Submicrometer tunnel ferromagnetic Josephson junctions with transmon energy scale

Abstract

We have realized submicron tunnel ferromagnetic Al/AlO$_x$/Al/Ni$_{80}$Fe$_{20}$/Al Josephson junctions (JJs) in Manhattan-style configuration for qubit applications. These junctions have been designed to lie within the energy range of transmons. The current-voltage characteristics of these junctions are comparable with those of standard JJs implemented in state-of-the-art transmons, thus confirming the high quality of the devices and marking a significant step toward the realization of the ferrotransmon. Low-frequency characterization confirms that our junctions operate in the quantum phase diffusion limit, as tunnel JJs in conventional transmons with similar characteristic energies. Ultimately, mitigation of quantum phase fluctuations will represent a key for advancing the entire field of superconducting quantum circuit architectures.

Figures (5)

• Figure 1: a) Design of the multilayer Superconductor/ Insulator/ thin superconductor/ Ferromagnet/ Superconductor Josephson junctions (SIsFS JJs). The pads (in blue) allow separate measurements of the SIs, sFS, and multilayer SIsFS junctions. b) Zoom-in of the junction area: the SIs part is shown in light blue, the window for the FS deposition in orange, and the aluminum connection between the junction and the pad in red. c) Scanning Electron Microscope (SEM) image of the fabricated multilayer SIsFS JJ.
• Figure 2: Current-voltage (I–V) characteristics of Al (20 nm) /AlO$_x$(2 nm) /Al (25 nm) /Ni$_{80}$Fe$_{20}$ (3 nm)/ Al (70 nm) Josephson junction (Sample A) with a lateral size of 500 nm as a function of the temperature. At low-bias current, the IV characteristic at 10 mK (blue curve) can be fitted with the tunnel junction microscopic (TJM) model (red curve).
• Figure 3: a) Finite slope of the supercurrent branch of the I-V curves $R_0$ as a function of the temperature $T$, where $R_0$ has been estimated through the linear fitting of the I-V superconducting branch in Figure \ref{['figure:IV']}. The yellow region indicates the quantum phase diffusion regime, while the light blue one indicates the phase diffusion regime activated by thermal fluctuations. b) Evidence of a resistive branch in the I-V characteristics and its modulation by applying a magnetic field for sample SIsFS B.
• Figure 4: $R_0$ versus the ratio $E_C/E_{J}$. The plot shows a comparison of our experimental data (dots) with theoretical Wentzel–Kramers–Brillouin (WKB) simulations. The red curve, adapted from Ref. [Iansiti1987], models quantum tunneling through a potential with a barrier height equal to the Josephson energy ($E_J$), while the blue curve uses the renormalized binding energy ($E_B$). Experimental data from literature are also included for comparison, as detailed in the legend.
• Figure A1: a) Measurements of the I–V characteristics for the sFS trilayer within the SIsFS multilayer JJs as a function of the temperature T. b) The inset shows the temperature dependence of the I–V characteristics for SIsFS JJ A. From these curves, the values of the superconducting voltage gap $2 \Delta/e$ [panel b)] and the characteristic voltage $I_{sw} R_{N}$ [panel c)] have been determined. In both panels b) and c), the experimental data (red points) have been fitted (red curves) by using the Bardeen-Cooper-Schrieffer (BCS) equation for the superconducting gap [panel b)] and the Ambegaokar-Baratoff (AB) relation for the $I_{sw} R_{N}$ product [panel c)].