The ability to generate entangled photons states on demand is an essential resource for scaling up the complexity of the current quantum communication technologies. Epitaxial quantum dots have emerged as a leading candidate for overcoming the limit of probabilistic photon generation. Thanks to this advantage, their use is being pushed in an increasing number of applications. However, the potential of this technology has yet to be fully exploited.
The objective of this project is to experimentally investigate the possibility of generating different entangled states by exciting different charge complexes in semiconductor quantum dots. Entangled photons are usually generated in a specific polarization-entangled state via the mechanism of the ground biexciton-exciton radiative cascade. However, theoretical and experimental studies have shown that different spin complexes can be created inside a quantum dot, a resource which in principle could be harnessed to generate different entangled states. This could be used to actively switch operation mode and increase the functionalities of the source.
A combination of state-of-the-art skills and know-how in the fields of materials science and quantum optics will be crucial to tackle the challenges of this project. Based on the current state of research, GaAs/AlGaAs nanostructures grown by droplet etching epitaxy are selected as the entangled photon emitters. An experimental apparatus will be set up to spectrally isolate specific emission lines corresponding to different excitonic complexes and to investigate the presence and quality of entanglement in polarization via two-photon quantum state tomography.
The primary goal of the project is to address the lack of studies in the literature which investigate the presence of entanglement when a QD system is initiated in a non-trivial biexciton state. As QDs consolidate their position as one of the most viable candidates for entangled photon generation in quantum communication and photonic simulation applications, it becomes paramount to improve our understanding of the full capabilities of the technology. Previous studies devoted to the characterization of excitonic complexes in various materials systems suggest that the goal is realistic and that excited biexciton states could exist with configurations of angular momentum which can lead to the emission of different polarization entangled states.
This research direction has interesting implications for the use of these photon sources in several quantum information schemes. The more appealing perspective is the potential ability of actively switching the emitted entangled photon state at the source level. This could be achieved for example by adopting multiplexing and ultrafast optical switching on the excitation laser, which is a strategy preferrable with respect to performing the same on the entangled photons. Indeed, in spite of viable optimizations [1], inserts of this kind come with a certain amount of attenuation, and the efficiency of quantum information protocols always relies on the ability to preserve single photon signals from losses. Simpler implementations of the proposed concept have been realized to demonstrate ultrafast switching of single photon sources based on single QDs [2].
Another potential impact concerns the standard entanglement generation process as well. Even if the system can be initiated in the ground biexciton state with very high fidelity, above 90% [3], it is known that parasitic emission lines can be a limiting factor for efficiency. These can be due precisely to the unwanted formation of excited biexciton complexes [4]. Understanding and characterizing better such processes can therefore help to further improve the already excellent deterministic nature of entangled pair generation in QDs.
Based on the unrivaled figures of entanglement fidelity achieved so far and on the rich list of already observed excitonic complexes, I identify droplet etching GaAs QDs as the most suitable photon source. An additional advantage is the convenient wavelength of emission, which is tunable around 785 nm. This is a standard value for quantum optics in the near infrared, matches the spectral window of high detection efficiency of silicon single-photon avalanche photodiodes, and it is compatible in prospect with a rubidium-based quantum memory, a promising pathway towards the landmark realization of a quantum repeater [5].
Having access to bright QD devices through existing scientific collaborations and combining experimental expertise in materials science and in advanced quantum optics, the research group I work in possesses the optimal and uncommon set of skills required to undertake the goal of the project. It will be possible to perform the analysis described so far by including specific elements in an advanced photoluminescence spectroscopy setup. In particular, starting from the optical excitation of the system, to have control over the excitation of specific multi-excitonic states, a custom-made pulse shaper and an unbalanced Mach-Zehnder delay line will be included. In the collection path, an apparatus will be devised to spectrally separate the various emission lines in an efficient way. Cutting edge equipment will be critical to reduce any losses due to the setup. In particular, the following will be used: high-performance spectral filters, fiber collimators and polarization elements, such as waveplates and beam splitters.
[1] Hall, M. A., ... & Kumar, P. (2011). All-optical switching of photonic entanglement. New Journal of Physics, 13(10), 105004.
[2] Muñoz-Matutano, G., ... & Gurioli, M. (2020). All optical switching of a single photon stream by excitonic depletion. Communications Physics, 3(1), 1-9.
[3] Basso Basset, F., ... & Trotta, R. (2021). Quantum key distribution with entangled photons generated on demand by a quantum dot. Science Advances, 7(12), eabe6379.
[4] Senellart, P., ... & Bloch, J. (2005). Few particle effects in the emission of short-radiative-lifetime single quantum dots. Physical Review B, 72(11), 115302.
[5] Schimpf, C., ..., Basso Basset, F., ... & Rastelli, A. (2021). Quantum dots as potential sources of strongly entangled photons: Perspectives and challenges for applications in quantum networks. Applied Physics Letters, 118(10), 100502.