Development of Terahertz Single-Photon Detectors Based on Hafnium-Aluminum Transmon Resonator Qubits for Future NASA Far-Infrared and Submillimeter Missions

The exploration of the far-infrared (FIR) and terahertz (THz) spectral regimes remains one of the highest priority frontiers in modern astrophysics, planetary science, and Earth observation. Observations in this wavelength range are critical for tracing the cosmic history of star formation, probing the interstellar medium, characterizing the atmospheres of outer planets, and monitoring Earth’s atmospheric composition. However, realizing the full scientific potential of future NASA flagship and probe-class space observatories demands revolutionary leaps in detector sensitivity. Current state-of-the-art technologies, such as Transition Edge Sensors (TES) and Microwave Kinetic Inductance Detectors (MKIDs), are rapidly approaching fundamental noise limits. To surpass these limitations and achieve true background-limited performance or single-photon counting capabilities in the THz regime, novel detection paradigms must be explored. This FY26 Small Scale Strategic Technology Development IRAD proposes the design, fabrication, and cryogenic characterization of an ultra-sensitive, quantum-coherent THz single-photon detector based on transmon resonator qubits. Building upon recent breakthroughs in superconducting quantum information science and adapting mechanisms detailed in recent literature (e.g., arXiv:2601.16261 and arXiv:2103.06803), we propose a hybrid superconducting architecture that seamlessly integrates a broadband THz antenna with a highly sensitive qubit-based readout. The proposed detector architecture consists of a macroscopic Aluminum (Al) superconducting film that serves a dual purpose: it acts as a broadband antenna tailored to couple incident THz radiation, and it functions as the primary absorber. At the heart of the antenna structure lies a nanoscale Hafnium (Hf) Josephson junction, which provides the nonlinear inductance characteristic of a transmon qubit. The choice of materials is highly deliberate and leverages the superconducting gap engineering principle. Because Hafnium possesses a significantly lower superconducting energy gap compared to Aluminum, it naturally creates a potential well for quasiparticle dynamics. The operational principle of the proposed detector is fundamentally distinct from standard calorimetric or kinetic inductance methods. When a THz photon is successfully coupled into the Aluminum antenna structure, its energy is absorbed, breaking Cooper pairs and generating a cascade of nonequilibrium quasiparticles. Because of the engineered superconducting gap gradient between the Al film and the Hf junction, these quasiparticles rapidly diffuse toward the center of the structure where they become deeply trapped in the lower-gap Hafnium film. Once trapped within the localized Hf region, these quasiparticles tunnel across the Josephson junction. This localized quasiparticle tunneling dynamically alters the quantum state and the effective Josephson inductance of the transmon circuit. Because the junction is embedded within a microwave superconducting resonator, this shift in the nonlinear inductance translates directly into a dispersive shift of the structure’s resonant frequency. By capacitively coupling this transmon resonator to a standard microwave transmission line, we can continuously monitor the phase and amplitude of a microwave probe tone. A single THz photon absorption event will therefore register as a discrete, measurable quantum jump in the microwave readout signal. This coherent, non-dissipative readout mechanism isolates the fragile quantum sensor from amplifier noise, theoretically allowing for unprecedented Noise Equivalent Power (NEP) and resolving individual THz photons with high fidelity. During this FY26 IRAD effort, our team will execute a rapid, small-scale technology maturation plan consisting of three primary phases: (1) Electromagnetic and quantum circuit co-design to optimize the broadband Al antenna coupling and Hf junction parameters for maximal THz absorption and readout fidelity; (2) Nanofabrication of the hybrid Al/Hf transmon structures utilizing the advanced lithography and thin-film deposition capabilities at GSFC’s detector development facilities; and (3) Ultra-low temperature (millikelvin) dilution refrigerator testing to characterize the device’s dark count rates, quasiparticle lifetimes, and response to heavily attenuated thermal THz sources. Successful demonstration of this transmon-based detector concept will establish a foundational capability at NASA Goddard Space Flight Center for quantum-enhanced electromagnetic sensing. By transitioning qubit architectures from quantum computing applications into the realm of astronomical instrumentation, this IRAD will uniquely position GSFC to lead the development of next-generation focal plane arrays. The scalability of the microwave multiplexing readout inherent to transmon resonators further ensures that this technology can eventually be expanded into large-format arrays, directly aligning with the strategic technology needs of NASA’s future FIR/THz space missions.

Design, fabrication, and cryogenic characterization of ultra-sensitive THz single-photon detector; unprecedented Noise Equivalent Power (NEP) and individual THz photon counting; foundational quantum-enhanced electromagnetic sensing capability at GSFC; scalable microwave multiplexing readout for large-format focal plane arrays.

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