It is widely suspected that harnessing the quantum properties of photons could provide one of the best pathways towards building quantum technologies on an industrial scale, but there are a few significant hurdles in the way.

First, we need a way to reliably generate photons with specific characteristics. Second, we need to be able to generate these photons in a material that can be miniaturized and embedded on computer chips. And third, we need a way to eliminate experimental ‘noise’ from the process, such as unwanted photons.

Now, Perimeter researcher Agata Brańczyk and collaborators Luke Helt and Michael Steel in Australia and Marco Liscidini in Italy have put forward a powerful suggestion of how to do just that, using techniques from conventional glass optics.

In a new paper, “Parasitic photon-pair suppression via photonic stop-band engineering,” published today in Physical Review Letters, the team proposes a way to craft a silicon medium that dramatically improves the quality of the generated photons.

Their technique takes a silicon medium and builds into it a periodic variation of the refractive index, called a ‘Bragg grating’. Bragg gratings are often used to filter out unwanted light that already exists, but this proposal adds a novel twist: it uses a Bragg grating to prevent the generation of unwanted photons before they are even created.

And the idea could soon be put to the test: the proposal is achievable using today’s technology.

How to make quantum light

There are a few methods of generating quantum light. One method, which has been used in single-photon experiments for decades, is spontaneous parametric down conversion (SPDC). In SPDC, a high-powered laser is fired at a specially designed medium (a crystal), inside which a photon splits to create two different, often spectrally entangled, photons. SPDC is popular because it is relatively easy to implement, but it is bulky and extremely challenging to integrate with computer chips.

Another option, which has gained a lot of researcher attention in recent years, is spontaneous four-wave mixing (SFWM). In SFWM, a high-powered laser is fired at a different type of medium, such as silicon. Inside the silicon, two photons combine into a pair. That pair then splits to create two different photons, which are also typically entangled. Such sources are attractive because they could be made in silicon fabrication facilities, just as we do for computer chips.

Through SFWM, a laser pump of one specific frequency, or energy level, will produce a photon pair of split frequencies: one photon in the final pair will be at a higher frequency than the original beam, and one will be lower. That’s because the law of conservation of energy dictates that the final total energy must equal the initial energy. (See ‘a’ in the diagram below.)

Various SFWM processes, with down and up arrows representing the ingoing pump and generated frequencies of photon pairs generated by: (a) a single pump; (b) an idealized dual pump; and (c) a dual pump with undesired noise from parasitic photon pairs.[/caption]

More useful for many experimentalists are photon pairs of the same frequency, because those types of photons can interact with each other. To create these, one can start with two lasers set at different frequencies. A photon from each laser will combine to make a pair, and that spectrally entangled pair will emerge at a frequency between the origin beams, at the same frequency as each other. Again, this is thanks to the conservation of energy: one laser pump will have higher energy than the other: a photon from each will combine, then come out at the frequency between the original pumps. (See ‘b’ in the diagram.)

However, this two-pump system also generates ‘noisy’ photons; sometimes photons from each laser pump will break up in a different way, creating unwanted pairs. Much like in the first example, one photon will be higher energy and one will be lower than its origin pump beam. In the case of a two-pump system, one photon in the unwanted, or “parasitic,” pair will inevitably be close to the spectrum of desired photon-pairs. (See ‘c’ in the diagram.)

These parasitic photons have been a headache for theorists and experimentalists alike, because they contaminate experimental efforts.

Then Brańczyk and her co-authors had a sudden insight. Trying to block both of those parasitic photons is problematic, because it risks blocking the photon pairs you actually do want.

Instead, they wondered if you could just suppress the ‘outer’ photon of the parasitic pair, i.e. the one that lies furthest away, in energy, from the desired photon pairs. Because the photon pairs are spectrally entangled, perhaps just blocking one half would prevent the other from emerging.

Inspired by Bragg gratings in classical glass optics, they designed a structure with photonic stop-bands that does just that: it creates desired photon pairs, suppresses undesired pairs, and could be made from glass or fibre optics.

“It’s a pretty simple idea. We were surprised no-one has done it yet,” said Brańczyk, who is also a PSI Fellow at Perimeter and an Adjunct Assistant Professor at the University of Waterloo.

“Sometimes that happens. Sometimes you just get lucky, and no-one has checked a really simple thing. Usually the problems you’re trying to solve have complicated solutions. Here, the hard work went into checking how well it works, and under what circumstances, but the idea is quite straightforward.”

Nothing easy about simple solutions

While the solution might have been simple, getting there was not easy, said Krister Shalm, a quantum physicist and experimentalist who works at the U.S. National Institute of Standards and Technologies, and who knows Brańczyk from his previous position as a postdoctoral researcher at the Institute for Quantum Computing at the University of Waterloo.

“What I’ve discovered is sometimes the best ideas are the obvious ones. But those types of insights are very difficult and take a long time to get to. If it had been obvious, someone else would have figured it out,” he said.

Currently, a lot of effort is expended in labs trying to filter out unwanted photons. The new proposal holds great potential to improve experiments involving quantum light by efficiently reducing, or removing, contamination, he said.

“They’ve imposed a larger structure [on the glass], so that certain regions behave one way and other regions behave another way,” Shalm said. “That creates interference that cancels things out.”

Brańczyk is energized by the experimental pathways this idea might open up, but what really drives her is the satisfaction of solving a perplexing puzzle.

“The combination of a quantum light source with a periodic structure was a natural system to look at from a theoretical point of view, but we didn’t expect that this combination in its simplest form could be so effective at improving today’s photon sources,” she said. “These kind of surprise benefits are what makes curiosity-driven research so exciting and worthwhile.”