Recent content by Morbert

PS to tie my last post to Bohmian mechanics you could presumably include detector and environmental degrees of freedom so the states becomes a pure states and hence pilot waves for the total system's trajectory.

As the conversation has continued I'll post here. In Ma's actual experiment, the BiSA apparatus is interpreted as performing a measurement at the incoming spatial modes. I.e. the detector clicks that occur near modes b'' and c'' are interpreted as a measurement at the prior modes b and c. This...

I may save my responses to @PeterDonis for that thread as they are about the experiment in general rather than a Bohmian accounting.

Two points here: i) A run where no swap is attempted will leave 2 entangled with 1 and 3 entangled with 4. But a run where a swap is attempted and fails will break entanglement between 1 and 2 (and between 3 and 4) and leave them classically correlated. This is because a failed measurement is...

The less important issue: The state of A does change as it passes through the PBS The polarization degree of freedom becomes correlated with the "which port" degree of freedom. But if we ignore this additional degree of freedom, it is fair to say the state of A's polarization does not change...

It's common in consistent histories literature, where you are often chaining many projectors together. See e.g. equation (2) in https://journals.aps.org/pra/abstract/10.1103/PhysRevA.96.032110 (preprint: https://arxiv.org/pdf/1704.08725 ).

To explicitly model* the measurement of 2 and 3 first, we would include the apparatus degrees of freedom and trace over them post-measurement, which would yield$$\rho_{1234} =...

I forgot about B. Before A is detected, the state of B is the usual ##\mathrm{tr}_\mathrm{A}\ket{\Phi^+}_\mathrm{AB}##. After A is detected and the measurement outcome V is obtained, then B is in the state ##\ket{V}##. Though there is some subtlety here as Bob, before learning of Alice's...

The state of A before it passes through the PBS would be ##\mathrm{tr}_\mathrm{B}\ket{\Phi^+}_\mathrm{AB} = \frac{1}{2}(\ket{H}\bra{H}+\ket{V}\bra{V})##. The state of A after is passes through the PBS, but before it is detected, would be ##\frac{1}{2}(\ket{H,h}\bra{H,h}+\ket{V,v}\bra{V,v})##...

The difference in ordinary QM would be in theirs states. If the state of A+B before they arrive at their respective detectors is ##\ket{\Phi^+}_\mathrm{AB}## then the state of A before it arrives is ##\mathrm{tr}_\mathrm{B}\ket{\Phi^+}_\mathrm{AB}## and the state of B before it arrives is...

If you are carrying out the textbook standard QM calculation of unitary evolution -> measurement -> collapse (preparation) -> unitary evolution, and if, in the lab reference frame, measurements on A and D happen before measurement on B and C, then the wavefunction collapses upon measurements on...

Write down the initial wavefunction of the entangled particles and observing environment. ##|\Psi(0)\rangle = |\psi,M_\mathrm{ready}^A,M_\mathrm{ready}^B\rangle## and evolve it unitarily, yielding some form...

It looks like a generalization of the "dictionary" account of the stochastic matrix. Similar to the general form here: [edit] - Not to imply this is all the paper is. It's a rigorous exploration of this kind of representation.

Disabusing people of notions like everything is made of energy is well within the wheelhouse of physicists.

fwiw a skim of literature shows papers that resolve Bohmian trajectories from weak measurements. E.g. https://arxiv.org/abs/0706.2522