Unknown quantum circuit symbol

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)".

• I don't recognize it. Have you tried emailing the authors? If they answer make sure to put it here. – Craig Gidney Dec 15 '20 at 4:38
• Could it be some sort of "plug"? The idea being that the same circuit can be applied to parts (b) and (c) where the filled circles at the end of each wire would plug in to the open circle parts on (a)? – DaftWullie Dec 15 '20 at 7:37
• @DaftWullie I think this may be along the right lines. – rjh324 Dec 15 '20 at 17:47

The picture has two parts: The first goes until the dots. It is simply three $$|0\rangle$$ states. (The ground state.)