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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.

Figure 3a

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)".

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    $\begingroup$ I don't recognize it. Have you tried emailing the authors? If they answer make sure to put it here. $\endgroup$ Dec 15 '20 at 4:38
  • $\begingroup$ 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)? $\endgroup$
    – DaftWullie
    Dec 15 '20 at 7:37
  • $\begingroup$ @DaftWullie I think this may be along the right lines. $\endgroup$
    – rjh324
    Dec 15 '20 at 17:47
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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.

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