Electrical post-fabrication tuning of aluminum Josephson junctions at room temperature

Abstract

Josephson junctions are a key element of superconducting quantum technology, serving as the core building blocks of superconducting qubits. We present an experimental study on room-temperature electrical tuning of aluminum junctions, showing that voltage pulses can controllably increase their resistance and adjust the Josephson energy while maintaining qubit quality factors above 1 million. We find that the rate of resistance increase scales exponentially with pulse amplitude during manipulation, after which the spontaneous resistance increase scales proportionally to the amount of manipulation. We show that this spontaneous increase halts at cryogenic temperatures, and resumes again at room temperature. Using our stepwise protocol, we achieve up to a 270% increase in junction resistance, corresponding to a reduction of nearly 2 GHz of the qubit transition frequency. These results establish the achievable range, relaxation behavior, and practical limits of electrical tuning, enabling post-fabrication mitigation of frequency crowding in quantum processors.

Figures (14)

• Figure 1: Active resistance manipulation of a Josephson junction using two different sequences of voltage pulses. (a) Schematic illustration of the active resistance manipulation protocol. (b) An example resistance trace of a junction manipulated up to the time $t_\mathrm{Stop}$ with the scheme in (a), showing the initial resistance drop, active manipulation, and subsequent relaxation. The inset shows a magnified view of the initial resistance drop. (c) Schematic illustration of the stepped resistance manipulation protocol in which the active resistance manipulation is repeated after waiting for some relaxation time $t_\mathrm{Relax}$. During relaxation, we repeatedly measure the junction's resistance with a period of $t_\mathrm{Probe}$ between measurements. (d) Measured resistance evolution during stepped manipulation in (c). The insert in (d) highlights the beginning of the third relaxation event.
• Figure 2: Natural aging of low-dose and medium-dose junctions of different sizes, denoted as Lo and Me, respectively. (a) Resistance increase as a function of time measured at different probe currents and measurement intervals; 1/d: daily, 1/w: weekly: 2/w: biweekly, 2/m: bimonthly. The legend indicates the measurement current density $I/A$. (b) Resistance increase of low-dose and medium-dose junctions as a function of junction electrode width. The low-dose data were measured $2$ days after fabrication, and the medium-dose data after $19$ days. When normalized to the earliest data point, the medium-dose junctions appear to age more slowly, likely reflecting them being $17$ days older when measured.
• Figure 3: Resistance increase during active manipulation at multiple pulse amplitudes for (a) low-dose 1, (b) low-dose 2, (c) high-dose 1, (d) high-dose 2, and (e) medium-dose 1 junctions. Manipulation at voltages exceeding the highest value shown in each panel resulted in junction failure. The time-dependent resistance change is dominated by a linear term $\alpha(V)$ with a quadratic contribution $\beta(V)$, as defined in equation (\ref{['eq:alpha_beta_fit_formula']}). (f) and (g) show the duration of the manually excluded initial resistance drop for each device set in (a-e), as a function of the manipulation voltage (f) and the maximum resistance change (g).
• Figure 4: Extracted parameters (a) $\alpha(V)$ and (b) $\beta(V)$ obtained by fitting equation (\ref{['eq:alpha_beta_fit_formula']}) to the resistance traces in figure \ref{['fig:active_manipulation_room_temperature']}. Panels (c-e) compare the voltage dependence of $\alpha(V)$ and $\beta(V)$. The linear coefficient $\alpha(V)$ exhibits an exponential dependence on voltage. The resistance-area product $R_W A~$[nΩ□m] for the five junction variants is indicated in the legends of (a) and (b).
• Figure 5: Linear fit of the total resistance manipulation as a function of the active resistance manipulation for (a) low-dose 1, (b) medium-dose 1, (c) high-dose 1, and (d) high-dose 2 junctions. The total manipulation is evaluated 30min after active manipulation ended; the vertical axes show $R(t_{\mathrm{Stop}}+30min)/R(0)-1$. A slope of unity indicates that the relaxation contributes a fixed resistance increase offset independent of the actively induced resistance change.