  Crosstalk is a major concern in the implementation of large-scale quantum
computation since it can degrade the performance of qubit addressing and cause
gate errors. Finding the origin of crosstalk and separating contributions from
different channels are essential prerequisites for figuring out crosstalk
mitigation schemes. Here, by performing circuit analysis of two coupled
floating transmon qubits, we demonstrate that, even if the stray coupling,
e.g., between a qubit and the drive line of its nearby qubit, is absent,
microwave crosstalk between qubits can still exist due to the presence of a
spurious crosstalk channel. This channel arises from free modes, which are
supported by the floating structure of transmon qubits, i.e., the two
superconducting islands of each qubit with no galvanic connection to the
ground. For various geometric layouts of floating transmon qubits, we give the
contributions of microwave crosstalk from the spurious channel and show that
this channel can become a performance-limiting factor in qubit addressing. This
research could provide guidance for suppressing microwave crosstalk between
floating superconducting qubits through the design of qubit circuits.
