[ExI] DIY Quantum Eraser

[Jason Resch]

Excellent experiment!

I disagree with ChatGPT's #4. You can't consistently "keep the math of a
unitary evolution" and still deny that math's implied ontology of the
multiverse (short of saying the math doesn't refer to anything that's
"really real" -- which is just absurd IMO).

Jason

On Sun, Nov 30, 2025, 3:25 AM Stuart LaForge via extropy-chat <
extropy-chat at lists.extropy.org> wrote:

> https://www.youtube.com/watch?v=sAm7iVdAvTA
>
> Dr. Mithuna Yoganathan performed a very elegant quantum eraser
> experiment at home for her Youtube channel Looking Glass Universe. I
> find her setup to to be much simpler and easier to understand than the
> published experimental protocol with the beam splitters and
> interferometers. In any case, this experiment seems to suggest that
> measurement (such as which way info) can be done reversibly and does not
> necessarily cause any form of wave function collapse. That poses serious
> conceptual issues for objective collapse theories. Epistemic collapse
> theories can say that collapse, and the wavefunction itself, was just in
> your head all along and skirt the problem that way. MWI predicts
> preservation of phase information and super-position even in decohered
> systems.
>
> Here is ChatGPT's take on it:
>
> What theory offers the simplest explanation of the quantum eraser?
>
> ChatGPT said:
>
> --------------------------------------
>
> If you’re asking strictly about conceptual simplicity—fewest moving
> parts, least metaphysical overhead—then the interpretation that gives
> the cleanest, least-tortured account of the quantum eraser is:
>
> Decoherence + No-collapse (Everett-ish)
>
> (a minimal Everett/relative-state ontology, without full Many-Worlds
> metaphysics if you don’t want it)
>
> Here’s why.
>
> 1. Why decoherence + no-collapse is simplest for the quantum eraser
>
> In these experiments:
>
> The “interference pattern” never really disappears.
>
> The which-path detector never truly destroys the phase information.
>
> The environment simply entangles with the photon and makes the
> interference terms unobservable locally.
>
> When you “erase,” you just re-entangle or re-rotate bases so the phase
> relationships become readable again.
>
> So the explanation is:
>
> The interference never goes away; access to it does.
> Erasing restores access to correlations that were always there.
>
> There is zero collapse in this story—every term stays in the universal
> wavefunction. All the “mystery” dissolves into ordinary linear quantum
> mechanics.
>
> The quantum eraser is literally just unitary evolution + a basis
> rotation.
> That’s as simple as it gets.
>
> No extra axioms.
> No special rules for measurement.
> No epistemic leaps.
> No metaphysical carve-outs for consciousness or observation.
>
> Just Schrödinger evolution everywhere, always.
>
> 2. How collapse-based theories handle it
>
> Collapse models can explain it, but they have to wiggle around the
> timing:
>
> You get “partial collapse” or “conditional collapse,”
>
> or collapse that is undone if the which-path marking is later reversed,
>
> or collapse that only triggers on certain kinds of information
> extraction.
>
> All of these are possible, but none are simple.
>
> A real, physical, irreversible collapse is incompatible with truly
> recovering interference — unless you say:
>
> “Well, it wasn’t a true collapse; it was a pre-collapse pseudo-update of
> information.”
>
> Which is just Copenhagen hand-waving with extra steps.
>
> Collapse theories can match the data, but not elegantly.
>
> 3. Why Everett/decoherence wins on simplicity
>
> Because the quantum eraser is just:
>
> photon + atom become entangled, carrying phase in correlations;
>
> environment decoheres the branches so interference is hidden;
>
> eraser recombines or rotates the entangled basis, re-exposing the
> correlations.
>
> No collapse ever happens.
> Nothing is destroyed.
> Nothing is “un-destroyed.”
> Nothing requires special rules.
>
> You treat the system + detectors + environment as one big quantum object
> evolving unitarily, and the whole experiment becomes a trivial exercise
> in linear algebra.
>
> 4. If you don’t like multiverses
>
> You can drop the “many worlds” ontology and keep only the math:
>
> Pure unitary evolution
>
> Reduced density matrices for subsystems
>
> Decoherence explains why classical outcomes are stable
>
> Erasure restores coherence when subsystems are recombined
>
> This is sometimes called:
>
> decoherent histories (without Everett commitment)
>
> minimal Everett
>
> no-collapse operational quantum mechanics
>
> It still gives the same simplicity: collapse never happens.
>
> Bottom line
>
> If the criterion is simplicity of explanation, meaning the quantum
> eraser becomes a straightforward, mechanistic result with no extra
> postulates, then:
>
> The simplest theory explaining the quantum eraser is a no-collapse,
> unitary-only interpretation (Everett/decoherence or minimal
> relative-state theory).
>
> Because in that framework:
>
> Erasure = basis rotation exposing correlations that always existed.
> Nothing is reversed because nothing ever collapsed.
>
> If you want, I can sketch the full entangled state evolution
> step-by-step to show exactly where the “lost” phase lives at each stage.
> ------------------
>
>
> Stuart LaForge
> _______________________________________________
> extropy-chat mailing list
> extropy-chat at lists.extropy.org
> http://lists.extropy.org/mailman/listinfo.cgi/extropy-chat
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.extropy.org/pipermail/extropy-chat/attachments/20251130/3e9a749b/attachment.htm>