabricating cloaking shells to guide acoustic waves around a certain object — an entire building, perhaps — so that whatever is inside the shell is invisible to the sound waves.

Martin Maldovan, of the Georgia Institute of Technology. (Credit: Rob Felt)

The future possibilities for phonons might also solve the biggest challenges in energy consumption and buildings today. Understanding and controlling the phononic properties of materials could lead to novel technologies to thermally insulate buildings, reduce environmental noise, transform waste heat into electricity and develop earthquake protection, all by developing new materials to manipulate sound and heat. These ideas are all possible in theory, but to make them a reality, phononics will have to inspire the same level of scientific innovation as electronics, and today that’s not the case.

“People know about electrons because of computers, and electromagnetic waves because of cell phones, but not so much about phonons,” said Martin Maldovan, a research scientist in the School of Chemical and Biomolecular Engineering at the Georgia Institute of Technology.

Technologies, such as sonic and thermal diodes, optomechanical crystals, acoustic and thermal cloaking, hypersonic phononic crystals, thermoelectrics and thermocrystals, “herald the next technological revolution in phononics,” he said. All of these areas share a common theme: manipulating mechanical vibrations, but at different frequencies.

The hottest fields in phononics, Maldovan said, is the development of acoustic and thermal metamaterials. These materials are capable of cloaking sound waves and thermal flows. The phononics approach to cloaking is based on electromagnetic cloaking materials that are already in use for light.