Quantum Bayesianism (QBism) and the double slit experiment How is the following experimental fact interpreted in Quantum Bayesianism (QBism)?
In a double slit experiment we measure position directly after the slits. Because of that the interference pattern on the screen changes to a ballistic pattern.
 A: For the QBist the only thing of relevance is the state of knowledge of the observer about a quantum system. The "state of knowledge" is code for the quantum density matrix, $\rho$. One might wonder if the QBist could get away without using a density matrix and simply resorting to the wavefunction, $|\psi \rangle$ and simply keeping track of all terms and entanglement that is generated as the wavefunction evolves.
The answer is no because the QBist demands a complete description of the system under study independent of other physical systems that in fact may be implicated in the problem such as the measurement device you imagine measuring the particles position directly after the double slit. In general the quantum system under study becomes entangled with the first measurement device and any other system that interacts with the particle over time. This means if we want a thorough account of the state of the particle independent of other systems we must use a density matrix to describe it.
One final note before the direct answer. For a good smart QBist we must of course allow them to KNOW that there IS a detector in the two-slit experiment. Just like a manyworldsist or a Copenhagenist the QBist would predict an interference pattern if they didn't know there was a detector in the experiment. They would all then run the experiment and get results which disagree with their predictions (no interference pattern). If they were good physicists they would inspect their apparatus and see that there is in fact a measurement device and try their predictions again. With this point aside, the point is now to ask how does the QBist, knowing there is a detector, account for the disappearance of the interference pattern?
The answer isn't too bad. The QBist has evolution equations for their state of knowledge about the system. In this case the total system consists of two subsystem. $\rho_S$ and $\rho_M$ where $\rho_S$ is the particle density matrix and $\rho_M$ is the 1st measurement apparatus density matrix. After the particle passes through the slit and is measurement there is an interaction/mixing/entanglement between $\rho_M$ and $\rho_S$. There are two possibilities now.
If the QBist doesn't look at the result of this measurement then her evolution equations tell her she needs to trace over the degrees of freedom of the measurement apparatus to get the state of the system after the first measurement, $\rho_S'$ which will be a mixed state density matrix. She would then calculate that the expected pattern from this state $\rho_S'$ would create no interference pattern!
The second option is that the QBist does look at the result of the measurement. In that case her evolution equations tell her that instead of tracing over the degrees of freedom of the apparatus she should actually update her state of knowledge to a new conditional density matrix, either $\rho_{S,c}^1$ or $\rho_{S,c}^2$ depending on which measurement outcome she saw. The equations then tell her that if the state is $\rho_{S,c}^1$ she expects the particle in a Gaussian spot near slit 1 and likewise for $\rho_{S,c}^2$ and slit 2.
The short answer to your question about how the QBist accounts for the disappearance of the interference in a double slit experiment is that the evolution equations for the density matrix take into account the effects of the entangling interaction of the system with ancillary systems over the course of its evolution. The result of these entangling effects is that the terms in the system's density matrix that would lead to interference disappear.
A: I just wrote this as a comment, but I suppose it's better given as an answer:
If you're the one doing the measurement, then it's interpreted no different to standard quantum mechanics.
QBism is unique in its solution to the EPR paradox being its throwing out of universal probability - hence 'quantum Bayesianism'. Thus, wave function collapse is relative to the observer; if you affect the result of a double slit experiment by covering one of the slits, and I've not interacted with you or the apparatus, then from my frame of reference you and the apparatus are in a superposition (à la Schrödinger's cat) - this includes the status of the slits being covered.
This isn't a subjective description on degrees of knowledge; QBism posits that this is the real state of the system.
