This strange material could reveal the link between classical and quantum physics.


Classical and quantum physics are defined by what makes them so different, but an even bigger question has plagued physicists for decades: what links these two opposing views together? Why do the fundamental laws of classical physics fail at the quantum level, and can they be reconciled?

Now, thanks to a newly developed material, scientists might be closing in on the answer, because they’ve devised a way to see quantum mechanics occur on a scale visible to the naked eye.

“We found a particular material that is straddling these two regimes,” says team leader N. Peter Armitage, from Johns Hopkins University.

“Usually we think of quantum mechanics as a theory of small things, but in this system quantum mechanics is appearing on macroscopic length scales. The experiments are made possible by unique instrumentation developed in my laboratory.”

The material in question is a type of topological insulator. This type of material was first predicted back in the 1980s, and scientists have been producing different variations of it since 2007.

Topological insulators are special because they’re conductive on their outer layer but, internally, it’s an insulator. This means that electrons can only flow along the outside of the material, causing them to display some really weird behaviours.

For their experiment, Armitage and his team created topological insulators made from pieces of bismuth and selenium that were about the size of finger nail clippings of various thicknesses.

They revealed for the first time that these two elements offer a way for physicts to see the quantum phenomena on a much larger scale than usual.

To figure this out, they sent a beam of terahertz radiation (sometimes called THz or T-rays – an invisible spectrum of light) through these insulators, measuring the beam as it travelled.

The team found that the beam changed as it passed through the material by rotating slightly – an effect that’s usually only observed at the atomic scale.

Even better, the amount of change they saw could be accurately predicted using the same complex mathematics that govern at the quantum level. This is the first time researchers have witnessed quantum mechanics occurring on the macro scale in a topological insulator.

That might not sound like a big deal, but the insulator has given the team a rare opportunity to reproduce a quantum effect in a larger object, and it shows a promising link between the world of quantum and classical mechanics.

This link is something scientists have been chasing for decades, as part of the hunt for the elusive ‘theory of everything‘.

To put it simply, scientists know that the rules of the quantum world – which explain how atoms operate on an extremely small scale – have to somehow be linked to the everyday classical world – the rules of bigger systems, like how a ball rolls or a rocket is launched.

But the problem is, this link is elusive. Many of the rules of classical physics break down at the quantum level. For example, gravity, which is crucial to our world, doesn’t seem to affect quantum systems at all, and the rules of classical physics can’t explain the ‘spooky action at a distance‘ of quantum entanglement.

This experiment suggests that topological insulators could be the way we finally see that link once and for all, if we can continue to manipulate them further.

Though the new experiment is definitely a step in the right direction and “a piece of the puzzle”, according to Armitage, there’s still a lot of work for researchers to do before the link between the two different physical worlds is fully understood.

The hope is that one day we’ll have a completed picture of physics, and new materials like the team’s topological insulator might be the way we get there.

Team develops compact, high-power terahertz source at room temperature


Terahertz (THz) radiation—radiation in the wavelength range of 30 to 300 microns—is gaining attention due to its applications in security screening, medical and industrial imaging, agricultural inspection, astronomical research, and other areas. Traditional methods of generating terahertz radiation, however, usually involve large and expensive instruments, some of which also require cryogenic cooling. A compact terahertz source—similar to the laser diode found in a DVD player —operating at room temperature with high power has been a dream device in the terahertz community for decades.

Manijeh Razeghi, Walter P. Murphy Professor of Electrical Engineering and Computer Science at Northwestern University‘s McCormick School of Engineering and Applied Science, and her group has brought this dream device closer to reality by developing a compact, room-temperature source with an of 215 microwatts.

Razeghi presented the research October 7 at the International Conference and Exhibition on Lasers, Optics & Photonics in San Antonio, and will also present at the European Cooperation in Science and Technology conference in Sheffield, England on October 10. The findings were published July 1 in the journal Applied Physics Letters and was presented at the SPIE Optics + Photonics conference in August in San Diego.

Razeghi’s group is a world leader in developing quantum cascade lasers (QCL), compact semiconductor lasers typically emitting in the mid-infrared spectrum ( of 3 to 16 microns).

Terahertz radiation is generated through nonlinear mixing of two mid-infrared wavelengths at 9.3 microns and 10.4 microns inside a single quantum cascade laser. By stacking two different QCL emitters in a single laser, the researchers created a monolithic nonlinear mixer to convert the mid-infrared signals into , using a process called difference frequency generation. The size is similar to standard , and a wide spectral range has already been demonstrated (1 to 4.6 THz).

“Using a room-temperature mid-infrared laser to generate terahertz light bypasses the temperature barrier, and all we need to do is to make the output power high enough for practical applications,” said Razeghi, who leads Northwestern’s Center for Quantum Devices (CQD). “Most applications require a minimum of microwatt power levels, but, of course, the higher the better.”

The achieved output power, 215 microwatts, is more than three times higher than earlier demonstrations. This dramatic boost is due to a number of novelties, including Cherenkov phase matching, epilayer down mounting, symmetric current injection, and anti-reflection coating.