Tag Archives: #insulators

Insulators Turn Up The Heat On Quantum Bits (Quantum)

Quantum technologies are based on quantum properties of light, electrons, and atoms. In recent decades, scientists have learned to master these phenomena and exploit them in applications. Thus, the construction of a quantum computer for commercial applications is also coming within reach. One of the emerging technologies that is currently being advanced very successfully is ion trap quantum computers. Here, charged particles are trapped with electromagnetic fields in a vacuum chamber and prepared in such a way that they can serve as carriers for information and be used for computing, which includes cooling them to the lowest temperatures permitted by quantum mechanics. However, the quantum mechanical properties exploited in this process are highly error prone. Even the smallest deficiencies can heat up the strongly cooled particles and thereby lead to errors in the processing of quantum information. Possible sources of such faults are weakly conducting or non-conducting materials, which are used, for example, as insulators in a metallic ion trap, or optics, which are necessary for coupling ions with laser light. “Even for ion traps made exclusively of metal, oxide layers on the metals would cause such failures,” explains Tracy Northup at the Department of Experimental Physics of the University of Innsbruck in Austria. Northups team together with collaborators in Innsbruck and in the U.S. have found a way to determine the influence of dielectric materials on the charged particles in ion traps.

Experimentally confirmed

This was achieved because the Innsbruck quantum physicists have an ion trap in which they can precisely set the distance between the ions and dielectric optics. Based on an earlier proposal by Rainer Blatt’s group, the physicists computed the amount of noise caused by the dielectric material for this ion trap and compared it with data from experiment. “Theory and experiment agree very well, confirming that this method is well suited for determining the influence of dielectric materials on the ions,” explains Markus Teller from the Innsbruck team. To calculate the noise, the so-called fluctuation-dissipation theorem from statistical physics was used, which mathematically describes the response of a system in thermal equilibrium to a small external perturbation.

View into the vacuum chamber where the ion trap is isolated from external noise. © University of Innsbruck

“In quantum computers, there are many possible sources of noise, and it is very difficult to sort out the exact sources,” says Tracy Northup. “Our method is the first to quantify the influence of dielectric materials in a given ion trap on the charged particles. In the future, designers of ion trap quantum computers will be able to assess this effect much more accurately and design their devices to minimize these perturbations.” After having successfully demonstrated the method on their own ion trap, the Innsbruck physicists now want to apply it to the ion traps of collaborators in the U.S. and Switzerland.

The research was financially supported by the Austrian Science Fund FWF and the European Union, among others. The results have been published in the journal Physical Review Letters.

Featured image: In the ion trap, the distance between the ions and optics can be precisely adjusted. © University of Innsbruck

Publication: Heating of a trapped ion induced by dielectric materials. Markus Teller, Dario A. Fioretto, Philip C. Holz, Philipp Schindler, Viktor Messerer, Klemens Schüppert, Yueyang Zou, Rainer Blatt, John Chiaverini, Jeremy Sage, and Tracy E. Northup. Phys. Rev. Lett. 126, 230505 doi: 10.1103/PhysRevLett.126.230505

Provided by University of Innsbruck

Magnetism Drives Metals to Insulators in New Experiment (Physics)

Study provides new tools to probe novel spintronic devices

Like all metals, silver, copper, and gold are conductors. Electrons flow across them, carrying heat and electricity. While gold is a good conductor under any conditions, some materials have the property of behaving like metal conductors only if temperatures are high enough; at low temperatures, they act like insulators and do not do a good job of carrying electricity. In other words, these unusual materials go from acting like a chunk of gold to acting like a piece of wood as temperatures are lowered. Physicists have developed theories to explain this so-called metal–insulator transition, but the mechanisms behind the transitions are not always clear.

“In some cases, it is not easy to predict whether a material is a metal or an insulator,” explains Caltech visiting associate Yejun Feng of the Okinawa Institute for Science and Technology Graduate University. “Metals are always good conductors no matter what, but some other so-called apparent metals are insulators for reasons that are not well understood.” Feng has puzzled over this question for at least five years; others on his team, such as collaborator David Mandrus at the University of Tennessee, have thought about the problem for more than two decades.

Now, a new study from Feng and colleagues, published in Nature Communications, offers the cleanest experimental proof yet of a metal–insulator transition theory proposed 70 years ago by physicist John Slater. According to that theory, magnetism, which results when the so-called “spins” of electrons in a material are organized in an orderly fashion, can solely drive the metal–insulator transition; in other previous experiments, changes in the lattice structure of a material or electron interactions based on their charges have been deemed responsible.

“This is a problem that goes back to a theory introduced in 1951, but until now it has been very hard to find an experimental system that actually demonstrates the spin–spin interactions as the driving force because of confounding factors,” explains co-author Thomas Rosenbaum, a professor of physics at Caltech who is also the Institute’s president and the Sonja and William Davidow Presidential Chair.

“Slater proposed that, as the temperature is lowered, an ordered magnetic state would prevent electrons from flowing through the material,” Rosenbaum explains. “Although his idea is theoretically sound, it turns out that for the vast majority of materials, the way that electrons interact with each other electronically has a much stronger effect than the magnetic interactions, which made the task of proving the Slater mechanism challenging.”

The research will help answer fundamental questions about how different materials behave, and may also have applications in technology, for example in the field of spintronics, in which the spins of electrons would form the basis of electrical devices instead of the electron charges as is routine now. “Fundamental questions about metal and insulators will be relevant in the upcoming technological revolution,” says Feng.

Interacting Neighbors

Typically, when something is a good conductor, such as a metal, the electrons can zip around largely unimpeded. Conversely, with insulators, the electrons get stuck and cannot travel freely. The situation is comparable to communities of people, explains Feng. If you think of materials as communities and electrons as members of the households, then “insulators are communities with people who don’t want their neighbors to visit because it makes them feel uncomfortable.” Conductive metals, however, represent “close-knit communities, like in a college dorm, where neighbors visit each other freely and frequently,” he says.

Likewise, Feng uses this metaphor to explain what happens when some metals become insulators as temperatures drop. “It’s like winter time, in that people—or the electrons—stay home and don’t go out and interact.”

In the 1940s, physicist Sir Nevill Francis Mott figured out how some metals can become insulators. His theory, which garnered the 1977 Nobel Prize in Physics, described how “certain metals can become insulators when the electronic density decreases by separating the atoms from each other in some convenient way,” according to the Nobel Prize press release. In this case, the repulsion between the electrons is behind the transition.

In 1951, Slater proposed an alternate mechanism based on spin–spin interactions, but this idea has been hard to prove experimentally because the other processes of the metal–insulator transition, including those proposed by Mott, can swamp the Slater mechanism, making it hard to isolate.

Challenges of Real Materials

In the new study, the researchers were able at last to experimentally demonstrate the Slater mechanism using a compound that has been studied since 1974, called pyrochlore oxide or Cd2Os2O7. This compound is not affected by other metal–insulator transition mechanisms. However, within this material, the Slater mechanism is overshadowed by an unforeseen experimental challenge, namely the presence of “domain walls” that divide the material into sections.

“The domain walls are like the highways or bigger roads between communities,” says Feng. In pyrochlore oxide, the domain walls are conductive, even though the bulk of the material is insulating. Although the domain walls started out as an experimental challenge, they turned out to be essential to the team’s development of a new measurement procedure and technique to prove the Slater mechanism.

“Previous efforts to prove the Slater metal–insulator transition theory did not account for the fact that the domain walls were masking the magnetism-driven effects,” says Yishu Wang (PhD ’18), a co-author at the Johns Hopkins University who has continuously worked on this study since her graduate work at Caltech. “By isolating the domain walls from the bulk of the insulating materials, we were able to develop a more complete understanding of the Slater mechanism.” Wang had previously worked with Patrick Lee, a visiting professor at Caltech from MIT, to lay the basic understanding of conductive domain walls using symmetry arguments, which describe how and if electrons in materials respond to changes in the direction of a magnetic field.

“By challenging the conventional assumptions about how electrical conductivity measurements are made in magnetic materials through fundamental symmetry arguments, we have developed new tools to probe spintronic devices, many of which depend on transport across domain walls,” says Rosenbaum.

“We developed a methodology to set apart the domain-wall influence, and only then could the Slater mechanism be revealed,” says Feng. “It’s a bit like discovering a diamond in the rough.”

The paper, titled, “A continuous metal-insulator transition driven by spin correlations,” was funded by the Okinawa Institute, with subsidy funding from the Cabinet Office, Government of Japan; the National Science Foundation; the Air Force Office of Scientific Research; and the U.S. Department of Energy. Other authors include Daniel M. Silevitch of Caltech and Scott E. Cooper of the Okinawa Institute of Science and Technology. Mandrus is also affiliated with the Oak Ridge National Laboratory.

Featured image: An illustration of two domains (blue and orange) divided by a domain wall (white area) in a material. The magnetic order is designated with organized arrows (electron spins) while the colors represent two different domains (but the same magnetic order). In the material pictured here, the domain walls are conductive and the domains are insulating. © Yejun Fang

Reference: Feng, Y., Wang, Y., Silevitch, D.M. et al. A continuous metal-insulator transition driven by spin correlations. Nat Commun 12, 2779 (2021). https://doi.org/10.1038/s41467-021-23039-6

Provided by Caltech

Topological Protection Vs. Degree of Entanglement of Two-photon Light in Photonic Topological Insulators (Material Science)

In a joint effort, researchers from the Humboldt-Universität (Berlin), the Max Born Institute (Berlin) and the University of Central Florida (USA), have revealed the necessary conditions for the robust transport of entangled states of two-photon light in photonic topological insulators, paving the way the towards noise-resistant transport of quantum information. The results have appeared in Nature Communications.

Originally discovered in condensed matter systems, topological insulators are two-dimensional materials that support scattering-free (uni-directional) transport along their edges, even in the presence of defects and disorder. In essence, topological insulators are finite lattice systems where, given a suitable termination of the underlying infinite lattice, edge states are formed that lie in a well-defined energy gap associated with the bulk states, i.e. these edge states are energetically separated from the bulk states, see Fig 1.

Fig 1: Topological insulators are finite-sized lattice systems (a) that exhibit eigenspectra where (b) the eigenenergies of bulk states (c) exhibit a band gap that (d) contains the eigenenergies of so-called edge states. © MBI Berlin

Importantly, single-particle edge states in such systems are topologically protected from scattering: they cannot scatter into the bulk due to their energy lying in the gap, and they cannot scatter backwards because backward propagating edge states are either absent or not coupled to the forward propagating edge states.

The feasibility of engineering complex Hamiltonians using integrated photonic lattices, combined with the availability of entangled photons, raises the intriguing possibility of employing topologically protected entangled states in optical quantum computing and information processing (Science 362, 568, (2018), Optica 6, 955 (2019)).

Achieving this goal, however, is highly nontrivial as topological protection does not straightforwardly extend to multi-particle (back-)scattering. At first, this fact appears to be counterintuitive because, individually, each particle is protected by topology whilst, jointly, entangled (correlated) particles become highly susceptible to perturbations of the ideal lattice. The underlying physical principle behind this apparent ‘discrepancy’ is that, quantum-mechanically, identical particles are described by states that satisfy an exchange symmetry principle.

In their work the researchers make several fundamental advances towards understanding and controlling topological protection in the context of multi-particle states:

  • First, they identify physical mechanisms which induce a vulnerability of entangled states in topological photonic lattices and present clear guidelines for maximizing entanglement without sacrificing topological protection.
  • Second, they stablish and demonstrate a threshold-like behavior of entanglement vulnerability and identify conditions for robust protection of highly entangled two-photon states.

To be precise, they explore the impact of disorder onto a range of two-photon states that extend from the fully correlated to the fully anti-correlated limits, thereby also covering a completely separable state. For their analysis, they consider two topological lattices, one periodic and one aperiodic. In the periodic case they consider the Haldane model, and for the aperiodic case a square lattice, whose single-particle dynamics corresponds to the quantum Hall effect, is studied.

The results offer a clear roadmap for generating robust wave packets tailored to the particular disorder at hand. Specifically, they establish limits on the stability of entangled states up to relatively high degrees of entanglement that offer practical guidelines for generating useful entangled states in topological photonic systems. Further, these findings demonstrate that in order to maximize entanglement without sacrificing topological protection, the joint spectral correlation map of two-photon states must fit inside a well-defined topological window of protection, Fig. (2).

Figure 2: In order to identify the topological window of protection, the researchers considered a spectrally broad product state as initial state and propagate it through an ensemble of 1000 random Haldane lattices. (a) Depicts the spectral correlation map for the initial state and in (b) the ensemble-average of the spectral correlation maps inside the edge-edge subspace after the propagation through the ensemble of disordered lattices is shown. It is found that the only two-photon amplitudes that survive the scattering induced by the disorder lie in the region indicated by the black square which is the protection window. Finally, (c) and (d) display, respectively, the edge-mode content E and the product of the edge-mode content with the Schmidt-number E · SN as a function of the variances of the initial states. © MBI Berlin

Featured Image credit: MBI/ BERLIN

Original publication

Topological protection versus degree of entanglement of two-photon light in photonic topological insulators

Konrad TschernigÁlvaro Jimenez-Galán, Demetrios N. Christodoulides, Misha IvanovKurt Busch, Miguel A. Bandres, Armando Perez-LeijaNature Communications 12, Article number: 1974 (2021) URL, DOI or PDF

Provided by MBI Berlin

Discovery of Quantum Behavior in Insulators Suggests Possible New Particle (Quantum)

In a surprising discovery, Princeton physicists have observed an unexpected quantum behavior in an insulator made from a material called tungsten ditelluride. This phenomenon, known as quantum oscillation, is typically observed in metals rather than insulators, and its discovery offers new insights into our understanding of the quantum world. The findings also hint at the existence of an entirely new type of quantum particle.

A team led by Princeton physicists discovered a surprising quantum phenomenon in an atomically thin insulator made of tungsten ditelluride. The results suggest the formation of completely new types of quantum phases previously hidden in insulators. Image designed by Kai Fu for the Wu Lab, Princeton University

The discovery challenges a long-held distinction between metals and insulators, because in the established quantum theory of materials, insulators were not thought to be able to experience quantum oscillations.

“If our interpretations are correct, we are seeing a fundamentally new form of quantum matter,” said Sanfeng Wu, assistant professor of physics at Princeton University and the senior author of a recent paper in Nature detailing this new discovery. “We are now imagining a wholly new quantum world hidden in insulators. It’s possible that we simply missed identifying them over the last several decades.”

The observation of quantum oscillations has long been considered a hallmark of the difference between metals and insulators. In metals, electrons are highly mobile, and resistivity — the resistance to electrical conduction — is weak. Nearly a century ago, researchers observed that a magnetic field, coupled with very low temperatures, can cause electrons to shift from a “classical” state to a quantum state, causing oscillations in the metal’s resistivity. In insulators, by contrast, electrons cannot move and the materials have very high resistivity, so quantum oscillations of this sort are not expected to occur, no matter the strength of magnetic field applied.

The discovery was made when the researchers were studying a material called tungsten ditelluride, which they made into a two-dimensional material. They prepared the material by using standard scotch tape to increasingly exfoliate, or “shave,” the layers down to what is called a monolayer — a single atom-thin layer. Thick tungsten ditelluride behaves like a metal. But once it is converted to a monolayer, it becomes a very strong insulator.

“This material has a lot of special quantum properties,” Wu said.

The researchers then set about measuring the resistivity of the monolayer tungsten ditelluride under magnetic fields. To their surprise, the resistivity of the insulator, despite being quite large, began to oscillate as the magnetic field was increased, indicating the shift into a quantum state. In effect, the material — a very strong insulator — was exhibiting the most remarkable quantum property of a metal.

“This came as a complete surprise,” Wu said. “We asked ourselves, ‘What’s going on here?’ We don’t fully understand it yet.”

Wu noted that there are no current theories to explain this phenomenon.

Nonetheless, Wu and his colleagues have put forward a provocative hypothesis — a form of quantum matter that is neutrally charged. “Because of very strong interactions, the electrons are organizing themselves to produce this new kind of quantum matter,” Wu said.

But it is ultimately no longer the electrons that are oscillating, said Wu. Instead, the researchers believe that new particles, which they have dubbed “neutral fermions,” are born out of these strongly interacting electrons and are responsible for creating this highly remarkable quantum effect.

Fermions are a category of quantum particles that include electrons. In quantum materials, charged fermions can be negatively charged electrons or positively charged “holes” that are responsible for the electrical conduction. Namely, if the material is an electrical insulator, these charged fermions can’t move freely. However, particles that are neutral — that is, neither negatively nor positively charged — are theoretically possible to be present and mobile in an insulator.

“Our experimental results conflict with all existing theories based on charged fermions,” said Pengjie Wang, co-first author on the paper and postdoctoral research associate, “but could be explained in the presence of charge-neutral fermions.”

The Princeton team plans further investigation into the quantum properties of tungsten ditelluride. They are particularly interested in discovering whether their hypothesis — about the existence of a new quantum particle — is valid.

“This is only the starting point,” Wu said. “If we’re correct, future researchers will find other insulators with this surprising quantum property.”

Despite the newness of the research and the tentative interpretation of the results, Wu speculated about how this phenomenon could be put to practical use.

“It’s possible that neutral fermions could be used in the future for encoding information that would be useful in quantum computing,” he said. “In the meantime, though, we’re still in the very early stages of understanding quantum phenomena like this, so fundamental discoveries have to be made.”

Reference: Wang, P., Yu, G., Jia, Y. et al. Landau quantization and highly mobile fermions in an insulator. Nature (2021). https://www.nature.com/articles/s41586-020-03084-9 https://doi.org/10.1038/s41586-020-03084-9

Provided by Princeton University