Quest for Entropy #18: we ran Wheeler's late choice on a quantum computer. The order didn't matter. This episode shows that on the ledger toy from #17 the delayed-choice quantum eraser runs as a strict sequence of meetings and two settlements, with no step that waits and no message to the past, and that a real quantum computer gives the same sorted fringes whether the marker is turned and read before the photon reaches the join or after it has landed. It does not derive the arrows: the turns, the delay and the meetings are standard quantum arithmetic, imported. It does not say why a detector settles and a marker does not. It is not the first delayed-choice circuit run on a cloud quantum computer; it is ours, with the code public so that you can break it.
Quest for Entropy #18: we ran Wheeler's late choice on a quantum computer. The order didn't matter. This episode shows that on the ledger toy from #17 the delayed-choice quantum eraser runs as a strict sequence of meetings and two settlements, with no step that waits and no message to the past, and that a real quantum computer gives the same sorted fringes whether the marker is turned and read before the photon reaches the join or after it has landed. It does not derive the arrows: the turns, the delay and the meetings are standard quantum arithmetic, imported. It does not say why a detector settles and a marker does not. It is not the first delayed-choice circuit run on a cloud quantum computer; it is ours, with the code public so that you can break it.
I discuss the delayed choice quantum eraser experiment by drawing an analogy to a Bell-type measurement and giving a straightforward account in standard quantum mechanics. The delayed choice quantum eraser experiment turns out to resemble a Bell-type scenario in which the resolution of the paradox is rather trivial, and so there really is no mystery. At first glance, the experiment suggests that measurements on one part of an entangled photon pair (the idler) can be employed to control whether the measurement outcome of the other part of the photon pair (the signal) produces interference fringes at a screen after being sent through a double slit. Significantly, the choice whether there is interference or not can be made long after the signal photon encounters the screen. The results of the experiment have been alleged to invoke some sort of backwards in time influence. I argue that this issue can be eliminated by taking into proper account the role of the signal photon. Likewise, in the de Broglie–Bohm picture the trajectories of the particle can be given a well-defined description at any instant of time during the experiment. Thus, it is again clear that there is no need to resort to any kind of backwards in time influence. Quanta 2019; 8: 44-56.
Introduction In recent years, numerous quantum optics experiments have demonstrated novel quantum effects based on quantum amplitude superposition. Some of these empirical observations raise disturbing questions about the nature of reality, particularly concerning quantum nonlocality. In this chapter we discuss some experiments that suggest (to us, at least) that the information void, that uncharted regime between signal preparation and outcome detection, is not well modeled by classical spacetime. Something strange seems to be lurking there. For the purposes of exposition, the sequence of experiments we discuss in this chapter does not follow the historical sequence of those experiments, We discuss first the delayed-choice quantum eraser experiment of Kim et al. (Kim et al., 2000), referred to here as KIM. This leads us to define a heuristic measure of which-path information that is central to the theme of this chapter. This leads to a discussion of Wheeler's thought experiments on the conundrum of delayed choice in physics, particularly evident in the phenomenon of galactic lensing (Wheeler, 1983), and its empirical implementation, the delayed-choice interferometer experiment of Jacques et al., referred to as JACQUES. Our final experiment in this sequence is the double-slit quantum eraser of Walborn et al. (Walborn et al., 2002), referred to as WALBORN. That experiment gives a significant challenge for quantized detector networks (QDN) to reproduce its empirical results. Such experiments have led to suggestions that interference patterns formed by particles impacting on a screen may be influenced in some way by decisions made long after those particles had landed on that screen. Our objective in this chapter is to show by a detailed QDN analysis of those experiments that quantum principles do not support these suggestions. Nevertheless, something bizarre seems to be there. There are two complementary aspects in these experiments that have deeply worried physicists. The first involves spatial nonlocality and is the point of Wheeler's concerns. Historically, it has been a great source of debate among physicists.
The current Paper 59 benchmark does not claim that a full laboratory optical delayed-choice experiment, exact interferometric amplitudes, or the complete quantum measurement formalism have already been derived from first principles. It does show that the verified QGEFT Monte Carlo engine now supports a direct algorithmic analogue of delayed choice with explicit left/right slits and a macroscopic detector region. In the reference `N=64` run, the engine opens `97` pending virtual branches, records `19` detector hits, resolves `5` left-slit and `4` right-slit virtual passages, but commits only `2` left and `2` right histories while discarding `32` alternatives. The defensible conclusion is therefore that delayed choice can be reinterpreted inside QGEFT as late branch-cache commit plus deletion of unused histories, rather than retrocausal rewriting of an already completed past.
I discuss the delayed choice quantum eraser experiment (DCQE) by drawing an analogy to a Bell-type measurement and giving a straightforward account in standard quantum mechanics. The delayed choice quantum eraser experiment turns out to resemble a Bell-type scenario in which the paradox's resolution is rather trivial, and so there really is no mystery. At first glance, the experiment suggests that measurements on one part of an entangled photon pair (the idler) can be employed to control whether the measurement outcome of the other part of the photon pair (the signal) produces interference fringes at a screen after being sent through a double slit. Significantly, the choice whether there is interference or not can be made long after the signal photon encounters the screen. The results of the experiment have been alleged to invoke some sort of 'backwards in time influence'. I argue that this issue can be eliminated by taking into proper account the role of the signal photon. Likewise, in the de Broglie-Bohm picture the particle's trajectories can be given a well-defined description at any instant of time during the experiment. Thus, it is again clear that there is no need to resort to any kind of 'backwards in time influence'.
Quest for Entropy #17: the proofs say quantum probability must be a square, never how. On the ledger a fact needs two signatures, and two lengths make an area. That is the Born rule. This episode shows that a settlement rule in which two independent books each sign for an outcome in proportion to its amplitude length, and a fact needs both signatures on the same outcome, reproduces the Born rule, for one particle and for any number of entangled particles settled one pair at a time, while one signature or three do not. The same rule leaves a distant partner's odds untouched and passes the Bell exam; the numbers are on the appendix page. It does not derive the wave: the table and its turns are standard quantum arithmetic, imported. It does not prove that nature settles this way; the two is picked out by counting, and that nature runs the same mechanism is the postulate. It does not say when a settlement occurs, or why a detector settles and a partner particle does not. It is not a claim that nature keeps books, runs a loop, or hashes records; it is a claim that a small deterministic machine with those parts passes these exams, and the code is public so that you can break it.
The delayed-choice thought experiment proposed by Wheeler has been demonstrated over the last several decades for the wave-particle duality of a single photon. The delayed-choice quantum eraser proposed by Scully and Druhl has also been intensively studied for the violation of the cause-effect relation of a single photon as well as a pair of entangled photons in an interferometric system. Here, a coherence interpretation is conducted for the nonlocal realism of the space-like separated photons observed in Phys. Rev. Lett. 84, 1 (2000). As a result, coherence solutions of the observed nonlocal fringes are deterministically derived from coincidence detection-caused selective measurements, where the resulting product-basis superposition becomes the origin of the otherwise quantum mystery of the nonlocal fringes. For this, a fixed sum-phase relation between entangled photons is a prerequisite, which cannot be explained by conventional particle nature-based quantum mechanics.