Optics · Cross-listed from quant-ph
A Quantum-Enhanced Feedback Oscillator
Hudson A. Loughlin, Daniel M. DeSantis, Nergis Mavalvala et al. (+1 more)
Feedback oscillators, such as
lasers and masers, serve as time references in modern computing, communication, and measurement.
Quantum fluctuations ultimately limit their phase stability and ability to keep time precisely; in the absence of
quantum engineering, their phase stability is bounded by a standard
quantum limit (SQL). Techniques to improve the frequency stability of feedback oscillators beyond the SQL have been theorized, but have not yet been demonstrated. We demonstrate an opto-electronic oscillator (OEO), a type of feedback oscillator, with phase stability approaching the SQL. We then engineer the OEO's
quantum state to improve its phase stability, thereby demonstrating the essential principle
quantum-enhancement of feedback oscillators. Similar techniques may be employed to evade the SQL in other feedback oscillators such as masers and
lasers.
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All authors Hudson A. Loughlin, Daniel M. DeSantis, Nergis Mavalvala, Vivishek Sudhir
Optics
Active Control of Extreme Ultraviolet Photon Flux and Resolution with a Plane Ruled Reflection Grating Spectrometer that Measures Diverging High-Harmonics
Caleb H. DeWitt, Nnamdi C. Okey, Griffin W. Hancock et al. (+1 more)
High harmonic generation is a versatile tabletop source for the investigation of ultrafast dynamics with atomic fidelity and few-femtosecond time resolution. However, intensity modulations in the high harmonic spectra due to an alternating peak-and-valley structure in femtosecond extreme ultraviolet (XUV) sources can lead to significant distortions in measured XUV absorption spectra. We report a broadband extreme ultraviolet (XUV) spectrometer with a plane ruled reflection grating that can be spectrally adjusted on the fly using a motorized iris aperture in the entrance slit to control the divergence of the XUV beam prior to its dispersive detection. The effective divergence is tuned between 0.03 to 1.9 milli-radian with concomitant variation in the detected XUV
photon counts of 1.0 $\times$ 10$^6$
photons/s/eV to 1.5 $\times$ 10$^7$ near 40 eV. Metallic samples of Fe (10 nm) and Ti (10 nm) deposited on Si$_3$N$_4$ membranes (100 nm) are measured under these conditions, with a spectral coverage of > 30 eV and a resolution of up to 500 meV. A volcano plot between signal-to-noise ratio (SNR) and spectral resolution determines the trade-off point in measuring elemental near-edge absorption spectra of thin film samples. Consistent with ray tracing simulations, we show that this design parameter offers on-demand high resolution or high SNR configurations for the rapid acquisition of XUV absorption spectra of thin film solid-state samples, especially strongly correlated materials.
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All authors Caleb H. DeWitt, Nnamdi C. Okey, Griffin W. Hancock, Aditi Bhattacherjee
Optics
Harnessing bound states in the continuum for quantum and nonlinear photonics in silicon nitride microresonators
Eduardo C. Lima, Marco Clementi, Alice Viola et al. (+4 more)
In this work we theoretically and experimentally investigate bound states in the continuum (BICs) in a reconfigurable integrated photonic structure, focusing on spontaneous four-wave mixing. We consider a configuration in which only one mode, the idler, is tuned to the BIC condition, and show that suppressing the radiative loss of this single mode profoundly reshapes the overall nonlinear dynamics. In particularly, we realize electrically tunable Friedrich-Wintgen BICs with Q > 10^6 in an interferometrically coupled silicon-nitride microresonator. Although inaccessible through linear excitation, the BIC is populated by four-wave mixing and read out through its radiative
photon partner. In the low-gain regime, we investigate the spectral, correlation, and coherence properties of the generated
photon pairs, revealing distinctive signatures in the signal-idler correlations and an extracted signal whose coherence time approaches the intrinsic-loss-limited lifetime of the dark idler. In the high-gain regime, making one mode participating in optical parametric amplification a BIC enhances the parametric gain by 12 dB and lowers the oscillation threshold by 1.6 dB. These results establish BICs as a tool for mode-selective lifetime engineering, enabling control over both biphoton wavepackets and the threshold dynamics of integrated parametric devices.
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All authors Eduardo C. Lima, Marco Clementi, Alice Viola, Matteo Galli, Daniele Bajoni, Marco Liscidini, Massimo Borghi
Optics
Optomechanical Levitation and Control of High Aspect Ratio Silicon Nanorods
Zhenan Chen, Sophie Sanghera, Yanhui Hu et al. (+7 more)
Nano- and micro-particles levitated by optical, electrical or
magnetic fields are a new frontier in precision sensing and for tests of fundamental physics. For optically levitated anisotropic particles it is possible to control their translation,
alignment and rotation. We report on the levitation and characterization of nanofabricated, high uniformity, high aspect ratio, high refractive index silicon cylinders, with diameters as low as 50 nm and lengths up to 1500 nm. We are able to tune their oscillation frequencies from 10 kHz to over 1 MHz, and exert huge optical torque to generate high rotation rates. These optically levitated silicon nanorods will enable precision torque sensing, and when pushed to smaller sizes, tests of
quantum physics through the generation of angular momentum superposition states.
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All authors Zhenan Chen, Sophie Sanghera, Yanhui Hu, James Sabin, Maryam Nikkhou, Alex Gee, David Burt, Qiongyuan Wu, Joseph Kelly, James Millen
Optics
Dynamically reconfigurable THz quantum walk comb laser through subharmonic excitation
Valerio Digiorgio, Robert Matthew Gray, Marco Raffa et al. (+5 more)
On-chip frequency combs are increasingly relevant to both
laser science and applications. Broad bandwidths and flat-top spectral envelopes are especially desirable for precision spectroscopy and dense wavelength-division multiplexed communications. Toward these goals, active microwave modulation has emerged as a powerful strategy for generating, stabilizing, and reconfiguring frequency combs at the source. However, practical challenges associated with high-frequency modulation imposes an upper bound on the accessible cavity free spectral ranges. Here, we demonstrate a subharmonic locking scheme in a
quantum walk comb
laser, a recently introduced platform for broadband and highly controllable comb states. Using a THz ring
quantum cascade
laser, we realize
quantum walk comb formation under strong microwave injection at successive subharmonics of the cavity round-trip frequency, tuning the comb spacing from 15.8 to 1.58 GHz. The resulting states arise from fast-gain dynamics and nonlinear microwave mixing in the
laser cavity. Through two-tone injection, we exploit this mixing to dynamically control the comb bandwidth and spectral shape. These results establish subharmonic excitation as a route to broadband, reconfigurable
semiconductor comb generation in high-FSR cavities.
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All authors Valerio Digiorgio, Robert Matthew Gray, Marco Raffa, Paolo Micheletti, Alexander Dikopoltsev, Mattias Beck, Jérôme Faist, Giacomo Scalari
Optics · Cross-listed from quant-ph
Demonstration of an LLO CV-QKD system over 12 km of optical fiber
Christiano M. S. Nascimento, Artur A. Matoso, Gustavo C. Amaral et al. (+1 more)
Continuous-variable
quantum key distribution (CV-QKD) promises high rates and seamless integration with classical beams within a single optical fiber. Over the years, implementations have been performed by transmitting a local oscillator reference along with the
quantum channel, opening security loopholes for eavesdroppers and limiting potential applications. Here, we report on a Gaussian CV-QKD implementation using fully independent transmitter and receiver
lasers (local-oscillator sources) over a 12 km fiber spool. The system was experimentally evaluated using logical frames containing approximately $10^7$ coherent states, each composed of ten independently processed subframes of approximately $10^6$ states, and security was assessed in both asymptotic and finite-size regimes under a trusted-device model. The full-fledged classical post-processing is capable of recovering the channel parameters and extracting secret key rates of 5.11 Mbit/s in the asymptotic regime and 4.67 Mbit/s in the finite-size regime, showing good agreement with theoretical predictions. This work establishes the foundation for metropolitan fiber deployment of CV-QKD under strict security constraints.
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All authors Christiano M. S. Nascimento, Artur A. Matoso, Gustavo C. Amaral, Guilherme P. Temporão
Optics · Cross-listed from cond-mat.quant-gas
Crossing over universal scaling laws in two-dimensional driven dissipative condensates
Q. Fontaine, F. Helluin, M. Escalera et al. (+8 more)
In low dimensional systems, fluctuations are enhanced and prevent the spontaneous breaking of continuous
symmetries. As a result, spatial and temporal correlation functions decay at large distances and long times. A well established example is given by two-dimensional bosonic condensates at
equilibrium, which do not display long-range order of the coherence but algebraic decay belonging to the Berezinski-Kosterlitz-Thouless universality class. In contrast, the universal behaviors of non-
equilibrium bosonic condensates are more diverse and many open questions remain. Here, we explore the spatio-temporal coherence properties of two-dimensional driven-dissipative polariton condensates in
semiconductor optical microcavities. By tuning microscopic parameters, we observe a cross-over between two scaling laws that we attribute to the Edwards-Wilkinson (EW) and the Kardar-Parisi-Zhang (KPZ) universality classes. We demonstrate the collapse of the measured first-order correlations onto the EW and KPZ universal scaling functions and obtain critical exponents, well matching the values predicted theoretically. Our results highlight the intrinsic non-
equilibrium nature of polariton condensates and establish them as a platform of choice for controlled exploration of the two-dimensional KPZ universality class.
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All authors Q. Fontaine, F. Helluin, M. Escalera, D. Pinto Dias, A. Lemaître, M. Morassi, M. Wouters, A. Minguzzi, L. Canet, S. Ravets, J. Bloch
Optics
Giant backward Brillouin interaction in generic InP integrated photonics
Riley te Morsche, Lukas Puts, Weiming Yao et al. (+1 more)
We report the first measurement of backward stimulated Brillouin scattering (SBS) in generic InP waveguides, supporting enhanced Brillouin gain of $g_B/Q_m = 3.5 \pm 0.6~\text{W}^{-1} \text{m}^{-1}$ mediated by weakly-guided pressure waves in InGaAsP. This leads to SBS gain coefficients as high as $737 \pm 54~\text{W}^{-1} \text{m}^{-1}$, observed in a mature, foundry-accessible photonic integration platform.
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All authors Riley te Morsche, Lukas Puts, Weiming Yao, David Marpaung
Optics · Cross-listed from cond-mat.mes-hall
Perfect absorption by metal-contacted two-dimensional systems with ultra-proximate reflectors
Kirill Kapralov, Vladislav Atlasov, Alina Khisameeva et al. (+2 more)
Electromagnetic absorbance by most two-dimensional electron systems is typically well below unity, which hinders both practical applications in photodetection and fundamental studies of their optical properties. Here, we show that a periodic structure comprised of narrow two-dimensional sections connected with wide perfectly conducting metal sections enables large absorbance. It reaches 50 % provided the filling factor by the two-dimensional system $f$ equals its dimensionless conductivity $\eta=\sigma Z_0/2$, where $Z_0$ is the free-space impedance. The absorbance is further raised to 100 % if the periodic structure is placed above a perfectly conducting electromagnetic reflector, and provided $f=2\eta$. Surprisingly, the optimal distance between two-dimensional system and reflector may fall well below the quarter of incident wavelength $\lambda_0/4$, which was assumed as conventional absorption enhancement condition in optics. For low filling factors $f\ll1$, large dielectric constants of the substrate, and grating periods comparable with $\lambda_0$, the optimal distance to reflector tends to zero. Above the critical values of the grating geometrical parameters, the absorbance maximum ceases to exist. The critical behavior manifests as a large-amplitude resonance in 'dirty' two-dimensional system with purely real conductivity, while enhancement of carrier momentum relaxation time lowers the resonant peak. Such resonance mimics the plasmonic one, but does not rely on high electron mobility.
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All authors Kirill Kapralov, Vladislav Atlasov, Alina Khisameeva, Viacheslav Muravev, Dmitry Svintsov
Optics
Fast Ptychographic Near-Field Computed Tomography
Sami Wirtensohn, Silja Flenner, Imke Greving et al. (+13 more)
Near-field X-ray ptychography enables quantitative three-dimensional imaging with nanometer-scale resolution, but its broader application is limited by long acquisition times - often necessitating cryogenic or
vacuum environments - and demanding sample preparation procedures. In this paper, we present a flexible near-field ptychography setup that operates under ambient conditions and features an adjustable geometry for different sample sizes and resolution requirements. By replacing stepwise angular acquisition with a fly-rotation sample motion, the scanning overhead is reduced by a factor of 11, drastically lowering the tomographic scan time. To address the bottleneck in sample preparation, we also introduce a sample milling machine that enables fast preparation of specimen pillars with diameters down to 20 um. By lowering both preparation effort and measurement time while maintaining high spatial resolution, the presented system overcomes key limitations of near-field ptychography and makes quantitative X-ray nanotomography substantially more accessible for biological research.
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All authors Sami Wirtensohn, Silja Flenner, Imke Greving, Jens Brehling, Hilmar Burmester, Dominik John, Sara Baggio, Franziska Hinterdobler, Maximiliane Wojke, Kritika Singh, Benedikt J. Daurer, Johannes Hagemann, Frank Seiboth, Sara Savatovic, Fritz Vollrath, Julia Herzen
Optics
Non-monotonic dependence of OAM Schmidt spectrum on crystal thickness
Harshal Jain, Suman Karan, Radhika Prasad et al. (+1 more)
The orbital angular momentum (OAM) of
photons provides a high-dimensional resource for
quantum information protocols. The dimensionality of OAM-entangled states generated via spontaneous parametric down-conversion (SPDC) is quantified by the angular Schmidt spectrum. Here, we experimentally investigate the dependence of the angular Schmidt spectrum on the thickness of the nonlinear
crystal. Contrary to previous studies reporting a monotonic decrease in the Schmidt number with increasing
crystal thickness, we report the first experimental observation of a nonmonotonic behavior, as we demonstrate an increase in the Schmidt number beyond a certain
crystal thickness. We attribute this to the spatial walk-off effect in the anisotropic nonlinear
crystal and explain it using a theoretical model that is devoid of standard phase-matching approximations. These findings can have important implications for high-dimensional entangled state generation.
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All authors Harshal Jain, Suman Karan, Radhika Prasad, Anand K. Jha
Optics
Enhanced Scalability of Horseshoe-and-Spur Networks by Exploiting Hollow-Core Fiber
Mohammad M. Hosseini, Joao Pedro and Antonio Napoli
The convergence of metro and access networks into unified optical infrastructures requires cost-effective alternatives to conventional solid-core fiber (SCF). This paper examines using hollow-core fiber (HCF) with digital subcarrier multiplexing (DSCM) transceivers in horseshoe-and-spur filterless optical architectures. Leveraging HCF's ultra-low nonlinearity, we optimize amplifier placement to maximize power budgets under realistic constraints. Our results show that HCF shifts the main limitation from nonlinearity to amplifier output power, enabling up to a 20 dB spur power-budget gain over SCF. Considering balanced and unbalanced couplers, we find that increasing amplifier density boosts reach only up to a saturation point (about 11-13 units in a 5-transit-node network). A techno-economic break-even analysis of hybrid SCF-HCF deployments shows that targeted HCF use provides intermediate performance gains and can fully recover its fiber cost premium through amplifier reductions.
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Optics
Characterising Interleaver Length in a Turbulent Deployed Terrestrial Free-Space Optical Link
Kadir Gümüş, Vincent van Vliet, Menno van den Hout et al. (+3 more)
We investigate the correlation between interleaver length and
turbulence for a deployed terrestrial free-space optical link spanning 4.6 km. Using 24 hours of measurements, we show an indicative distribution of interleaver length for capacities up to 350 Gb/s/pol and outage probabilities down to $10^{-4}$.
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All authors Kadir Gümüş, Vincent van Vliet, Menno van den Hout, Thomas Bradley, Eduward Tangdiongga, Chigo Okonkwo
Optics
Real-Time Dynamic Crosstalk Tracking and Compensation via Photonic Blind Source Separation
Dongliang Wang, Yikun Nie, Dingqian Zhao et al. (+4 more)
Space-division multiplexing in few-mode fibers can substantially increase optical-link capacity, but its practical deployment is hindered by dynamically varying inter-modal crosstalk induced by environmental perturbations. Although silicon photonic reconfigurable mode processors have been widely demonstrated, continuous adaptation during high-speed intensity-modulation direct-detection (IM/DD) transmission remains challenging. Here, we experimentally demonstrate real-time tracking and compensation of dynamically varying inter-modal crosstalk using a hybrid photonic–electronic framework that combines an integrated silicon photonic processor with an FPGA-based control backend. The photonic processor performs signal separation in the optical domain, while the FPGA extracts signal statistics and implements a blind source separation (BSS)-based feedback
algorithm without requiring dedicated training sequences. A two-stage optimization strategy combines rapid crosstalk suppression with stable continuous tracking under dynamic channel variations. We demonstrate photonic blind source separation for signals up to 100 GBaud and FPGA-enabled continuous adaptation for two 64 GBaud data channels in a few-mode-fiber transmission system. The system maintains bit-error rates below $10^{-4}$ under dynamic crosstalk, with a per-update control latency of 4 ms. These results establish a practical route toward low-latency, hardware-efficient adaptive photonic front ends for dynamic high-speed IM/DD space-division-multiplexed links.
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All authors Dongliang Wang, Yikun Nie, Dingqian Zhao, Dan Yi, Gaolei Hu, Hon Ki Tsang, Chaoran Huang
Optics
Unveiling charge dynamics on the generation of high extinction wide pulse generation on thin-film lithium tantalate
Ayed Al Sayem, Shiekh Zia Uddin, Ting-Chen Hu et al. (+6 more)
We experimentally demonstrate high-extinction optical pulse generation in thin-film lithium tantalate (TFLT), achieving a wide range of pulse widths with sharp rise and fall edges, without measurable distortion or long temporal tails. We further provide a direct one-to-one comparison with thin-film lithium niobate (TFLN) modulators under identical measurement conditions. Our results reveal that the distorted pulse response observed in TFLN originates from charge activation and transport, whereas the distortion-free response in TFLT is enabled by the substantially larger activation energy of defect-related charge carriers. This larger activation barrier suppresses leakage-current-induced charging dynamics, thereby enabling stable and distortion-free pulse generation. The successful generation of high-extinction, distortion-free optical pulses in TFLT can play a significant role in integrated
quantum photonic technologies, particularly for
qubit preparation, measurement, and fast feedback
circuits.
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All authors Ayed Al Sayem, Shiekh Zia Uddin, Ting-Chen Hu, Tam Huynh, Bongjun Choi, Alaric Tate, Mark Cappuzzo, Rose Kopf, Mark Earnshaw
Optics · Cross-listed from eess.SP
Comparative Analysis of Spread-Spectrum Codes for Fibre-Optic Distributed Acoustic Sensing
Maria Freire-Hermelo and Elie Awwad
This paper presents a comprehensive performance comparison of several coded sequences for distributed acoustic sensing systems through numerical simulations. Correlation analysis reveals that perfect autocorrelation sequences provide accurate channel estimation. Phase noise sensitivity varies significantly with frequency diversity, with high-diversity sequences maintaining lower estimation errors for phase and intensity despite increased
laser linewidths. For a coded distributed acoustic sensing system with \qty{10}{\hertz}
laser linewidth, high-diversity sequences achieve $2.7\times10^{-3}$ rad phase error compared to $5.3\times10^{-3}$ rad for low-diversity codes for a \qty{1.5}{\kilo\meter} fibre length. The findings establish performance trade-offs between correlation quality, phase noise resilience, and practical implementation constraints for next-generation fibre-optic distributed acoustic sensing systems.
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Optics · Cross-listed from cond-mat.soft
Particle Contacts Generate Fractional Density Relaxation
Hu Cang
Dense-
liquid relaxation evolves from local particle collisions to cooperative structural rearrangements. While hard-sphere kinetics determines an early $t^{3/2}$ fractional decay in density correlation functions, collective theories describe the subsequent structural relaxation. A central open question has been how short-time contact physics supplies an exact starting point for the
memory kernel governing later times without being modified by subsequent many-body rearrangements. Here we resolve this problem for a broad class of reversible Brownian systems. We prove that hard particle contacts act as reflecting boundaries in configuration space, uniquely dictating the amplitude of the leading $t^{3/2}$ density relaxation. Mechanistically,
diffusion samples a contact boundary layer of thickness $O(\sqrt{t})$, which combines with the local density response to produce the fractional signal. We derive an explicit surface formula expressing this amplitude in terms of
equilibrium contact probability, normal mobility, and density sensitivity. For monodisperse hard spheres, this yields an exact, fit-free prediction determined entirely by static structure $S(k)$, radial contact value $g(\sigma^+)$, and short-time
diffusion $D_0$. Extending the construction, we determine the corresponding normalization for soft interfaces and prove via a Gram–Schur projection hierarchy that regular collective variables leave the leading contact amplitude strictly invariant. The resulting formulation connects microscopic collision kinetics directly to caging and glass-like structural relaxation, providing an exact microscopic boundary condition for scattering experiments, molecular simulations, and
memory-kernel reconstructions.
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Optics
Geometric Control of Visible Emitter Creation in Hexagonal Boron Nitride by Oblique Ion Irradiation
Sagar Chowdhury, Bhaveshkumar Kamaliya, Ramachandra Bangari et al. (+5 more)
Ion irradiation creates optically active defects in wide-bandgap van der Waals materials, yet most approaches tune defect formation by varying the ion species, energy, or fluence while leaving the incidence geometry fixed. The ion-incidence angle is established here as a
geometric control parameter for engineering visible emitters in hexagonal boron nitride (hBN). The angle and ion fluence of a
plasma-focused heavy-ion (Xe+) beam are varied across hBN flakes of different thickness, and the resulting photoluminescence is quantified. In thick flakes, oblique irradiation shifts the fluence for maximum emission by nearly two orders of magnitude relative to normal incidence, whereas thin flakes exhibit an angle-independent optimum. Ion-trajectory simulations attribute this thickness dependence to lateral redistribution of the collision cascade and enhanced oblique sputtering. Atomic force microscopy identifies distinct processing regimes that delineate the useful defect-creation window. Post-irradiation annealing quenches the emission and shifts the spectral weight toward the green-yellow band while preserving the angle-dependent activation trends. Spectrally resolved lifetime measurements show comparable biexponential dynamics for normal and oblique incidence, consistent with emission from related defect families rather than a geometry-specific emitter species. These results establish ion-incidence geometry as a materials-level knob for programming optical defect activation and spatial defect distributions in van der Waals photonic materials.
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All authors Sagar Chowdhury, Bhaveshkumar Kamaliya, Ramachandra Bangari, Caleb Whittier, Joseph Spielbauer, Nabil D. Bassim, Thomas G. Folland, Ravitej Uppu