A necklace of fractional vortices

Researchers at Chalmers University of Technology have arrived at how what is known as time-reversal symmetry can break in one class of superconducting material. The results have been published in the highly ranked Nature Physics journal, which also put the Chalmers researchers' study on the cover.

“Symmetries are an important aspect when describing nature”, says Mikael Fogelström, who is a professor of theoretical physics at Chalmers University of Technology. “A ball is round and looks the same regardless of how we rotate it; thus, it has rotational symmetry. In the same way, most materials have symmetries that describe what the materials look like and what their properties are. If one or more symmetries breaks, this signals a phase transition to a new state. When a material becomes magnetic, a more abstract symmetry, what is known as time-reversal symmetry, is broken”.

Superconducting materials conduct electric current without loss of energy. In 1986, researchers discovered that a family of perovskite materials – that have two-dimensional copper-oxide planes – becomes superconductive at relatively high temperatures. It could also thereafter be fairly quickly ascertained by experiments that the superconducting phase also broke the crystal symmetry, and that the material was unusual in this respect.

Theoreticians pondered whether the materials could also break time-reversal symmetry and produce spontaneous magnetisation. Experiments, primarily related to electron transport, showed that this was the case, while another category of experiments aimed at directly measuring the spontaneous magnetisation demonstrated no effect.

“Our work has arrived at a new mechanism for breaking time-reversal symmetry in high-temperature superconductors” says Tomas Löfwander, who is one of the researchers behind the new results. “We maintain that this has probably already been observed and that the two sets of experiments do not contradict one another.”

The Chalmers researchers' discovery is based on a software package that researcher Mikael Håkansson developed while completing his licentiate thesis at the Division of Applied Quantum Physics at MC2, in order to produce a theoretical model of small mesoscopic superconducting grains. The software package utilises massive parallelisation of the numeric work, which can then be processed in graphics processing units, or GPUs.

“The time required to perform the fairly demanding computations was significantly reduced, and we were able to focus more on the physics and simulate more realistic systems”, explains Mikael Håkansson. “At the same time, I developed a tool to process and visualise the large amounts of data that the software produced. The cover of the September issue of Nature Physics shows how the electron state is distributed in energy along a surface of a high-temperature superconductor when it has broken the time-reversal symmetry.”

The computational tool has allowed the Chalmers researchers to investigate cases where the ring of a superconducting crystal affects the force of the superconducting phase. A periodic pattern of vortices spontaneously forms in the shape of a necklace along the surface as soon as the temperature is lower than a limit temperature. These vortices in turn cause a spontaneous magnetic flux that alternates direction on a length scale of a few dozen nanometers.

“We believe that new results with what are known as nanosquids, which are magnetometers with extremely good resolution, will be able to give immediate experimental verification of our results,” says Mikael Fogelström.

Caption: A route to a time-reversal symmetry-broken state for d-wave superconductors is shown to occur via the formation of a necklace of fractional vortices around the perimeter of the material, where neighbouring vortices have opposite current circulation. This vortex pattern is a result of a spectral rearrangement of current-carrying states near the edges. Image: Mikael Håkansson

Read the article Spontaneously broken time-reversal symmetry in high-temperature superconductors in Nature Physics

Chalmers University of Technology

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Interacting Ion Qutrits

Enlisting symmetry to protect quantum states from disruptions

Symmetry permeates nature, from the radial symmetry of flowers to the left-right symmetry of the human body. As such, it provides a natural way of classifying objects by grouping those that share the same symmetry. This is particularly useful for describing transitions between phases of matter. For example, liquid and gas phases have translational symmetry, meaning the arrangement of molecules doesn’t change regardless of the direction from which they are observed. On the other hand, the density of atoms in a solid phase is not continuously the same — thus translational symmetry is broken.

In quantum mechanics, symmetry describes more than just the patterns that matter takes — it is used to classify the nature of quantum states. These states can be entangled, exhibiting peculiar connections that cannot be explained without the use of quantum physics. For some entangled states, the symmetry of these connections can offer a kind of protection against disruptions.

Here, the word protection indicates that the system is robust against non-symmetry breaking changes. Like an island in the middle of an ocean, there is not a direct road leading to a symmetry-protected phase or state. This means that the only way to access the state is to change the symmetry itself. Physicists are interested in exploring these classes of protected states because building a useful quantum device requires its building blocks to be robust against outside disturbances that may interfere with device operations.

Recently, JQI researchers under the direction of Christopher Monroe have used trapped atomic ions to construct a system that could potentially support a type of symmetry-protected quantum state. For this research they used a three-state system, called a qutrit, and demonstrated a proof-of-principle experiment for manipulating and controlling multiple qutrits. The result appeared in Physical Review X, an online open-access journal, and is the first demonstration of using multiple interacting qutrits for doing quantum information operations and quantum simulation of the behavior of real materials.

To date, almost all of the work in quantum information science has focused on manipulating "qubits," or so-called spin-1/2 particles that consist of just two energy levels.  In quantum mechanics, multilevel systems are analogous to the concept of "spin," where the number of energy levels corresponds to the number of possible states of spin. This group has used ion spins to explore a variety of topics, such as the physics of quantum magnetism and the transmission speed of quantum information across a spin-crystal. Increasingly, there is interest in moving beyond spin-½ to control and simulations of higher order spin systems, where the laws of symmetry can be radically altered. “One complication of spin-1 materials is that the added complexity of the levels often makes these systems much more difficult to model or understand. Thus, performing experiments in these higher [spin] dimensional systems may yield insight into difficult-to-calculate problems, and also give theorists some guidance on modeling such systems, ” explains Jake Smith, a graduate student in Monroe’s lab and author on the paper.

To engineer a spin-1 system, the researchers electromagnetically trapped a linear crystal of atomic ytterbium (Yb) ions, each atom a few micrometers from the next. Using a magnetic field, internal states of each ion are tailored to represent a qutrit, with a (+) state, (-) state and (0) state denoting the three available energy levels (see figure). With two ions, the team demonstrated the basic techniques necessary for quantum simulation: preparing initial states (placing the ions in certain internal states), observing the state of the system after some evolution, and verifying that the ions are entangled, here with 86% fidelity (fidelity is a measure of how much the experimentally realized state matches the theoretical target state).

To prepare the system in certain initial states, the team first lowers the system into its ground state, the lowest energy state in the presence of a large effective magnetic field. The different available spin chain configurations at a particular magnetic field value correspond to different energies. They observed how the spin chain reacted or evolved as the amplitude of the magnetic field was lowered. Changing the fields that the ions spins are exposed to causes the spins to readjust in order to remain in the lowest energy configuration.  

By adjusting the parameters (here laser amplitudes and frequencies) the team can open up and follow pathways between different energy levels. This is mostly true, but for some target states a simple trajectory that doesn’t break symmetries or pass through a phase transition does not exist. For instance, when the team added a third ion, they could not smoothly guide the system into its ground state, indicating the possible existence of a state with some additional symmetry protections.

“This result is a step towards investigating quantum phases that have special properties based on the symmetries of the system,” says Smith. Employing these sorts of topological phases may be a way to improve coherence times when doing quantum computation, even in the face of environmental disruptions. Coherence time is how long a state retains its quantum nature. Quantum systems are very sensitive to outside disturbances, and doing useful computation requires maintaining this quantum nature for longer than the time it takes to perform a particular calculation.

Monroe explains, "These symmetry-protected states may be the only way to build a large-scale stable quantum computer in many physical systems, especially in the solid-state.  With the exquisite control afforded atomic systems such as trapped ions demonstrated here, we hope to study and control how these very subtle symmetry effects might be used for quantum computing, and help guide their implementation in any platform."

To further investigate this protected phase, the researchers next intend to address the problem of creating antisymmetric ground states. Smith continues, “The next steps are to engineer more complicated interactions between the effective spins and implement a way to break the symmetries of the interactions.”

http://www.nanotechnologyworld.org/#!Interacting-Ion-Qutrits/c89r/55b65d7e0cf2d0bb156af20f 

New Results from Daya Bay: Tracking the Disappearance of Ghostlike Neutrinos

Daya Bay neutrino experiment releases high-precision measurement of subatomic shape shifting and new result on differences among neutrino masses

The international Daya Bay Collaboration has announced new results about the transformations of neutrinos - elusive, ghostlike particles that carry invaluable clues about the makeup of the early universe.  The latest findings include the collaboration's first data on how neutrino oscillation – in which neutrinos mix and change into other "flavors," or types, as they travel – varies with neutrino energy, allowing the measurement of a key difference in neutrino masses known as "mass splitting."  

"Understanding the subtle details of neutrino oscillations and other properties of these shape-shifting particles may help resolve some of the deepest mysteries of our universe," said Jim Siegrist, Associate Director of Science for High Energy Physics at the U.S. Department of Energy (DOE), the primary funder of U.S. participation in Daya Bay. 

U.S. scientists have played essential roles in planning and running of the Daya Bay experiment, which is aimed at filling in the details of neutrino oscillations and mass hierarchy that will give scientists new ways to test for violations of fundamental symmetries. For example, if scientists detect differences in the way neutrinos and antineutrinos oscillate that are beyond expectations, it would be a sign of charge-parity (CP) violation, one of the necessary conditions that resulted in the predominance of matter over antimatter in the early universe.  The new results from the Daya Bay experiment about mass-splitting represent an important step towards understanding how neutrinos relate to the structure of our universe today.  

"Mass splitting represents the frequency of neutrino oscillation," says Kam-Biu Luk of the U.S. Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab), the Daya Bay Collaboration's Co-spokesperson, who identified the ideal site for the experiment. "Mixing angles, another measure of oscillation, represent the amplitude. Both are crucial for understanding the nature of neutrinos." Luk is a senior scientist in Berkeley Lab's Physics Division and a professor of physics at the University of California (UC)  Berkeley.  

The Daya Bay Collaboration, which includes more than 200 scientists from six regions and countries, is led in the U.S. by DOE's  Berkeley Lab and Brookhaven National Laboratory (BNL). The Daya Bay Experiment is located close to the Daya Bay and Ling Ao nuclear power plants in China, 55 kilometers northeast of Hong Kong.  The latest results from the Daya Bay Collaboration will be announced at the XVth International Workshop on Neutrino Factories, Super Beams and Beta Beams in Beijing, China. 

"These new precision measurements are a great indication that our efforts will pay off with a deeper understanding of the structure of matter and the evolution of the universe – including why we have a universe made of matter at all," says Steve Kettell, a Senior Scientist at BNL and U.S. Daya Bay Chief Scientist. 

U.S. contributions to the Daya Bay experiment include coordinating detector engineering; perfecting the recipe for the liquid used to track neutrinos in the Daya Bay detectors; overseeing the photo-detector systems used to observe neutrino interactions and muons; building the liquid-holding acrylic vessels and the detector-filling and automated calibration systems; constructing the muon veto system; developing essential software and data analysis techniques; and managing the overall project. 

Measuring neutrino mass and flavors 

Neutrinos come in three "flavors" (electron, muon, and tau) and each of these exists as a mixture of three masses. Measuring oscillations of neutrinos from one flavor to another gives scientists information on the probability of each flavor occupying each mass state (the mixing angles) and the differences between these masses (mass splitting). 

Daya Bay measures neutrino oscillation with electron neutrinos – actually antineutrinos, essentially the same as neutrinos for the purpose of these kinds of measurements. Millions of quadrillions of them are created every second by six powerful reactors. As they travel up to two kilometers to underground detectors, some seem to disappear.  

The missing neutrinos don't vanish; instead they have transformed, changing flavors and becoming invisible to the detectors. The rate at which they transform is the basis for measuring the mixing angle, and the mass splitting is determined by studying how the rate of transformation depends on the neutrino energy. 

Daya Bay's first results were announced in March 2012 and established the unexpectedly large value of the mixing angle theta one-three, the last of three long-sought neutrino mixing angles. The new results from Daya Bay put the precise number for that mixing angle at sin22θ13=0.090 plus or minus 0.009. The improvement in precision is a result of having more data to analyze and having the additional measurements of how the oscillation process varies with neutrino energy. 

The energy-dependence measurements also open a window to the new analysis that will help scientists tease out the miniscule differences among the three masses. From the KamLAND experiment in Japan, they already know that the difference, or "split," between two of the three mass states is small. They believe, based on the MINOS experiment at Fermilab, that the third state is at least five times smaller or five times larger.  Daya Bay scientists have now measured the magnitude of that mass splitting,  |Δm2ee|, to be (2.54±0.20)x10-3 eV2.

The result establishes that the electron neutrino has all three mass states and is consistent with that from muon neutrinos measured by MINOS. Precision measurement of the energy dependence should further the goal of establishing a "hierarchy," or ranking, of the three mass states for each neutrino flavor.

MINOS, and the Super-K and T2K experiments in Japan, have previously determined the complementary effective mass splitting (Δm2μμ) using muon neutrinos. Precise measurement of these two effective mass splittings would allow calculations of the two mass-squared differences (Δm232 and Δm231) among the three mass states. KamLAND and solar neutrino experiments have previously measured the mass-squared difference Δm221 by observing the disappearance of electron antineutrinos from reactors about 100 miles from the detector and the disappearance of neutrinos from the sun. 

UC Berkeley and Berkeley Lab's Bill Edwards, Daya Bay's U.S. Project and Operations Manager, says, "The ability to measure these subtle effects with greater and greater precision is a testament to the scientific and engineering team that designed and built this exceptional experiment." 

U.S. scientists are also laying the groundwork for a future neutrino project, the Long-Baseline Neutrino Experiment (LBNE). This experiment would use high intensity accelerators at Fermi National Accelerator Laboratory to produce high-energy muon neutrinos and aim them at detectors 1,300 kilometers away in South Dakota, a distance from neutrino source to detector needed to observe the transformations of high-energy muon neutrinos. LBNE would detect the appearance of the other two flavors at the far-away detector in addition to the disappearance of one flavor of neutrino as evidence of oscillation.  The combined results from LBNE and other global neutrino experiments will give scientists new ways to test for violations of fundamental symmetries, and open other avenues to understanding the structure of the universe today.  

http://www.bnl.gov/newsroom/news.php?a=11568