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the right impedance transformation to allow for long energy relaxation times T 1
while still allowing for a sufficiently strong bias.
Given these two restrictions, values of 180 pH for the coils inductance and 2 pH
for the mutual inductance between the flux coil and the qubit loop seem to work
well.
4.1.3 Readout Squid Parameters
It turns out that the design of the readout squid is actually one of the most
interesting and complex parts of the circuit design process. This is due to the
fact that the squid needs to be strongly coupled to the qubit during the readout
process but preferably uncoupled during the qubit operation.
The readout squid consists of an inductive loop containing three Josephson
junctions. These junctions are arranged as shown in Figure 4.2a. The loop shares
a mutual inductance with the qubit loop. This allows the qubit to apply a state
dependent current bias I Q to the squid loop. In the branch of the squid containing
one junction, this current bias adds to any externally applied current bias I bias
while in the other branch it subtracts.
This changes the external bias needed
to exceed the critical current of the squid’s junctions at which point the squid
switches to the voltage state and thus generates a measurable signal. Just like
for a single junction, this voltage signal is large enough to create quasi-particle
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