# Quantum tunnelling - what is real?

1. Dec 19, 2013

### Jilang

Is it correct that that the wavefunction of a particle is well defined in the classically forbidden region? This would seem to suggest that there would be a chance of finding it there. But I think that is not allowed. Have any experiments actually discovered particles in the barrier or do they just find them coming out the other side?

2. Dec 19, 2013

### tom.stoer

Yes

In principle - yes

Why?

I think this is impossible practically b/c the barrier is too small to contain a measuring device. Look at the applications section in http://en.wikipedia.org/wiki/Quantum_tunnelling

3. Dec 19, 2013

### Jilang

Thanks Tom, I didn't think it was allowed as it is classically forbidden so even if there was a method of detecting it, it would not be possible. Are there any quantum barriers that would be sufficiently large to allow detection or are they all really tiny?

4. Dec 19, 2013

### ZapperZ

Staff Emeritus
What if I put, say, atoms in with magnetic moment in the barrier itself when electrons are tunneling across it. And what if these electrons came from the superconducting supercurrent?

Such an experiment has been done way back in the 70s and 80s, and showed that the presence of such magnetic moments can increase the inelastic scattering of these electrons as they tunnel through the barrier[1].

Look, quantum tunneling may sound "weird" or "unfamiliar". However, please be aware that we know it so well, its validity is no longer questioned, so much so that we are using it in the study of other things (STM, tunnel diodes, etc.. etc.).

Zz.

[1] J.R. Kirtley and D.J. Scalapino, Phys. Rev. Lett. v.65, p.798 (1990).

Last edited: Dec 19, 2013
5. Dec 19, 2013

### tom.stoer

I think what is questioned is not the effect, but the mathematics regarding a non-vanishing wave function inside the barrier. Of course this sounds natural mathematically I is a valid question to ask for an experiment to measure this non-vanishing probability inside the barrier. Unfortunately all effects I am aware of are not able to look into the barrier.

6. Dec 19, 2013

### Jilang

Thanks Zz I don't find the idea of tunnelling weird, just the idea of being able to detect them inside the barrier. Has this actually been possible? I just don't know and have been unable to find this out so far.

7. Dec 19, 2013

### ZapperZ

Staff Emeritus
I don't understand this. We can PROBE what's going on in the barrier, the same way we probe what's going on inside any material that we study.

Without changing the barrier height and width/shape (which is all the parameters that we care about in this tunneling phenomena), can we do something inside the barrier to see if the particle does interact with something inside the barrier? We can. In a superconductor tunneling experiment, we can include, say, magnetic barrier in which the barrier topology doesn't change, but magnetic fluctuation inside the barrier can be present. And when we do that, it definitely changes the tunneling current and spectrum. In fact, certain type of barriers will induce inelastic scattering inside the barrier itself!

So have I "seen" what's going on inside the barrier to sufficiently conclude that, for example, the electron did pass through the barrier? If I have a theoretical description that can accurately describe the tunneling spectra, is that a convincing description of what's going on inside the barrier?

Please note that there are a couple of very old papers by Bardeen and Harrison that set up the tunneling matrix element inside the barrier itself for a superconducting tunneling phenomenon. I can dig up my old, dusty thesis and find those citations for you if you want them.

Zz.

8. Dec 19, 2013

### Maui

Detectors tend to kill quantum behavior, so it will likely not detect anything. Mind you how the interaction with the barrier isn't collapsing or decohering the tunneling particle's wavefunction.

9. Dec 19, 2013

### Jilang

I am happy that they go through the barrier, but there seems to be some uncertainty as to how long they actually spend in there. My question is more about whether you could actually in principle devise an experiment to detect them inside it.

10. Dec 19, 2013

### tom.stoer

ZapperZ, I don't think that we disagree on the tunneling effect. All I am saying is that I don't know / are not familiar with experiments measuring the wave function inside the barrier. So if you say that there are experiments where the barrier is large enough to look inside experimentally, then everything is fine.

11. Dec 19, 2013

### Jilang

Thanks Maui, but I can't quite make sense of your second sentence. If you are saying there should be an interaction with the barrier that is not necessarily true as the barrier is not like a wall. It might be just a potential gradient of some kind.

12. Dec 19, 2013

### Maui

Not really. Quantum tunneling has been a practical hurdle for quite some time in processor designs and the barrier's intended purpose is to really be a wall:

"Gate thickness, another important dimension, is reduced to as little as 1.2 nm (Intel). Only a few atoms insulate the "switch" part of the transistor, causing charge to flow through it. This undesired effect, leakage, is caused by quantum tunneling. The new chemistry of high-k gate dielectrics must be combined with existing techniques including substrate bias and multiple threshold voltages to prevent leakage from prohibitively consuming power."

http://en.wikipedia.org/wiki/65_nanometer

The point was both environmentally induced decoherence and interactions cause collapse do not seem to be on solid footing.

13. Dec 19, 2013

### Staff: Mentor

It's possible. The total energy of the system (particle plus detector) has to be conserved, but in the interaction between detector and particle, energy can be transferred to the particle allowing a measurement that localizes the particle in the classically forbidden region.

14. Dec 19, 2013

### ZapperZ

Staff Emeritus
Do you know of any experiment that "measures the wave function" anywhere?

Zz.

15. Dec 19, 2013

### tom.stoer

Not ψ, but |ψ|, e.g. in interference experiments.

16. Dec 19, 2013

### ZapperZ

Staff Emeritus
How do interference experiments measure that?

Zz.

17. Dec 19, 2013

### tom.stoer

ZZ, this is silly.

You measure e.g. individual particles on screens, in detectors etc. and you derive |ψ| from the measured intensity, distribution etc. This is known since more than a century.

The only question is how to construct a measuring device which can be located in the barrier and which could do something similar. It's about observing an effect which appears in the forbidden region, not outside.

The only technical problem I see is the size and the material of the barrier.

18. Dec 20, 2013

### Jilang

Thanks for this. I think you are saying that to detect it you need to give it enough energy to climb up above the barrier so in that sense it's not still in there. This article discusses an experiment like that
http://cds.cern.ch/record/386067/files/9904098.pdf
It's quite amusing as by the end they seem to have talked themselves out of it actually performing it. One issue appears to be that the disturbance created will destroy any information you are likely to receive. Perhaps it's one of those situations where the uncertainty principle wins every time so there is no way even in principle to devise such an experiment?

19. Dec 20, 2013

### tom.stoer

I don't think so. I think it's a technical problem.

20. Dec 20, 2013

### ZapperZ

Staff Emeritus
It is not silly. I'm trying to find your "threshold" in what you consider to be a sufficient level of information that you can actually deduce for the validity of your |ψ|.

Now first of all, note that interference measurements were known LONG before QM came along. If it is such an obvious experiment that shows such a description, we would have had QM from the very beginning. So already the interference experiment is NOT an obvious observation.

Secondly, do you get the full information about your ψ from interference experiment? I will argue that you do not! You lose information about position time evolution, and you also lose information not only along the longitudinal direction, but also the direction parallel to the slits. You also can only make inference about the phase of the wavefunction. In other words, the interference experiment may give you SOME info, but not a complete info about the wavefunction.

Thirdly, note what you had to do. You compared the results of experiments to the theoretical formulation, and thus, deduced the validity of that formulation. How is this different than what I had stated earlier? I mentioned about the theoretical description of the phenomenon. I even mentioned about Bardeen/Harrison's setup of matrix element in the barrier (W.A. Harrison,Phys. Rev.123, 85 (1961)). The Kirtley/Scalapino paper that I cited showed the physics of introducing magnetic impurities inside the barrier itself, and its description was used in the tunneling spectroscopy analysis. This is no different than your argument of what you can get out of the interference experiment!

So what's different, and why the unusual requirement that one must have some detector of some kind inside the barrier? We do different experiments for different things based on what Nature allows.

Zz.

21. Dec 20, 2013

### Jilang

Tom, thank you for your input and opinion. You seem to understand the question I am asking. I have been reading how the momentum ends up being complex in the barrier. Does that mean that the normal idea of a particle needs to be abandoned altogether in that region?

22. Dec 20, 2013

### George Jones

Staff Emeritus
What do mean by complex? Do you mean the the wave number $k$ is imaginary in the classically forbidden region (say $x>0$)? In this case, write $k = i \kappa$ where $\kappa$ is real. Then

$$e^{i\left(kx - \omega t\right)} = e^{ikx}e^{i\omega t} = e^{-\kappa x} e^{i \omega t}.$$

The probability density then is

$$\left| e^{-\kappa x} e^{i \omega t} \right|^2 = e^{-2\kappa x}$$

Thus, probability density decays exponentially; the deeper into the classically forbidden region, the smaller the chance of finding the particle.

23. Dec 20, 2013

### George Jones

Staff Emeritus
24. Dec 20, 2013

### Jilang

Thank you Mr Jones, I am happy with how an imaginary momentum leads to an exponential decay of the wavefunction but less clear about what it means for the velocity. If the momentum is imaginary is the velocity also?

25. Dec 21, 2013

### phion

You can see the consequences of quantum tunneling today in damn near anything with an IC; [bipolar] transistors, diodes, thyristors, MOSFET, CMOS, and more recently TFET.

Device physicists are starting to take advantage of thinner barriers to induce the quantum tunneling affect and control the the flow of current by keeping the energy high, switching on or off according to the likelihood of of electrons materializing on the other side of a barrier.

I believe the barrier-lowering strategy is still the most widely used current control mechanism, though there is a voltage problem that introduces a physical limit which arises from the fact that electrons and holes are constantly in motion due to their thermal energy.

I'd be willing to bet that these new junctions play a big role in the next 12-18 months of "shrink" in the transistor industry.