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