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Figure 8.1: Squid I/V: Axis scales are given in terms of the values applied and
measured outside the DR. These relate to the values at the squid as follows:
I ≈ SQ bias /10 kΩ and SQ V out ≈ 1, 000×V . – a) X-T and Y-T plots: The squid is
biased with an oscillating drive (top) to which it responds hysteretically (bottom).
b) X-Y plot: The I/V response shows the hysteretic switching to the voltage state
close to the center and the entry of the squid’s normal conductance at 2∆.
current due to the random flux bias. At sufficiently large biases the 2∆-rise should
be visible as explained in Chapter 4.1.3.
Just like a single junction, the squid can conduct a small amount of current
without generating a significant voltage. Once the current bias exceeds this flux
bias dependent critical current I c (φ), the squid switches to the voltage state and
begins generating a voltage. The critical current can therefore be measured by
slowly increasing the bias and recording the point at which the squid’s voltage
jumps up. Since for the qubit readout only the point of the jump is relevant,
but not the exact behavior of the response voltage, the jump can be encoded into
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