We have designed a fully integrated 5-bit flash-type single-flux quantum (SFQ) analog-to-digital converter (ADC), in which an error correction and a bit-interleaving circuit are integrated with complementary quasi-one-junction SQUID (CQOS) comparators, and we verified its operation in experiments. Two types of look-back error correction circuits with different clock schemes were designed for the integrated ADCs to avoid the gray zone in comparators and to convert gray code to binary code. The interleaving circuit was also used in the ADC to add one extra bit. A binary 5-bit A/D converter was integrated into an IC chip. Then, input waveforms at low frequencies were successfully retrieved from the binary data of the A/D converter without any errors. We also confirmed that the A/D converter operated properly by conducting tests in our cryocooling system using a 4K-GM cryocooler after the chip was flip-chip bonded onto a multi-chip-module (MCM) carrier. We also used the beat frequency method to test and confirm that the CQOS comparator operated effectively at high frequency, i.e., 3 bits at 15 GHz in the binary code operation and 4 bits at 15 GHz in the gray code operation. Furthermore, operations at sampling frequencies of up to 32 and 50 GHz with a low-frequency analog input signal were experimentally confirmed for a 4-bit comparator circuit with a critical current density Jcof 2.5 kA/cm2and 10 kA/cm2, respectively.
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.
We demonstrate the efficient modulation of flux-tunable superconducting resonators (FTRs) using flip-chip or on-chip-based input coils. The FTRs we use are aluminum-based quarter-wave coplanar waveguide resonators terminated with 100um or 200um-wide square loop dc superconducting quantum interference devices (SQUIDs) employing 1um-sized Josephson junctions. We employ SQUIDs with a geometric loop inductance of up to 0.7nH to increase the flux transfer efficiency. The geometric inductance of the SQUID results in a non-zero screening parameter
βL, whose branch switching effect is mitigated by using asymmetric junctions. We achieve flux modulation of the FTRs by more than one GHz and flux responsivities of up to tens of GHz/
Φ0 with uA-scale on-chip currents. We compare flip-chip with on-chip input-coil-based flux modulation, where the former is realized through galvanically connected and closely spaced chips, while the latter is achieved through superconducting air-bridge connections. We achieve a flux-transfer efficiency from the input coil to the SQUID loop of up to 20%. Our work paves the way for efficient low current flux modulation of FTRs and sensitive measurement of flux signals.