I Does a qubit break interference in the double slit experiment?

[pines-demon]

According to recent podcast between Jacob Barandes and Sean Carroll, Barandes claims that putting a sensitive qubit near one of the slits of a double slit interference experiment is sufficient to break the interference pattern. Here are his words from the official transcript:

the simplest quantum mechanical system you could imagine, a single quantum bit or qubit right near the holes, and the qubit can either be on or off. It's a binary switch, a quantum binary switch, if you think of it classically. Quantum mechanically, it can sort of be blends of those, but classically it's like on or off. And you program it so that it's definitely off and it stays off if the particle goes down through the lower slit, but it switches to on if it goes to the upper slit. So you might think this is the most rudimentary way you could possibly... 'Cause then what you do is after the particle lands, you go and you look at the qubit, and if it was off, you know that the particle must have gone through the lower hole, and if it's on, it must have gone to the upper hole in each one of the experiment. But this is enough to ruin the interference pattern, this is enough to actually ruin the interference pattern. So decoherence has happened and it's happened through an interaction of the particle with the simplest possible kind of qubit system.

Is that true?

Caveats I see:

• The qubit is a quantum object, so if the particle was in a superposition of up and down, the qubit can be in a superposition too. Measuring the qubit in an orthogonal direction might preserve the particle "coherence".
• Wherever the particle falls in the screen tells you nothing, because you cannot see an interference pattern with a single particle. So is there anything to conclude from this?

Is there a way to verify this calculation with usual quantum mechanics? Maybe using some simplified model?

Disclaimer: at least in principle, this question has nothing to do with Barandes interpretation of quantum mechanics. Note also this is part of an unreleased paper that Barandes promised a while ago but I don't think it is out yet.

 

[.Scott]

What the OP quoted from the podcast is correct. They are only using the qubit to capture a bit of information. They are using the qubit as a classical bit - just to indicate a minimal condition.

It is worth noting that when the podcast says "you program it so that it's definitely off and it stays off if the particle goes down through the lower slit, but it switches to on if it goes to the upper slit", they are fast-forwarding past some significant issues. In fact, that phrase challenges their point of using something very minimal (just a qubit) to capture the information.

When you ask "is there any way to verify this calculation", I am uncertain which "calculation" you are referring to.
When you say "measuring the qubit in an orthogonal direction", you are mixing terms.
You can use a particle to store a qubit, but the qubit itself has no orthogonal direction. If you don't measure the host particle in the direction of the qubit, you not get the qubit.

If the photon lands in what would be a dark band of the interference pattern, it is very likely that it lost its "wave" characteristics.

 

[PeterDonis]

when @pines-demon says "you program it...

That isn't a quote from @pines-demon. It's @pines-demon quoting from the transcript of the podcast he linked to.

 

[PeterDonis]

Is that true?

Not as stated, no. If all you have is the qubit, you've just entangled the qubit with the particle going through the slits. Decoherence doesn't happen just from that interaction between the qubit and the particle. Decoherence only happens if you measure the qubit. And then you no longer have just the qubit--you have whatever measuring device you're using to measure the qubit, and you have a much more complicated interaction with that measuring device.

 

[PeterDonis]

Is there a way to verify this calculation with usual quantum mechanics?

There are already plenty of calculations in the literature showing how which-way information can destroy interference. There are also plenty of calculations showing that "just put something that interacts with the particle at the slit", even if it's just a qubit, isn't enough--you have to have a measurement that decoheres the alternatives for the particle going through the slit. For example, in an experiment with photons, if you put polarizers at each slit, whether or not they destroy the interference depends on how they're oriented; you can get any amount of "interference destruction" from 0% to 100% by adjusting the orientations of the polarizers. And the polarizers are more or less treated like qubits in this analysis. I think @DrChinese has linked to such calculations in previous threads.

 

[pines-demon]

Not as stated, no. If all you have is the qubit, you've just entangled the qubit with the particle going through the slits. Decoherence doesn't happen just from that interaction between the qubit and the particle. Decoherence only happens if you measure the qubit. And then you no longer have just the qubit--you have whatever measuring device you're using to measure the qubit, and you have a much more complicated interaction with that measuring device.

The screen will measure the particle, but does that imply a given result for the qubit?

 

[pines-demon]

What you quoted from @pines-demon is correct. He is only using the qubit to capture a bit of information. He is using the qubit as a classical bit - just to indicate a minimal condition.

It is worth noting that when @pines-demon says "you program it so that it's definitely off and it stays off if the particle goes down through the lower slit, but it switches to on if it goes to the upper slit", he is fast-forwarding past some significant issues. In fact, that phrase challenges his point of using something very minimal (just a qubit) to capture the information.

I’m not quoting (myself). I am quoting Jacob Barandes in a podcast.

When you say "measuring the qubit in an orthogonal direction", you are mixing terms.
You can use a particle to store a qubit, but the qubit itself has no orthogonal direction. If you don't measure the host particle in the direction of the qubit, you not get the qubit.

No. Imagine that the qubit is given by the spin of a neutron. Let’s imagine that when particle passes through the upper slit the spin of neutron is up, if not it remains spin down. Then the spin can also be in a superposition of up and down, thus not giving away any information about which slit the particle went through. The orthogonal direction to vertical (up/down) can be any horizontal projection of the spin (left/right).

 

[PeterDonis]

The screen will measure the particle, but does that imply a given result for the qubit?

Only if the result of measuring the particle is only consistent with one given result for the qubit. Whether that's the case will depend on how the experiment is set up; but if the qubit's only interaction with the particle is to potentially provide "which slit" information, then no, detecting the particle at a particular point on the screen doesn't correlate with either slit and so doesn't imply a given result for the qubit.

 

[.Scott]

Sorry for mis-attributing the quotes. I believe I have corrected them.