I was reading DiCarlo, L., Reed, M., Sun, L. et al. Preparation and measurement of three-qubit entanglement in a superconducting circuit. Nature 467, 574–578 (2010). https://doi.org/10.1038/nature09416 (open access preprint: https://arxiv.org/abs/1004.4324) and came across a quantum circuit symbol that I did not recognize and couldn't find online.
I am wondering about the symbols after the zero kets, before the rotation gates. The caption reads, "Gate sequences producing states with increasing number of entangled qubits: (a) the ground state (no entanglement)".