April 2025

Abstracts of the Quantum Center Lunch Seminar

Date: Thursday, April 3, 2025
Place: ETH Zurich, Hönggerberg, HPF G 6
Time: 12:00 - 13:30

Neural network quantum states for many body physics

Matija Medvidovic - Carrasquilla Group (Carrasquilla Group), ETH Zurich

Variational quantum calculations have borrowed many tools and algorithms from the machine learning community in the recent years. Leveraging great expressive power and efficient gradient-based optimization, researchers have shown that trial states inspired by deep learning problems can accurately model many-body correlated phenomena in spin, fermionic and qubit systems. In this talk, central equations of different flavors variational quantum calculations will be discussed, including ground state search, real time evolution and overlap optimization. An emphasis is put on the geometry of the variational manifold as well as bottlenecks in practical calculations.

Perovskite QDs as novel quantum light sources

Chenglian Zhu - Kovalenko Lab - Functional Inorganic Materials (Kovalenko Group), ETH Zurich

Lead halide perovskite (LHP) quantum dots (QDs), the new generation of colloidal QDs, have garnered peculiar attention since its first synthesis by Protesescu et al.1 in 2015 due to their compelling optoelectronic properties, combined with the low cost, facile synthesis. Their versatility as building blocks in various optoelectronic applications2, particularly as efficient quantum emitters, motivates systematic studies into the mechanism underlying their excellent optical properties. LHP QDs, such as CsPbBr3 QDs, have been shown to generate coherent single photons even under non-resonant excitation and without an external cavity3. This talk will focus on the mechanisms responsible for their coherent photon emission, with optical coherence time (T₂) and radiative lifetime (T₁) approaching the Fourier limit. On the one hand, the primary reason for the optical decoherence (T_2^*), exciton-phonon coupling, is quantified via the magnitude of phonon replicas in the photoluminescence (PL) spectra of individual QDs at cryogenic temperatures4. Specifically, we revealed the highly size-dependent coupling strength to low-energy surface-enhanced phonon modes, which is governed by a deformational-potential mechanism. This reflects the reduced dephasing in larger QDs, crucial to achieve a long optical coherence time (T_2). On the other hand, achieving rapid radiative decay (shorter T_1) is highly desired. Instead of the commonly applied technique of combining emitters with plasmonic or dielectric microcavities to boost the local density of state (LDOS) via the Purcell effect, we demonstrated enhanced oscillator strength in weakly confined CsPbX3 (X = Br, Br/Cl) QDs, attributed to strong electron/hole correlations5. This leads to a sub-100 ps radiative lifetime, comparable to the reported optical coherent time3. These findings provide comprehensive insight into the mechanisms that enable coherent single-photon emission from LHP QDs, paving the way for future optimization and their integration into quantum photonic devices.

References

1. Protesescu, L. et al. Nanocrystals of cesium lead halide perovskites (CsPbX3, X= Cl, Br, and I): novel optoelectronic materials showing bright emission with wide color gamut. Nano Lett. 15, 3692-3696 (2015). 
2. Kovalenko, M. V., Protesescu, L. & Bodnarchuk, M. I. Properties and potential optoelectronic applications of lead halide perovskite nanocrystals. Science 358, 745-750 (2017). 
3. Utzat, H. et al. Coherent single-photon emission from colloidal lead halide perovskite quantum dots. Science 363, 1068-1072 (2019).
4. Zhu, C. et al. Quantifying the Size‐Dependent Exciton‐Phonon Coupling Strength in Single Lead‐Halide Perovskite Quantum Dots. Adv. Opt. Mater., 2301534 (2024). 
5. Zhu, C. et al. Single-Photon Superradiance in Individual Caesium Lead Halide Quantum Dots. Nature 626, 535-541 (2024).