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 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 at 4.2 K.

Extremely large area (88 mm × 88 mm) superconducting integrated circuit (ELASIC)

“…Additionally, increasing the integration scale through packaging is a larger driver of SCE, for which on-carrier memory offers a significant benefit. By adding an active junction layer within the carrier, this approach favors active-to-active bonding with short distances between active elements such as logic and memory (see Supplemental Materials S1), enabling higher bandwidth and lower latency communications than an equivalent active-to-passive platform 28 , 29 .…”

“…The technology further demonstrated 28 synchronous communications between eight superconducting Reciprocal Quantum Logic (RQL) chips powered by a passive large-area (32 mm × 32 mm) circuit with a resonant clock distribution network at a data rate of up to 8 GHz with 3 fJ/bit dissipation. Furthermore, the passive superconducting circuit technology extended 29 isochronous data links across RQL-passive carrier-superconducting (niobium) flex operated with a clock margin of 3 dB @ 3.6 GHz with 5 fJ/bit dissipation. Heterogeneous integration 30 – 32 used various superconducting interconnect materials for microbumps to integrate up to 20 mm × 20 mm SICs.…”

Disentangling superconductor and dielectric microwave losses in sub-micron $\rm Nb$/$\rm TEOS-SiO_2$ interconnects using a multi-mode microstrip resonator

“…Measurements were taken at 4.2K in a LHe Dewar using an RF dip probe equipped with a 32-contact-pad test fixture. 11,18,19,21 To provide for fast sample exchange, a flip-chip press-contact technology is used, where the chip contact pads are pressed against the fixture non-magnetic Cu/Au bumps. The fixture PCB interfaces the bumps to the probe semi-rigid coaxial cables.…”

“…On one hand, they can provide a basic building block for GHz clock and power distribution systems. 11,18,19 On the other hand, they can form a passive transmission line, to propagate low-bandwidth (35-50 GHz) data with 7-10 SFQ pulses per bit, 19 or to propagate high-bandwidth (350 GHz) data with single SFQ pulse per bit, 20,21 or to produce a delay-line memory. 22 The power dissipation is governed by the superconductor and dielectric loss, and is inversely proportional to a transmission line resonator figure of merit, quality factor (Q-factor).…”

Synchronous chip-to-chip communication with a multi-chip resonator clock distribution network *

“…Synchronous communication can be used in systems where the fabrication-related skew between wires is small relative to the timing window of the receiver, about one phase of the RQL clock. For larger systems involving board-to-board communication isochronous protocol is required, as described in [25].…”