News
Congrats to 2026 QNN Thesis Defenders!
In May 2026, we congratulate Dr. Emma Batson, Dr. Camron Blackburn, and Dr. Matteo Castellani on their succesful thesis defenses!
Emma Batson successfully defended her PhD thesis entitled “Counting Events in Superconducting Magnesium Diboride and Niobium Nitride Thin-Film Strips” on May 6, 2026.
Abstract:
Detection of single infrared photons at wavelengths beyond the silicon bandgap is necessary to enable quantum communication, dark matter search, deep space communication, and deep imaging of biological tissues. Superconducting nanowire single-photon detectors (SNSPDs) could fill this technological need. However, existing SNSPDs require low operating temperatures which limit their utility in environments with constraints on space, power, or operating cost. Moving to material platforms with higher transition temperatures could open up a broader application space, but fabrication challenges and poorly-understood material and device physics have been major obstacles to reproducible and scalable operation of SNSPDs at elevated operating temperatures.
In this thesis, we study novel fabrication techniques for SNSPDs in magnesium diboride (\MgB) and reveal a simple mechanism for unintuitive device behaviors at high fractions of \Tc, demonstrating paths forward to 1) more reliably fabricate \MgB\ devices with operating temperatures from 10-20~K and 2) engineer materials to operate at higher fractions of \Tc.
Towards the first goal, we simulate and perform irradiation of a micron-wide \MgB\ detector with 30-keV helium ions. The device is at first insensitive to photons, but after irradiation it detects 1550-nm light at 1000~cps at 10~K with few-photon sensitivity. Towards the second goal, we incorporate a realistic thermal Langevin term and spatial variations of \Tc\ based on grain distribution measurements into a numerical TDGL solver and simulate dark count rates in devices of different geometries, then compare the results to experimental data. We demonstrate quantitatively that spatial inhomogeneity can explain switching current suppression and that including simple Langevin mechanics in a carefully applied time-dependent Ginzburg-Landau (TDGL) model can explain the unintuitive decrease of switching rate at high temperatures. This model can be used to make quantitative predictions about material engineering for improved device performance, potentially enabling increases in both operating current and temperature with respect to their critical values. Finally, we present results on the usage of different etching strategies for producing nanoscale \MgB\ devices, and show that irradiation of 400-nm-wide \MgB\ constrictions with 125-keV electrons modifies sheet resistance and retrapping current in a way similar to helium ion irradiation.
With these advances, we lay the groundwork to operate SNSPDs at higher temperatures in order to expand their feasibility in different applications.
Abstract:
The world’s artificial intelligence data centers are on track to consume more electricity than many industrialized nations, but the silicon transistors powering them dissipate energy 10,000 times above the physical minimum dictated by thermodynamics. In contrast, the Adiabatic Quantum Flux Parametron (AQFP) is a superconducting digital logic device capable of switching at energies near this fundamental thermodynamic limit, i.e. AQFP dissipates about 10^-21 J per operation at 5 GHz. Even accounting for a ~1000 W/W cryogenic cooling overhead to maintain superconducting operation at 4 K, this represents roughly 100× lower energy dissipation than the ~10^-16 J switching energy of modern CMOS transistors. Yet superconducting digital logic has long struggled to translate device-level efficiency into practical system-level gains, hindered by the absence of dense superconducting memory, complex clocking networks that limit scalability, and, until recently, the continued dominance of Moore’s law CMOS scaling.
This dissertation demonstrates how AQFP circuits can move beyond device-level promise toward system-level viability through contributions at four levels of the hardware abstraction stack. First, I develop and experimentally characterize synchronizer circuits that relax AQFP’s rigid timing requirements, enabling scalable multi-clock domain designs. Second, I design, fabricate, and test a compact AQFP SR-Loop register file for low-level on-chip memory and characterize Long Josephson Junction devices as a candidate technology for future high-capacity cryogenic delay-line memories. Third, I introduce a high-throughput dataflow microarchitecture that exploits AQFP’s clocked pipeline structure to achieve high compute utilization on AI workloads. Fourth, I extend established CMOS accelerator modeling tools to superconducting electronics, enabling the first quantitative full-stack comparison between AQFP and CMOS systems on real-world AI workloads. Taken together, these contributions chart a practical path toward scalable superconducting computing with the potential for orders-of-magnitude improvement in energy performance.
Matteo Castellani successfully defended his PhD thesis entitled “Superconducting Nanowire Electronics for Single-Photon Detector Control and Readout” on May 1, 2026.
Abstract:
Quantum technologies are driving the development of large-scale cryogenic systems for communication, computation, and sensing. Superconducting nanowire single-photon detectors (SNSPDs), with their exceptional sensitivity and timing resolution, are central to many of these platforms, including photonic quantum circuits and classical imaging systems for space exploration and biological analysis. However, scaling SNSPD arrays and any other cryogenic architectures introduces major challenges: managing the complexity and heat load of readout cables connecting devices to room-temperature stages; enabling low-latency edge processing; and distributing power efficiently within the cryostat.
Superconducting electronics, with ultra-low energy consumption and intrinsic cryogenic compatibility, can help address these challenges. In particular, superconducting nanowires, beyond sensing, offer a promising alternative or complement to Josephson junctions, the gold-standard device for superconducting circuits. Owing to their simple fabrication, resilience to magnetic flux noise, and CMOS-compatible impedance, nanowire electronics are attractive for building co-processors monolithically integrated with SNSPDs.
This thesis presents four nanowire-based circuits for integrated signal processing, control, and power handling. A photon counter is developed for pixel-level digitization in SNSPD arrays, and a reconfigurable logic gate is introduced to enable on-chip photon-coincidence detection for quantum photonic feedforward. A comparator is integrated with a photon-number-resolving SNSPD to discriminate single-photon events for heralding operations, and superconducting diodes and bridge rectifiers are demonstrated for cryogenic power distribution. Together, these results highlight the potential of a unified superconducting platform for sensing and electronics to scale quantum and classical architectures.
Malick Sere receives the 2026 Frederick C. Hennie III Teaching Award
QNN grad student Malick Sere was awarded the 2026 Frederick C. Hennie III Teaching Award in recognition of his outstanding contributions to teaching within the EECS department. He worked with our own Donnie Keathley as a TA in the Spring semester for 6.2410 – Laboratory in Quantum Systems Engineering: Quantum Engineering. Platforms. Throughout that class, many students mentioned how they appreciated his guidance and instruction in the class.
For more on Frederick C. Hennie III, for whom this award is named, see this EECS memorial article. Unsurprisingly, he also won an award for his work in teaching while a graduate student.
New Publication: Field-resolved observation of exciton coherence in a van der Waals magnet
A new publication written from the group using nanoantenna field-sampling techniques to observe femtosecond-scale coherent exciton dynamics in CrSBr was recently published in Nature Materials.
Matthew Yeung, Alexander von Hoegen, Emil Viñas Boström, Fangzhou Zhao, Felix Ritzkowsky, Jack B. Maier, Gian Luca Dolso, Christian Heide, Daniel G. Chica, Xavier Roy, Karl K. Berggren, Angel Rubio, Philip D. Keathley, and Nuh Gedik. “Field-resolved observation of exciton coherence in a van der Waals magnet.” Nat. Mater., p. 1-7, May 2026
The emergence of coherence among electronic quasiparticles underlies collective quantum phenomena from superconductivity to superradiance. In semiconductors, exciton coherence is generally thought to decay rapidly due to scattering and dephasing, limiting its persistence on ultrafast timescales. Here we demonstrate a light-field-driven mechanism that creates and stabilizes exciton coherence in the layered antiferromagnet CrSBr. We directly record the coherent optical field emitted by excitons and track in real time how a deterministic phase, imprinted by the excitation laser, drives incoherent excitons to synchronize into a collective state. This ensemble remains phase coherent for more than 2 ps, whereas its resonance energy undergoes an ultrafast modulation mediated by spin and lattice interactions. The time-resolved field evolution indicates that the multiple peaks seen in conventional spectra originate from a single excitonic resonance subject to dynamic energy modulation. Our findings establish optical phase imprinting as a mechanism to control and sustain collective order in semiconducting magnets, bridging light-driven dynamics with excitonic and magnetic correlations in layered quantum materials.
QNN Newsletter from IAP 2026
Dear QNN Group Members, Alums and Affiliates,
Hope everyone is enjoying the start to Spring! We are excited to share updates from the QNN group over the last months.
It has been a while since our last newsletter, so a lot of activity to report. In particular we want to highlight that Dip Joti Paul received the 2026 Claude Shannon Award from RLE, and Adina Bechhofer the 2026 Samsung Semiconductor Fellowship from EECS. Also, last November we held our annual group retreat at Endicott house and have included a picture from the event. As usual, we provide a quick summary of comings and goings, awards, research activity and events below.
Best regards,
Karl and Donnie
Comings and Goings
The last few months we’ve welcomed the following new group members:
- Dong min Kim, Visiting Scientist
- Zhenyang Xiao, Postdoctoral Associate
- Eileen Xiu, UROP
- Jaden McKee, UROP
- Berkan Tarak, UROP
The following members have now left and become alumni group members:
- Josh Piety, UROP, graduated!
- Hanson Nguyen, Undergrad researcher, MSRP
- Giorgia Ciuffarella, Polytechnic of Turin & École Polytechnique Fédérale de Lausanne, graduated!
- Gabriel LeGuay, Federal Polytechnique School of Zurich, graduated!
- Owen Medeiros, working as a Superconducting Electronics Architect at MIT Lincoln Lab
Awards
- Dip Joti Paul has received the 2026 Claude E. Shannon Award!
- Adina Bechhoffer has received the Samsung Semiconductor Fellowship!
Theses!
G. Ciuffarella, “Infrared-Enhanced Electron Emission from Nanoantennas for Optical Detection,” Ecole Polytechnique Federale de Lausanne and Massachusetts Institute of Technology, 2025.
G. Le Guay, “Reconfigurable superconducting logic gate: From photon-driven operation to electro-optic modulation,” Master’s Thesis, Swiss Federal Institute of Technology, Zurich and Massachusetts Institute of Technology, 2025.
Publications (6/1/25-1/31/26)
R. A. Foster, S. Kandeh, O. Medeiros, A. Simon, M. Castellani, and K. K. Berggren, “Time-tagging data acquisition system for testing superconducting electronics based on an RFSoC and custom analog frontend,” J. Inst., vol. 20, no. 09, p. P09018, Sep. 2025, doi: 10.1088/1748-0221/20/09/P09018.
R. Jing et al., “Bolometric Superconducting Optical Nanoscopy (BOSON),” Phys. Rev. X, vol. 15, no. 3, p. 031027, Jul. 2025, doi: 10.1103/f13d-dpdn.
V. Karam et al., “Parameter extraction for a SPICE model of an hTron superconducting thermal switch,” Phys. Rev. Appl., vol. 24, no. 2, p. 024020, Aug. 2025, doi: http://dx.doi.org/10.1103/jdzc-7l2x.
O. Medeiros et al., “A scalable superconducting nanowire memory array with row–column addressing,” Nat Electron, pp. 1–9, Jan. 2026, doi: 10.1038/s41928-025-01512-0.
H. Nguyen, A. Simon, R. A. Foster, and K. K. Berggren, “Modeling electrothermal feedback of superconducting nanowire single photon detectors in SPICE,” IEEE Trans. Appl. Supercond., vol. 35, no. 9, pp. 1–5, Dec. 2025, doi: 10.1109/TASC.2025.3625542.
D. J. Paul, T. X. Zhou, and K. K. Berggren, “Determination of mid-infrared refractive indices of superconducting thin films using Fourier transform infrared spectroscopy,” Appl. Phys. Lett., vol. 126, no. 25, p. 252601, Jun. 2025, doi: 10.1063/5.0268308.
D. J. Paul, T. X. Zhou, and K. K. Berggren, “Photolithography-compatible three-terminal superconducting switch for driving CMOS loads,” Phys. Rev. Appl., vol. 24, no. 2, p. 024060, Aug. 2025, doi: 10.1103/261b-37xx.
F. Ritzkowsky, M. Yeung, G. L. Dolso, L.-T. Chou, and P. D. Keathley, “High-repetition rate, CEP-stable shortwave infrared source with two-cycle pulses for field-resolved spectroscopy,” J. Opt. Soc. Am. B, JOSAB, vol. 43, no. 1, pp. 207–212, Jan. 2026, doi: 10.1364/JOSAB.580422.
Z. Scherübl et al., “Multimode Operation of a Superconducting Nanowire Switch in the Nanosecond Regime,” ACS Nano, vol. 19, no. 32, pp. 29207–29215, Aug. 2025, doi: 10.1021/acsnano.5c03718.
A. Simon et al., “Ab initio modeling of nonequilibrium dynamics in superconducting detectors and qubits,” Phys. Rev. B, vol. 112, no. 17, p. 174512, Nov. 2025, doi: 10.1103/3m2k-mzr6.
J. W. Simonaitis, J. A. Alongi, B. Slayton, W. P. Putnam, K. K. Berggren, and P. D. Keathley, “Electron energy loss spectroscopy of two-dimensional materials in a scanning electron microscope,” Phys. Rev. B, vol. 112, no. 23, p. 235421, Dec. 2025, doi: 10.1103/tdfh-1ppp.<
Conferences & Proceedings (6/1/25-1/31/26)
K. K. Berggren, “Superconducting Nanowire Single-Photon Detectors for Quantum Sensing: From Photon-Number Resolution to Dark-Matter Detection,” presented at the Munich Conference on Quantum Science & Technology, Kufstein, Austria, Jun. 05, 2025. [Online]. Available: https://www.youtube.com/watch?v=-DgP-F5uu1A
K. K. Berggren, “Superconducting Nanostrip Single-Photon Detectors,” presented at the Low-Temperature Quantum Detectors 2025, Helsinki, Finland, Aug. 06, 2025.
K. K. Berggren, “Superconducting Nanowire Single-Photon Detectors,” presented at the Optica Incubator on Cryogenic Integrated Photonics for Classical and Quantum Systems, Washington, DC, Sep. 11, 2025.
K. K. Berggren, “Superconducting Cryotron-Like Electronic Devices and Circuits,” presented at the EUCAS 2025, Porto, Portugal, Sep. 23, 2025.
K. K. Berggren, “Superconducting Nanowire Single-Photon Detectors for Quantum Sensing: From Photon-Number Resolution to Dark-Matter Detection,” presented at the Q-FARM Seminar, Stanford University, California., Oct. 01, 2025.
K. K. Berggren, “Superconducting Detectors and Electronics for Quantum Sensing and Applications in Space,” presented at the Detector Technology Seminar, NASA Goddard Space Flight Center, Virtual, Jan. 14, 2026.
L. C. Blackburn, “SuperLoop: Architecture Modeling for Superconducting AI Accelerators,” presented at the EUCAS 2025, Porto, Portugal, Sep. 22, 2025.
M. Castellani, “Superconducting Nanowire Electronic Devices and Circuits,” presented at the International Conference on Superconductor Materials and Metama- terials for Quantum Hardware: Devices, Circuits, and Systems, Glasgow, UK, Nov. 2025.
R. A. Foster, “Fast numerical methods for the Usadel equation,” presented at the EUCAS 2025, Porto, Portugal, Sep. 22, 2025.
R. A. Foster, “Fast numerical methods for the Usadel equation,” TU Graz, Oct. 15, 2025.
F. Incalza, “Superconducting Nanowire Single-Photon Detectors Fabricated on Epitaxial NbN Thin Films Grown by Sputtering,” presented at the European Conference on Applied Superconductivity (EUCAS 2025), Munich, Germany, Sep. 2025.
Reed A. Foster, “Scalable superconducting multilayer process for digital imager readout with nanocryotrons,” presented at the MARC, Brettn Woods, NH, Jan. 27, 2026.
A. Simon, “Ab initio modeling of superconducting devices,” presented at the EUCAS, Porto, Portugal, Aug. 14, 2025.
A. Simon, “Ab initio modeling of superconducting devices,” TU Graz, Oct. 15, 2025.
F. Ritzkowsky, M. Yeung, G. L. Dolso, L.-T. Chuo, and P. D. Keathley, “High-Repetition Rate 2.3-Cycle Shortwave-Infrared Source for Next-Generation Field-Resolved Spectroscopy,” in 2025 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Jun. 2025. doi: 10.1109/CLEO/Europe-EQEC65582.2025.11109943.
A. Bechhofer, J. Simonaitis, F. Ritzkowsky, L. Daniel, K. K. Berggren, and P. D. Keathley, “Exploring Parasitics and Coupling between Optically Driven Nanoantennas and Interconnects in Petahertz Electronic Circuits,” in 2025 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Munich, Germany, Jun. 2025. doi: 10.1109/CLEO/Europe-EQEC65582.2025.11109477.
P. D. Keathley, “Nanoscale Petahertz-Electronics for Field-Resolved Spectroscopy,” in 2025 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), Munich, Germany: IEEE, Jun. 2025. doi: 10.1109/CLEO/Europe-EQEC65582.2025.11109586.
Preprints (6/1/25-1/31/26)
F. Incalza et al., “Fast-Recovery Epitaxial NbN Superconducting Nanowire Single-Photon Detectors with Saturated Efficiency at 1550 nm in Liquid Helium,” Dec. 19, 2025, arXiv:2512.18063. doi: 10.48550/arXiv.2512.18063.
From left to right: Gian Luca Dolsa, Karl Berggren, Joey Alongi, Reed Foster, Alejandro Simon, Malick Sere, DJ Paul, Emma Batson, Gabriel La Guay, Ari Willner, Francesca Incalza, Donnie Keathley, Evan Golden, Dorothy Fleischer, Ben Mazur, Daniel Graham, Matteo Casellani, and Dong-min Kim.
New Publication: Electron energy loss spectroscopy of two-dimensional materials in a scanning electron microscope
A new publication written by the group on the characterization of 2D materials in a scanning electron microscope was published in Physical Review B.
John W. Simonaitis, Joseph A. Alongi, Benjamin Slayton, William P. Putnam, Karl K. Berggren, and Phillip D. Keathley, “Electron energy loss spectroscopy of two-dimensional materials in a scanning electron microscope,” Phys. Rev. B, 112, 23, p. 235421, December 19, 2025. | ArXiV
This work demonstrates electron energy loss spectroscopy of 2D materials in the 1–20 keV incident electron energy range, observing 50 times stronger electron-matter scattering relative to 125 keV microscopes. We observe that the universal curve relating beam energy to scattering holds for the transition from bulk graphite to graphene, albeit with a scale factor. We calculate that optimal coupling for most 2D materials and optical nanostructures falls in this range, concluding that spectroscopy of such systems could greatly benefit from use of this energy regime.