Isochronous Data Link Across a Superconducting Nb Flex Cable with 5 femtojoules per Bit

Abstract

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 RQL 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 onchip through a transmission line, and on a multi-chip module (MCM) 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 at 4.2 K.Superconducting digital circuits have the potential to enable digital systems with very high computation density [1]. The combination of low power logic [2] and low loss interconnects (e.g. [3] and references thereof) shifts the design trade-off towards heterogeneous distributed architectures with multiple chips on multiple boards. Volumetric cooling with efficient heat transfer allows such a system to be physically small, with packing density limited only by the physical dimensions of the connectors on the boards. Superconducting systems trade modest chip integration density for high packaging density.Efficient distributed heterogeneous architectures require tremendous communication bandwidth on-board and board-to-board. Design challenges involve both the interconnects themselves, and synchronization between driver and receiver. Nb interconnects having 700 GHz analog bandwidth can cover distance with appropriate data encoding and geometry of the wires [4]. A bandwidth-efficient driver producing 10 SFQ pulses per bit, with an analog bandwidth of 35 GHz and 0.58 fJ per bit has been reported in [5]. This bandwidth-efficient driver has been used to demonstrate synchronous communication between 10×10 mm chips on an MCM with interconnect length up to 54 mm. Superconducting Nb flex tape on Polyimide has been proposed for board-toboard communication [6], [7]. Tape enables longer range communication relative to MCM because of very low dielectric losses, tan δ ≈ 10 −4 , at cryo temperatures [8] and due to relatively large dimensions for the traces and * This research is based upon work supported in part by the ODNI, IARPA, via ARO. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the ODNI, IARPA, or the U.S. Government.