The rapid rise of artificial intelligence has led to an unsustainable growth in energy consumption. This has motivated progress in neuromorphic computing and physics-based training of learning machines as alternatives to digital neural networks. Many theoretical studies focus on simple architectures like all-to-all or densely connected layered networks. However, these may be challenging to realize experimentally, e.g. due to connectivity constraints. In this work, we investigate the performance of the widespread physics-based training method of equilibrium propagation for more realistic architectural choices, specifically, locally connected lattices. We train an XY model and explore the influence of architecture on various benchmark tasks, tracking the evolution of spatially distributed responses and couplings during training. Our results show that sparse networks with only local connections can achieve performance comparable to dense networks. Our findings provide guidelines for further scaling up architectures based on equilibrium propagation in realistic settings.
Many challenges arising in Quantum Technology can be successfully addressed using a set of machine learning algorithms collectively known as reinforcement learning (RL), based on adaptive decision-making through interaction with the quantum device. After a concise and intuitive introduction to RL aimed at a broad physics readership, we discuss the key ideas and core concepts in reinforcement learning with a particular focus on quantum systems. We then survey recent progress in RL in all relevant areas. We discuss state preparation in few- and many-body quantum systems, the design and optimization of high-fidelity quantum gates, and the automated construction of quantum circuits, including applications to variational quantum eigensolvers and architecture search. We further highlight the interactive capabilities of RL agents, emphasizing recent progress in quantum feedback control and quantum error correction, and briefly discuss quantum reinforcement learning as well as applications to quantum metrology. The review concludes with a discussion of open challenges -- such as scalability, interpretability, and integration with experimental platforms -- and outlines promising directions for future research. Throughout, we highlight experimental implementations that exemplify the increasing role of reinforcement learning in shaping the development of quantum technologies.
Toward In Situ Monitoring of the Precipitation of Gold Nanoparticles Using In-Fiber Absorption Spectroscopy
Florian Schorn,
Markus Binder,
Cornelia Damm,
Marco Haumann,
Nicolas Joly
The use of hollow-core photonic crystal fibers in operando spectrometry of chemical reactions is a relatively unexplored technology. It can be used in different ways and offers a variety of advantages compared with conventional operando spectrometry, such as a significantly increased path length with a simultaneously reduced volume. We apply fiber absorption spectroscopy here to the synthesis of gold nanoparticles. We measured the rate of formation of gold nanoparticles at different initial concentrations. We show that much higher resolution is possible with this technique in comparison with a conventional measurement technique using cuvettes.
A 25 THz bandwidth THz spectroscopy system exploiting BNA crystals and a tunable single-ring-fiber pulse compressor
Wei Cui,
Aswin Vishnuradhan,
Markus Lippl,
Eeswar Kumar Yalavarthi,
Angela Gamouras,
Nicolas Joly,
Jean-Michel Ménard
We present a terahertz time-domain spectroscopy (THz-TDS) system which accesses a broadband spectrum, efficiently covering the so-called "new THz gap" between 5 and 15 THz and extending beyond 25 THz. The system exploits nonlinear interactions within the organic crystal BNA (N-benzyl-2-methyl-4-nitroaniline) to generate and detect THz radiation upon excitation by a near-infrared (NIR) pulse centered at 1.03 um. To enable broadband THz spectral monitoring, the NIR pulse from a Yb-based solid-state laser undergoes spectral broadening in a gas-filled single-ring hollow-core photonic crystal fiber, followed by pulse compression to achieve durations as short as 31 fs. This approach paves the way for broadband spectroscopy in hard-to-access THz regions using widely available near-infrared ultrafast sources.
Thin-film Al0.30Ga0.70As (111) as a ‘flat’ source of high-purity orthogonally polarized entangled photons
Simon Stich,
Vitaliy Sultanov,
Trevor Blaikie,
Qingyu Shi,
Zbig Wasilewski,
Mikhail A. Belkin,
Maria Chekhova
Optics Express
34
1664-1673
(2026)
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Flat-optics platforms offer new opportunities for the generation of entangled photons by relaxing traditional phase-matching constraints, enabling the use of a broader range of nonlinear materials. Among these, gallium arsenide and aluminum gallium arsenide stand out for their exceptionally high second-order nonlinearities, but their conventional orientation (001) has limited their applicability for photon-pair generation. By transitioning to crystals with (111) surface orientation, we overcome these limitations. We demonstrate a flat-optics-based telecom-range SPDC source using Al0.30Ga0.70As that achieves a high photon-pair generation rate per pump power and bandwidth of up to 0.24 Hz/mW/nm. The choice of 30% aluminum concentration allowed us to reduce pump absorption and photoluminescence background for photon-pair generation at telecom wavelengths by at least an order of magnitude compared to that of GaAs. The specific layer orientation facilitates the generation of orthogonally polarized entangled photons, a prerequisite for polarization-entangled states. Rather than directly probing entanglement, we observe the effect of hidden polarization. Our results highlight AlGaAs (111) as a promising platform for scalable quantum photonic sources and shed light on nonclassical polarization effects accessible through flat-optics engineering.
Unitary fault-tolerant encoding of Pauli states in surface codes
Luis Colmenarez,
Remmy Zen,
Jan Olle,
Florian Marquardt,
Markus Müller
In fault-tolerant quantum computation, the preparation of logical states is a ubiquitous subroutine, yet significant challenges persist even for the simplest states required. In the present work, we present a unitary, scalable, distance-preserving encoding scheme for preparing Pauli eigenstates in surface codes. Unlike previous unitary approaches whose fault-distance remains constant with increasing code distance, our scheme ensures that the protection offered by the code is preserved during state preparation. Building on strategies discovered by reinforcement learning for the surface-17 code, we generalize the construction to arbitrary code distances and both rotated and unrotated surface codes. The proposed encoding relies only on geometrically local gates, and is therefore fully compatible with planar 2D qubit connectivity, and it achieves circuit depth scaling as O(d), consistent with fundamental entanglement-generation bounds. We design explicit stabilizer-expanding circuits with and without ancilla-mediated connectivity and analyze their error-propagation behavior. Numerical simulations under depolarizing noise show that our unitary encoding without ancillas outperforms standard stabilizer-measurement-based schemes, reducing logical error rates by up to an order of magnitude. These results make the scheme particularly relevant for platforms such as trapped ions and neutral atoms, where measurements are costly relative to gates and idling noise is considerably weaker than gate noise. Our work bridges the gap between measurement-based and unitary encodings of surface-code states and opens new directions for distance-preserving state preparation in fault-tolerant quantum computation.
Octave-spanning frequency comb from a single-diode-pumped 1 GHz Ti:sapphire laser
Ewan Allan,
Abdullah Alabbadi,
Pablo Castro-Marín,
Hanna Ostapenko,
Pascal Del'Haye,
Derryck T. Reid
Broadband frequency combs with mode spacings of 1 GHz provide a valuable resource for optical frequency metrology and astrophotonics. Significant average powers are often needed to reach the pulse energies required for supercontinuum generation at 1 GHz repetition rates, putting this beyond the reach of most simple ultrafast lasers. Here, by using dispersion-engineered Si₃N₄ waveguides, we report octave-spanning comb generation from 539 to 1078 nm (−20 dB bandwidth) pumped with a three-element 1 GHz Ti:sapphire laser powered by a single laser diode. Laser repetition-rate stability of 790 mHz is achieved over a 1-hour duration, and carrier-envelope-offset control and stabilization to a single-frequency cw laser is presented. The system offers a simple route to a coherent, broadband supercontinuum spanning the visible to the near-infrared, with potential as an enabling technology for optical frequency metrology, quantum timekeeping, and astrophysical spectrograph calibration.
Twisted single-ring hollow-core fiber for broadband chiral detection in nanoliter volumes
Christof Helfrich,
Sonia Maniappan,
Michael Frosz,
Raju Adhikary,
Sandro Colagioia,
Nicolas Joly,
Andrea Marini,
Francesco Tani
Journal of Physics: Photonics
8
015035
(2026)
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The ongoing evolution of hollow-core fibers continues to inspire the development of optofluidic platforms with enhanced sensitivity and minimal sample requirements. Here, we utilize the intrinsic advantages of anti-resonant reflection hollow-core fibers—such as low optical loss and broadband transmission—to realize a twisted single-ring hollow-core fiber (SR-HCF) tailored for polarization-sensitive chiral detection. We optimize the fiber geometry to ensure single-mode operation by strongly attenuating higher-order modes (>50 dB/m) while maintaining low loss for the fundamental mode (<0.1 dB/m) and reducing the sample volume to only ~660 nanoliters per 34 cm fiber length. By applying a constant twist along the fiber length, we minimize birefringence and ensure stable transmission of linear polarization states with polarization extinction ratios (PER) surpassing 38 dB. After injecting an aqueous solution of an optically active molecule, we measure its optical rotation (OR) at different wavelengths with millidegree-level sensitivity and remarkable robustness against misalignment. Measurements with different enantiomeric excess concentrations are in good agreement with independent liquid chromatography characterization.
Solar-pumped lasers, predominantly based on neodymium gain media, offer a promising route to renewable laser-energy conversion and space-based photonics; however, their performance has been constrained by thermal loading and limited power scalability. Here, we propose and numerically investigate a solar-pumped ytterbium thin-disk gain medium in combination with a dome concentrator that enables multipass solar pumping and enhanced absorption. The design yields comparably low lasing thresholds for neodymium- and ytterbium-doped media, while ytterbium provides superior power scalability, enabling up to threefold higher output power. We further identify ytterbium-doped medium combined with a spherical concentrator as a viable solar-pumped, radiation-balanced configuration, achieving self-cooled lasing at solar pump intensities of 28.5 kW cm-2 within the 1020-1033 nm window of the solar spectrum. The spherical concentrator increases the averaged fluence of the solar pump while permitting anti-Stokes fluorescence to escape efficiently. These results establish multi-pass, solar-pumped thin-disk ytterbium lasers as a compact, scalable, and sustainable platform for high-performance solar-pumped lasers
Color symmetry breaking in a nonlinear optical microcavity
Luca O. Trinchão,
Alekhya Ghosh,
Arghadeep Pal,
Haochen Yan,
Toby Bi,
Shuangyou Zhang,
Nathalia B. Tomazio,
Flore K. Kunst,
Lewis Hill, et al.
Spontaneous symmetry breaking leads to diverse phenomena across the natural sciences, from the Higgs mechanism in particle physics to superconductors and collective animal behavior. In photonic systems, the symmetry of light states can be broken when two optical fields interact through the Kerr nonlinearity, as shown in early demonstrations with counterpropagating and cross-polarized modes. Here, we report the first observation of color symmetry breaking in an integrated silicon nitride microring, where spontaneous power imbalance arises between optical mode at different wavelengths, mediated by the Kerr effect. The threshold power for this effect is as low as 19 mW. By examining the system's homogeneous states, we further demonstrate a Kerr-based nonlinear activation-function generator that produces sigmoid-, quadratic-, and leaky-ReLU-like responses. These findings reveal previously unexplored nonlinear dynamics in dual-pumped Kerr resonators and establish new pathways towards compact, all-optical neuromorphic circuits.
Brillouin–Mandelstam scattering-based cooling of traveling acoustic waves from cryogenic temperatures
Lisa Fischer,
Laura Blázquez Martínez,
Changlong Zhu,
Robin Chenevière,
Johann Troles,
Birgit Stiller
Thermal phonons are a major source of decoherence in quantum mechanical systems. Operating in the quantum ground state is therefore often an experimental prerequisite. In addition to passive cooling in a cryogenic environment, active laser cooling enables the reduction of phonons at specific acoustic frequencies. Brillouin cooling has been used to show efficient reduction of the thermal phonon population in waveguides at GHz frequencies down to 74 K. In this Letter, we demonstrate the cooling of a 7.608 GHz acoustic mode by combining Brillouin active cooling with precooling from 77 K using liquid nitrogen. We show a 69% reduction in the phonon population, resulting in a final temperature of 24.3 +/- 1.9 K, 50 K lower than previously reported.
Soliton self-frequency shift in hollow-core fiber for bright femtosecond radiation tunable across the short-wavelength infrared
Markus Lippl,
Martin Butryn,
Nicolas Joly,
Francesco Tani
We report a fiber-based source of femtosecond radiation that is spectrally tunable in the short-wavelength infrared region, delivering average powers at the multi-watt level. The system utilizes self-soliton frequency shifting in a hydrogen-filled hollow-core fiber, producing pulse trains at 1.1 MHz with integrated relative intensity noise below 0.3% and a polarization extinction ratio of 30 dB. This source constitutes an efficient and valid fiber-based alternative to optical parametric amplifiers for a variety of applications, including THz generation, multiphoton imaging, and high-harmonic generation.
Red Blood Cell-derived Extracellular Vesicles as biomaterials: the opportunity of freezing-induced accelerated aging
Lucia Paolini,
Miriam Romano,
Valentina Mangolini,
Selene Tassoni,
Shuhan Jiang,
Elena Laura Mazzoldi,
Angelo Musicò,
Andrea Zendrini,
Anna Kashkanova, et al.
Biomaterials Science
14
122-139
(2026)
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Red blood cell-derived extracellular vesicles (RBC-EVs) are emerging as promising biomaterials for next-generation drug delivery, owing to their intrinsic biocompatibility, immune evasion properties, and minimal oncogenic risk. However, their broader application is currently limited by unresolved challenges related to heterogeneity, reproducibility, and long-term storage stability. By combining discontinuous sucrose density gradient separation with high-resolution interferometric nanoparticle tracking analysis, we identified a sharp bimodal size distribution of the vesicles in freshly prepared samples. We then tracked how long-term storage at −80 °C drove its conversion into a monomodal distribution. To reproduce these conditions in a shorter time frame, we developed an “accelerated-ageing” protocol based on freeze–thaw cycles that generates RBC-EV samples with homogeneous density, size distribution, and biological activity, effectively replicating the properties of preparations stored for six months at −80 °C. This new vesicle population results stable and retains membrane integrity and cellular internalization capacity, as confirmed by surface-associated enzymatic activity assays and uptake tests in cancer cell lines. These results suggest that freezing-induced “accelerated ageing” represents an effective method for the optimization and standardization of RBC-EVs as building blocks for biomaterial and bioengineering applications.
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