arXiv subjects

Alejandro Simon

Publications and source records attributed to Alejandro Simon.

11 recordsLinked to original sources

arXiv2026

Dark matter searches with a 13 meV threshold superconducting sensor array

Many well-motivated dark matter models predict meV-scale energy deposits in interactions with terrestrial experiments, but this regime is challenging to probe due to a lack of mature single-quantum detectors. Here we report results from QUALIPHIDE (QUAntum LImited PHotons In the Dark Experiment), a cryogenic dark matter search using a $41$-pixel array of energy-resolving microwave kinetic inductance detectors with a $13$ meV threshold, simultaneously used to look for both conversion photons from THz wavelength hidden photon dark matter and phonons from particle-like light dark matter interactions. The experimental design, with on- and off-focus pixels for the hidden photon search, allows for a data-driven background model, giving the experiment discovery potential. A blind analysis of $22$ hours of data shows no significant excess, setting the strongest constraints on the hidden photon kinetic mixing parameter $\chi$ over the mass range of $13$-$90$ meV/$c^2$, reaching $1.5\times10^{-12}$ at $50$ meV/$c^2$. These data also yield among the first terrestrial limits on dark matter scattering off nuclei and electrons, down to $5$ MeV/$c^2$ and $20$ keV/$c^2$, respectively. The low threshold also enables future study of the low-energy excess limiting cryogenic detectors and, as we project, will allow for a terahertz-scale QCD axion search with a magnetic field.

hep-ex↗

arXiv2026

Reconfigurable Superconducting Logic for On-Chip Photon Coincidence Detection

Scaling photonic quantum-information platforms requires arrays of superconducting nanowire single-photon detectors (SNSPDs) for feedforward control, in which optical operations are conditioned on Bell-state measurements relying on photon-coincidence detections. On-chip superconducting cryotron electronics, performing logic on detector outputs and driving optical modulators, could reduce latency and room-temperature interconnect complexity for feedforward schemes. To date, no cryotron circuits designed for this purpose have been demonstrated. We demonstrate a bias-programmable logic gate based on three nanocryotrons (nTrons) that implements selectable AND (coincidence), XOR (odd-parity), and OR functions. It operates on two electrical pulses at 4.2 K, with bit-error rates below $10^{-3}$, bias margins up to $\pm21.7\%$, and operation extending to 25 MHz over narrower bias windows. It performs coincidence and odd-parity detection on two SNSPDs' outputs with bit-error rates below $3.2 \times 10^{-2}$. As proof-of-concept, we show that nTrons can drive capacitive loads up to 1.15 V, potentially enabling compatibility with electro-optic modulators in feedforward schemes.

physics.app-ph↗

arXiv2026

Ultrafast dynamics and light-induced superconductivity from first principles

Experiments on superconducting materials have unveiled unique emergent properties when they are driven far from equilibrium. However, a quantitative first-principles treatment that describes experimental observations is lacking. In this work, we develop an ab-initio model for the nonequilibrium response of optically irradiated superconducting films within the framework of conventional electron-phonon-mediated superconductivity, leveraging new numerical techniques to solve the Migdal-Eliashberg equations directly on the real-frequency axis. This enables us to quantitatively reproduce the optical response of superconducting films in pump-probe experiments and validate our approach on measurements of the differential reflectance of Pb and LaH$_{10}$ in response to a pump excitation. Similar calculations performed on the alkali-doped fulleride K$_3$C$_{60}$ reveal that a photo-induced superconducting state is generated after irradiation by an ultrafast mid-infrared pulse of sufficient intensity, as reported in prior experimental work. The enhancement in this framework is attributed to the excitation of quasiparticles to energies resonant with the strongest electron-phonon coupling in K$_3$C$_{60}$, in close analogy to the mechanism for enhancement of superconductivity under microwave irradiation, explaining the nature of the photo-induced superconducting state and elucidating the subsequent quasiparticle and phonon dynamics. Our results suggest that photo-induced superconductivity is accessible in more materials than previously recognized. We demonstrate this by performing calculations on calcium-intercalated graphite, CaC$_6$, and predict a similar photo-induced superconducting gap.

cond-mat.supr-con↗

arXiv2026

Fast Real-Axis Eliashberg Calculations: Full-bandwidth solutions beyond the constant density of states approximation

Experimentally relevant signatures of superconductivity require access to real-frequency quantities, such as the spectral functions, optical response, and transport properties, yet Migdal-Eliashberg calculations are commonly performed on the imaginary axis and then analytically continued, a step that is numerically delicate and can obscure physically relevant spectral features. Here we present a practical route to solving the finite-temperature Migdal-Eliashberg equations directly on the real-frequency axis, while retaining the effects from the full-bandwidth electronic structure. Our formulation accounts for particle-hole asymmetry through an energy-dependent electronic density of states, avoiding the constant density of states approximation often used in real-axis calculations, and includes a static screened Coulomb contribution. We introduce an efficient numerical technique to solve the Migdal-Eliashberg integrals whose computational cost scales linearly with the real-frequency grid, making high-resolution, full-bandwidth real-axis calculations feasible and providing direct access to the interacting Green's function and derived observables without analytic continuation. As an illustration, we apply the method to H$_{3}$S, where a van-Hove singularity near the Fermi level produces strong particle-hole asymmetry. The full-bandwidth solution yields noticeably different spectra than the constant density of states approximation and brings the superconducting gap and lineshapes into closer agreement with experiment, highlighting when band-structure details are essential. Furthermore, the methods presented here open the door to time-dependent, nonequilibrium simulations within Eliashberg theory.

cond-mat.supr-con↗

arXiv2025

Fast-Recovery Epitaxial NbN Superconducting Nanowire Single-Photon Detectors with Saturated Efficiency at 1550 nm in Liquid Helium

Achieving both high internal efficiency and fast reset times at elevated temperatures remains challenging due to limited understanding of how film properties govern SNSPD performance. We demonstrate that epitaxial NbN films on sapphire enable simultaneous high efficiency and rapid response. We fabricate and characterize SNSPDs based on these films deposited via DC magnetron sputtering on c-cut sapphire. High-quality epitaxial growth preserves a low electron diffusion coefficient and promotes strong electron-phonon coupling, yielding a high critical temperature and efficient hotspot formation in the dirty limit. X-ray diffraction and transmission electron microscopy confirm epitaxial alignment and lattice order. Nanowires of 20 nm width exhibit saturated internal efficiency at 1550 nm wavelength and short reset times at 4.2 K, enabled by lattice matching and high thermal conductance of the sapphire interface. Ab initio modeling reproduces photon count rates, linking device performance quantitatively to film properties such as diffusivity and electron-phonon coupling.

physics.app-ph↗

arXiv2025

Modeling electrothermal feedback of superconducting nanowire single photon detectors in SPICE

Superconducting nanowire single-photon detectors (SNSPDs) exhibit complex switching behaviors due to electrothermal feedback during the detection process. Modeling and understanding these behaviors is integral for designing superconducting devices; however, many models often prioritize accuracy over computational speed and intuitive integration for circuit designers. Here, we build upon a growing architecture of SPICE tools for superconducting nanowire devices by capturing complex residual heating effects in a compact thermal model of an SNSPD. We demonstrate that our model is comparable to more complicated thermal models of superconducting nanowire devices, including finite-element simulations, and is applicable for the fast development of SNSPD circuits.

physics.app-ph↗

arXiv2025

Time-tagging data acquisition system for testing superconducting electronics based on an RFSoC and custom analog frontend

Novel electronic devices can often be operated in a plethora of ways, which makes testing circuits comprised of them difficult. Often, no single tool can simultaneously analyze the operating margins, maximum speed, and failure modes of a circuit, particularly when the intended behavior of subcomponents of the circuit is not standardized. This work demonstrates a cost-effective time-domain data acquisition system for electronic circuits that enables more intricate verification techniques than are practical with conventional experimental setups. We use high-speed digital-to-analog converters and real-time multi-gigasample-per-second waveform processing to push experimental circuits beyond their maximum operating speed. Our custom time-tagging data capture firmware reduces memory requirements and can be used to determine when errors occur. The firmware is combined with a thermal-noise-limited analog frontend with 50 dB of dynamic range. Compared to currently available commercial test equipment that is seven times more expensive, this data acquisition system was able to operate a superconducting shift register at a nearly three-times-higher clock frequency (200 MHz vs. 80 MHz).

physics.ins-det↗

arXiv2025

Scalable Superconducting Nanowire Memory Array with Row-Column Addressing

Developing ultra-low-energy superconducting computing and fault-tolerant quantum computing will require scalable superconducting memory. While conventional superconducting logic-based memory cells have facilitated early demonstrations, their large footprint poses a significant barrier to scaling. Nanowire-based superconducting memory cells offer a compact alternative, but high error rates have hindered their integration into large arrays. In this work, we present a superconducting nanowire memory array designed for scalable row-column operation, achieving a functional density of 2.6$\,$Mb/cm$^{2}$. The array operates at $1.3\,$K, where we implement and characterize multi-flux quanta state storage and destructive readout. By optimizing write and read pulse sequences, we minimize bit errors while maximizing operational margins in a $4\times 4$ array. Circuit-level simulations further elucidate the memory cell's dynamics, providing insight into performance limits and stability under varying pulse amplitudes. We experimentally demonstrate stable memory operation with a minimum bit error rate of $10^{-5}$. These results suggest a promising path for scaling superconducting nanowire memories to high-density architectures, offering a foundation for energy-efficient memory in superconducting electronics.

physics.app-ph↗

arXiv2025

Ab initio modeling of nonequilibrium dynamics in superconducting detectors and qubits

Nonequilibrium quasiparticle (QP) and phonon dynamics are central to the operation of superconducting devices. Superconducting detectors, such as the superconducting nanowire single-photon detector, perform best when a large QP population is generated in response to small perturbations. Conversely, for superconducting qubits and topologically protected Majorana fermions, even relatively small QP densities can lead to significant performance degradation, and thus, ideal materials are less susceptible to QP poisoning. However, existing models of these devices lack a rigorous description of the QP and phonon dynamics, relying on approximations and phenomenology. In this article, we combine kinetic equations with density functional theory to model the nonequilibrium dynamics of a superconducting film ab initio. To demonstrate the universality of our model, we illustrate two examples: (1) we develop a model for the detection of single photons in superconducting nanowires, and (2) we calculate the energy-relaxation rate of a transmon qubit due to the presence of excess QPs. Our examples demonstrate from first principles that NbN is well-suited for single-photon detection and that Ta transmon qubits possess reduced sensitivity to QP poisoning relative to other materials, which is likely in part responsible for their longer coherence times. In contrast to previous models, our ab initio approach makes these predictions without experimental input and thus can be used to accelerate progress in device development. Moreover, by considering the full-bandwidth electron-phonon coupling, our approach can incorporate strong-coupling effects. Our methods effectively integrate ab initio materials modeling with nonequilibrium theory of superconductivity to perform practical modeling of superconducting devices, providing a comprehensive approach that connects fundamental theory with device applications.

cond-mat.supr-con↗

arXiv2024

Characterizing and modeling the influence of geometry on the performance of superconducting nanowire cryotrons

The scaling of superconducting nanowire-based devices to larger arrays is often limited by the cabling required to interface with each device. Cryogenic integrated circuits constructed from nanowire cryotrons, or nanocryotrons, can address this limitation by performing signal processing on chip. In this study, we characterize key performance metrics of the nanocryotron to elucidate its potential as a logical element in cryogenic integrated circuits and develop an electro-thermal model to connect material parameters with device performance. We find that the performance of the nanocryotron depends significantly on the device geometry, and trade-offs are associated with optimizing the gain, jitter, and energy dissipation. We demonstrate that nanocryotrons fabricated on niobium nitride can achieve a grey zone less than 210 nA wide for a 5 ns long input pulse corresponding to a maximum achievable gain of 48 dB, an energy dissipation of less than 20 aJ per operation, and a jitter of less than 60 ps.

cond-mat.supr-con↗

arXiv2023

The QUATRO Application Suite: Quantum Computing for Models of Human Cognition

Research progress in quantum computing has, thus far, focused on a narrow set of application domains. Expanding the suite of quantum application domains is vital for the discovery of new software toolchains and architectural abstractions. In this work, we unlock a new class of applications ripe for quantum computing research -- computational cognitive modeling. Cognitive models are critical to understanding and replicating human intelligence. Our work connects computational cognitive models to quantum computer architectures for the first time. We release QUATRO, a collection of quantum computing applications from cognitive models. The development and execution of QUATRO shed light on gaps in the quantum computing stack that need to be closed to ease programming and drive performance. Among several contributions, we propose and study ideas pertaining to quantum cloud scheduling (using data from gate- and annealing-based quantum computers), parallelization, and more. In the long run, we expect our research to lay the groundwork for more versatile quantum computer systems in the future.

cs.CE↗