Thermal budget of superconducting digital circuits at sub-kelvin temperatures

Superconducting single-flux-quantum (SFQ) circuits have so far been developed and optimized for operation at or above helium temperatures. The SFQ approach, however, should also provide potentially viable and scalable control and read-out circuits for Josephson-junction qubits and other applications with much lower, milli-kelvin, operating temperatures. This paper analyzes the overheating problem which becomes important in this new temperature range. We suggest a thermal model of the SFQ circuits at sub-kelvin temperatures and present experimental results on overheating of electrons and silicon substrate which support this model. The model establishes quantitative limitations on the dissipated power both for "local" electron overheating in resistors and "global" overheating due to ballistic phonon propagation along the substrate. Possible changes in the thermal design of SFQ circuits in view of the overheating problem are also discussed.Comment: 10 pages, 8 figures, submitted to J. Appl. Phy

Characterization of a fabrication process for the integration of superconducting qubits and RSFQ circuits

In order to integrate superconducting qubits with rapid-single-flux-quantum (RSFQ) control circuitry, it is necessary to develop a fabrication process that fulfills at the same time the requirements of both elements: low critical current density, very low operating temperature (tens of milliKelvin) and reduced dissipation on the qubit side; high operation frequency, large stability margins, low dissipated power on the RSFQ side. For this purpose, VTT has developed a fabrication process based on Nb trilayer technology, which allows the on-chip integration of superconducting qubits and RSFQ circuits even at very low temperature. Here we present the characterization (at 4.2 K) of the process from the point of view of the Josephson devices and show that they are suitable to build integrated superconducting qubits