Decoherence in Josephson Qubits from Dielectric Loss

Dielectric loss from two-level states is shown to be a dominant decoherence source in superconducting quantum bits. Depending on the qubit design, dielectric loss from insulating materials or the tunnel junction can lead to short coherence times. We show that a variety of microwave and qubit measurements are well modeled by loss from resonant absorption of two-level defects. Our results demonstrate that this loss can be significantly reduced by using better dielectrics and fabricating junctions of small area $\lesssim 10\mu {\mathrm{m}}^2$. With a redesigned phase qubit employing low-loss dielectrics, the energy relaxation rate has been improved by a factor of 20, opening up the possibility of multiqubit gates and algorithms.

Figure 1

Microwave dielectric loss for materials used in superconducting qubit fabrication. The amorphous thin-film insulators were grown by chemical vapor deposition and form the dielectric of the resonator’s parallel-plate capacitor. The $a\mathrm{\text{−}}{\mathrm{SiO}}_2\mathrm{\text{−}}1$ (circles) and $a\mathrm{\text{−}}{\mathrm{SiO}}_2\mathrm{\text{−}}2$ (triangles) data refer to amorphous ${\mathrm{SiO}}_2$ deposited at $T=13$ and 250 °C, respectively, using silane and oxygen as precursers. The $a\mathrm{\text{−}}{\mathrm{SiN}}_x$ data refer to silicon nitride deposited at $T=100°\mathrm{C}$ by reacting silane and nitrogen.

Figure 2

Measurement of junction resonances and their size distribution. (a) Spectroscopy of the $0\to 1$ qubit transition as a function of junction bias for two representative phase qubits with junction areas $13\mu {\mathrm{m}}^2$ and $70\mu {\mathrm{m}}^2$ (shifted for clarity). (b) Size distribution of splittings. The arrows indicate the cutoff $S_{\max }$.

Figure 3

Rabi Oscillations for a phase qubit using CVD ${\mathrm{SiO}}_2$ (top trace, offset) and SiN (bottom trace) as a dielectric for the crossover wiring. Microwaves at the qubit frequency are pulsed for a time $t_R$, and subsequent measurement of the qubit state shows an oscillation of the probability of the excited state. The decay of the Rabi oscillations is consistent with the measured relaxation time of $T_1=0.5\mu \mathrm{s}$, which is about 20 times better than previous experiments with a ${\mathrm{SiO}}_2$ dielectric. From Ramsey fringes, we determined a dephasing time of $T_2=150\mathrm{ns}$ (data not shown).