[ExI] DIY Quantum Eraser

[Stuart LaForge]

On 2025-11-30 03:05, Jason Resch via extropy-chat wrote:
> 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).

LOL. I know, right? Once you have drawn a random marble from the bag, 
you can pretend the bag doesn't exist . . . until you need another 
marble. Rinse and repeat.

Stuart LaForge




> 
> 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
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