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I have recently read a lot about the BB84 protocol, I have used three primary sources, the original work, a QK book, and a diploma thesis.

My questions refer to the photons sent by Alice, the base of Bob and a possible attack on the protocol (MITM attack).

My first question is about sending the photons from Alice to Bob. The original paper states that Alice sends a single photon in one out of four polarization directions, my question. How does Alice know the direction of the photon? I imagine that I look at a single unpolarized photon, this is sent through a random filter. But there are cases in which a photon does not come through a filter (photon has 90° and the filter is 0°). So how does Alice know if a photon is ever sent? How can she even produce a single say we make a vertically polarized photon?

My second question is based on Bob, in the original paper, only Bob is measured in one of two possible bases (0°, 90°) or (45° and -45°). In my second source (the book), however, it is stated that Bob simply uses a filter from the respective base. I'll explain it a bit more carefully: The statement of the book is that Bob measures (always) at 90° or 45°. But now the diploma thesis, which I use as a third source, says that Bob uses 0°, 90° 45° -45° as a filter for the detection at random. I understand both possibilities, because assuming a photon comes in 90° and I measure in 0° then I can indeed from the non-detection conclude that the photon has been polarized in 90° direction. So I suspect the statements are equivalent, both in the book and in the thesis. Is it correct that way? and what does measuring in a base mean?

My third question relates to a possible attack. I have read a paper in which a MITM (man-in-the-middle) attack is carried out. My book source also lists this attack. But in what way is that an attack scenario, if only I have to authenticate the connection? Then the actual attack is still witless?

I hope my questions are understandable. It is important for me to understand that. I am looking forward to your answers. If I should explain one or the other in more detail, then, of course, I would correct my question and specify. Thanks so far!

1Bob's measurement with 2 random or 4 random directions will lead to the same result, since anyway he can get 4 possible measurement results on each photon. – XXDD – 2018-11-21T04:32:36.327

Perfect, I almost thought so. Thanks also for your contribution @XXDD – None – 2018-11-21T08:51:09.163