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1. The Born Versus Heisenberg Quantum-Vacuum Controversy and Beyond

   https://doi.org/10.1007/978-3-031-55463-6_8  

   Roso, Luis; Lera, Roberto; Ravichandran, Smrithan; Longman, Andrew; He, Calvin Z; Pérez-Hernández, José Antonio; Apiñaniz, Jon I; Mahnot, Rohan; Mateu, Vicent; Fedosejevs, Robert; et al (May 2024, Springer Nature Switzerland)

   Yamanouchi, K; DiMauro, L F; Hill_III, W T (Ed.)

   Photon-photon collisions, as one of the fundamental processes in quantum physics, have attracted a lot of attention. However, most effort has been focused on photons energetic enough to create particle-antiparticle pairs. The low energy limit—e.g., optical photons—has attracted less attention because of their extremely low collision cross section. By optical photons we mean UV, visible and infrared, although the cutting edge of extreme lasers is in the near infrared. The Schwinger critical field for pair generation seems not possible, at least directly, with the current laser technology. This often is considered as a problem, but we view this as an asset; the near impossibility of pair production via photon-photon scattering in the infrared is a perfect scenario to study virtual pairs that characterize Dirac’s quantum vacuum. Moreover, it is remarkable that this scenario of photon-photon collisions was already studied in the 1930s by two of the fathers of Quantum Mechanics, among others, at the dawn of this theory. In their respective papers, however, Born and Heisenberg arrived to different conclusions regarding the birefringence of vacuum. This controversy is still an open question that will be solved soon, we hope, with upcoming experiments. Here, we discuss a possible photon-photon collision experiment with extreme lasers, and will show that it can provide measurable effects, allowing fundamental information about the essence of Quantum Electrodynamics and its Lagrangian to be extracted. A possible experimental scenario with two ultra-intense pulses for detecting photon-photon scattering is analyzed. This would need a high-precision measurement, with control of temporal and spatial jitter, and noise. We conclude that such an experiment is barely feasible at $$10^{23}$$ W/cm$^2$ (today’s intensity record) and very promising at 1$$0^{24}$$ W/cm$^2$. 

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2. InGaP χ(2) integrated photonics platform for broadband, ultra-efficient nonlinear conversion and entangled photon generation

   https://doi.org/10.1038/s41377-024-01653-5  

   Akin, Joshua; Zhao, Yunlei; Misra, Yuvraj; Haque, A_K_M Naziul; Fang, Kejie (December 2024, Light: Science & Applications)

   Abstract Nonlinear optics plays an important role in many areas of science and technology. The advance of nonlinear optics is empowered by the discovery and utilization of materials with growing optical nonlinearity. Here we demonstrate an indium gallium phosphide (InGaP) integrated photonics platform for broadband, ultra-efficient second-order nonlinear optics. The InGaP nanophotonic waveguide enables second-harmonic generation with a normalized efficiency of 128, 000%/W/cm²at 1.55μm pump wavelength, nearly two orders of magnitude higher than the state of the art in the telecommunication C band. Further, we realize an ultra-bright, broadband time-energy entangled photon source with a pair generation rate of 97 GHz/mW and a bandwidth of 115 nm centered at the telecommunication C band. The InGaP entangled photon source shows high coincidence-to-accidental counts ratio CAR > 10⁴and two-photon interference visibility > 98%. The InGaP second-order nonlinear photonics platform will have wide-ranging implications for non-classical light generation, optical signal processing, and quantum networking. 

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3. Staring at the Sun with the Keck Planet Finder: An Autonomous Solar Calibrator for High Signal-to-noise Sun-as-a-star Spectra

   https://doi.org/10.1088/1538-3873/ad0b30  

   Rubenzahl, Ryan A; Halverson, Samuel; Walawender, Josh; Hill, Grant M; Howard, Andrew W; Brown, Matthew; Ida, Evan; Tehero, Jerez; Fulton, Benjamin J; Gibson, Steven R; et al (December 2023, Publications of the Astronomical Society of the Pacific)

   Abstract Extreme precision radial velocity (EPRV) measurements contend with internal noise (instrumental systematics) and external noise (intrinsic stellar variability) on the road to 10 cm s⁻¹“exo-Earth” sensitivity. Both of these noise sources are well-probed using “Sun-as-a-star” RVs and cross-instrument comparisons. We built the Solar Calibrator (SoCal), an autonomous system that feeds stable, disk-integrated sunlight to the recently commissioned Keck Planet Finder (KPF) at the W. M. Keck Observatory. With SoCal, KPF acquires signal-to-noise ratio (S/N) ∼ 1200,R= 98,000 optical (445–870 nm) spectra of the Sun in 5 s exposures at unprecedented cadence for an EPRV facility using KPF’s fast readout mode (<16 s between exposures). Daily autonomous operation is achieved by defining an operations loop using state machine logic. Data affected by clouds are automatically flagged using a reliable quality control metric derived from simultaneous irradiance measurements. Comparing solar data across the growing global network of EPRV spectrographs with solar feeds will allow EPRV teams to disentangle internal and external noise sources and benchmark spectrograph performance. To facilitate this, all SoCal data products are immediately available to the public on the Keck Observatory Archive. We compared SoCal RVs to contemporaneous RVs from NEID, the only other immediately public EPRV solar data set. We find agreement at the 30–40 cm s⁻¹level on timescales of several hours, which is comparable to the combined photon-limited precision. Data from SoCal were also used to assess a detector problem and wavelength calibration inaccuracies associated with KPF during early operations. Long-term SoCal operations will collect upwards of 1000 solar spectra per six-hour day using KPF’s fast readout mode, enabling stellar activity studies at high S/N on our nearest solar-type star. 

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4. Low-noise frequency-agile photonic integrated lasers for coherent ranging

   https://doi.org/10.1038/s41467-022-30911-6  

   Lihachev, Grigory; Riemensberger, Johann; Weng, Wenle; Liu, Junqiu; Tian, Hao; Siddharth, Anat; Snigirev, Viacheslav; Shadymov, Vladimir; Voloshin, Andrey; Wang, Rui Ning; et al (December 2022, Nature Communications)

   Abstract Frequency modulated continuous wave laser ranging (FMCW LiDAR) enables distance mapping with simultaneous position and velocity information, is immune to stray light, can achieve long range, operate in the eye-safe region of 1550 nm and achieve high sensitivity. Despite its advantages, it is compounded by the simultaneous requirement of both narrow linewidth low noise lasers that can be precisely chirped. While integrated silicon-based lasers, compatible with wafer scale manufacturing in large volumes at low cost, have experienced major advances and are now employed on a commercial scale in data centers, and impressive progress has led to integrated lasers with (ultra) narrow sub-100 Hz-level intrinsic linewidth based on optical feedback from photonic circuits, these lasers presently lack fast nonthermal tuning, i.e. frequency agility as required for coherent ranging. Here, we demonstrate a hybrid photonic integrated laser that exhibits very narrow intrinsic linewidth of 25 Hz while offering linear, hysteresis-free, and mode-hop-free-tuning beyond 1 GHz with up to megahertz actuation bandwidth constituting 1.6 × 10¹⁵Hz/s tuning speed. Our approach uses foundry-based technologies - ultralow-loss (1 dB/m) Si₃N₄photonic microresonators, combined with aluminium nitride (AlN) or lead zirconium titanate (PZT) microelectromechanical systems (MEMS) based stress-optic actuation. Electrically driven low-phase-noise lasing is attained by self-injection locking of an Indium Phosphide (InP) laser chip and only limited by fundamental thermo-refractive noise at mid-range offsets. By utilizing difference-drive and apodization of the photonic chip to suppress mechanical vibrations of the chip, a flat actuation response up to 10 MHz is achieved. We leverage this capability to demonstrate a compact coherent LiDAR engine that can generate up to 800 kHz FMCW triangular optical chirp signals, requiring neither any active linearization nor predistortion compensation, and perform a 10 m optical ranging experiment, with a resolution of 12.5 cm. Our results constitute a photonic integrated laser system for scenarios where high compactness, fast frequency actuation, and high spectral purity are required. 

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5. A General Strategy for Enhancing Sensitivity and Suppressing Noise in Infrared Organic Photodetectors Using Non‐Conjugated Polymer Additives

   https://doi.org/10.1002/adfm.202314210  

   Bills, Tyler; Liu, Chih‐Ting; Lim, Jasmine; Eedugurala, Naresh; Mahalingavelar, Paramasivam; Seo, Bogyeom; Hanna, Ethan_T; Ng, Tse_Nga; Azoulay, Jason_D (February 2024, Advanced Functional Materials)

   Abstract Photodetectors operating across the near‐ to short‐wave infrared (NIR–SWIR,λ= 0.9–1.8 µm) underpin modern science, technology, and society. Organic photodiodes (OPDs) based on bulk‐heterojunction (BHJ) active layers overcome critical manufacturing and operating drawbacks inherent to crystalline inorganic semiconductors, offering the potential for low‐cost, uncooled, mechanically compliant, and ubiquitous infrared technologies. A constraining feature of these narrow bandgap materials systems is the high noise current under an applied bias, resulting in specific detectivities (D^*, the figure of merit for detector sensitivity) that are too low for practical utilization. Here, this study demonstrates that incorporating wide‐bandgap insulating polymers within the BHJ suppresses noise by diluting the transport and trapping sites as determined using capacitance‐frequency analysis. The resultingD^*of NIR–SWIR OPDs operating from 600–1400 nm under an applied bias of −2 V is improved by two orders of magnitude, from 10⁸to 10¹⁰ Jones (cm Hz^1/2 W⁻¹), when incorporating polysulfone within the blends. This broadly applicable strategy can reduce noise in IR‐OPDs enabling their practical operation and the realization of emerging technologies. 

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