Development of SFQ Multi-Chip Modules for Quantum Bits

We developed a multi-chip module (MCM) system of superconducting quantum bits with single-flux quantum (SFQ) interface circuits. The MCM system consists of chips with quantum bits and chips with SFQ circuits mounted on a base chip by flip-chip bonding (FCB) with superconducting solder. We report on a reliability test of the MCM system. The test was done using a resistance measurement of an MCM with 30 mum diameter bumps. Tested samples showed good durability against thermal cycles between room temperature and 4.2 K. At 4.2 K, the resistance measured through more than two hundred bumps in a series was less than 1 mOmega. Along with achieving a high reliability, reducing thermal interference from SFQ circuits to quantum bits is important. We must understand the thermal conductance between and within the chips to design a system with minimum interference. An MCM on-chip thermometer which is based on the Johnson noise thermometry, has been designed and fabricated to achieve this desired level. The preliminary results of the thermal measurement are reported.