Highly-linear flux-to-voltage transducer based on superconducting quantum interference proximity transistors

Abstract

Superconducting quantum interference devices (SQUIDs) are state-of-the-art in ultra-sensitive magnetometry; however, conventional SQUID devices are fundamentally limited by the inherently nonlinear and periodic nature of their transfer function. Although flux-locked loop (FLL) configurations can mitigate this issue, they introduce electronic complexity and bandwidth constraints that hinder scalability in quantum circuits. In this work, we present an experimental demonstration of the bi-SQUIPT, a flux transducer that modulates the density of states in a proximitized superconducting weak link. The device employs a dual-loop architecture with differential readout, which enables cancellation of non-linearities typical of individual elements, achieving a voltage swing of approximately 120 $μ$V. Measurements yield a spurious-free dynamic range (SFDR) of up to 60 dB, consistent with theoretical predictions and comparable to that of SQUID arrays, while maintaining power dissipation in the femtowatt range. The results further highlight a remarkable operational stability up to 600 mK, positioning the bi-SQUIPT as an enabling technology for high-density cryogenic quantum electronics.

Figures (4)

• Figure 1: Circuit and IV characterization of single SQUIPT:(a) False color micrograph of a SQUIPT. The device consists of a tunnel-probe lead (blue), flux lines (yellow), a loop and a ground lead (purple), and a nanowire (red). (b) Circuit schematic of a bi-SQUIPT. Two SQUIPTs are connected in parallel, thereby grounding the loop and enabling measurement of the voltage drop between their tunnel probes. Each SQUIPT has a current bias that is used to set it at the correct point on the IV curve to maximize the voltage swing. Different magnetic fluxes pierce both loops. (c) IV curves of a typical SQUIPT as a function of the magnetic flux in its loop. (d) Cuts of the color plot shown in (c) at $\Phi = \Phi_0$ and $\Phi = \Phi_0/2$.
• Figure 2: Voltage swing and sensitivity of a bi-SQUIPT: (a) Theoretical normalized voltage response under normalized DC biases $I_{b,x}=0.1 \Delta_0 / e R_{x}$ of an ideal (same loop areas and tunnel probe resistivity) bi-SQUIPT as a function of the CMF, for selected values of the DMF. These curves were obtained by setting $T=0.01 T_C$, where $T_C$ is the critical temperature of the superconductor, and for a Dynes parameter $\Gamma=10^{-4} \Delta_0$, where $\Delta_0$ is the zero-temperature superconducting energy gap. (b) Voltage swing of a bi-SQUIPT measured at different differential magnetic fluxes as a function of the common mode magnetic field for the same selected values of DMF, as shown in panel a. (c) Sensitivity to magnetic flux of a bi-SQUIPT relative to the curves shown in panel b. The portion of the plot highlighted in red represents the dynamic range for $DMF = 1/9\cdot \Phi_0$. The dots indicate the optimal operating points used to calculate the SFDR.
• Figure 3: Study of the SFDR:(a) SFDR calculated for fixed input amplitude as a function of the offset from the optimal working points. (b) SFDR calculated at the optimal working points as a function of the input amplitude. Inset: Sketch for the calculation process of the SFDR. A monochromatic wave (blue) is fed into the system, which transforms it via its transfer function (violet), yielding an output signal (red) whose Fourier transform we study.
• Figure 4: Temperature performance: SFDR calculated by taking the optimal working point at base temperature for $DMF = 4/9\cdot \Phi_0$ as a function of temperature. The dashed black line is a guide for the eye. Inset: TF measured as a function of temperature for fixed DMF. The curves are shifted along the voltage axis for clarity. The vertical dashed line indicates the CMF point where the SFDR is calculated in the inset.