In a beam splitter with one vacuum input, the output looks like ##\frac{1}{\sqrt 2}(|1_A\rangle +i|0_B \rangle)\frac{1}{\sqrt 2}(-i|1_A\rangle +|0_B \rangle)##.(adsbygoogle = window.adsbygoogle || []).push({});

If there is some further processing, the vacuum ##i|0_B\rangle##, along with the i , could end up multiplied with some non-vacuum term.

Do we need to keep track of that "i" that originates from such a vacuum term? If so, what is the physical interpretation?

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# I Phase of |0> when it appears in a product

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