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1. Spatio-spectral quantum state estimation of photon pairs from optical fiber using stimulated emission

   https://doi.org/10.1364/OPTICAQ.547491  

   Kim, Dong Beom; Hu, Xiye; U’Ren, Alfred B; Garay-Palmett, Karina; Lorenz, Virginia O (January 2025, Optica Quantum)

   Developing a quantum light source that carries more than one bit per photon is pivotal for expanding quantum information applications. Characterizing a high-dimensional multiple-degree-of-freedom source at the single-photon level is challenging due to the large parameter space as well as limited emission rates and detection efficiencies. Here, we characterize photon pairs generated in optical fiber in the transverse-mode and frequency degrees of freedom by applying stimulated emission in both degrees of freedom while detecting in one of them at a time. This method may be useful in the quantum state estimation and optimization of various photon-pair source platforms in which complicated correlations across multiple degrees of freedom may be present. 

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2. Spectral-temporal-spatial customization via modulating multimodal nonlinear pulse propagation

   https://doi.org/10.1038/s41467-024-46244-5  

   Qiu, Tong; Cao, Honghao; Liu, Kunzan; Yu, Li-Yu; Levy, Manuel; Lendaro, Eva; Wang, Fan; You, Sixian (December 2024, Nature Communications)

   Abstract Multimode fibers (MMFs) are gaining renewed interest for nonlinear effects due to their high-dimensional spatiotemporal nonlinear dynamics and scalability for high power. High-brightness MMF sources with effective control of the nonlinear processes would offer possibilities in many areas from high-power fiber lasers, to bioimaging and chemical sensing, and to intriguing physics phenomena. Here we present a simple yet effective way of controlling nonlinear effects at high peak power levels. This is achieved by leveraging not only the spatial but also the temporal degrees of freedom during multimodal nonlinear pulse propagation in step-index MMFs, using a programmable fiber shaper that introduces time-dependent disorders. We achieve high tunability in MMF output fields, resulting in a broadband high-peak-power source. Its potential as a nonlinear imaging source is further demonstrated through widely tunable two-photon and three-photon microscopy. These demonstrations provide possibilities for technology advances in nonlinear optics, bioimaging, spectroscopy, optical computing, and material processing. 

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3. Continuous entanglement distribution from an AlGaAs-on-insulator microcomb for quantum communications

   https://doi.org/10.1364/OPTICAQ.510032  

   Steiner, Trevor J.; Shen, Maximilian; Castro, Joshua E.; Bowers, John E.; Moody, Galan (January 2023, Optica Quantum)

   Using an aluminum gallium arsenide microring resonator, we demonstrate a bright quantum optical microcomb with >300 nm (>40 THz) bandwidth and more than 20 sets of time–energy entangled modes, enabling spectral demultiplexing with simple, off-the-shelf commercial telecom components. We report high-rate continuous entanglement distribution for two sets of entangled-photon pair frequency modes exhibiting up to 20 GHz/mW²pair generation rate. As an illustrative example of entanglement distribution, we perform a continuous-wave time-bin quantum key distribution protocol with 8 kbps sifted key rates while maintaining less than 10% error rate and sufficient two-photon visibility to ensure security of the channel. When the >20 frequency modes are multiplexed, we estimate >100 kbps entanglement-based key rates or the creation of a multi-user quantum communications network. The entire system requires less than 110 µW of on-chip optical power, demonstrating an efficient source of entangled frequency modes for quantum communications. As a proof of principle, a quantum key is distributed across 12 km of deployed fiber on the University of California Santa Barbara (UCSB) campus and used to encrypt a 21 kB image with <9% error. 

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4. Probing electronic decoherence with high-resolution attosecond photoelectron interferometry

   https://doi.org/10.1140/epjd/s10053-022-00438-y  

   Busto, David; Laurell, Hugo; Finkelstein-Shapiro, Daniel; Alexandridi, Christiana; Isinger, Marcus; Nandi, Saikat; Squibb, Richard J.; Turconi, Margherita; Zhong, Shiyang; Arnold, Cord L.; et al (July 2022, The European Physical Journal D)

   Abstract Quantum coherence plays a fundamental role in the study and control of ultrafast dynamics in matter. In the case of photoionization, entanglement of the photoelectron with the ion is a well-known source of decoherence when only one of the particles is measured. Here, we investigate decoherence due to entanglement of the radial and angular degrees of freedom of the photoelectron. We study two-photon ionization via the 2s2p autoionizing state in He using high spectral resolution photoelectron interferometry. Combining experiment and theory, we show that the strong dipole coupling of the 2s2p and 2p $$^2$$ 2 states results in the entanglement of the angular and radial degrees of freedom. This translates, in angle-integrated measurements, into a dynamic loss of coherence during autoionization. Graphic Abstract 

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5. Growing Quantum Communication Capacity with Spatial Modes of Optical Fiber

   M. Vasilyev (November 2021, IEEE MetroCon conference, Fort Worth, TX, November 3, 2021)

   Quantum communication links and networks are needed for secure information exchange and for interconnecting future quantum computers. However, their capacity decreases exponentially with distance due to the effect of fiber attenuation that cannot be undone by amplification (although quantum repeaters are an active area of research, they are almost as hard to build as quantum computers). Hence, the only way to increase the quantum communication capacity is by employing more degrees of freedom (optical modes) over which this information can be encoded and transmitted. While frequency (WDM), temporal, and polarization modes have already been exploited for this purpose, the use of many spatial modes has only recently become possible owing to the development of low-loss few-mode fibers (FMFs). This talk will present the work of Prof. M. Vasilyev’s research group on the development of two key enablers of quantum communication over spatial modes of FMFs: 1) generator of spatially-entangled photon pairs and 2) receiver sub-system that can perform projective measurements that alternate between two sets of mutually unbiased bases in a given spatial mode space (this sub-system can also perform dynamically reconfigurable de-multiplexing of spatial modes of the FMF). Both of the above devices / sub-systems are based on the spatial-mode-selective quantum frequency conversion process, implemented in a medium with either second-order nonlinearity (multimode lithium niobate waveguide) or third-order nonlinearity (custom-made FMF). The talk will introduce the principles of their operation, as well as the recent experimental results obtained in both media. 

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