Definitive Measurement of Spin-Triplet Superconductivity

I must be out of it, because I thought that there had already been a definitive measurement of such spin-triplet superconductivity before this. But based on this latest report, it appears that NMR measurement of such superconductivity has not been "definitive" until now.

To clarify, the Cooper pair states in most superconducting material are formed due to the pairing of electrons forming what is known as the singlet state. This is when the spin of one electron pairs with another electron that has an opposite direction of spin. Via Pauli Exclusion principle (or rules for fermions), these two electrons can be quite close to one another in space because they do not occupy the same spin states.

However, the physics also allows for the two electrons to pair up with their spins in the same direction. Unfortunately, again due to Pauli, because they have the same spin state, their spatial state cannot be the same, i.e. they can't be too close to one another. This is a more difficult way to sustain this pair. The reason why this is called the triplet state, as opposed to the single state of the more common pairing, is because quantum mechanics allows for three possible ways for these electrons to pair up (see figure below) when we make a measurement along a particular direction.

 

I think that this is a very good exercise to be included in an intro QM class because it is showing something very basic that could be used in such a class and it represents an application to a very important and advanced idea in condensed matter.

 

Zz. 

 

Are Quasiparticles Real?

I'm posting this because someone here (@dougnatelson) can respond to this. This is based on this article that asks if quasiparticle are real.

In much the same way, "quasiparticles can only exist within some medium or material, because they are built up from the response of that material's constituents or building blocks," Natelson said; in contrast, particles such as electrons and protons "can exist in free space."

In other words, quasiparticles can't exist in a vacuum and they can't exist on their own. They're reliant on particles acting together and being in a material where they can emerge, just as a wave carried out in a stadium can only exist when there is a group of people there who can perform it.

As a condensed matter physicist, I'm quite familiar with the concept of "quasiparticles", since we deal with electrons, "holes", etc. that undergo many-body interactions. It is why, for example, charge carriers in solids have "effective mass", which is typically not the "bare mass" of the charge carrier, but the "normalized" mass due to such many-body interactions. In essence, we have reduced one many-body problem into many one-body problem by lumping the complicated many-body interactions into this new particle's mass. This particle is often called the "quasiparticle".

But the above is a highly simplified picture. Some people would say that this is the Fermi Liquid model of quasiparticles where the strength of the coupling between the particle and the many-body interactions are "weak", allowing for such simplification. This is no longer true for particles with strong interactions where the Fermi Liquid model breaks down (marginal Fermi Liquid and so on). Therefore, it can get very complicated, especially when you add particles in reduced dimensions (2D and 1D).

I like this article because it highlights a very basic concept in condensed matter physics that does not get the appropriate publicity. However, the one issue that is missing is the explanation on what we mean by "real". Is an electron "real" based on what we measured the "bare electron" properties? After all, if we buy into the idea of quantum field theory, even a bare electron has its own vacuum interactions. This leads to the crazy idea that particles actually are all the result of some kind of many-body interactions themselves, i.e. they are quasiparticles!

Instead of worrying whether something is "real", whatever that means, we should be focusing on fact that we can measure such-and-such properties of such-and-such a thing. The measurements, or rather, the outcome of the measurements are real for they can be reproduced. The measurements represent the characteristics and properties of the entity that we are measuring. Maybe this is carrying empiricism to the extreme, but it beats trying to convince people that it is "real".

😊

Zz. 

 

Entanglement and Experiment

The AIP has produced one of the most informative historical account of the history of quantum entanglement after EPR, especially on its development before Bell came up with his infamous inequality test. It is a two-parter, so you definitely want to read both.

Entanglement and experiment, part 1: Before Bell

Entanglement and experiment, part 2: Oral history of the first Bell tests

What I was not aware of was the early experiment by Chien-Shiung Wu in this area. She is definitely one of the giants of physics that should have been awarded the Nobel Prize. I'm glad this article finally gives her the recognition that she deserves, and it certainly gives me even more reason to admire her accomplishments.

Zz. 

2025 Nobel Prize in Physics

It's unusual that a Nobel Prize in Physics is given to physicists working in the field that was the same as my PhD research work. It finally happened this year.

I did research work in tunneling spectroscopy in cuprate superconductors, and we did both superconductor-insulator-normal metal and superconductor-insulator-superconductor tunnel junctions, the latter of which is where we observe the Josephson tunneling current. Therefore, the work cited here is something that I'm quite familiar with. I just never realized till now that it was such a major discovery to be awarded a Nobel Prize. I know that one of my colleagues had John Clarke as his PhD advisor at Berkeley.

Interesting that this is such an old and well-established phenomenon and technique that is only now being recognized.

Zz. 

A Century of Quantum Mechanics

CERN Courier has a special issue this month celebrating what they consider as the 100th anniversary of Quantum Mechanics.

Of course, the focus here is predominantly on elementary/particle physics. And yet, many of the most obvious demonstration and manifestation of quantum mechanics can be found not in particle physics, but in condensed matter physics. The Schrodinger-Cat type demonstration using SQUIDs, and the clearest manifestation of the effect of coherence can be seen in condensed matter experiment. To quote Carver Mead's article[1]:

Although superconductivity was discovered in 1911, the recognition that superconductors manifest quantum phenomena on a macroscopic scale (4) came too late to play a role in the formulation of quantum mechanics. Through modern experimental methods, however, superconducting structures give us direct access to the quantum nature of matter. The superconducting state is a coherent state formed by the collective interaction of a large fraction of the free electrons in a material. Its properties are dominated by known and controllable interactions within the collective ensemble. The dominant interaction is collective because the properties of each electron depend on the state of the entire ensemble, and it is electromagnetic because it couples to the charges of the electrons. Nowhere in natural phenomena do the basic laws of physics manifest themselves with more crystalline clarity.

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 [1] C.A. Mead, PNAS v.94, p.6013 (1997); or you may be able to access it here.

100 Years of Quantum Mechanics

I mentioned earlier of an article on the Davisson-Germer's experiment as part of the commemoration of 100 anniversary of Quantum Mechanics (QM). This is an article describing a bit more of the celebration and the importance of QM. Hint: without QM, none of your modern electronics (computers, smartphones, etc.) will work.

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2022 Nobel Prize In Physics Lectures

If you are bored over the holidays, here's something to keep you occupied for 2 hours.

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Three Ways You Use Quantum Physics Everyday

Most of you know this already, but it is always helpful to remind people on how quantum physics, as esoteric of a subject as it is, is the key to understanding many of the devices that we use everyday and take for granted.

The only drawback here is that the article listed only three, when there could be plenty more.

Zz.

Seeing A Single Atom With The Naked Eye?

This is not a critique of the winning photo. Rather, it is an example of a "click bait", where the news report tries to entice you to read it because the title is so astounding. I guess it worked on me.

This news report, purportedly from Popular Mechanics, is highlighting a winning science/engineering photo of a single strontium atom being held in an ion trap. But what it says is a bit misleading:

Now, we have a photograph that shows a single atom floating in an electric field, and it's large enough to see without any kind of microscope.

This is wrong. It is not "large enough" to be seen.

They corrected this somehow later in the article, but it still does not dispel the error that this has nothing to do with size, and it requires a bit of elementary knowledge of atomic energy level to realize that the earlier description is a mistake.

The strontium atom in the photo is hit by a high-powered laser, which causes the electrons orbiting the strontium atom to become more energized. Occasionally, these energized electrons will give off light. With enough energized electrons giving off enough light, it's possible for an ordinary camera to image the atom.

In other words, the strontium atom was excited and this then causes it to emit light. This process is no different than the light that you see from neon signs or your fluorescent light bulb that has mercury vapor. The unique part about this setup is that you are seeing light from a single atom, whereas in your neon signs, you are seeing the light from many, many atoms. But the process is identical! Yet, we don't go ga-ga and proclaim that we can see an atom with our naked eye.

Just be clear, you are not seeing the atom in the normal sense. You are seeing the light from an atomic transition of this strontium atom. The fact that this is made by a captured single atom is remarkable. The fact that we can detect light from this atom with our "naked eye" does not mean that we are "seeing" the atom in the normal sense that most people understand it.

Zz.

Quantum Mechanics and the Double-Slit Experiment

The double-slit experiment continues to be of interest with respect to quantum mechanics, even after so many years. I've mentioned about this many times, with this one being the most relevant here to this particular post. And note that I made that blog entry back in 2013!

This time, Don Lincoln of Fermilab has released a video on the topic of the double-slit experiment and how it is relevant to QM.

BTW, has he lost weight? If he has, I hope it is on purpose and not due to an illness.

In any case, watch the video and check out the link that I gave. This issue doesn't look like it will be resolved anytime soon unless some new experiment comes up.

Zz.

Quantum Entanglement

I made a post quite a while back on "Quantum Entanglement for Dummies" that tried to describe what it is. I emphasized the fact that this phenomenon is different than classical physics because of one every important characteristics of quantum mechanics, which is the superposition principle that is built into the quantum wavefunction. So to be able to understand why quantum entanglement exists and why it is so "spooky", one must first understand the superposition concept.

Don Lincoln has produced a video on quantum entanglement, and if you pay attention closely, he starts off with describing the superposition concept and how that made a quantum system not "predetermined" before a measurement. He also give a good overview on a Bell-type measurement that shows how experiments agree with QM description but not the hidden variables scenario.



A good video to start you off on understanding this phenomenon.

Zz.

State of the Art of MRI

This is a very good article from Physics Today on the history and development of Magnetic Resonance Imaging, which has become ubiquitous in medical diagnostics. Of course, this came out of the discovery of the nuclear magnetic resonance phenomenon, a technique that itself came out of our understanding of quantum mechanics.

When you read this article, pay attention to how it is continuing to be developed, to evolve, and its continuing improvement. Medical physicists are still actively improving this, and other aspect of the medical field by incorporating things that physicists already know and use. Without advancement in physics, both theoretically and experimentally, there is nothing to trickle down from to the medical field.

Zz.

Charge Fluctuation at a Quantum Critical Point.

This is a fascinating paper[1] (which I'll be reading more of in the next several weeks). But for now, I'll just highlight it here.

The authors found that charge fluctuation in a "strange metal" antiferromagnetic compound exhibit a scaling of f/T (frequency over temperature) in the optical conductivity, which often indicates the presence of a quantum critical point.

If anyone has done MBE before, you'll know how tedious and difficult it is to synthesize a material such as this, and have it be pristine enough to produce these effects that can be measured, at a THz level, no less!

There are many implications here, not the least of which is that the cuprate high-Tc superconductors share the same "parent" or undoped state, being antiferromagnetic perovskites themselves. There have been experiments indicating that the cuprates superconductors are also influenced by their proximity to a quantum critical point.

This is another example where some of the most fundamental aspects of quantum mechanics, in this case the concept of quantum criticality, can often be clearly manifested in a condensed matter system, not in elementary particle physics experiment.

Zz.

[1] L. Prochaska et al., "Singular charge fluctuations at a magnetic quantum critical point." Science v.367, p.285 (2020). ArXiv version of the paper can be found here.

What Is Quantum Mechanics Really All About?

Don Lincoln tries to explain what QM is to non-expert. Do you understand, and buy it?



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First Image Of Entangled Photons

We have the first image ever of photons in an entangled state. You may read the actual paper at the Science Advances page.

Of course, you can't tell that there is any entanglement going on just by looking at the image shown. You have to read the entire thing to see why there is a clear violation of Bell-type inequality here, or more specifically, the CHSH inequality that was meaning measured.

Neat stuff!

Zz.

Quantum Entanglement of 10 Billion Atoms!

Not only is the Schrodinger Cat getting fatter, but the EPR/Bell bulldog is also putting on mass.

New report out of Delft University has shown the successful demonstration of quantum entanglement of two strips of silicon resonators, consisting of roughly 10 billion atoms!

They demonstrated quantum entanglement and violations of Bell’s inequality—a canonical test of the principle that all influences on a particle are local and that particle states exist independently of the observer. They used two mechanical resonators, each containing roughly 10 billion atoms.

If you do not have access to the PRL paper, you may read the arXiv version here.

This is quite a feat, and I think that things can only get bigger, literally and figuratively.

Zz.

The Quantum Form of General Relativity's Equivalence Principle?

This is an interesting approach to one of the dilemma being faced in physics, which is trying to reconcile General Relativity, or gravity in particular, with the quantum mechanical picture. We have had String Theory and Loop Quantum Gravity, etc. going through this effort. But in this paper that just got published in Nature[1], the authors tackled it in a different way, by examining the Einstein's equivalence principle and formulating the QM's version of it, which is different than the classical version.

The ArXiv version of the paper can be found here. However, I have not verified if it is identical to the published version. The ArXiv manuscript was submitted in 2015, while the version in Nature Physics has only been published recently (2018). There doesn't appear to be any updates to this version since its submission to ArXiv.

The best part about this is that the predictions are testable (gives dirty look at String Theory).

I'll let you explore this and see what you think.

Zz.

[1] Magdalena Zych, Caslav Brukner, Nature Physics, https://www.nature.com/articles/s41567-018-0197-6

Loop Quantum Gravity

This is one of those still-unverified theory that tries to reconcile quantum mechanics with General Relativity. I'm not in this field, so I have no expertise in it. But I know that for many people who have read about it, they are aware of String theory and it's competition, Loop Quantum Gravity.

In this video, Fermilab's Don Lincoln tries to explain LQG to the masses.



Keep in mind that this idea is still lacking in experimental support. The gamma ray burst observation that he mentioned in the video has been highlighted here quite a while back.

Without experimental verification, both String theory and LQG continue to have issues with their credibility as a science.

Zz.

The Big Bell Test

Hey, I'm missing all the fun here!

A new paper to be published in Nature appears to have closed the "freedom-of-choice" loophole in the standard Bell-type experiment.

The BIG Bell Test asked human volunteers, known as Bellsters, to choose the measurements, in order to close the so-called "freedom-of-choice loophole" -- the possibility that the particles themselves influence the choice of measurement. Such influence, if it existed, would invalidate the test; it would be like allowing students to write their own exam questions. This loophole cannot be closed by choosing with dice or random number generators, because there is always the possibility that these physical systems are coordinated with the entangled particles. Human choices introduce the element of free will, by which people can choose independently of whatever the particles might be doing.
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Participants contributed with more than 90 million bits, making possible a strong test of local realism, as well as other experiments on realism in quantum mechanics. The obtained results strongly disagree Einstein's worldview, close the freedom-of-choice loophole for the first time, and demonstrate several new methods in the study of entanglement and local realism.

I have not read the actual paper yet, so if you have, I'd like to hear about it.

From my personal point of view, I no longer consider that the loopholes of Bell tests are anything significant anymore. This is due to the NUMEROUS consistent and non-contradictory results that we have obtained so far. In terms of the physics, Mother Nature seems to already let us know what she really is in this regards.

But I guess, until all of the loopholes are closed, we will always have to find a way to close them.

Zz.

Quantum Entanglement Just Got "Big"

The "big" news of the week so far is the two papers published in nature that increased the size of entities that can be in an quantum entanglement.

It looks like we've approached the size of a human hair, which, by any quantum mechanical standards, is humongous.

This is beginning to approach the the scale size of the Schrodinger Cat. Well, not quite, but it is in the right direction. The Schrodinger Cat-type states, which is more of a demonstration of quantum superposition (and a vital ingredient in quantum entanglement), is also getting to be huge.

Zz.