Superconducting Technology Assessment - nitrd

Examples of what has been achieved through each approach are given in Table 5-5 below. Josephson output
interfaces have demonstrated Gbps communication of data to room-temperature electronics. Actual bit error rates
are better than listed for cases where no errors were found in the data record. Stacked SQUIDs use DC power and
produce Non-Return to Zero (NRZ) outputs, which are significant advantages, but at the cost of many more JJs per
output. Latching interfaces develop higher voltages than SQUIDs, but require double the signal bandwidth for their
RZ output. The latching interface must also be synchronized to the on-chip data rate.
Higher performance is expected for higher-current-density junctions; speed increases linearly for latching circuits
and increases as the square root of current density for non-latching circuits. The Advanced Technology Program
demonstration at NGST (then TRW) successfully integrated semiconductor amplifiers (10 mW dissipation) onto the
same multi-chip module with Suzuki stack outputs.
Both coaxial cable and ribbon cable have been used to carry signals from 4 K to room-temperature electronics.
In the NSA crossbar program, GaAs HBT amplifiers (10-30 mW dissipation) were operated at an intermediate
temperature, approximately 30 K.
TABLE 5-5. RESULTS OF OUTPUT TECHNIQUES
JJ output type
JJ
count
dc
power
Vout
(mV)
NRZ
Rate
(Gbps)
BER
(max)
Jc
(kA/cm 2 )
Stacked SQUIDs 60 Yes 1.3 Yes 1.0 1e-07 1
SFQ/Latch 5 No 2 No 3.3 1e-08 8
Suzuki Stack (6X) 17 No 12 No 10 1e-07 8
Suzuki (4X) + 12 No 10 No 2 1e-09 2
GaAs Amplifier
5.3.2 OUTPUT: 4 K RSFQ TO ROOM TEMPERATURE ELECTRONICS
– READINESS AND PROJECTIONS
Commercial off-the-shelf (COTS) fiber optic components provide much of the basic infrastructure for input and
output between room-temperature and 4 K electronics. For example, 40 Gbps (OC-768) transceiver parts are
becoming widely available. However, as discussed in Section 5.1, they are very expensive and are not designed to
be compatible with word-wide, low-power, short-range applications. A significant effort must be put into tailoring
COTS systems to meet the requirements of an RSFQ computer.
The challenge for output interfaces is raising the signal level by 60 dB from 200 µV to 200 mV at 50 Gbps. HEMT
amplifiers have demonstrated low power (4 mW) operation at 12 K with low noise figures of 1.8 dB in the band
4-8 GHz. Modern High Electron Mobility Transistor amplifiers are capable of the 35 GHz bandwidth needed for
NRZ outputs at 50 Gbps, and will operate at 4 K.
A significant effort must be put into tailoring COTS
systems to meet the requirements of an RSFQ computer.
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