Interconnect properties position superconducting digital circuits to build large, high performance, power efficient digital systems. We report a board-to-board communication data link, which is a critical technological component that has not yet been addressed. Synchronous communication on chip and between chips mounted on a common board is enabled by the superconducting resonant clock/power network for Reciprocal Quantum Logic circuits. The data link is extended to board-to-board communication using isochronous communication, where there is a common frequency between boards but the relative phase is unknown. Our link uses over-sampling and configurable delay at the receiver to synchronize to the local clock phase. A single-bit isochronous data link has been demonstrated on-chip through a transmission line, and on a multi-chip module through a superconducting tape between driver and receiver with variable phase offset. Measured results demonstrated correct functionality with a clock margin of 3 dB at 3.6 GHz, and with 5 fJ bit−1 at 4.2 K.

The true-differential superconductor on-chip amplifier has complementary outputs that float with respect to chip ground. This improves signal integrity and compatibility with the receiving semiconductor stage. Both source-terminated and non-source-terminated designs producing 4 mV demonstrated rejection of a large common mode interference in the package. Measured margins are ±8.5% on the output bias, and ±28% on AC clock amplitude. Waveforms and eye diagrams are taken at 2.9–10 Gb  s−1 . Direct measurement of bit-error rates are better than the resolution limit of 1×10−12 at 2.9 Gb  s−1 , and better than 1×10−9 at 10 Gb  s−1 .

Superconducting digital circuits are a promising approach to build integrated systems with high energy-efficiency and computational density of the packaged chips. In such systems, performance of the data link between chips mounted on a multi-chip-module (MCM) is a critical driver of performance. In this work we report a synchronous data link using reciprocal quantum logic enabled by resonant clock distribution on-chip and on the MCM carrier. The simple physical link has only four Josephson junctions and 3 fJ/bit dissipation, including a 300 W/W cooling overhead. The driver produces a signal with 35 GHz analog bandwidth and connects to a single-ended receiver via 20 Ω Nb passive transmission line. To validate this link, we have designed, fabricated, and tested two 32 × 32 mm 2 MCMs with eight 5 × 5 mm 2 chips connected serially and powered by a traveling-wave clock, and with four 10 × 10 mm 2 chips powered with a 2 GHz resonant clock. The traveling-wave clock MCM validates performance of the data link components and achieves a 5.4 dB AC bias margin with no degradation relative to individual chip tests. The resonator MCM validates synchronization between chips, with a measured AC bias margin up to 4.8 dB between two chips. The resonator MCM is capable of powering circuits of 4 million Josephson junctions across the four chips with a projected 10 Gbps serial data rate.

Interconnects are a major discriminator for superconducting digital technology, enabling energy efficient data transfer and high-bandwidth heterogeneous integration. We report a method to simulate propagation of picosecond pulses in superconducting passive transmission lines (PTLs). A frequency-domain propagator model obtained from the Ansys High Frequency Structure Simulator (HFSS) field solver is incorporated in a Cadence Spectre circuit model, so that the particular PTL geometry can be simulated in the time-domain. The Mattis-Bardeen complex conductivity of the superconductor is encoded in the HFSS field solver as a complex-conductivity insulator. Experimental and simulation results show that Nb 20 Ohm microstrip PTLs with 1um width can support propagation of a single-flux-quantum pulse up to 7mm and a double-flux-quantum pulse up to 28mm.

We demonstrate experimentally the operation of a double-balanced mixer, a phase shifter, and an I/Q modulator, built with Josephson junction active elements in a superconducting integrated ciruit. The devices operate at cryogenic temperatures with no power dissipation on chip, making them suitable for qubit control.