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possible to read out all qubits correctly, the squids and coupling strengths need
to be redesigned to reduce the crosstalk.
9.2 Always-On Capacitive Coupling
Qubits that are coupled with a simple capacitor suffer the most from the above
described crosstalk effects. In fact, if the coupling is too strong ( 30 MHz), it
might not be possible at all to find an order in which to ramp the squids such
that their switching does not randomize the measured state of all qubits.
9.2.1 Measurement Crosstalk and Timing
One of the biggest issues for a capacitively coupled qubit system is the problem
of measurement crosstalk [Kofman et al., 2007, McDermott et al., 2005]. Measurement
crosstalk is the process by which the tunneling of one qubit causes a
tunneling of other qubits even if they were in the | 0 〉-state. This happens because
the tunneling process leaves the tunneled qubit in a highly excited state
in the neighboring minimum.
As the qubit decays to the ground state of this
minimum, it radiates photons of progressively decreasing frequency into the circuit.
These photons can couple via the coupling capacitor to the other qubits
and excite them into higher states in the operating minimum. If this excitation
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