The prospect of quantum networks is pushing technical advances in entangled photon generation, and different solutions have now the potential to coexist. After reviewing the development that led to implement entanglement-based quantum key distribution using a quantum dot—a technology motivated by the goal of on-demand operation—in an urban free-space optical link, we present its extension to a three-node quantum network including a source based on spontaneous parametric down conversion. Using separable measurements, we combine intrinsically independent sources to demonstrate a significant violation of a Bell-like inequality associated to nonlocal correlations in a tripartite hybrid network.

Boron vacancy color centers in hBN have drawn substantial interest over the past year because of the potential for quantum sensing in van der Waals heterostructures. Here, we describe strong localized enhancement and redshifting of boron vacancy luminescence at creases in an hBN flake measured with correlative photoluminescence and cathodoluminescence microscopies. These results are consistent with density functional theory calculations showing boron vacancy migration toward regions with uniaxial compressive strain, and they are essential for the development of new quantum devices that leverage the optically accessible spin state in boron vacancy color centers in hBN.

Over the past three decades, graphene has become the prototypical platform for discovering topological phases of matter. Both the Chern C∈Z and quantum spin Hall υ∈Z2 insulators were first predicted in graphene, which led to a veritable explosion of research in topological materials. We introduce a new topological classification of two-dimensional matter – the optical N-phases N∈Z. This topological quantum number is connected to polarization transport and captured solely by the spatiotemporal dispersion of the susceptibility tensor χ. We verify N ≠ 0 in graphene with the underlying physical mechanism being repulsive Hall viscosity. An experimental probe, evanescent magneto-optic Kerr effect (e-MOKE) spectroscopy, is proposed to explore the N-invariant. We also develop topological circulators by exploiting gapless edge plasmons that are immune to back-scattering and navigate sharp defects with impunity. Our work indicates that graphene with repulsive Hall viscosity is the first candidate material for a topological electromagnetic phase of matter.

Due to the constantly increasing demand for single photon detection at telecom wavelengths, superconducting micron-scale bridges (SMSPDs) are attracting attention as a feasible alternative to superconducting nanowire superconducting detectors (SNSPDs). Simple geometry, combined with tunable wavelength response and compatibility with affordable, large-scale fabrication processes, make SMSPDs an alternative to conventional single photon detectors. SMSPDs exhibit short recovery times and good temporal resolution, enabling integration into photonic circuits where high response rate is essential for reliable operation. We realized 2 µm wide NbTiN microbridge single photon detector integrated with a SiO2/TiO2 Bragg resonator, for the telecom C-band. Furthermore, we correlate the microbridge geometry with observed behavior to validate the quality of fabricated structures and monitor its effect on detector performance.

Color centers in silicon carbide have key properties required for quantum networking, simulation and computing applications. We explore how their integration with nanophotonic devices can enable scalable and high performing hardware.

This talk will review our recent studies on the enhanced emission near ENZ thin film. Our study employs thin homogeneous titanium nitride (TiN) films with ENZ wavelengths in the visible spectrum that match a wide excitation bandwidth of quantum dots and 2D emitters to probe ENZ-emitter interaction. Our results show that the ENZ substrate enhances excitation fields in the MoS2 monolayer and hence enhances its spontaneous emission. This study will enrich the fundamental understanding of spontaneous emission on ENZ substrates that might be useful for the development of advanced nanophotonic light sources.

Coupling optical or vibrational transitions to a single mode of an optical cavity has the potential to enable nonlinear-optical applications, quantum transduction, and control of chemical pathways. However, previous experiments have been limited to cryogenic temperatures. By coupling single colloidal quantum dots to plasmonic nanocavities, we have demonstrated induced transparency and strong coupling at room temperature. Second-harmonic-generation experiments on single gold nanorods strongly coupled to a monolayer transition metal dichalcogenide point towards strong nonlinear-optical effects in these systems.

Cavity quantum electrodynamics describes coupled systems comprising of optically active emitters with atom-like transitions, such as single-photon emitters (SPEs), and optical cavities that underlies various fundamental quantum properties. In the strong coupling regime, SPEs and cavity photons coherently exchange energy leading to two new hybrid states that significantly modify the optical responses of the originally uncoupled or weakly coupled states. As a result, various intriguing effects could be realized such as Rabi oscillations, nonlinearities, photon blockade when the system is in strong coupling conditions. To achieve strong coupling between SPEs and photonic cavities, SPE transition energies have to coincide with cavity resonances (spectral overlapping), SPEs are precisely positioned at the electric field antinodes (spatial overlapping) and SPE emission decay is slower than coupling strength (narrow linewidth). Such stringent conditions impose serious challenges for experimental realization of SPE-cavity strong coupling particularly at room temperature due to the lack of stable single-photon sources with high color purity at high temperatures and controllability on positioning SPE in cavity. Strong coupling has been reported for single semiconducting quantum-dots and photonic crystal cavities at cryogenic temperature1,2 and for single molecules and plasmonics cavities at room temperature.3 Here, we introduce a new approach using point-defects purposely created in a two-dimensional few-layer-thick hexagonal boron nitride (hBN) film as single-photon emitters and a robust photonic cavity with arbitrarily high quality-factor based on bound-state-in-the-continuum (BIC) concept.4 We observe, at room temperature and in ambient conditions, strong coupling between SPEs and BIC photons characterized by a Rabi splitting of ∼7 meV. The coupling strength can be tuned by varying detuning energy that is strongly supported by theoretical calculation. Our findings unveil new opportunities for exploiting the BIC cavity to realize the long-sought strong coupling with SPEs, ultimately for the development of quantum-based devices operating at ambient conditions. 1Reithmaier, J. P. et al. Nature 432, 197-200 (2004) 2Yoshie, T. et al. Nature 432, 200-203 (2004) 3Chikkaraddy, R. et al. Nature 535, 127-130 (2016) 4Do, T. T. H. et al., manuscript in preparation (2022)

Tunable entangled photon emitters based on cavity-enhanced GaAs quantum dots on micromachined piezoelectric substrates was recorded at SPIE Optics + Photonics held in San Diego, California, United States 2022.

In this talk, I will present several design concepts for nanolasers based on collective resonances of dielectric nanoantennas. The interference of collective resonances associated with the bound state in the continuum (BIC) or Van Hove singularity will be discussed in detail based on Mie theory analysis. I will show experimentally and theoretically various directional nanolasers made out of GaAs at cryogenic temperature. By using a more efficient gain material such as CdSe/CdxZn1-xS nanoplatelets or InGaP multi-quantum well, room temperature lasing operation is also illustrated. This work presents design guidelines for high-performance in-plane and out-of-plane lasers, which may find broad applications in nanophotonics.

Interference on thin-film and metamaterial absorbers enables coherent control and processing of quantum light. Recently, this phenomenon was used to demonstrate deterministic control of photon absorption probability, quantum states filtering, anti-Hong-Ou-Mandel interference, and application of geometric (Berry) phase for remote control of light dissipation. Here, we expand these ideas by introducing the regime of distributed coherent absorption where light quanta are absorbed within spatially separated active layers. We show that this scheme allows photon number discriminating detection free from the limitations of conventional temporal and spatial multiplication approaches. Free space and integrated designs are discussed.

We propose a scheme for the generation of highly indistinguishable single photons using semiconductor quantum dots and demonstrate its performance and potential. The scheme is based on the resonant two-photon excitation of the biexciton followed by stimulation of the biexciton to selectively prepare an exciton. Quantum-optical simulations and experiments are in good agreement and show that the scheme provides significant advantages over previously demonstrated excitation methods. Specifically, the scheme allows for ultra-low multi-photon error rates, high indistinguishability, high brightness and programmable linear polarization.

In recent years, Superconducting Nanowire Single-Photon Detectors (SNSPDs) have obtained tremendous attention as a possible key technology for photonic quantum processing and faint light detection. Here, we present our recent progress on engineering of the properties of NbTiN SNSPDs fabricated on various substrates measured. We discuss approaches to simultaneously improve the most important figures of merit (DCR, dead time, timing jitter, efficiency) as well as efficient characterization methods. Specifically, for the latter we investigate the impact of the substrate material on the performance parameters as well as the fundamental interrelation of the resulting voltage pulse properties, such as pulse height, rise time and timing jitter, and their dependence on the applied bias current.

Semiconductor quantum dots (QDs) are able to confine single charges on the nanoscale in all three dimensions of space, making them excellent systems for exploring quantum phenomena. In particular, QDs have demonstrated outstanding performance as sources of entangled and indistinguishable photon pairs, properties highly desired in the fields of quantum communication and -information processing. Here I report on the advances of QDs as potential resources for photonic quantum networks, which allow to overcome the fundamental range limitations of single photon-based applications. After an introduction to the underlying mechanisms of entangled photon pair generation, I demonstrate several building blocks of quantum networks, with quantum key distribution as a prime application.

We fabricate multiple circular Bragg gratings (CBGs) operating in the telecom O-band for Purcell-enhanced single photon emission and investigate the reproducibility with a rigorous characterization. Furthermore, we present a design optimization of CBGs operating around 1550 nm and propose a modified device design directly compatible with electric field control, reporting Purcell factors up to 20 and collection efficiency in NA=0.65 close to 70% for the whole telecom C-band. These results pave the way for the large-scale fabrication of highly efficient semiconductor quantum light sources for long-distance quantum communication systems.

We propose a new paradigm for the synthesis of new materials consisting in modifying the motion of valence and conduction electrons in atomically thin materials through the image interaction with neutral structures placed in their vicinity.