About

Karl Berggren is the Faculty Head of Electrical Engineering at MIT and holds the title of Julius A. Stratton Professor in Electrical Engineering and Physics. His research focuses on electronic, magnetic, optical, and quantum materials and devices, as well as nanoscale materials, devices, and systems. His work involves designing systems that sense, process, and transmit energy and information, leveraging computational, theoretical, and experimental tools to develop groundbreaking sensors, energy transducers, and new physical substrates for computation. Berggren's contributions are integral to advancing the understanding and development of nanoscale systems and quantum materials, addressing shared challenges facing humanity through innovative engineering and scientific approaches.

Research topics

• Physics
• Computer Science
• Electrical engineering
• Engineering
• Optics
• Artificial Intelligence
• Quantum mechanics
• Condensed matter physics
• Embedded system
• Electronic engineering
• Particle physics
• Operating system
• Engineering physics
• Optoelectronics
• Materials science
• Computer architecture
• Nanotechnology
• Telecommunications

Selected publications

• Reconfigurable Superconducting Logic for On-Chip Photon Coincidence Detection

  arXiv (Cornell University) · 2026-04-23

  preprintOpen accessSenior author

  Scaling photonic quantum-information platforms requires arrays of superconducting nanowire single-photon detectors (SNSPDs) for feedforward control, in which optical operations are conditioned on preceding Bell-state measurements that typically rely on photon coincidence detections. On-chip superconducting cryotron electronics, performing logic directly on detector outputs and subsequently driving optical modulators, could substantially reduce latency and room-temperature interconnect complexity for feedforward schemes. To date, no cryotron logic gates specifically designed to process SNSPD outputs for quantum applications have been demonstrated. We demonstrate a bias-programmable logic gate based on three nanocryotrons (nTrons), fabricated using the same thin-film technology as SNSPDs. The circuit implements selectable AND (coincidence), XOR (odd-parity), and OR functions on two externally generated electrical pulses at 4.2 K, with bit-error rates below $10^{-3}$, bias margins up to $\pm24\%$, and operation extending to 25 MHz over narrower bias windows. Moreover, it performs coincidence and odd-parity detection on two co-fabricated SNSPDs' outputs with bit-error rates below $3.2 \times 10^{-2}$. As a 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.

  PublisherDOI

• Ultrafast dynamics and light-induced superconductivity from first principles

  arXiv (Cornell University) · 2026-03-18

  preprintOpen access

  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.

  PublisherDOI

• Enhanced Mid-Infrared Single-Photon Detection with Antenna-Coupled Superconducting Nanowires

  arXiv (Cornell University) · 2026-04-20

  preprintOpen accessSenior author

  Scaling the photon-detection area of superconducting nanowire single-photon detectors (SNSPDs) has traditionally been achieved by nanowire meandering. However, material inhomogeneities and fabrication-induced defects, such as line-edge roughness, increase with nanowire length, leading to reduced internal photon-detection efficiency and elevated dark-count rates. This trade-off becomes increasingly pronounced as nanowires are scaled to sub-100 nm widths and sub-5 nm thicknesses required for mid- to far-infrared sensitivity. Here, we demonstrate an antenna-coupled SNSPD architecture that enhances the effective photon-detection area without increasing nanowire length. A crossed bowtie antenna integrated with an 80 nm-wide, 3 nm-thick WSi nanowire yields 15.7$\times$ increase in effective detection area at 7.4 $μ$m compared to a bare nanowire of identical geometric footprint, while maintaining the same internal detection efficiency and dark-count rate. Antenna coupling improves noise-equivalent power and provides a more scalable route to increasing photon-detection area than conventional meander geometries, offering performance benefits for applications in astronomy, biological imaging, and molecular spectroscopy.

  PublisherDOI

• A scalable superconducting nanowire memory array with row-column addressing

  Figshare · 2026-01-01

  datasetOpen accessSenior author

  Figure 1. Device architecture and operation concept of the superconducting nanowire memory (SNM). This figure illustrates the structure and operating principle of the SNM cell. It includes a schematic representation of the nanowire circuit layout, highlighting key components such as the storage loop, write and read paths, and control lines. The figure also includes a simplified circuit diagram and a conceptual illustration of the bistable states corresponding to stored logical ‘0’ and ‘1’. These states are defined by the presence or absence of a persistent current in the storage loop, enabling non-volatile memory behavior.

  PublisherDOI

• Ultrafast dynamics and light-induced superconductivity from first principles

  ArXiv.org · 2026-03-18

  articleOpen access

  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.

  PublisherOA PDF

• A scalable superconducting nanowire memory array with row-column addressing

  Figshare · 2026-01-01

  datasetOpen accessSenior author

  Figure 1. Device architecture and operation concept of the superconducting nanowire memory (SNM). This figure illustrates the structure and operating principle of the SNM cell. It includes a schematic representation of the nanowire circuit layout, highlighting key components such as the storage loop, write and read paths, and control lines. The figure also includes a simplified circuit diagram and a conceptual illustration of the bistable states corresponding to stored logical ‘0’ and ‘1’. These states are defined by the presence or absence of a persistent current in the storage loop, enabling non-volatile memory behavior.

  PublisherDOI

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

  ArXiv.org · 2026-03-18

  articleOpen access

  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.

  PublisherOA PDF

• Reconfigurable Superconducting Logic for On-Chip Photon Coincidence Detection

  ArXiv.org · 2026-04-23

  articleOpen accessSenior author

  Scaling photonic quantum-information platforms requires arrays of superconducting nanowire single-photon detectors (SNSPDs) for feedforward control, in which optical operations are conditioned on preceding Bell-state measurements that typically rely on photon coincidence detections. On-chip superconducting cryotron electronics, performing logic directly on detector outputs and subsequently driving optical modulators, could substantially reduce latency and room-temperature interconnect complexity for feedforward schemes. To date, no cryotron logic gates specifically designed to process SNSPD outputs for quantum applications have been demonstrated. We demonstrate a bias-programmable logic gate based on three nanocryotrons (nTrons), fabricated using the same thin-film technology as SNSPDs. The circuit implements selectable AND (coincidence), XOR (odd-parity), and OR functions on two externally generated electrical pulses at 4.2 K, with bit-error rates below $10^{-3}$, bias margins up to $\pm24\%$, and operation extending to 25 MHz over narrower bias windows. Moreover, it performs coincidence and odd-parity detection on two co-fabricated SNSPDs' outputs with bit-error rates below $3.2 \times 10^{-2}$. As a 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.

  PublisherOA PDF

• A scalable superconducting nanowire memory array with row–column addressing

  Nature Electronics · 2026-01-06 · 3 citations

  articleSenior author
  PublisherDOI

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

  arXiv (Cornell University) · 2026-03-18

  preprintOpen access

  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.

  PublisherDOI

Recent grants

• Templated Self-Assembly for Nanomanufacturing

  NSF · $405k · 2012–2015

• Collaborative Research: Kinetic Inductance in Superconducting Nanowire Microwave Devices

  NSF · $383k · 2020–2024

• Collaborative research: Understanding and Engineering the Timing Precision of Superconducting Nanowire Single Photon Detectors

  NSF · $381k · 2015–2019

• Engineering and Physics of Superconducting Nanowire Single-Photon Detectors

  NSF · $370k · 2011–2014

• Single Photon Detection in the Near-and Mid-Infrared by Using Superconductive Nanowires

  NSF · $350k · 2008–2011

Frequent coauthors

• Marco Colangelo

  Massachusetts Institute of Technology

  143 shared

• Di Zhu

  National University of Singapore

  93 shared

• Yujia Yang

  École Polytechnique Fédérale de Lausanne

  89 shared

• Richard G. Hobbs

  Advanced Materials and BioEngineering Research

  75 shared

• Phillip D. Keathley

  Massachusetts Institute of Technology

  72 shared

• Franz X. Kärtner

  71 shared

• Andrew E. Dane

  70 shared

• Eric A. Dauler

  MIT Lincoln Laboratory

  63 shared

Awards & honors

• SPIE Frits Zernike Award for Microlithography