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During the measurement, the | 1 〉 state is tunneled out of the operating minimum
into a neighboring minimum. After the tunneling, the state needs to decay
in the neighboring minimum to “latch” the measurement before it has time to tunnel
back to the operating minimum. The energy relaxation time scales roughly as
1/n, where n is the number of the starting level. Therefore it is desirable to have
as many states as possible in the neighboring minimum at the time of tunneling.
This number should be in the several hundred states and needs to be increased as
T 1 is improved.
The qubit will most likely be subject to 1/f flux noise, the origin of which is
still being investigated [Sendelbach et al., 2008]. This flux noise will be converted
to a bias noise by the qubit’s inductor leading to dephasing of the qubit’s state.
A larger inductance will reduce this effect and is therefore preferred.
These considerations taken together can lead to critical current values around
2 µA, inductances of around 720 pH and capacitances of around 1 pF, but none of
these values need to be hit exactly.
4.1.2 Biasing Circuit Parameters
The biasing circuit consists of three parts: The inductive coil in the phase
qubit integrated circuit, the filtering and wiring inside the dilution refrigerator,
and the electronics that will generate the required biasing sequences. This chapter
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