While several research groups have already demonstrated tunable single-photon emission from a few quantum dots when confined in an optical waveguide cavity or a modified LED structure, researchers at the U.S. Naval Research Laboratory (NRL; Washington, DC) have tuned the emission wavelength of three indium arsenide (InAs) quantum dots inside a waveguide to be exactly the same by “squeezing” them.1 This enabled a demonstration of an entangled, superradiant state through the emission process of quantum dots embedded in the same waveguide.
These InAs dots are considered to be at the forefront of solid-state single-photon sources and can be precisely positioned, making them promising for on-chip networks for quantum technologies such as computing and communication in a miniature, scalable platform with low power consumption. However, the inability to create a network of dots on the same chip with identical emission wavelength has been a barrier to realizing these quantum technologies.
Squeezing the dots
The ability to fabricate quantum dots with homogeneous physical size and emission properties is extremely challenging. Furthermore, tuning of quantum dots has only been marginally successful through the application of strain using piezoelectric actuators, dielectric capping layers, laser annealing, and electrical bias. Unfortunately, these methods cannot independently tune different quantum dots within the same photonic structures or can lead to degradation of the surrounding semiconductor materials or the quantum dots themselves.
These issues have now been addressed by the NRL researchers through introduction of controlled mechanical strain in specific areas of a photonic structure—with submicron spatial resolution—using laser modification of a thin (around 40 nm) encapsulating layer of hafnium oxide (HfO2). The local strain tailors the emission wavelength of individual quantum dots to a preselected value and can be monitored in real time.
To induce squeezing, a 532 nm laser illuminates a section of the waveguide, crystallizing the initially amorphous HfO2, which compresses the semiconductor and blue-shifts the quantum-dots’ emission wavelength without degrading their optical qualities or affecting quantum dots outside the illuminated region. The laser heating power, spot size, and duration can be adjusted to modify how much the quantum-dot emission energy is tuned. Importantly, this technique allows bringing quantum dots with very narrow emission linewidths into mutual resonance, enabling a quantum-entangled superradiant state of three dots coupled to the same waveguide (see figure).
“This technique creates new possibilities for exploring phenomena that emerge from quantum interactions between nanoscale emitters,” says Joel Grim, physicist at NRL. “It’s an exciting capability because of its potential to enable the qubit scaling necessary to realize future quantum technologies on a chip.”
1. J. Q. Grim et al., Nat. Mater. (2019); https://www.nature.com/articles/s41563-019-0418-0.