Fig. 
illustrates the basic idea of RSFQ - to pass the
information about the flux state from one loop to another using the
dynamics of the flux transients. A 
jump in Josephson phase
across the junction (see Eq. ()) corresponds to one
flux quantum passing through the junction.
Figure: Single flux quantum entering a J1-L1-J2 stage
of a Josephson transmission line (JTL) through junction J1.
Fig. has many essential elements of a typical RSFQ
circuit: overdamped Josephson junctions J1 and J2 break the
superconducting loop formed by the inductance L1 and the ground plane
(horizontal bars indicate connections to the ground). Both junctions
are biased by current sources I1 and I2 (a shorthand notation for
current sources is used which only shows the direction and entry point
of the applied bias current) so that the current flowing through each
junction is close but smaller than the critical value 
. A flux
quantum arriving from the input ``in'' of the circuit drives the
current through the junction J1 above the critical value and, for a
short time (of the order of 
), a
voltage pulse is formed across the junction J1. The height of the
voltage peak may be estimated from Faraday's law ( 
) as
. After that the
junction J1 returns to the superconductive state and the flux quantum
is now trapped inside the ``J1 - L1 - J2 - ground plane''
loop. What happens next is determined by the parameters of the
circuit. If the total inductance of the loop L (determined mostly by L1)
is large enough so that the induced change in current 
through the junction J2 ( 
) is not sufficient
to exceed its critical value, the state with a trapped flux is stable
and the loop stores the information. Inductance L1 in this case is
called ``quantizing''. If the inductance of the loop is smaller, so
that current through the junction J2 exceeds its critical value, the
junction flips, making a turn in phase and the flux
quantum leaves the loop through J2 in the same manner as it entered it
through J1.
Another basic idea of RSFQ is illustrated in Fig.
showing a two-junction comparator controlled by a quantizing
inductance L1 (note a bar over the inductor symbol).
Figure: Two-junction comparator controlled by a
quantizing inductance
The parameters of the circuit are chosen so that when an SFQ voltage
pulse arrives to the input ``read'' of the circuit, the flux stored in
the loop to the left of the junction J1 is released and appears on the
output ``out''. When there is no stored flux, the current through J1
is smaller and further from its critical value so with the arrival of
the ``read'' pulse junction J2 flips and there is no output. Removal
of the controlling inductance creates a simple diode (or ``buffer
stage'') when nothing from input ``read'' goes into output ``out''
(see also Fig. and Fig. in the next
section). Two-junction comparators (both in the form of a simple
``buffer stage'' and controlled by a quantizing inductance) are at the
basis of every RSFQ design and have been the subject of extensive
theoretical and experimental research [8, 9].