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Latest articles for Reports on Progress in Physics
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Temporal solitons in hybrid-driven active resonators
Solitons, as coherent structures that maintain their shape while traveling at constant velocity, are ubiquitous across various branches of physics, from fluid dynamics to quantum fields. However, it is within the realm of optics where solitons have not only served as a primary testbed for understanding solitary wave phenomena but have also transitioned into applications ranging from telecommunications to metrology. In the optical domain, temporal solitons are localized light pulses, self-reinforcing via a delicate balance between nonlinearity and dispersion. Among the many systems hosting temporal solitons, active optical resonators stand out due to their inherent gain medium, enabling to actively sustain solitons. Unlike conventional mode-locked lasers, active resonators offer a richer landscape for soliton dynamics through hybrid driving schemes, such as coupling to passive cavities or under external optical injection, affording them unparalleled control and versatility. We discuss key advantages of these systems, with a particular focus on quantum cascade lasers as a promising soliton technology within the class of active resonators. By exploring diverse architectures from traditional Fabry–Perot cavities to racetrack devices operated under external injection, we present the current state-of-the-art and future directions for soliton-based sources in the realm of semiconductor lasers and hybrid integrated photonic systems.
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Pu 5f population: the case for n = 5.0
The quantitative determination of the 5f population in α-Pu and δ-Pu is reconsidered in detail. Trends across the 5f series are discussed, including atomic sizes, 5f populations and computational modeling. A recently developed and novel approach, based upon a thermodynamical evaluation of entropies, is presented. Finally, a detailed spectroscopic analysis of the original Pu N4,5 and O4,5 x-ray absorption spectroscopy has been performed, including the correction of a fundamental flaw in the Electron Energy Loss Spectroscopy (EELS) measurements. Thus, the determination of the 5 f occupation (n) in elemental Pu has been re-evaluated with the result that n = 5.0 ± 0.1 for αPu and n = 4.9 ± 0.2 for δPu. These values are significantly lower than the value of ∼5½ that was propagated earlier.
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Impurities and polarons in bosonic quantum gases: a review on recent progress
This review describes the field of Bose polarons, arising when mobile impurities are immersed into a bosonic quantum gas. The latter can be realized by a Bose–Einstein condensate of ultracold atoms, or of exciton polaritons in a semiconductor, which has led to a series of experimental observations of Bose polarons near inter-species Feshbach resonances that we survey. Following an introduction to the topic, with references to its historic roots and a presentation of the Bose polaron Hamiltonian, we summarize state-of-the-art experiments. Next we provide a detailed discussion of polaron models, starting from the ubiquitous Fröhlich Hamiltonian that applies at weak couplings. Already this highly simplified model allows insights into ultra-violet divergencies, logarithmic and power-law, that need to be properly regularized. To capture the physics near a Feshbach resonance, two-phonon scattering terms on the impurity as well as phonon-phonon interactions need to be included. We proceed by a survey of concurrent theoretical methods used for solving strongly interacting Bose polaron problems, ranging from Lee–Low–Pines mean-field theory, Chevy-ansatz, Gross–Pitaevskii-equation to diagrammatic Monte Carlo approaches. The subsequent sections are devoted to the large bodies of work investigating strong coupling Bose polarons, including detailed comparisons with radio-frequency spectra obtained in ultracold atom experiments; to investigations of universal few-body and Efimov states associated with a Feshbach resonance in atomic mixtures; to studies of quantum dynamics and polarons out of equilibrium; Bose polarons in low-dimensional 1D and 2D quantum systems; induced interactions among polarons and bipolaron formation; and to Bose polarons at non-zero temperatures. We end our review by detailed discussions of closely related experimental setups and systems, including ionic impurities, systems with strong light-matter interactions, and variations and extensions of the Bose polaron concepts e.g. to baths with topological order or strong interactions relevant for correlated electrons. Finally, an outlook is presented, highlighting possible future research directions and open questions in the field as a whole.
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A homogenous polymer design with widely tunable work functions for high-performance two-dimensional photodetectors
Contact electrodes, which significantly influence the Schottky barrier and interfacial quality with two-dimensional (2D) materials, are key to boosting the performance of 2D photodetectors. However, it is challenging to fabricate electrically conducting films with sufficiently high or low work functions (WF2) in homogenous electrodes for 2D devices due to the fixed WF of traditional metallic and semi-metallic electrodes, which restricts their adaptability for 2D metal-semiconductor-metal (MSM) structured photodetectors. Here, we utilize a homogenous PEDOT:PSS electrode designed with adjustable WF ranging from 5.1 to 3.2 eV in 2D MSM photodetectors, achieving a high rectification ratio of ∼105 and superior performance metrics: responsivity up to 1.8 A W−1, an Ilight/Idark of 108, and an ultrafast response time of 3.2 μs. Meanwhile, the excellent transparency of PEDOT:PSS electrode extends the 2D device’s response to the near-infrared (NIR) region, overcoming the semiconductor bandgap limitation. The universality of polymer electrode is proven across various 2D photodetectors, and its flexibility enables the creation of durable, wearable 2D devices. This work paves the way for the development of flexible, self-powered photodetectors, heralding a new era of next-generation intelligent interactive systems.
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Repeater-like asynchronous measurement-device-independent quantum conference key agreement
Quantum conference key agreement (QCKA) enables secure communication among multiple parties by leveraging multipartite entanglement, which is expected to play a crucial role in future quantum networks. However, its practical implementation has been severely limited by the experimental complexity and low efficiency associated with the requirement for synchronous detection of multipartite entangled states. In this work, we propose a measurement-device-independent QCKA protocol that employs asynchronous Greenberger–Horne–Zeilinger state measurement. Our protocol enables a linear scaling of the conference key rate among multiple parties, demonstrating performance comparable to that of the single-repeater scheme in quantum networks. Additionally, we achieve intercity transmission distances with composable security under finite-key conditions. By adopting the generalized asynchronous pairing strategy, our approach eliminates the need for complex global phase locking techniques. Furthermore, by integrating asynchronous pairing with ring-interference network structure, our method provides insights for various quantum tasks beyond quantum communication, including multiparty computing and quantum repeaters.