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Realizing a strong interaction between individual photons is an important objective of research in quantum science and technology. It requires an optical medium in which light experiences a phase shift that depends nonlinearly on the photon number. Once the additional two-photon phase shift reaches π, such an ultra-strong nonlinearity could enable the implementation of high-fidelity quantum logic operations. However, the nonlinear response of standard optical media is orders of magnitude too weak. Here, we demonstrate a fibre-based nonlinearity that realizes an additional two-photon phase shift close to the ideal value of π. We employ a whispering-gallery-mode resonator, interfaced by an optical nanofibre, where the presence of a single rubidium atom in the resonator mode results in a strongly nonlinear response. We show that this results in entanglement of initially uncorrelated incident photons. This demonstration of a fibre-integrated, ultra-strong nonlinearity is a decisive step towards photon-based scalable quantum logics.

Laser cooling based on dark states, i.e. states decoupled from light, has proven to be effective to increase the phase-space density of cold trapped atoms. Dark-states cooling requires open atomic transitions, in contrast to the ordinary laser cooling used for example in magneto-optical traps (MOTs), which operate on closed atomic transitions. For alkali atoms, dark-states cooling is therefore commonly operated on the D transition nS  → nP . We show that, for Rb, thanks to the large hyperfine structure separations the use of this transition is not strictly necessary and that \"quasi-dark state\" cooling is efficient also on the D line, 5S  → 5P . We report temperatures as low as (4.0 ± 0.3) μK and an increase of almost an order of magnitude in the phase space density with respect to ordinary laser sub-Doppler cooling.

In recent years, optical nanofibres have become a promising platform for trapping, manipulating and controlling atomic systems. In this work, I will highlight our recent work on the demonstration of multiphoton processes using optical nanofibres embedded in a Rb MOT for the generation of entangled photons and the excitation of Rydberg atoms for all-fibred quantum networks.

An external cavity diode laser, equipped with a cat-eye reflector setup, an interference filter, and a polarizing beam splitter, has been demonstrated to produce a laser wavelength of approximately 483 nm. This ECDL is based on the affordably priced commercial laser diode, GH04850B2G. Critical characteristics, such as fluctuations in optical power and output wavelength in response to injected current, were investigated. In anticipation of potential experiments involving rubidium Rydberg atoms, a simulation of the electromagnetically induced transparency spectrum was conducted using this laser setup in conjunction with a 780-nm laser.

Glioblastoma (GBM) is a highly aggressive brain tumor with limited treatment options, mainly due to challenges such as incomplete resection and blood–brain barrier limitations. Rubidium, a naturally occurring alkali metal with favorable biocompatibility, widely used in myocardial and tumor perfusion imaging as a blood flow tracer, is repurposed in this study to investigate its potential therapeutic effects and mechanisms against GBM. The impacts of rubidium ions (Rb⁺) on GBM cells were assessed through functional assays evaluating proliferation, migration, invasion, and apoptosis. RNA sequencing and Western blot analyses were employed to investigate molecular mechanisms, while in vivo models were used to evaluate therapeutic efficacy and safety. Rb⁺ treatment significantly suppressed GBM cell proliferation, migration, and invasion, while inducing apoptosis and cell cycle arrest at the G2/M phase. Mechanistic studies revealed that Rb⁺ downregulated the phosphatidylinositol 3-kinase/protein kinase B/mammalian target of rapamycin (PI3K/AKT/mTOR) pathway, contributing to the induction of apoptosis in tumor cells. In vivo, Rb⁺ exhibited potent anti-tumor activity with no detectable adverse effects on major organs, physiological functions, or behavior in mice. Our findings highlight Rb⁺ as a promising and innovative candidate for GBM therapy, leveraging the PI3K/AKT/mTOR pathway to inhibit tumor growth and promote apoptosis. This study underscores the potential of Rb⁺ in addressing the urgent need for novel GBM treatments, warranting further preclinical and clinical investigations.

Experimental realization of a quantum degenerate gas of molecules would provide access to a wide range of phenomena in molecular and quantum sciences. However, the very complexity that makes ultracold molecules so enticing has made reaching degeneracy an outstanding experimental challenge over the past decade. We now report the production of a degenerate Fermi gas of ultracold polar molecules of potassium-rubidium. Through coherent adiabatic association in a deeply degenerate mixture of a rubidium Bose-Einstein condensate and a potassium Fermi gas, we produce molecules at temperatures below 0.3 times the Fermi temperature. We explore the properties of this reactive gas and demonstrate how degeneracy suppresses chemical reactions, making a long-lived degenerate gas of polar molecules a reality.

Hybrid perovskite solar cells often use the more thermally stable formamidinium (FA) cation rather than methylammonium, but its larger size can create lattice distortion that results in an inactive yellow phase. Turren-Cruz et al. show that by using iodide instead of bromide as the anion (to create a redder bandgap) and an optical mix of cesium, rubidium, and FA cations, they can make solar cells with a stabilized efficiency of more than 20%. No heating steps above 100°C were needed to create the preferred black phase. Science , this issue p. 449 Avoidance of bromide anions and methylammonium cations allows optimal tuning of perovskite bandgaps. Currently, perovskite solar cells (PSCs) with high performances greater than 20% contain bromine (Br), causing a suboptimal bandgap, and the thermally unstable methylammonium (MA) molecule. Avoiding Br and especially MA can therefore result in more optimal bandgaps and stable perovskites. We show that inorganic cation tuning, using rubidium and cesium, enables highly crystalline formamidinium-based perovskites without Br or MA. On a conventional, planar device architecture, using polymeric interlayers at the electron- and hole-transporting interface, we demonstrate an efficiency of 20.35% (stabilized), one of the highest for MA-free perovskites, with a drastically improved stability reached without the stabilizing influence of mesoporous interlayers. The perovskite is not heated beyond 100°C. Going MA-free is a new direction for perovskites that are inherently stable and compatible with tandems or flexible substrates, which are the main routes commercializing PSCs.

A wide range of strongly correlated quantum materials, including some high-temperature superconductors, exhibit “electronic nematic” phases, in which the electronic properties spontaneously break the rotational symmetry of the crystal. However, the role that the corresponding nematic fluctuations play in these complicated systems is unclear, motivating the search for simpler model systems. Here, we identify a particular 4 f intermetallic material which exhibits ferroquadrupole order, Y b R u 2 G e 2 , as just such a model system. We also provide a robust and accurate method to probe the divergence of an important associated quantity, the quadrupole-strain susceptibility. The temperature dependence of this quantity provides insight into the nature of the interactions that lead to ferroquadrupole order, in this case, magnetoelastic coupling. Ferroquadrupole order associated with local 4 f atomic orbitals of rare-earth ions is a realization of electronic nematic order. However, there are relatively few examples of intermetallic materials which exhibit continuous ferroquadrupole phase transitions, motivating the search for additional materials that fall into this category. Furthermore, it is not clear a priori whether experimental approaches based on transport measurements which have been successfully used to probe the nematic susceptibility in materials such as the Fe-based superconductors will be as effective in the case of 4 f intermetallic materials, for which the important electronic degrees of freedom are local rather than itinerant and are consequently less strongly coupled to the charge-carrying quasiparticles near the Fermi energy. In the present work, we demonstrate that the intermetallic compound Y b R u 2 G e 2 exhibits a tetragonal-to-orthorhombic phase transition consistent with ferroquadrupole order of the Yb ions and go on to show that elastoresistivity measurements can indeed provide a clear window on the diverging nematic susceptibility in this system. This material provides an arena in which to study the causes and consequences of electronic nematicity.

Statistical mechanics relies on the maximization of entropy in a system at thermal equilibrium. However, an isolated quantum many-body system initialized in a pure state remains pure during Schrödinger evolution, and in this sense it has static, zero entropy. We experimentally studied the emergence of statistical mechanics in a quantum state and observed the fundamental role of quantum entanglement in facilitating this emergence. Microscopy of an evolving quantum system indicates that the full quantum state remains pure, whereas thermalization occurs on a local scale. We directly measured entanglement entropy, which assumes the role of the thermal entropy in thermalization. The entanglement creates local entropy that validates the use of statistical physics for local observables. Our measurements are consistent with the eigenstate thermalization hypothesis.