Superconducting Technology Assessment - nitrd

Single flux quantum is the latest generation of superconductor devices and circuits. The I-V curve for SFQ operation
is single-valued and the devices are DC powered. A fundamental property of JJs in the voltage state is that the junction
produces precisely reproducible SFQ pulses at a frequency proportional to the voltage:
(2)
where Φ 0 = 2.07 mV-ps is the magnetic flux quantum. Each pulse represents one quantum of magnetic flux, 2.07
x10 -15 Webers, passing through the junction. At 100 µV, the SFQ frequency is 50 GHz. Thus, invisible in the DC
characteristics, the junction DC voltage is the result of generating identical SFQ pulses according to Eq. (2). A 2 ps
pulse is approximately 1 mV. In SFQ circuits, each switching junction is associated with a small inductor L that can
compress and store a flux quantum. A parameter β L~ 1 defines the relation between β L and I C,
(3)
Switching time is a critical factor for digital applications; the minimum pulse width and maximum frequency are
limited by parameters J C,I C, C’, and R, where C’ is the specific capacitance of the junction and R is generally an
external shunt resistance. SFQ junctions are designed for optimal speed (i.e., near critical damping). An external
shunt resistor is used to insure that β C ~ 1. Then, the SFQ pulse width is:
(4)
Thus, the maximum operating frequency scales as J C 1/2 . Figure 3 shows that the measured speed of asynchronous
flip-flops, the simplest SFQ logic circuit, follows this rule.
Figure 3. Measured speed of static dividers varies as J C 1/2 .
f max (GHz)
1000
900
800
700
600
500
400
300
200
100
1 10 100
J c (kA/cm 2 )
149