A First Demonstration of the SQUAT Detector Architecture: Direct Measurement of Resonator-Free Charge-Sensitive Transmons

TL;DR

The paper reports the first experimental validation of the SQUAT detector architecture, a direct-feedline, resonator-free sensor that leverages a weakly charge-sensitive transmon to transduce quasiparticle-tunneling parity events into measurable signals. Through a three-SQUAT Al/AlOx/Al prototype on sapphire, it characterizes steady-state transmission, charge dispersion, and pulsed qubit dynamics, establishing the link between $f_0$, $2\chi$, and readout power for high-fidelity parity readout. The parity measurements demonstrate both amplitude and phase readout modes, quantify the parity-switching rate and fidelity, and identify IR loading, readout-photon effects, and vibrations as major background sources requiring shielding and filtering. The results confirm SQUAT as a viable, high-density, resonator-free platform for meV/THz sensing, with a clear path toward gap-engineered trapping, energy calibration, and scalable microwave multiplexing for large detector arrays. These advances open avenues for single THz photon counting and phonon detection with potential applications in dark matter searches, nuclear monitoring, and quantum networks, while also providing a versatile platform for studying quasiparticle dynamics in superconducting devices.

Abstract

The Superconducting Quasiparticle-Amplifying Transmon (SQUAT) is a new sensor architecture for THz (meV) detection based on a weakly charge-sensitive transmon directly coupled to a transmission line. In such devices, energy depositions break Cooper pairs in the qubit capacitor islands, generating quasiparticles. Quasiparticles that tunnel across the Josephson junction change the transmon qubit parity, generating a measurable signal. In this paper, we present the design of first-generation SQUATs and demonstrate an architecture validation. We summarize initial characterization measurements made with prototype devices, comment on background sources that influence the observed parity-switching rate, and present experimental results showing simultaneous detection of charge and quasiparticle signals using aluminum-based SQUATs.

Figures (17)

• Figure 1: Top Left: Micrograph of one of three SQUATs on a prototype chip. The SQUAT is centered in the image, with fin-shaped islands (purple and blue) surrounded by a ground plane (green). The SQUAT is strongly coupled to the transmission line (yellow) and weakly coupled to the charge bias line (red). The transmission line can be used to measure transmission past the SQUAT ($S_{21}$), while the charge line can be used to measure transmission through the SQUAT ($S_{23}$), making this a 3-port equivalent network. An SEM of the junction is shown in the inset image. Top Right: The circuit diagram for a single SQUAT. The SQUAT characteristics are determined by the capacitance between the islands, to ground, and to the feedline, as well as the Josephson energy ($E_J$) of the junction. The SQUAT can be DC biased through either the transmission line or the charge line, with voltages $V_t$ and $V_b$ respectively. Bottom: Diagrammatic representation of the SQUAT mechanism for detecting photon and phonon events. Phonons from the crystalline substrate or directly absorbed photons can break Cooper pairs in either island, increasing the quasiparticle density. Quasiparticles that drift near the junction have some probability to tunnel, producing measurable signal.
• Figure 2: Top: The magnitude of transmission ($S_{21}$) for three SQUATs on a single chip at multiple readout powers. Bottom Left: Complex $S_{21}$ for one of the above SQUATs with fits at each power. The fit interrogation rate ($\Gamma_n$) is used to calculate the inferred on-chip readout powers ($P_r$) for the color axis. Compression of the resonance loop due to increased qubit excitation probability ($P_1$) is visible for higher powers. Bottom Center: Fit $\Gamma_n$ normalized to the total qubit decoherence rate ($\gamma$) as a function of readout power from the VNA. Bottom Right:$P_1$ as a function of readout frequency and power, calculated from Eq. \ref{['eq:P1']} with fit results from Eq. \ref{['eq:S21Squat']}.
• Figure 3: The magnitude of transmission ($S_{23}$) as a function of island charge. A DC bias voltage is applied on the transmission line, and a frequency scan is taken in the vicinity of the qubit. The SQUAT dispersion ($2\chi$) varies periodically as a function of induced charge. The voltages shown here are applied at the input to the fridge, along wiring shown in Fig. \ref{['fig:readout_chain']}.
• Figure 4: Rabi oscillations in emitted field plotted as a function of drive pulse duration and amplitude. The pulse frequency is matched to $f_0$, and each point consists of 50,000 averages.
• Figure 5: A representative parity-switching measurement. Top: Raw and filtered data plotted as a function of time. The data is a small slice of a longer time-domain acquisition and has been rotated to the basis of maximum signal. Bottom: PSD of the full dataset with a fit to Eq. \ref{['eq:fidelity_main']} to extract the characteristic switching rate.