The 'forbidden gap' is prison for qubits, but scientists found a way to take advantage of that

May 14, 2021
The "forbidden gap" has at last been opened. (COPS/University of Twente)

The "forbidden gap" has at last been opened. (COPS/University of Twente)

The forbidden gap of a photonic crystal is the ideal area to safely isolate a single photon or electron, which can be used to store quantum information. On the flip side, however, the gap is a victim of its own perfection because humans have been locked out, too — until now.

Scientists took a new approach to accessing the so-called forbidden gap, detailed in a paper published April 27 in Physical Review Letters. Instead of trying to alter a crystal to send lightwaves inside — as previous attempts have done — this method molds the lightwave itself.

Study authors Ravitej Uppu and Willem L. Vos spoke with The Academic Times about the unique features of the gap, why it's so difficult to touch with lightwaves and the implications of finding a viable way to do so.

Vos, the first author of the paper and a professor at the University of Twente in the Netherlands, explained that photonic crystals — those with materials spaced by one order of the wavelength of light — have defects, the deepest of which can be thought of as within the forbidden gap. He used silica crystal as an example.

"If there's even one silicon atom … sitting in wrong place, that is already what we would call a defect because we would notice," Vos said. "No matter how [much] you try to be careful or do your best, it will have very tiny variations."

Though jewelers may find them appalling, from a physicist's perspective the defects aren't a bad thing. That's because they allow for a singular atom to be broken away from the group, meaning a singular particle can be, too — a crucial concept for the development of quantum computers, as they require quantum particles sitting in isolation to function.

However, an issue with the forbidden gap is that it's so protected, to the point where anything placed inside becomes completely inaccessible.

"If you have the perfect cavity sitting inside the perfect crystal, then no light is ever going to reach this cavity, because it's shielded," explained Uppu, an assistant professor in the Department of Physics and Astronomy at the University of Iowa who is formerly affiliated with the University of Twente.

For instance, if the particle sitting inside the crystal is meant to be a qubit, a quantum bit that stores computer language for quantum programs, the qubit will need to transmit information at some point. That transfer can't happen if the qubit is trapped. 

Uppu invoked an analogy to paint a picture of the conundrum. 

"I shine light on the wall," he said. "How can you control this light to go and talk to a particular position inside the wall? Imagine that my wall is a mirror — the light is not going to talk with anything on the other side of the wall."

That happens because of the way lightwaves inevitably interact with each other, Vos explained. When light starts to shine onto a crystal structure, the waves form what is known as Bragg interference. They compound and start reflecting back to the outside. This interference happens because of the perfectly spaced materials.

"You have your photonic crystal, and by interference, the light that you send from the outside to your crystal is quickly decaying," he said. "It's basically a prison."

The team aimed to figure out how to enter the forbidden gap and harness the unparalleled properties it offers. 

First, the researchers generated photonic crystals and placed an emitter within a cavity, where a quantum particle may reside. Uppu explained that to push light beyond the typical distance it can go within a crystal, called Bragg length, he and his colleagues adjusted the lightwave's shape such that it targeted certain defects on the way in.

"Using these little bits of imperfections, what we do is we push all this light to couple only to those imperfections, so that the light can go to the other side," he said. "Earlier, all the information that I'm trying to send it is just thrown back by the crystal, but now I can push this information into the crystals and drag it to that."

Vos said the team essentially "molded the waves to steer them to where we want." In the long run, he says, this instrumentation can be used to control light in a way that hasn't been done before. 

"Basically, I can then store light in a tiny box that can hop to the next one, can hop to the next one, can hop to bottom, can hop to top — this is completely different from our light traveling in free space," he said. "These extra bits of light basically bouncing from defect to defect is what we can target."

Such a mechanism is precisely what would be needed to connect quantum computers in the future, with the quantum internet

The study, "Spatially shaping waves to penetrate deep inside a forbidden gap," published April 27 in Physical Review Letters, was authored by Ravitej Uppu, Manashee Adhikary, Cornelis A. M. Harteveld and Willem L. Vos, University of Twente. 

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