Using new materials that have been widely studied as potential new solar photovoltaic devices, MIT researchers have demonstrated that nanoparticles of these materials can emit a single, identical stream of photons.

The researchers say that while the work is currently a fundamental exploration of materials’ capabilities, it could eventually pave the way for new optical quantum computers as well as possible quantum teleportation devices for communication. The results were published June 22 in the journalism. Nature Photonics in a paper by graduate student Alexander Kaplan, chemistry professor Mungi Bawendi, and six others from MIT.

Most quantum computing concepts use ultracold atoms or spins of individual electrons that act like quantum bits or qubits, which form the basis of such devices. But nearly two decades ago, some researchers proposed the idea of ​​using light instead of physical objects as the main qubits. Among other advantages, this would eliminate the need for complex and expensive hardware to control qubits and to enter and extract data from them. Instead, conventional mirrors and optical detectors will be required.

“With these qubit-like photons,” Kaplan explains, “you can build a quantum computer with just homemade linear optics, provided you prepare the photons properly.”

The preparation of these photons is the key thing. Each photon must exactly match the quantum properties of the previous one, and so on. Once that perfect match is achieved, “the really big paradigm shift is going from needing very fancy optics, very fancy hardware to just needing simple hardware. It’s the light itself that has to be special.”

Then, Bawendi explains, they take these individual photons, which are identical and indistinguishable from each other and interact with each other. This indistinguishability is crucial: if you have two photons and they are all the same and you can’t distinguish between number one and number two, you can’t track them that way. That’s what allows them to interact in certain non-classical ways.”

“If we want a photon to have this very specific property, very well defined in terms of energy, polarization, spatial mode, time, everything we can encode in a quantum mechanical way, the source has to be very well defined quantum mechanics,” Kaplan said.

The source they use is a form of lead-halide perovskite nanoparticles. Because thin films of lead halide perovskites can be much lighter and easier to process than today’s standard silicon-based photovoltaic devices, they are widely viewed as potential next-generation photovoltaic devices, among other things. Lead halide perovskites in the form of nanoparticles are distinguished by extremely high rates of cryogenic radiation, which distinguishes them from other colloidal semiconductor nanoparticles. The faster the light propagates, the more likely the result will have a distinct wave function. Thus, high emission rates allow lead halide perovskite nanoparticles to emit quantum light.

A standard test to test whether the photons they produce do indeed have this distinctive feature is to detect a particular type of interference between two photons, known as Hong-Ou-Mandelian interference. Kaplan says this phenomenon is key to many quantum technologies, and so its demonstration of its existence is “a confirmation that a photon source can be used for these purposes.”

Few materials can emit light that meets this test, he said. “You can almost count on one hand.” While their new source isn’t yet perfect, generating HOM interference only half the time, other sources are having significant problems maintaining scalability. “The reason other sources are consistent is because they are made of the purest materials and are made one by one, atom by atom. So scalability and reproducibility are very poor,” says Kaplan.

In contrast, perovskite nanoparticles are made in solution and simply deposited onto a substrate material. “We actually spin them on a surface, in this case just a normal glass surface,” Kaplan says. “And we’re seeing them engage in behaviors previously only seen in the toughest training conditions.”

So while these materials aren’t perfect yet, “They’re very scalable, we can do a lot from them. And they’re not very optimized right now. We can integrate them into devices and develop them further,” Kaplan says. At this stage, he said, this work is a “very interesting fundamental discovery” that demonstrates the possibilities of these materials. “The significance of the work is that it can encourage people to look for ways to further improve themselves across different device architectures.” Source