This breakthrough, say the researners, opens a range of practical applications, including quantum communication, quantum metrology, medical imaging and diagnostics, and clandestine labeling.
"The demonstration of high single-photon purity in the infrared," says Victor Klimov, lead author of a paper on the research, "has immediate utility in areas such as quantum key distribution for secure communication."
The researchers say they developed an elegant approach to synthesizing the colloidal-nanoparticle structures based on their prior work on visible light emitters that used a core of cadmium selenide encased in a cadmium sulfide shell. By inserting a mercury sulfide interlayer at the core/shell interface, the scientists turned the quantum dots into highly efficient emitters of infrared light that can be tuned to a specific wavelength.
“This new synthesis," says Vladimir Sayevich, the lead chemist on this project, "allows for highly accurate, atomic-level control of the thickness of the emitting mercury sulfide interlayer. By changing it in increments of a single atomic layer, we can tune the wavelength of the emitted light in discrete quantized jumps, and further adjust it in a more continuous fashion by tuning the cadmium selenide core size."
The new quantum dots are far superior to existing near-infrared quantum dots, say the researchers, as the new structures show "blinking-free" emission at a single-dot level, nearly perfect single-photon purity at room temperature (which produces "quantum light"), and fast emission rates. They behave extremely well with both optical and electrical excitation.
Single photons can be used as qubits in quantum computing. In a cybersecurity application, single photons can protect a computer network through quantum key distribution, which provides ultimate security through "unbreakable" quantum protocols.
Another important application say the researchers is bio-imaging. The emission wavelength of the newly developed quantum dots is within the near-infrared bio-transparency window, which makes them well suited for deep tissue imaging.
While infrared light is not visible to the naked human eye, many modern technologies rely on it, from night-vision devices and remote sensing to telecommunications and biomedical imaging. Infrared light is also a big player in emerging quantum technologies that rely on the duality of light particles, or photons, which can also behave as waves. Exploiting this quantum property requires sources of "quantum light" that emit light in the form of individual quanta, or photons.
Zack Robinson, the project member focusing on quantum dot spectroscopy says, "There is also a cool chemical element in achieving single-atomic layer accuracy in making these dots. The thickness of the emitting mercury sulfide interlayer is identical across all dots in the samples. That's very unique, especially for a material made chemically in a beaker."
Looking ahead, in order to take full advantage of 'quantum light,' photon indistinguishability needs to be achieved where all emitted photons are quantum-mechanically identical. This is an extremely difficult task, which the researchers say they plan to tackle next in their project.