Unknown quantum circuit symbol

Question

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)".

Answer 1

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

You will recognize that the same picture -- but only until the dot -- is used in panel b) and c) of the same figure.

After the dot, there is a second part of the circuit -- starting with the open half-circles -- which describes the measurement/tomography of those three states. This is the second half of the experiment.

At first, this was just my interpretation of the pictures, but then I re-read the caption carefully, and indeed, later it says: "The state tomography sequence shown in a is also applied in b and c."

As to the symbol, this is simply the "connection" between the preparation and the measurement part, i.e., feeding the result of the preparation into the measurement device.