A cryogenic system always will have signal and power connections to
room temperature. The data bandwidth for this interface will translate
into hundreds of communications lines, each at gigahertz bandwidth.
For latency reasons, this physical separation must be kept short; but,
for heat transport reasons, should be kept long. Multi-gigahertz
bandwidth, multiconductor copper ribbon transmission line cables with
acceptable heat transfer to connect over a 6'' path (800 picosecond
delay) from room temperature to Kelvin have been built and
tested successfully as interconnect elements in a crossbar switch. This
microstrip cable provides impedance-matched communication from the room
temperature electronics to the cryogenic MCM and back to room
temperature.
A system level of interconnection has been demonstrated at a
preliminary stage by a recent set of measurements on a three-chip
superconductive crossbar cross section which operated at a 2.5
gigabytes/sec serial data rate from room temperature sources through
the cryoelectronics and back to room temperature receivers. The full
system, a , 2.5 gigabytes/sec per port crossbar is being
constructed for use in shared-memory computing or ATM switching
applications.
MITI's Electro-Technical Laboratory has built a second digital test and demonstration system, a four-chip computer consisting of these chips:
MITI's four-chip computer was tested at low speed in 1991 and shown to be fully functional; its power dissipation was 6.2 milliwatts. The difficulties of powering, packaging, and interconnecting for very high speeds have been addressed by a joint ETL-KYOCERA effort to develop a ceramic substrate with multilevel wiring of both normal and superconducting metals.
In addition to the anticipated difficulty of making electrical connection to room temperature is the well-known problem of providing the 1.0 volt level required by room temperature semiconductor electronics, given the millivolt levels generated by superconductive electronics. Several solutions have been demonstrated: amplification at room temperature by Fujitsu (Japan), State University of New York, Tektronix, and TRW; cryogenic temperature amplification by Fujitsu (Japan) and TRW; non-linear thresholding by Fujitsu (Japan) and Tektronix.