Elimination of two level fluctuators in superconducting quantum bits by an epitaxial tunnel barrier

Quantum computing based on Josephson junction technology is considered promising due to its scalable architecture. However, decoherence is a major obstacle. Here, we report evidence for improved Josephson quantum bits (qubits) using a single-crystal ${\mathrm{Al}}_2{\mathrm{O}}_3$ tunnel barrier. We have found an $\sim$80% reduction in the density of the spectral splittings that indicate the existence of two-level fluctators (TLFs) in amorphous tunnel barriers. The residual $\sim 20\%$ TLFs can be attributed to interfacial effects that may be further reduced by different electrode materials. These results show that decoherence sources in the tunnel barrier of Josephson qubits can be identified and eliminated.

Figure 1

(Color online) Reciprocal images and real-space figures of the growth sequence for the epitaxial-Re/epitaxial-${\mathrm{Al}}_2{\mathrm{O}}_3$/polycrystal-Al film. Refer to Ref. 30 for growth details. Under each $k$-space RHEED image we show a corresponding real-space figure depicting crystal ordering. (a) Epitaxial Re grown at $850°\mathrm{C}$ on a sapphire (0001) substrate. The streaks in the RHEED image indicate that the Re film is single crystalline. (b) Amorphous ${\mathrm{AlO}}_x$ tunnel barrier reactively evaporated onto the base Re film at room temperature. Absence of a diffraction pattern implies that the as-grown ${\mathrm{AlO}}_x$ layer is amorphous. (c) Epitaxial ${\mathrm{Al}}_2{\mathrm{O}}_3$ formed after $800°\mathrm{C}$ annealing of the amorphous ${\mathrm{AlO}}_x$. The streaks in the RHEED imply that the amorphous structure of (b) has transformed into a single crystal. (d) Polycrystalline Al top electrode evaporated onto the epitaxial ${\mathrm{Al}}_2{\mathrm{O}}_3$ tunnel barrier. The multiple diffraction rings indicate that the top Al electrode is polycrystalline.

Figure 2

(Color online) Optical and schematic views of the epitaxial barrier qubit. (a) Optical micrograph of the fabricated qubit device. It is comprised of a qubit junction ($70\mathrm{\mu }{\mathrm{m}}^2$ in area) on a quadrupole loop, a dc superconducting quantum interference device and a flux bias coil. (b) Cross-sectional view of the qubit showing all the component layers; ${\mathrm{Al}}^1\left({\mathrm{Al}}^2\right)$ stands for the UHV in situ evaporated (ex situ sputtered) aluminum. The amorphous ${\mathrm{SiO}}_2$ $\left(\mathrm{a}\text{−}{\mathrm{SiO}}_2\right)$ is used as an insulator between metallic wiring layers. (c) Schematic of the qubit and the measurement setup. For simplicity, some components such as filters are not shown here. On-chip components, which are cooled to $25\mathrm{mK}$, are enclosed in a dotted box. The circuit parameters are $I_0\approx 10\mathrm{\mu }\mathrm{A}$, $C\approx 2\mathrm{pF}$, $L\approx 115\mathrm{pH}$, $M\approx 1.1\mathrm{pH}$, and $M_S\approx 6.7\mathrm{pH}$.

Figure 3

(Color online) Qubit spectroscopy: probability of the qubit being in the $\mid 1⟩$ state after a long microwave $(f_{\mu w}\equiv {\omega }_{\mu w}∕2\pi )$ pulse is applied at each bias current; the brighter the color, the higher the probability. When the microwave energy $\left(h{\omega }_{\mu w}\right)$ is equal to the energy spacing between the lowest two levels of the qubit $\left(h{\omega }_{10}\right)$, this probability reaches a maximum. (a) an amorphous barrier qubit (polycrystal-Al/amorphous-${\mathrm{AlO}}_x$/polycrystal-Al) and (b) an epitaxial barrier qubit (epitaxial-Re/epitaxial-${\mathrm{Al}}_2{\mathrm{O}}_3$/polycrystal-Al). Both qubits have an identical design (qubit junction area of $70\mathrm{\mu }{\mathrm{m}}^2$) and the only difference is the trilayer structure. As reported previously, the amorphous barrier qubit (a) shows many spectral splittings. Each splitting can be seen more clearly in the zoomed-in image at the bottom inset. This situation is dramatically changed in the epitaxial barrier qubit (b). In (b), the density of splittings is substantially lower than that in (a). The bottom inset is a zoomed-in image of the spectrum. The top inset of (b) shows typical Rabi oscillation data of this epitaxial qubit measured at a bias point away from any splittings. Its exponentially fitted decay time is $\sim 90\mathrm{ns}$.