“Atomic and molecular configurations come in a near-infinite number of possible combinations, but the specific combinations found in any material determine its properties. Graphene, which is an individual, single-atom sheet of the material shown here, is the hardest material known to humanity, and in pairs-of-sheets it can create a type of material known as a superlattice, with many intricate and counterintuitive properties. (Credit: Max Pixel)”
VERIFYING the SCHWINGER EFFECT
70-year-old quantum prediction comes true, as something is created from nothing
by Ethan Siegel / September 13, 2022
“Whoever said, “You can’t get something from nothing” must never have learned quantum physics. As long as you have empty space — the ultimate in physical nothingness — simply manipulating it in the right way will inevitably cause something to emerge. Collide two particles in the abyss of empty space, and sometimes additional particle-antiparticle pairs emerge. Take a meson and try to rip the quark away from the antiquark, and a new set of particle-antiparticle pairs will get pulled out of the empty space between them. And in theory, a strong enough electromagnetic field can rip particles and antiparticles out of the vacuum itself, even without any initial particles or antiparticles at all.
“This chart of particles and interactions details how the particles of the Standard Model interact according to the three fundamental forces that Quantum Field Theory describes. When gravity is added into the mix, we obtain the observable Universe that we see, with the laws, parameters, and constants that we know of governing it. Mysteries, such as dark matter and dark energy, still remain. Credit: CPEP/DOE/SNF/LBNL”
Previously, it was thought that the highest particle energies of all would be needed to produce these effects: the kind only obtainable at high-energy particle physics experiments or in extreme astrophysical environments. But in early 2022, strong enough electric fields were created in a simple laboratory setup leveraging the unique properties of graphene, enabling the spontaneous creation of particle-antiparticle pairs from nothing at all. The prediction that this should be possible is 70 years old: dating back to one of the founders of quantum field theory, Julian Schwinger. The Schwinger effect is now verified, and teaches us how the Universe truly makes something from nothing.
In the Universe we inhabit, it’s truly impossible to create “nothing” in any sort of satisfactory way. Everything that exists, down at a fundamental level, can be decomposed into individual entities — quanta — that cannot be broken down further. These elementary particles include quarks, electrons, the electron’s heavier cousins (muons and taus), neutrinos, as well as all of their antimatter counterparts, plus photons, gluons, and the heavy bosons: the W+, W-, Z0, and the Higgs. If you take all of them away, however, the “empty space” that remains isn’t quite empty in many physical senses. For one, even in the absence of particles, quantum fields remain. Just as we cannot take the laws of physics away from the Universe, we cannot take the quantum fields that permeate the Universe away from it.
“Instead of an empty, blank, three-dimensional grid, putting a mass down causes what would have been ‘straight’ lines to instead become curved by a specific amount. No matter how far away you get from a point mass, the curvature of space never reaches zero, but always remains, even at infinite range. (Credit: Christopher Vitale of Networkologies and the Pratt Institute)”
For another, no matter how far away we move any sources of matter, there are two long-range forces whose effects will still remain: electromagnetism and gravitation. While we can make clever setups that ensure that the electromagnetic field strength in a region is zero, we cannot do that for gravitation; space cannot be “entirely emptied” in any real sense in this regard. But even for the electromagnetic force — even if you completely zero out the electric and magnetic fields within a region of space — there’s an experiment you can perform to demonstrate that empty space isn’t truly empty. Even if you create a perfect vacuum, devoid of all particles and antiparticles of all types, where the electric and magnetic fields are zero, there’s clearly something that’s present in this region of what a physicist might call, from a physical perspective, “maximum nothingness.”
“The Casimir effect, illustrated here for two parallel conducting plates, excludes certain electromagnetic modes from the interior of the conducting plates while permitting them outside of the plates. As a result, the plates attract, as predicted by Casimir in the 1940s and verified experimentally by Lamoreaux in the 1990s. (Credit: Emok/Wikimedia Commons)”
All you need to do is place a set of parallel conducting plates in this region of space. Whereas you might expect that the only force they’d experience between them would be gravity, set by their mutual gravitational attraction, what actually winds up happening is that the plates attract by a much greater amount than gravity predicts. This physical phenomenon is known as the Casimir effect, and was demonstrated to be true by Steve Lamoreaux in 1996: 48 years after it was calculated and proposed by Hendrik Casimir.
Similarly, in 1951, Julian Schwinger, already a co-founder of the quantum field theory that describes electrons and the electromagnetic force, gave a complete theoretical description of how matter could be created from nothing: simply by applying a strong electric field. Although others had proposed the idea back in the 1930s, including Fritz Sauter, Werner Heisenberg, and Hans Euler, Schwinger himself did the heavy lifting to quantify precisely under what conditions this effect should emerge, and henceforth it’s been primarily known as the Schwinger effect.
Normally, we expect there to be quantum fluctuations in empty space: excitations of any and all quantum fields that may be present. The Heisenberg uncertainty principle dictates that certain quantities cannot be known in tandem to arbitrary precision, and that includes things like: energy and time, position and momentum, orientation and angular momentum, voltage and free electric charge, as well as electric field and electric polarization density. While we normally express the uncertainty principle in terms of the first two entities, alone, the other applications can have consequences that are equally profound.
“This diagram illustrates the inherent uncertainty relation between position and momentum. When one is known more accurately, the other is inherently less able to be known accurately. Every time you accurately measure one, you ensure a greater uncertainty in the corresponding complementary quantity. (Credit: Maschen)”
Recall that, for any force that exists, we can describe that force in terms of a field: where the force experienced by a particle is its charge multiplied by some property of the field. If a particle passes through a region of space where the field is non-zero, it can experience a force, depending on its charge and (sometimes) its motion. The stronger the field, the greater the force, and the stronger the field, the greater the amount of “field energy” exists in that particular region of space. Even in purely empty space, and even in the absence of external fields, there will still be some non-zero amount of field energy that exists in any such region of space.
If there are quantum fields everywhere, then simply by Heisenberg’s uncertainty principle, for any duration of time that we choose to measure this region over, there will be an inherently uncertain amount of energy present within that region during that time period. The shorter the time period we’re looking at, the greater the uncertainty in the amount of energy in that region. Applying this to all allowable quantum states, we can begin to visualize the fluctuating fields, as well as fluctuating particle-antiparticle pairs, that pop in-and-out of existence due to all of the Universe’s quantum forces.
“Visualization of a quantum field theory calculation showing virtual particles in the quantum vacuum. Even in empty space, this vacuum energy is non-zero, but without specific boundary conditions, individual particle properties will not be constrained. In curved space, the quantum vacuum differs from flat space. Credit: Derek Leinweber”
Now, let’s imagine turning up the electric field. Turn it up, higher and higher, and what will happen? Let’s take an easier case first, and imagine there’s a specific type of particle already present: a meson. A meson is made of one quark and one antiquark, connected to one another through the strong force and the exchange of gluons. Quarks come in six different flavors: up, down, strange, charm, bottom, and top, while the anti-quarks are simply anti-versions of each of them, with opposite electric charges.
The quark-antiquark pairs within a meson sometimes have opposite charges to one another: either +⅔ and -⅔ (for up, charm, and top) or +⅓ and -⅓ (for down, strange, and bottom). If you apply an electric field to such a meson, the positively charged end and the negatively charged end will be pulled in opposite directions. If the field strength is great enough, it’s possible to pull the quark and antiquark away from one another sufficiently so that new particle-antiparticle pairs are ripped out of the empty space between them. When this occurs, we wind up with two mesons instead of one, with the energy required to create the extra mass (via E = mc²) coming from the electric field energy that ripped the meson apart in the first place.
“When a meson, such as a charm-anticharm particle shown here, has its two constituent particles pulled apart by too great an amount, it becomes energetically favorable to rip a new (light) quark/antiquark pair out of the vacuum and create two mesons where there was one before. A strong enough electric field, for long-enough lived mesons, can cause this to occur, with the needed energy for creating more massive particles coming from the underlying electric field, and with the amount of energy required to create these new particles (or particle-antiparticle pairs) described by E = mc². Credit: Particle Adventure/LBNL/Particle Data Group”
Now, with all of that as background in our minds, let’s imagine we’ve got a very, very strong electric field: stronger than anything we could ever hope to make on Earth. Something so strong that it would be like taking a full Coulomb of charge — around ~1019 electrons and protons — and condensing each of them into a tiny ball, one purely of positive charge and one purely of negative charge, and separating them by only a meter. The quantum vacuum, in this region of space, is going to be extremely strongly polarized. Strong polarization means a strong separation between positive and negative charges.
If your electric field in a region of space is strong enough, then when you create a virtual particle-antiparticle pair of the lightest charged particle of all (electrons and positrons), you have a finite probability of those pairs being separated by large enough amounts due to the force from the field that they can no longer reannihilate one another. Instead, they become real particles, stealing energy from the underlying electric field in order to keep energy conserved. As a result, new particle-antiparticle pairs come to exist, and the energy required to make them, from E = mc², reduces the exterior electric field strength by the appropriate amount.
“A visualization of QCD illustrates how particle-antiparticle pairs pop out of the quantum vacuum for very small amounts of time as a consequence of Heisenberg uncertainty. The quantum vacuum is interesting because it demands that empty space itself isn’t so empty, but is filled with all the particles, antiparticles, and fields in various states that are demanded by the quantum field theory that describes our Universe. The particle-antiparticle pairs illustrated here, however, are only a calculational tool; they are not to be confused with real particles. Credit: Derek B. Leinweber”
That’s what the Schwinger effect is, and unsurprisingly, it’s never been observed in a laboratory setting. In fact, the only places where it was theorized to occur was in the highest-energy astrophysical regions to exist in the Universe: in the environments surrounding (or even interior to) black holes and neutron stars. But at the great cosmic distances separating us from even the nearest black holes and neutron stars, even this remains conjecture. The strongest electric fields we’ve created on Earth are at laser facilities, and even with the strongest, most intense lasers at the shortest pulse times, we still aren’t even close.
Normally, whenever you have a conducting material, it’s only the “valence electrons” that are free to move, contributing to conduction. If you could achieve large enough electric fields, however, you could get all of the electrons to join the flow. In January of 2022, researchers at the University of Manchester were able to leverage an intricate and clever setup involving graphene — an incredibly strong material that consists of carbon atoms bound together in geometrically optimal states — to achieve this property with relatively small, experimentally accessible magnetic field. In doing so, they also witnesses the Schwinger effect in action: producing the analogue of electron-positron pairs in this quantum system.
“Graphene has many fascinating properties, but one of them is a unique electronic band structure. There are conduction bands and valence bands, and they can overlap with zero band gap, enabling both holes and electrons to emerge and flow. (Credit: K. Kumar and B. C. Yadav, Advanced Science, Engineering and Medicine, 2018)”
Graphene is an odd material in a lot of ways, and one of those ways is that sheets of it behave effectively as a two-dimensional structure. By reducing the number of (effective) dimensions, many degrees of freedom present in three-dimensional materials are taken away, leaving far fewer options for the quantum particles inside, as well as reducing the set of quantum states available for them to occupy. Leveraging a graphene-based structure known as a superlattice — where multiple layers of materials create periodic structures — the authors of this study applied an electric field and induced the very behavior described above: where electrons from not just the highest partially-occupied energy state flow as part of the material’s conduction, but where electrons from lower, completely filled bands join the flow as well.
Once this occurs, a lot of exotic behaviors arise in this material, but one was seen for the first time ever: the Schwinger effect. Instead of producing electrons and positrons, it produced electrons and the condensed-matter analogue of positrons: holes, where a “missing” electron in a lattice flows in the opposite directions to the electron flow. The only way to explain the observed currents were with this additional process of spontaneous production of electrons and “holes,” and the details of the process agreed with Schwinger’s predictions from all the way back in 1951.
@Caltech Assistant Professor Stevan Nadj-Perge and colleagues have discovered that when tungsten diselenide is placed on top of graphene bilayers, the untwisted graphene's superconductivity is greatly improved. https://t.co/ZeSfnjOkbR
— Caltech Division of Engineering & Applied Science (@CaltechEAS) February 27, 2023
There are many ways of studying the Universe, and quantum analogue systems — where the same mathematics that describes an otherwise inaccessible physical regime applies to a system that can be created and studied in a laboratory — are some of the most powerful probes we have of exotic physics. It’s very difficult to foresee how the Schwinger effect could be tested in its pure form, but thanks to the extreme properties of graphene, including its ability to withstand spectacularly large electric fields and currents, it arose for the very first time in any form: in this particular quantum system.
Five years after the discovery of superconductivity in twisted bilayer graphene, experimentalists and theorists are still trying to unravel the mechanism behind the phenomenon. From @DanGaristo for @PhysicsToday: https://t.co/CpDtcsRgmG
— American Institute of Physics (@AIP_HQ) May 5, 2023
As coauthor Dr. Roshan Krishna Kumar put it: “When we first saw the spectacular characteristics of our superlattice devices, we thought ‘wow … it could be some sort of new superconductivity’. Although the response closely resembles those routinely observed in superconductors, we soon found that the puzzling behavior was not superconductivity but rather something in the domain of astrophysics and particle physics. It is curious to see such parallels between distant disciplines.” With electrons and positrons (or “holes”) being created out of literally nothing, just ripped out of the quantum vacuum by electric fields themselves, it’s yet another way that the Universe demonstrates the seemingly impossible: we really can make something from absolutely nothing!”
“In theory, the Schwinger effect states that in the presence of strong enough electric fields, (charged) particles and their antiparticle counterparts will be ripped from the quantum vacuum, empty space itself, to become real. Theorized by Julian Schwinger in 1951, the predictions were validated in a tabletop experiment, using a quantum analogue system, for the first time. Credit: Matteo Ceccanti and Simone Cassandra”
SUPERCONDUCTIVITY or SCHWINGER EFFECT?
Schwinger effect seen in graphene
by Isabelle Dumé / 25 Mar 2022
“In theory, a vacuum is devoid of matter. In the presence of strong electric or magnetic fields, however, this void can break down, causing elementary particles to spring into existence. Usually, this breakdown only occurs during intense astrophysical events, but researchers at the UK’s National Graphene Institute at the University of Manchester have now brought it into tabletop territory for the first time, observing this so-called Schwinger effect in a device based on graphene superlattices. The work will be important for developing electronic devices based on graphene and other two-dimensional quantum materials. In graphene, which is a two-dimensional sheet of carbon atoms, a vacuum exists at the point (in momentum space) where the material’s conduction and valence electron bands meet and no intrinsic charge carriers are present.
Working with colleagues in Spain, the US, Japan and elsewhere in the UK, the Manchester team led by Andre Geim identified a signature of the Schwinger effect at this Dirac point, observing pairs of electrons and holes (the solid-state equivalent of positrons, which are positively-charged elementary particles) created out of the vacuum. A crucial component of the work was the team’s use of graphene superlattices. In these structures, graphene’s unit cell – that is, the simple repetition of carbon atoms in its crystal structure – expands to a huge extent, as if the 2D crystal were being stretched by a factor of 100 in all directions. This stretching dramatically changes the material’s properties, making it easier to reach the Schwinger limit.
In their work, the researchers focused on two types of graphene. In the first type, graphene is aligned crystallographically on top of another 2D material, hexagonal boron nitride, forming a material known as G/hBN. The second type, small-angle twisted bilayer graphene (TBG), is created by placing two sheets of graphene on top of each other and slightly misaligning them. Both superlattices were encapsulated in hBN to make sure they were of a high electronic quality and shaped into multi-terminal Hall bar devices using standard fabrication procedures. These devices consist of thin rectangular plates with current and Hall voltage contacts at their external boundaries that allow for subsequent measurements of current flow through them.
The researchers applied strong electrical currents through these graphene-superlattice-based-devices, with current densities of up to 0.1 mA mm−1 limited only by the need to avoid “burning” the devices. When they measured the devices’ current–voltage (I–V) characteristics, they observed signals associated with the production of electrons and holes, as well as another unusual high-energy process that has, as yet, no analogue in particle physics or astrophysics. The second process occurred when the researchers filled the vacuum in graphene with electrons and accelerated them to the maximum possible velocity allowed in the material (around 1/300 the speed of light). Under these conditions, the electrons appeared to become superluminous, providing an electric current much greater than that allowed by theory. The researchers attribute this effect to the spontaneous generation of additional holes.
And that was not all: the researchers say they also succeeded in unearthing conditions under which the Schwinger effect might be observed in ordinary graphene, without a superlattice. The material in its original form was first isolated in 2004 by Geim and Konstantin Novoselov (who shared the 2010 Nobel Prize for physics for their work) and has since been extensively studied by groups around the world. Geim and colleagues note that the I–V characteristics they associate with the Schwinger effect closely resemble those observed in superconductors. Indeed, they suggest that some previous reports of superconductivity in twisted graphene could have been observations of this type, rather than evidence for superconductivity.
Team member Roshan Krishna Kumar adds that graphene devices can now be made with extremely high quality, enabling the team to study the properties of graphene and its superlattices exactly at the material’s Dirac point. “Previously, this regime was dominated by charge inhomogeneity (essentially, a ‘dirt’ that is hard to analyse and interpret) but nowadays devices only contain thermal excitations of electrons and holes existing in this vacuum,” explains his colleague Alexey Berduygin, the lead author of a paper in Science describing the work. “This regime remains one of the last frontiers to explore graphene’s fundamental properties,” he tells Physics World, “and we expect a lot of interesting observations to be made here.”
SUPERCONDUCTIVITY at ROOM TEMPERATURE