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Figure 2.6: Squid Readout Scheme – a) Squid: A squid, to first order, behaves like
a junction with a critical bias current I biasc that can be tuned via an applied flux
bias current I φ . b) Coupling: The squid is coupled to the qubit via a mutual inductance
M. c) Phase qubit measurement: A measure pulse I Measure temporarily
lowers the barrier between the operating minimum and the neighboring minimum
to the point where the | 1 〉 state can tunnel, while the | 0 〉 state remains trapped.
This results in a flux difference in the qubit loop between the | 0 〉 and | 1 〉 state
of about one Φ 0 .
causes the junction to exceed its critical current and switch to the voltage state.
Squids can detect magnetic flux biases as small as fractions of a flux quantum,
making them useful for high-sensitivity applications like MRI or qubit readout.
In the flux qubit, the | 1 〉 state naturally causes a different flux bias in a
neighboring squid than the | 0 〉 state.
This allows this qubit to be read out
directly. If each qubit is provided with its own squid, the readout can be singleshot.
The switching of the squid to the voltage state releases a large amount of
energy into the circuit and thus randomizes the qubit state making it a non-QND
readout scheme.
Since the | 0 〉 and | 1 〉 state of the phase qubit correspond to fairly similar flux
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