[ExI] DIY Quantum Eraser

[Stuart LaForge]

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