Periodic Reporting for period 4 - SPQRel (Entanglement distribution via Semiconductor-Piezoelectric Quantum-Dot Relays)
Periodo di rendicontazione: 2020-11-01 al 2021-11-30
The SPQRel project focuses on the fabrication and study of near-ideal sources of non-classical light, which enable the construction of a quantum network for the distribution of quantum entanglement among distant parties. The sources are based on epitaxial quantum dots integrated onto innovative semiconductor-piezoelectric devices which allow for full control over the quantum-dot in-plane strain tensor. The sources can deterministically generate highly indistinguishable and strongly entangled photons with high efficiency and their emission wavelength can be finely adjusted via the application of voltages to the piezo-actuators without degrading the quality of the emitted photons. This unique feature enables the distribution of entanglement between distant parties and allows building up artificial-natural atomic interfaces where entangled photons are interfaced to absorption resonances in atomic vapors.
The findings of SPQRel have proven that it is possible to build up a quantum network in which photons from quantum dots are used to distribute entanglement over distant nodes. This research is extremely relevant for society, as it will find applications in the field of secure communication and long-distance quantum networking. Moreover, the hybrid-semiconductor piezoelectric technology developed during the SPQRel project has the potential to impact research fields beyond quantum communication, and in particular the field of strain-engineering of two-dimensional materials.
- We have developed a hybrid-semiconductor piezoelectric technology in which semiconductor nano- and micro-membranes are integrated onto micro-machined piezoelectric actuators. We have used this technology with different quantum emitters, including quantum dots and two-dimensional semiconductors, to fabricate non-classical light sources with tunable wavelength.
- We have fabricated and characterized several quantum dot devices and identified the system and the excitation scheme which give the best figures of merit. In particular, we showed that droplet-etched GaAs quantum dots driven under two-photon resonant excitation can deterministically generate indistinguishable entangled photons with tunable wavelength, with unprecedented single photon purity and near-unity degree of entanglement.
- We have shown that single and entangled photons generated on-demand by GaAs quantum dots can be used to implement successfully advanced quantum optics protocols, such as quantum teleportation and entanglement swapping.
- We have interfaced photons from quantum dots with clouds of natural atoms and studied the distortion of a single-photon wavepacket propagating though a dispersive and absorptive medium. Moreover, we have experimentally and theoretically investigated the possibility to interface single photons with a warm-atomic vapor quantum memory.
- We have posed the basis of a quantum-dot based quantum network: We have developed a free-space quantum communication channel and successfully performed quantum key distribution experiments in which entangled photons from quantum dots are distributed across two different buildings located few hundred meters apart. Then, we have combined photons from a quantum dot and non-linear crystal to demonstrate non-classical correlations among three nodes of primitive quantum network. Finally, we have successfully demonstrated quantum interference between photons from remote quantum dots and theoretically investigated the fidelities that can be achieved in a chain of quantum-dot relays that perform multiple swapping operations.
The results obtained within the SPQRel project are contained in several scientific publications (including Physical Review letters, Science Advances, Nature Communications, Nano Letters, etc.) and were presented at international conferences, workshops, and colloquia. They were also advertised in non-specialized journals as well as in a youtube video. The SPQRel also supported two international workshops as well as one summer school. Finally, the SPQRel project enabled the establishment of a new research group that is internationally recognized for its pioneering work on quantum optics with semiconductor nanostructures.
Our results related to near-ideal sources of entangled photons that can be used for quantum teleportation and entanglement swapping certainly represent the most important breakthroughs of the SPQRel project, which firstly showed that GaAs quantum dots are one of the most promising platforms for scalable photonic quantum technologies. In the last part of the project, we have also brought the technology outside the research laboratories, with a pioneering demonstration of free-space quantum key distribution protocol in which entangled photons are distributed between distant parties in an urban communication scenario. These achievements, combined with the successful interfacing of remote quantum dots, of quantum dots and non-linear crystals, as well as quantum dots and clouds of natural atoms, show that the SPQRel project has opened the path to the forthcoming realization of a practical solid-state based quantum network for long distance quantum communication.