Hyper-entanglement from ultra-bright photon pair sources (HYPER-U-P-S)

We will fabricate and exploit an entirely novel photonic device platform for the generation of highly indistinguishable and entangled photon pairs with near-unity extraction efficiency. The envisioned implementation consists of a quantum dot embedded in engineered photonic environment. We predict that this device will generate very high rate of polarization entangled photons pairs and, in combination with time-bin entanglement, hyper-entangled quantum states. Last but not the least, we will investigate the performance of this device in both free space and fibre based quantum networks, getting therefore closer to the establishment of an operating quantum system for real-life quantum communication.

Within the project "Hyper-entanglement from ultra-bright photon pair sources", which was initiated in 2017 in the QuantERA funding network together with other European universities, the researchers focused on increasing the yield of entangled quantum states of pairs of photons from the solid state. When two photons are entangled, they are correlated beyond the classical limit. For example, their direction of oscillation (polarization) is always the same (or opposite), regardless of the reference frame. This entanglement of particles can be used in novel technologies and to set up a quantum network that serves secure communication and transmission of quantum information. Small sets of semiconductor atoms embedded in another semiconductor material, so-called quantum dots, are used to generate entangled photon pairs. However, not all properties of such quantum dots are suitable for creating a network architecture. For example, many photon pairs are lost inside the chip, whereas the wavelength of the photons and the degree of entanglement varies from quantum dot to quantum dot. This complicates the compatibility to use them in a quantum network. JKU researchers are therefore working on the growth, as well as the integration of quantum dots made of gallium arsenide with optical resonators for increased light yield, as well as the connection with piezoelectric actuators to match the wavelength and increase the degree of entanglement. First, quantum dots were integrated into a planar antenna, which shows an increase in yield from 1% to 20%. However, to further increase the extraction efficiency of emitted light, other optical microstructures require precise placement of the quantum dot. For this purpose, a fluorescence microscope was built, enabling the extraction of quantum dot positions. Thus, the so-called circular "bullseye" structure, can be tailored to fit precisely around the quantum dot and transferred to the semiconductor in a plasma etching process. The chip was bonded to a piezoelectric actuator which allows fine tuning of the wavelength of quantum dots, as well as increasing the degree of entanglement of photon pairs to 96%. The yield of photons could be increased to 77% using the "bullseye" structure. In the course of this project, mechanisms resulting in a reduction of entanglement were also investigated. For example, a laser-induced effect was observed upon excitation of the quantum dot, which shifts internal energy levels and becomes especially evident at long laser pulses. A comparison to quantum dots made of indium gallium arsenide was also performed, providing evidence that the nuclear spin of indium atoms in the quantum dot can reduce the degree of entanglement. Exploring entanglement-reducing mechanisms in quantum dots combined with optical microstructures and piezoelectric actuators will help us to better understand the generation of entangled photon pairs and provide devices for the realization of quantum networks.