Researchers develop a new source of quantum light
(Nanowerk News) Using a new material that has been widely studied as a potential new solar photovoltaic, researchers at MIT have shown that nanoparticles of this material can emit streams of identical single photons.
While the current work represents a fundamental discovery of this material’s capabilities, it may eventually pave the way for new optical-based quantum computers, as well as possibly quantum teleportation devices for communication, the researchers said. The results appear in the journal Nature Photonics (“Hong-Ou-Mandel interference in CsPbBr Colloids3 Perovskite Nanocrystals”), in a paper by graduate student Alexander Kaplan, chemistry professor Moungi Bawendi, and six others at MIT.
Most concepts of quantum computing use ultracold atoms or individual electron spins to act as the quantum bits, or qubits, that form the basis of such devices. But about two decades ago some researchers proposed the idea of using light instead of physical objects as the basic qubit unit. Among other advantages, this will eliminate the need for complex and expensive equipment to control the qubits and input and extract data from them. Instead, only an ordinary mirror and an optical detector are needed.
“With these qubit-like photons,” Kaplan explains, “only with ‘household’ linear optics can you build a quantum computer, provided you prepare the photons properly.”
The preparation of those photons is key. Each photon must exactly match the quantum characteristic of the previous one, and so on. Once that perfect match was achieved, “a huge paradigm shift then went from needing very high end optics, very high end equipment, to needing only simple equipment. The thing that needs to be special is the light itself.”
Then, Bawendi explained, they take a single photon that is identical and indistinguishable from one another, and they interact with each other. That indistinguishability is very important: If you have two photons, and “everything is the same about them, and you can’t say number one and number two, you can’t trace them that way. That’s what allows them to interact in certain non-classical ways.”
Kaplan says that “if we want photons to have these very specific properties, which are very well defined in terms of energy, polarization, spatial mode, time, all of these things that we can encode quantum mechanically, we need a very well-defined quantum mechanical source. good, too.”
The source they ended up using was a form of lead-halite perovskite nanoparticles. Lead-halide perovskite thin films are being widely sought as potential next-generation photovoltaics, among other things, because they are much lighter and easier to process than today’s standard silicon-based photovoltaics. In nanoparticle form, lead-halide perovskite is noted for its extremely fast rate of cryogenic radiation, which distinguishes it from other colloidal semiconductor nanoparticles. The faster the light travels, the more likely the output will have a well-defined wave function. The fast radiation levels thus uniquely position lead-halide perovskite nanoparticles to emit quantum light.
To test whether the photons they produce really have this indistinguishable property, the standard test is to detect a certain type of interference between the two photons, known as Hong-Ou-Mandel interference. This phenomenon is critical to many quantum-based technologies, says Kaplan, and its presence therefore “has become a hallmark for ensuring that a photon source can be used for this purpose.”
Very few light-emitting materials meet this test, he said. “They can pretty much be listed on one side.” While their new source is rudimentary, generating HOM interference only about half the time, other sources have significant problems achieving scalability. “The reason other sources are coherent is because they are made with the purest materials, and they are made one at a time, atom by atom. So, there’s very poor scalability and very poor reproducibility,” said Kaplan.
In contrast, perovskite nanoparticles are created in solution and only deposited on the substrate material. “We’re basically just rotating it onto a surface, in this case just a normal glass surface,” says Kaplan. “And we saw them adopt behaviors previously seen only under the most stringent preparation conditions.”
So while these materials may be rudimentary, “It’s so scalable, we can make a lot of them. and they are currently very unoptimized. We can integrate it into the device, and we can improve it further,” said Kaplan.
At this stage, he says, the work is “very exciting fundamental findings”, which demonstrate what these materials are capable of. “The importance of this work is that hopefully it can encourage people to look for ways to further improve this across different device architectures.”
And, adds Bawendi, by integrating these transmitters into a reflective system called an optical cavity, as has been done with other sources, “we have full confidence that integrating them into an optical cavity will bring their properties to a competitive level. ”
