A resonator for electrons

Resonators are an important tool in physics. The curved mirrors inside the resonators usually focus light waves that act, for instance, on atoms. Physicists at ETH Zurich have now managed to build a resonator for electrons and to direct the standing waves thus created onto an artificial atom.

More than two thousand years ago the Greek inventor and philosopher Archimedes already came up with the idea of using a curved mirror to reflect light in such a way as to focus it into a point – legend has it that he used this technique to set fire to the ships of the Roman enemies. Today such curved or parabolic mirrors are used in a host of technical applications ranging from satellite dishes to laser resonators, where light waves are amplified between two mirrors. Modern quantum physics also makes use of resonators with curved mirrors. In order to study single atoms, for example, researchers use the light focused by the mirrors to enhance the interaction between the light waves and the atoms. A team of physicists at ETH Zurich, working within the framework of the National Centre of Competence in Research Quantum Science and Technology (NCCR QSIT), have now managed to build a resonator that focuses electrons rather than light waves. In the near future, such resonators could be used for constructing quantum computers and for investigating many-body effects in solids.

In their experiments the post-doctoral researchers Clemens Rössler and Oded Zilberberg used semiconductor structures in which electrons are free to move only in a single plane. At one end of that plane there is a so-called quantum dot: a tiny trap for electrons, only a hundred nanometers wide, in which owing to quantum mechanics the electrons exist in well-defined energy states similar to those of an atom. Such quantum dots are, therefore, also known as “artificial atoms”. At the other end, just a few micrometers away, a bent electrode acts as a curved mirror that reflects electrons when a voltage is applied to it.

Better materials

 

The possibility to focus electrons in this way was already investigated in 1997 at Harvard University. The ETH researchers, however, were now able to work with much better materials, which were produced in-house in Werner Wegscheider’s laboratory for Advanced Semiconductor Quantum Materials. “These materials are a hundred times cleaner than those used at the time”, explains Rössler, “and consequently the electrons can move undisturbed a hundred times longer.”

This, in turn, allows the quantum mechanical wave nature of the electrons to become very clearly visible, which was not the case in those earlier works.

In their experiments, the physicists detect this wave nature by measuring the current flowing from the quantum dot to the curved mirror. This current changes in a characteristic way as the applied voltage is varied. “Our results show that the electrons in the resonator do not just fly back and forth, but actually form a standing wave and thus couple coherently to the quantum dot”, stresses Rössler, who developed the experiment in the group of ETH professor Klaus Ensslin. Differently from light waves, the spin of the electrons also causes them to behave as tiny magnets. Indeed, the researchers were able to show that the interaction between the electrons in the quantum dot and the electronic wave in the resonator happens through the spin. “In the future, this spin-coherent coupling could make it possible to connect quantum dots over large distances”, says Zilberberg, who has developed a theoretical model for Rössler’s experiment in the group of ETH professor Gianni Blatter.

Suitable for quantum computers

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For some time now, quantum dots have been considered as possible candidates for making so-called quantum-bits or "qubits", which are used in quantum computers. Until now the quantum dots in such a computer needed to be very close to each other in order to achieve the necessary coupling for performing calculations. This, however, made it difficult to control and read out individual qubits. A long-distance coupling through an appropriately designed resonator could elegantly solve this problem.

Basic science could also benefit from the electron resonators realized by the ETH researchers, for instance in studies of the Kondo effect. This effect occurs when many electrons together interact with the magnetic moment of an impurity in a material. With the help of a resonator and a quantum dot simulating such an impurity, the physicists hope to be able to study the Kondo effect very precisely.

It took the young post-docs just over a year to go from the idea for their research – which grew out of discussions during a previous experiment – to the paper that has now been published. Zilberberg has a simple explanation for why this could happen so fast: “Within the QSIT network it’s easy to forge spontaneous collaborations across different groups as we are close both thematically and spatially, and we are often involved in common projects anyway. Plus, if one needs the opinion of an expert, there is usually one just down the corridor.”

ETH Zurich

Extends the lifetime of atoms using a mirror

Researchers at Chalmers University of Technology have succeeded in an experiment where they get an artificial atom to survive ten times longer than normal by positioning the atom in front of a mirror. The findings were recently published in the journal Nature Physics.

If we add energy to an atom, we say that the atom is excited, and it normally takes some time before the atom loses energy and returns to its original state. This time is called the lifetime of the atom.

Chalmers’ researchers have placed an artificial atom at a specific distance in front of a mirror.  By changing the distance to the mirror, they can get the atom to live longer, up to 10 times as long as if the mirror had not been there.

The artificial atom is actually a superconducting electrical circuit that the researchers make behave as an atom. Just like a natural atom, you can charge it with energy, excite the atom, which it then emits in the form of light particles. In this case, the light has a much lower frequency than ordinary light and in reality are microwaves.

“We have demonstrated how we can control the lifetime of an atom in a very simple way,” says Per Delsing, Professor of Physics and leader of the research team.

“We can vary the lifetime of the atom by changing the distance between the atom and the mirror. If we place the atom at a certain distance from the mirror the atom’s lifetime is extended by such a length that we are not even able to observe the atom. Consequently, we can hide the atom in front of a mirror,” he continues.

The experiment is a collaboration between experimental and theoretical physicists at Chalmers University of Technology, the latter have developed the theory for how the atom’s lifetime varies depending on the distance to the mirror.

“The reason why the atom “dies”, that is it returns to its original ground state, is that it sees the very small variations in the electromagnetic field which must exist due to quantum theory, known as vacuum fluctuations,” says Göran Johansson, Professor of Theoretical and Applied Quantum Physics and leader of the theory group.

When the atom is placed in front of the mirror it interacts with its mirror image, which changes the amount of vacuum fluctuations to which the atom is exposed. The system that the Chalmers researchers succeeded in building is particularly well suited for measuring the vacuum fluctuations, which otherwise is a very difficult thing to measure.

The findings are published in the highly ranked Nature Physics journal.

Read the article in Nature Physics:
I.-C. Hoi, A. F. Kockum, L. Tornberg, A. Pourkabirian, G. Johansson, P. Delsing and C. M.Wilson (2015) Probing the quantum vacuum with an artificial atom in front of a mirror. Nature Physics (2015). dx.doi.org/10.1038/NPHYS3484

Chalmers University of Technology

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The QUTIS Group creates a quantum simulator of impossible physics

The research group Quantum Technologies for Information Science (QUTIS) of the UPV/EHU-University of the Basque Country, led by the Ikerbasque professor Enrique Solano, in collaboration with an experimental group of the University of Tsinghua (Beijing, China) led by professor Kihwan Kim, has created a quantum simulator that is capable of creating unphysical phenomena in the atomic world, in other words, impossible physical phenomena.

The researchers in the two groups have succeeded in getting a trapped atom to imitate behaviours that contradict its own fundamental laws, thus taking elements of science fiction to the microscopic world.

"We have managed to get an atom to act as if it were infringing the nature of atomic systems, in other words, quantum physics and the theory of relativity. It is just like what happens in the theatre or in science fiction films in which the actors appear to display absurd behaviours that go against natural laws; in this case, the atoms are obliged to simulate absurd actions as if an actor in the theatre or in science fiction were involved," explained Prof Solano.

The results of this research have been published in the prestigious journal Nature Communications, in the article "Time reversal and charge conjugation in an embedding quantum simulator". The research team of the UPV/EHU's QUTIS group has been led by Prof Enrique Solano and has had the participation of Dr Lucas Lamata and Dr Jorge Casanova, currently at the University of Ulm, Germany.

In this experiment the researchers reproduced in the lab the theoretical proposal previously included in a previous piece of research led by the QUTIS group; it describes the possibility that a trapped atom can display behaviour that is incompatible with the fundamental laws of quantum physics. More specifically, we are talking about operations prohibited in microscopic physical systems, such as charge conjugation, which transforms a particle into an antiparticle, or time reversal, that reverses the direction of the time arrow.

To conduct the experiment it was necessary to use a charged atom trapped by means of electromagnetic fields under the action of an advanced laser system. We could describe symmetry operations of this type as prohibited ones, as they could only exist in a universe that is different from the one we know and governed by different laws. Yet in this experiment it has been possible to simulate the realisation of this set of impossible laws in an atomic system.

The UPV/EHU's QUTIS group is a world leader in quantum simulation and its influential theoretical proposals are often verified in the most advanced quantum technology laboratories. In this case, physical operations that are prohibited for the atomic world can be reproduced just as in science fiction, in other words, just as if they were taking place artificially in a quantum theatre.

Bibliographical reference

Xiang Zhang, Yangchao Shen, Junhua Zhang, Jorge Casanova, Lucas Lamata, Enrique Solano, Man-Hong Yung, Jing-Ning Zhang & Kihwan Kim. Time reversal and charge conjugation in an embedding quantum simulator. Nature Communications (2015),

http://dx.doi.org/10.1038/ncomms8917

University of the Basque Country

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Laser-wielding physicists seize control of atoms’ behavior

Physicists have wondered in recent years if they could control how atoms interact using light. Now they know that they can, by demonstrating games of quantum billiards with unusual new rules.

In an article published online Oct. 5 in Physical Review Letters, a team of University of Chicago physicists explains how to tune a laser to make atoms attract or repel each other in an exotic state of matter called a Bose-Einstein condensate. 

“This realizes a goal that has been pursued for the past 20 years,” said Cheng Chin, professor in physics, who led the team. “This exquisite control over interactions in a many-body system has great potential for the exploration of exotic quantum phenomena and engineering of novel quantum devices.

Many research groups in the United States and Europe have tried various ideas over the last decade. It was Logan Clark, a graduate student in Chin’s group, who came up with the first practical solution. He has now demonstrated the idea in the lab with cesium atoms chilled to temperatures just billionths of a degree above absolute zero, and the technique can be widely applied to other atomic species.

Clark compared the process to a billiards game, when one ball encounters another. “Normally, as soon as the surfaces touch, the balls repel each other and bounce away,” Clark said. In Chin’s lab, cesium atoms replace the billiard balls, and ordinarily they repel each other when they collide. But by turning up the laser while operating at a “magic” wavelength, Clark showed that the repulsion between atoms can be converted into attraction.

“The atoms exhibit fascinating behavior in this system,” he said. By exposing different parts of the sample to different laser intensities, “We can choose to make the atoms attract or repel each other, or pass right through each other without colliding.”

Alternatively, by oscillating their interactions, analogous to making the billiard balls rapidly grow and shrink while they roll, the atoms stick to each other in pairs.

The researchers explained two fundamental ways that lasers influence the atomic motion. One is to create potentials, like a bump or valley on the billiard table, proportional to laser intensity. The new way is to alter how billiard balls collide.

“We want our laser to control collisions, but we don’t want it to create any hills or valleys,” Clark said. When the laser is tuned to a “magic wavelength,” the beam creates no hills or valleys, but only affects collisions.

“This is because the magic wavelength happens to be in between two excited states of the atom, so they ‘magically’ cancel each other out,” he said.

Magic is a concept that has no place in science, though the word does enjoy fairly common use among atomic physicists. “Generally it is used to refer to a wavelength at which two effects cancel or are equal, in particular when this cancellation or equality is useful for some technological goal,” Clark said.

University of Chicago

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At the edge of a quantum gas

JQI physicists observe skipping orbits in the quantum Hall regime 

JQI scientists have achieved a major milestone in simulating the dynamics of condensed-matter systems – such as the behavior of charged particles in semiconductors and other materials – through manipulation of carefully controlled quantum-mechanical models.

Going beyond their pioneering experiments in 2009 (the creation of “artificial magnetism”), the team has created a model system in which electrically neutral atoms are coaxed into performing just as electrons arrayed in a two-dimensional sheet do when they are exposed to a strong magnetic field.

The scientists then showed for the first time that it is possible to tune the model system such that the atoms (acting as electron surrogates) replicate the signature “edge state” behavior of real electrons in the quantum Hall effect (QHE), a phenomenon which forms the basis for the international standard of electrical resistance.* The researchers report their work in the 25 September issue of the journal Science.

“This whole line of research enables experiments in which charge-neutral particles behave as if they were charged particles in a magnetic field,” said JQI Fellow Ian Spielman, who heads the research team at NIST’s Physical Measurement Laboratory.

“To deepen our understanding of many-body physics or condensed-matter-like physics – where the electron has charge and many important phenomena depend on that charge – we explore experimental systems in which the components have no electrical charge, but act in ways that are functionally equivalent and can be described by the same equations,” Spielman said.

Such quantum simulators are increasingly important, as electronic components and related devices shrink to the point where their operation grows increasingly dependent on quantum-mechanical effects, and as researchers struggle to understand the fundamental physics of how charges travel through atomic lattices in superconductors or in materials of interest for eventual quantum information processing.

Quantum effects are extremely difficult to investigate at the fundamental level in conventional materials. Not only is it hard to control the numerous variables involved, but there are inevitably defects and irregularities in even carefully prepared experimental samples. Quantum simulators, however, offer precise control of system variables and yield insights into the performance of real materials as well as revealing new kinds of interacting quantum-mechanical systems.

“What we want to do is to realize systems that cannot be realized in a condensed-matter setting,” Spielman said. “There are potentially really interesting, many-body physical systems that can’t be deployed in other settings.”

To do so, the scientists created a Bose-Einstein condensate (BEC, in which ultracold atoms join in a single uniform, low-energy quantum state) of a few hundred thousand rubidium atoms and used two intersecting lasers to arrange the atoms into a lattice pattern. 

Then a second pair of lasers, each set to a slightly different wavelength, was trained on the lattice, creating "artificial magnetism" — that is, causing the electrically neutral atoms to mimic negatively charged electrons in a real applied magnetic field.

Depending on the tuning of the laser beams, the atoms were placed in one of three different quantum states representing electrons that were either in the middle of, or at opposite edges of, a two-dimensional lattice.

By adjusting the properties of the laser beams, the team produced dynamics characteristic of real materials exhibiting the QHE. Specifically, as would be expected of electrons, atoms in the bulk interior of the lattice behaved like insulators. But those at the lattice edges exhibited a distinctive "skipping"motion.

In a real QHE system, each individual electron responds to an applied magnetic field by revolving in a circular (cyclotron) orbit. In atoms near the center of the material, electrons complete their orbits uninterrupted. That blocks conduction in the system’s interior, making it a “bulk insulator.” But at the edges of a QHE system, the electrons can only complete part of an orbit before hitting the edge (which acts like a hard wall) and reflecting off. This causes electrons to skip along the edges, carrying current.

Remarkably, the simulation's electrically neutral rubidium atoms behaved in exactly the same way: localized edge states formed in the atomic lattice and atoms skipped along the edge. Moreover, the researchers showed that by tuning the laser beams – that is, modifying the artificial magnetic field – they could precisely control whether the largest concentration of atoms was on one edge, the opposite edge, or in the center of the lattice.

“Generating these sorts of dynamical effects was beyond our abilities back in 2009, when we published our first paper on artificial magnetism,” Spielman said. “The field strength turned out to be too weak. In the new work, we were able to approach the high-field limit, which greatly expands the range of effects that are possible to engineer new kinds of interactions relevant to condensed-matter physics.”

* The Hall effect occurs when a current traveling in a two-dimensional plane moves through a magnetic field applied perpendicular to the plane. As electrons interact with the field, each begins to revolve in a circular (cyclotron) orbit. That collective motion causes them to migrate and cluster on one edge of the plane, creating a concentration of negative charge. As a result, a voltage forms across the conductor, with an associated resistance from edge to edge.

Much closer, detailed examination reveals the quantum Hall effect (QHE): The resistance is exactly quantized across the plane; that is, it occurs only at specific discrete allowed values which are known to extreme precision. That precision makes QHE the international standard for resistance.

An additional distinctive property of QHE systems is that they are “bulk insulators” that allow current to travel only along their edges.

Joint quantum Institute

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Physicists determine the three-dimensional positions of individual atoms for the first time

Atoms are the building blocks of all matter on Earth, and the patterns in which they are arranged dictate how strong, conductive or flexible a material will be. Now, scientists at UCLA have used a powerful microscope to image the three-dimensional positions of individual atoms to a precision of 19 trillionths of a meter, which is several times smaller than a hydrogen atom.

Their observations make it possible, for the first time, to infer the macroscopic properties of materials based on their structural arrangements of atoms, which will guide how scientists and engineers build aircraft components, for example. The research, led by Jianwei (John) Miao, a UCLA professor of physics and astronomy and a member of UCLA’s California NanoSystems Institute, is published Sept. 21 in the online edition of the journal Nature Materials.

For more than 100 years, researchers have inferred how atoms are arranged in three-dimensional space using a technique called X-ray crystallography, which involves measuring how light waves scatter off of a crystal. However, X-ray crystallography only yields information about the average positions of many billions of atoms in the crystal, and not about individual atoms’ precise coordinates.

“It’s like taking an average of people on Earth,” Miao said. “Most people have a head, two eyes, a nose and two ears. But an image of the average person will still look different from you and me.”

Because X-ray crystallography doesn’t reveal the structure of a material on a per-atom basis, the technique can’t identify tiny imperfections in materials such as the absence of a single atom. These imperfections, known as point defects, can weaken materials, which can be dangerous when the materials are components of machines like jet engines.

“Point defects are very important to modern science and technology,” Miao said.

Miao and his team used a technique known as scanning transmission electron microscopy, in which a beam of electrons smaller than the size of a hydrogen atom is scanned over a sample and measures how many electrons interact with the atoms at each scan position. The method reveals the atomic structure of materials because different arrangements of atoms cause electrons to interact in different ways.

However, scanning transmission electron microscopes only produce two-dimensional images. So creating a 3-D picture requires scientists to scan the sample once, tilt it by a few degrees and re-scan it — repeating the process until the desired spatial resolution is achieved — before combining the data from each scan using a computer algorithm. The downside of this technique is that the repeated electron beam radiation can progressively damage the sample.

Using a scanning transmission electron microscope at the Lawrence Berkeley National Laboratory’s Molecular Foundry, Miao and his colleagues analyzed a small piece of tungsten, an element used in incandescent light bulbs. As the sample was tilted 62 times, the researchers were able to slowly assemble a 3-D model of 3,769 atoms in the tip of the tungsten sample.

The experiment was time consuming because the researchers had to wait several minutes after each tilt for the setup to stabilize.

“Our measurements are so precise, and any vibrations — like a person walking by — can affect what we measure,” said Peter Ercius, a staff scientist at Lawrence Berkeley National Laboratory and an author of the paper.

The researchers compared the images from the first and last scans to verify that the tungsten had not been damaged by the radiation, thanks to the electron beam energy being kept below the radiation damage threshold of tungsten.

Miao and his team showed that the atoms in the tip of the tungsten sample were arranged in nine layers, the sixth of which contained a point defect. The researchers believe the defect was either a hole in an otherwise filled layer of atoms or one or more interloping atoms of a lighter element such as carbon.

Regardless of the nature of the point defect, the researchers’ ability to detect its presence is significant, demonstrating for the first time that the coordinates of individual atoms and point defects can be recorded in three dimensions.

“We made a big breakthrough,” Miao said.

Miao and his team plan to build on their results by studying how atoms are arranged in materials that possess magnetism or energy storage functions, which will help inform our understanding of the properties of these important materials at the most fundamental scale.

“I think this work will create a paradigm shift in how materials are characterized in the 21st century,” he said. “Point defects strongly influence a material’s properties and are discussed in many physics and materials science textbooks. Our results are the first experimental determination of a point defect inside a material in three dimensions.”

The study’s co-authors include Rui Xu, Chien-Chun Chen, Li Wu, Mary Scott, Matthias Bartels, Yongsoo Yang and Michael Sawaya, all of UCLA; as well as Colin Ophus of Lawrence Berkeley National Laboratory; Wolfgang Theis of the University of Birmingham; Hadi Ramezani-Dakhel and Hendrik Heinz of the University of Akron; and Laurence Marks of Northwestern University.

UCLA

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