11

The answer is structured as follows: I will first give the quantum circuit corresponding to a normal double slit (or interferometer), then the circuit where the which-way information has been recorded, a circuit where the which-way information is first recorded and then erased in a unitary way, and finally a circuit where the which-way information is ...


11

This question was cross-posted to physics forums word for word. I'll give the same basic answer I gave there. Consciousness is never part of any quantum mechanical explanation. Every experiment runs the same whether or not a person is in the room. Retrocausality is also not required here. For example, the Copenhagen interpretation explains the delayed ...


5

This experiment can be completely explained within classical physics. It must, because laser pointers produce coherent states which exactly match the predictions of classical electrodynamics. However, it is a very good analogy for the paradoxes you would face in a quantum eraser experiment with an electron beam. The reason the analogy is good is because ...


5

You're over-interpreting these sketches - they are only sketches, and their specific details can't really be used to make any real predictions. Here is a more accurate version of those sketches, with a proper underpinning on a solid model of the experiment's behaviour: Mathematica source via Import["http://halirutan.github.io/Mathematica-SE-Tools/decode.m"]...


4

The short answer is that you're right. The delayed choice experiment doesn't require backwards-in-time shenanigans, and all the pop-science articles implying this to be the case are basically garbage. For example, here is the example delayed choice eraser circuit from my quantum circuit simulator Quirk: The green boxes are state displays that show the ...


4

If "which route" information is collected, positively or negatively, the interference pattern will vanish. Thus, if you have a live bomb, you will only see a single band of light (and then the bomb will eventually explode when you get unlucky and the photon takes the bombed path). If you have a dud, you will see the interference pattern. Of course, this ...


4

Your interpretation is essentially correct, although the specific numbers you keep referencing are a red herring. (More to the point, the number of counts is just proportional to how long you run each experiment; it's the proportions that count.) Despite those failings in your analysis, the conclusion is mostly correct: the quantum eraser experiment is ...


4

Disclaimer: I am not a physicist, and don't know the details of QM, but have read a lot about the DCQE experiment, and feel I understand how it works intuitively. And I feel that a serious question like this one should not go unanswered. Your main mistake is assuming that anything changes on the D0 side, depending on what you do on the other side. The D0 ...


4

Photons are elementary particles, part of the SM, they are traveling at speed c in vacuum, when measured locally. The world line (or worldline) of an object is the path that object traces in 4-dimensional spacetime. It is an important concept in modern physics, and particularly theoretical physics. https://en.wikipedia.org/wiki/World_line Now ...


3

No, an interference pattern will not be detected. The universe does not care whether the which-way information is "detected" in any way ─ the only thing that matters is that the which-way information is available even in principle, and if it is (as is the case here) then interference will not be present. The only thing that can 'restore' the interference ...


3

The interference pattern only appears after the diagonal polarizer. I just performed the experiment with a Michelson interferometer. I place polarizers orthogonal to each other in each path of the interferometer. I also put a beam splitter in section b0. When the diagonal polarizer was position at B1 the interference pattern reappeared, but the beam splitter ...


3

You can think of the double slit pattern as caused by the interference of two branches of the wave function, say $|L\rangle$ and $|R\rangle$. Now assume that there is which way information because of the presence of some form of detection. The detection apparatus can also be in two states $|\cal L\rangle$ and $|\cal R\rangle$. If the detector is perfect, ...


3

I am doing some research about "time travelling" by shrinking humans to the quantum level. For educational purposes I will explain why this is impossible. Atoms and molecules are the sizes they are because of quantum physics. You cannot scale these up and down in the manner you want. Effectively you would end up with every atom in the human body ...


2

Because they're talking about two completely different things. The quantum no-deletion theorem tells you that there is no linear unitary transformation $U$ that will take two copies of the same state in a tensor product, $|\psi⟩\otimes|\psi⟩$, and turn one of them into a pre-specified blank, i.e. $$U|\psi⟩\otimes|\psi⟩=|\psi⟩\otimes|\mathrm{blank}⟩.$$ The ...


2

NOTE: This answer has now been merged into Understanding the quantum eraser from a quantum information stand point (part IV). Let me start by copying the first part of my previous answer which describes the circuit model of a double-slit or other interference experiment; then, I will try to describe the delayed choice setting (the way I understand it). I. ...


2

Am I missing an important detail in my understanding of how the delayed choice quantum eraser experiment is done? Yes. I'll respond to your list of your understanding. How does one account for what takes place in the experiment without using the concept of "retrocausality" (effect before cause)? This question doesn't make sense, if someone ...


2

I think this a good read: http://jamesowenweatherall.com/SCPPRG/EllermanDavid2012Man_QuantumEraser2.pdf There is a fallacy when we usually think of the quantum eraser, especially the delayed choice quantum eraser: we know the wavefunction doesn't collapse until it's measured, but for some reason we think there should be which-path information before our ...


2

Delayed_choice_quantum_eraser#The_experiment_of_Kim_et_al. [...] Now R1 and R2 shows an interference pattern These two patterns of coincidence counts both arise as a function of "x" (see the labelled arrow next to detector D0 in the schematic); i.e. as detector D0 is put in different places of the plane of convergence where the "Lens" produces a suitably ...


2

According to your description I agree with you that a prediction, that idler photon is more likely detected at D1 or D2, based on knowledge where signal photon hit D0, is possible. In your simplified experiment a 50% beam splitter (BSc) has remained. I think that reason why discussion about retrocausality has started, is the following: A 50% beam splitter ...


2

I wrote up a lot of this stuff a while ago on my github, if it helps -- not specifically what happens if we black-hole the second qubit, but all of the math you'll need to understand the basic results up through the delayed choice quantum eraser. When you look at your quantum-eraser double-slit experiment, you scan a photon-counter across some $x$-axis and ...


2

The Wikipedia webpage "Delayed choice quantum eraser" offers a simplified explanation of this experiment. "In the basic double slit experiment, a beam of light (usually from a laser) is directed perpendicularly towards a wall pierced by two parallel slit apertures. If a detection screen (anything from a sheet of white paper to a CCD) is put on the other ...


2

Here from an eagles eye perspective: No matter what Eve does will lead to a measurable difference in Bobs states, otherwise superluminal communication would be possible. Does Bob need to use (perform measurement on them, or let them directly interact with his qubits) the qubits which are at Eves? If yes then of course she can cripple Bob algorirhm by just ...


2

Imagine [...] auxiliary qubits [leak] to Eve. [Can she] cripple Bob's computation? No. This is guaranteed by the no-communication theorem. If Eve's measurements could affect whether Bob succeeded or not at finding factors, this would allow for a faster-than-light communication mechanism. However, there is Walborn's "Double-slit quantum erasure" ...


2

The setup you have drawn shows no 'eraser' part of the quantum eraser experiment. One way to add an eraser is using polarizing elements in the Mach-Zehnder setup you have drawn on the right. Try reading this article : http://www2.optics.rochester.edu/workgroups/lukishova/QuantumOpticsLab/homepage/snyderlapuma.pdf To answer why we need the quantum eraser ...


2

Your proposed configuration does not show any interference. In the Quantum Eraser experiment, what's getting "erased" is the which-way information, not the interference. The SPDC stage (which you term "photon splitting", though that's a confusing and misleading term that should not be used) creates which-way information, and the presence of that which-way ...


2

Wouldn't this cause the interference pattern of signal photons to disappear and reappear according to the bits of data I am trying to send? No. The fact that you're doing a quantum eraser experiment doesn't magically make interference appear on the other end. Instead, a quantum eraser experiment allows you to restore the interference patterns by taking ...


2

The diagrams represent light coming out of the screen towards us. There are therefore oscillating electric and magnetic fields in the plane of the screen. We don't need to consider the magnetic field, so I've left it out. I'm assuming (to make things easy) that linearly polarised light from one slit has an electric field increasing in the upward direction ...


2

In the quantum eraser experiment, nothing is measured: If the which-way information would be measured, the interference would indeed be irreversibly gone. Rather, what is happening is that the which-way information is copied to another quantum system. If that system is measured (or just ignored - which means it could still be measured), the interference ...


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