I want to ask a question regarding how spin state of an electron becomes entangled with the slit an electron travels through in the double slit experiment. So I understand that in the double slit experiment, we have two slits 1 and 2. We place a detector by slit 1 to measure whether or not an electron travels through slit 1. Now, this is where I am fuzzy to let me know where my thinking is wrong. The detector has a magnetic field oriented in a specific direction. So if the electron is spin down, it will not be flipped by the magnetic field and will not emit a photon. If the electron is spin up, its orientation will be flipped and it will emit a photon. Therefore, if a photon is emitted, we can say that the electron is spin up and went through slit 1. Now, the inverse statement is that if the electron is spin down, it must have went through slit 2. This gives us an entangled quantum state of |ψ> = 1/√2 (|u1> + |d2>) Now my real question is this: If an electron with spin down cannot be detected when it travels through slit 1, how do we distinguish between an electron with spin down that travels through slit 2 and an electron with spin down that travels through slit 1? I am sorry if I was unclear.