Building mental muscles through theoretical physics

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A grant from the D. J. Angus-Scientech Educational Foundation has made it possible for a student from a suburban Indianapolis high school to co-author, along with his mentor and two other scientists, a theoretical physics study in a top tier peer-reviewed scientific journal, a paper which has been selected for rapid communication due to its importance to the field.

"It is extremely rare for a high school student to be a co-author on aphysicspaper. Statistics on this aren't available, but it is likely less than 1 paper in 1,000, that's one tenth of one percent of physics research papers, has a high school co-author. It is unusual even for upper class undergraduates to publish in physics. This is usually the province of graduate students and faculty,"said Andrew Gavrin, Ph.D., chairman of the Department of Physics at the School of Science at Indiana University - Purdue University Indianapolis.

Published inPhysical Review A,"Robust and Fragile PT-symmetric Phases in a Tight-binding Chain,"was authored by Guerin Catholic High School senior Mark Babbey, along with IUPUI graduate student Derek Scott, Avadh Saxena, Ph.D. of Los Alamos National Laboratory and IUPUI assistant professor of physics Yogesh Joglekar, Ph.D., who led the study and mentored Babbey.

"Althoughtheoretical physicsresearch has traditionally been beyond the capability of beginning physics students and usually not tackled until the graduate level, the advent of new mathematicalcomputing softwarewith good user interfaces has enabled bright high-school and undergraduate students to carry out original research,"said Joglekar, who was the recipient of a 2009 Indiana University Trustee's Teaching Award.

Joglekar studied the properties ofquantum particlesthat hop from site to site on a chain in which one site can absorb them and another can emit them, technically known as a PT-symmetric chain. He uses a simplified analogy of a canal to explain the research.

A canal without tributaries has a constant flow of water. If a canal has one tributary carrying water in and one distributary carrying water out of it, the water flow in the canal will depend upon the distance between them and their sizes. With Babbey's assistance on the mathematical facets of the problem, Joglekar looked for the critical values of the tributary size and distance, below which the new system (a canal with a tributary and distributary) functions as the old one (just a canal).

With only a high-school level mathematical background, Babbey developed a MATLAB code under Joglekar's mentorship. Together they came up with a wide"U"shaped phase-diagram that, in the canal analogy, showed the relation between the constant-flow region and the distance between the tributary and the distributary. This diagram spurred further interest in and enabled theoretical analysis of the PT-symmetric chain problem.

"We constructed a new model with properties that had not been previously explored. Although we didn't do this with a practical application in mind, a possible application might include novel optical or electrical devices,"said Joglekar."Going back to the canal analogy, and to oversimplify, we were not just calculating water flow, we were also calculating a variety of flow characteristics such as the water velocities at the inlet and outlet."

"Although I only had one year of high-school physics and had to learn a lot of math on the fly over the summer to do the work, it was an amazing experience and I couldn't have asked for a better opportunity. Working in a real lab, on a real project that had never been attempted before sparked my interest. This wasn't a textbook lab exercise that every other physics student had done before; this was research. Both graduate student Derek Scott, who helped me understand the math and checked my work, and Dr. Joglekar, who patiently explained to me the concepts and the importance of what we were doing, were great mentors,"said Babbey, whose high school is located in Noblesville, Ind.

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Cold atoms make microwave fields visible

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Using clouds of ultracold atoms, a scientific team at the Max Planck Institute of Quantum Optics (Germany) have made microwave fields visible.

Microwaves are an essential part of modern communication technology. Mobile phones and laptops, for example, are equipped with integrated microwave circuits for wireless communication. Sophisticated techniques for measurement and characterization of microwave fields are an essential tool for the development of such circuits.

A novel technique developed by a group of scientists around Prof. Theodor W. Hansch (Max Planck Institute of Quantum Optics and Ludwig-Maximilians-Universitat Munich (LMU)) and Prof. Philipp Treutlein (University of Basel) allows for the direct and complete imaging of microwave magnetic fields with high spatial resolution. In this technique, clouds of ultracoldatomsserve as sensors for the microwave field. The technique is described in the cover story of the current issue ofApplied Physics Letters.

Modern wireless communication is based on the transmission of information throughradio wavesand microwaves. Integrated microwave circuits in devices such as mobile phones and computer laptops decode and process this information.Computer simulationsplay an important role in the development of these circuits. However, because of the large number of components in modernintegrated circuits, such simulations have to rely on approximations and are not always reliable. Therefore, measurements are required to test the circuits and to verify their performance.

To enable efficient testing and specific improvement, one would ideally like to measure all components of the microwave field directly and with very high spatial resolution. In existing techniques for measuring microwave fields, the field distribution has to be scanned point-by-point, so that this kind of data acquisition is slow. Moreover, most techniques only allow for a measurement of the amplitudes, but not of the phases of the microwave field. Furthermore, macroscopic probe heads used for the measurement can distort the microwave field and result in poor spatial resolution.

Scientists at the Max Planck Institute ofQuantum Optics, the LMU Munich and the University of Basel have now demonstrated a new technique for the imaging of microwave magnetic fields. As microwave field sensors, they use small clouds of ultracold atoms that hade been laser-cooled to a temperature of a few millionths of a degree above absolute zero. At these temperatures, the atoms obey the laws of quantum physics. Their quantum state is very sensitive to externally applied electromagnetic fields, which makes them ideal sensors. For the measurement, the atoms are positioned at the desired location above the microwave circuit with the help of static magnetic fields, and subsequently the microwave field is turned on.

"The internal state of the atoms changes if a microwave field is applied,"Pascal Bohi explains, who co-developed the technique as part of this doctoral thesis."We can image this change of internal state with a CCD camera with high spatial resolution. The stronger the microwave field at a given position, the faster the rate of change we observe."A unique feature of the new method is that it does not require the microwave field to be scanned point-by-point. Rather, a fully two-dimensional image of one component of the microwave field can be recorded in a single shot. This increases the data acquisition rate dramatically. In addition, the technique allows not only for a reconstruction of the amplitudes, but also of the phases of the microwave field components. As the atoms are truly microscopic objects, they do not distort the microwave circuit to be characterized, in contrast to macroscopic probe heads. The new method works for various frequencies in the gigahertz range.

"We have successfully demonstrated the new technique in our lab. Quite naturally, further development is necessary before it could be used in commercial applications,"says Philipp Treutlein, the leader of the project. However, a very compact and portable setup for experiments with ultracold atoms was recently built and could be of interest for such applications. The setup itself is at room temperature, but the atoms trapped inside are cooled within a few seconds with the help of laser light. Key components of such systems are now commercially available. Because of the potential for applications, the researchers have submitted a patent application describing the new technique.

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Pitt-led researchers to build foundation for quantum supercomputers

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A research team based at the University of Pittsburgh has received a five-year, $7.5 million grant from the U.S. Department of Defense to tackle some of the most significant challenges preventing the development of quantum computers, powerful devices that could solve problems too complex for all of the world's computers working together over the age of the Universe to crack. The project was one of 32 nationwide selected from 152 proposals to receive a grant from the Multi-University Research Initiative (MURI) program; a total of $227 million was distributed to institutions that include Harvard University, the Massachusetts Institute of Technology, the University of Illinois at Urbana-Champaign, and the University of Pennsylvania.

Jeremy Levy, a professor of physics and astronomy in Pitt's School of Arts and Sciences, will lead a team of researchers from Cornell University, Stanford University, the University of California at Santa Barbara, the University of Michigan, and the University of Wisconsin in combining the properties of semiconductors—such as those used to make computer processors, and superconductors—which allow for the perfect flow of electricity, into a single material suitable for the development of quantum computers. The team will use these superconducting semiconductors to develop new types ofquantum memory, perform quantum simulation, and create new methods for transferringquantum informationfrom one medium to another.

These functions are essential to realizing quantum computers—which are yet to exist in any practical form—but require a precise control of the laws ofquantum physicsthat has so far been difficult to achieve, Levy explained.

One of the most significant challenges with any approach to quantum computation is the inevitable loss of information. Group member Chetan Nayak, a physics professor at UC-Santa Barbara, has theorized that very thin sheets of certain types of superconductors have topological quantum excitations that can be used to make quantum memories highly immune to errors. The development of materials that can support these excitations will be undertaken by Chang-Beom Eom, a professor of materials science and engineering at Wisconsin; Harold Hwang, a professor of applied physics at Stanford; and Darrell Schlom, an engineering professor at Cornell. Xiaoqing Pan, a University of Michigan professor of materials science and engineering, will perform atomic-scale characterization of these structures.

A second research goal involves using superconducting semiconductors to perform quantum simulations of physical systems. To do this, the team will use a technique Levy developed that allows for atomic-scale devices such as transistors and computer processors to be created and erased on a single platform that functions like a microscopic Etch A SketchTM, the drawing toy that inspired Levy's idea; Levy reported on the platform in the Feb. 20, 2009, edition of Science. For the MURI project, Levy will create a new near-atomic scale lattice that will be used to experiment with new materials and search for superconducting phenomena.

The project's third thrust involves the transfer of quantum information from one physical system to another. Quantum bits are efficiently stored in nanoscale defects found in diamonds. David Awschalom, a professor of physics and electrical engineering at UC-Santa Barbara, will develop ways of transferring quantum information between these diamond defects and superconducting microwave resonators.

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On the path to quantum computers: Ultra-strong interaction between light and matter realized

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Researchers around the world are working on the development of quantum computers that will be vastly superior to present-day computers. Here, the strong coupling of quantum bits with light quanta plays a pivotal role. Professor Rudolf Gross, a physicist at the Technische Universitaet Muenchen, Germany, and his team of researchers have now realized an extremely strong interaction between light and matter that may represent a first step in this direction.

The interaction between matter andlightrepresents one of the most fundamental processes in physics. Whether a car that heats up like an oven in the summer due to the absorption of light quanta orsolar cellsthat extract electricity from light or light-emitting diodes that convert electricity into light, we encounter the effects of these processes throughout our daily lives. Understanding the interactions between individuallight particles- photons - and atoms is crucial for the development of a quantum computer.

Physicists from the Technische Universitaet Muenchen (TUM), the Walther-Meissner-Institute for Low Temperature Research of the Bavarian Academy of Sciences (WMI) and the Augsburg University have now, in collaboration with partners from Spain, realized an ultrastrong interaction between microwave photons and the atoms of a nano-structured circuit. The realized interaction is ten times stronger than levels previously achieved for such systems.

The simplest system for investigating the interactions between light andmatteris a so-called cavity resonator with exactly one light particle and one atom captured inside (cavity quantum electrodynamics, cavity QED). Yet since the interaction is very weak, these experiments are very elaborate. A much stronger interaction can be obtained with nano-structured circuits in which metals like aluminum become superconducting at temperatures just above absolute zero (circuit QED). Properly configured, the billions of atoms in the merely nanometer thick conductors behave like a single artificial atom and obey the laws of quantum mechanics. In the simplest case, one obtains a system with two energy states, a so-called quantum bit or qubit.

Coupling these kinds of systems with microwave resonators has opened a rapidly growing new research domain in which the TUM Physics, the WMI and the cluster of excellence Nanosystems Initiative Munich (NIM) are leading the field. In contrast to cavity QED systems, the researchers can custom tailor the circuitry in many areas.

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This is an electron microscopical picture of the superconducting circuit (red: Aluminum-Qubit, grey: Niob-Resonator, green: Silicon substrate). Credit: Thomasz Niemczyk, Technische Universitaet Muenchen

To facilitate the measurements, Professor Gross and his team captured the photon in a special box, a resonator. This consists of a superconducting niobium conducting path that is configured with strongly reflective"mirrors"for microwaves at both ends. In this resonator, the artificial atom made of an aluminum circuit is positioned so that it can optimally interact with the photon. The researchers achieved the ultrastrong interactions by adding another superconducting component into their circuit, a so-called Josephson junction.

The measured interaction strength was up to twelve percent of the resonator frequency. This makes it ten times stronger than the effects previously measureable in circuit QED systems and thousands of times stronger than in a true cavity resonator. However, along with their success the researchers also created a new problem: Up to now, the Jaynes-Cummings theory developed in 1963 was able to describe all observed effects very well. Yet, it does not seem to apply to the domain of ultrastrong interactions."The spectra look like those of a completely new kind of object,"says Professor Gross."The coupling is so strong that the atom-photon pairs must be viewed as a new unit, a kind of molecule comprising one atom and one photon.

Experimental and theoretical physicists will need some time to examine this more closely. However, the new experimental inroads into this domain are already providing researchers with a whole array of new experimental options. The targeted manipulation of such atom-photon pairs could hold the key to quanta-based information processing, the so-called quantum computers that would be vastly superior to today's computers.

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Quantum memory for communication networks of the future

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Researchers from the Niels Bohr Institute at the University of Copenhagen have succeeded in storing quantum information using two 'entangled' light beams. Quantum memory or information storage is a necessary element of future quantum communication networks. The new findings are published in<i>Nature Physics</i>.

Quantum networks will be able to protect the security of information better than the current conventional communication networks. The cornerstone of quantum communication is a phenomenon called entanglement between two quantum systems, for example, two light beams.Entanglementmeans that the two light beams are connected to each other, so that they have well defined common characteristics, a kind of common knowledge. Aquantum statecan– according to the laws of quantum mechanics, not be copied and can therefore be used to transfer data in a secure way.

In professor Eugene Polzik's research group Quantop at the Niels Bohr Institute researchers have now been able to store the two entangled light beams in two quantum memories. The research is conducted in a laboratory where a forest of mirrors and optical elements such as wave plates, beam splitters, lenses etc. are set up on a large table, sending the light around on a more than 10 meter long labyrinthine journey. Using the optical elements, the researchers control the light and regulate the size and intensity to get just the right wavelength and polarisation the light needs to have for the experiment.

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The illustration shows the two quantum memories. Each memory consists of a glass cell filled with caesium atoms, which are shown as small blue and red balls. The light beam is sent through the atoms and the quantum information is thus transferred from the light to the atoms. Credit: Quantop

The two entangled light beams are created by sending a single bluelight beamthrough a crystal where the blue light beam is split up into two red light beams. The two red light beams are entangled, so they have a common quantum state. The quantum state itself is information.

The two light beams are sent on through the labyrinth of mirrors and optical elements and reach the two memories, which in the experiment are two glass containers filled with a gas of caesium atoms. The atoms' quantum state contains information in the form of a so-called spin, which can be either 'up' or 'down'. It can be compared with computer data, which consists of the digits 0 and 1. When the light beams pass the atoms, the quantum state is transferred from the two light beams to the two memories. The information has thus been stored as the new quantum state in the atoms.

"For the first time such a memory has been demonstrated with a very high degree of reliability. In fact, it is so good that it is impossible to obtain with conventional memory for light that is used in, for example, internet communication. This result means that a quantum network is one step closer to being a reality", explains professor Eugene Polzik.

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German, Canadian scientists to open quantum physics center

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German and Canadian scientists on Monday announced plans to open an advanced research center on semiconductors, metals, magnetic materials and other aspects of quantum physics.

University of British Columbia in Vancouver will house the new research facility, the Max Planck-UBC Centre for Quantum Materials, which is to be funded by Germany's prestigious Max Planck Society.

The funding agreement includes annual exchanges by more than two dozen professors and graduate students from Canada and Germany, a center news release said.

"The knowledge and discoveries generated from these collaborations will profoundly change the lives of present and future generations,"said University of British Columbia President Stephen Toope.

The research center in this western Canadian city will be the third such center that the Max Planck Society funds outside Germany. Centers also exist in India and Spain; a fourth one is under construction in Florida.

The Max Planck Society for the Advancement of Science is an independent, non-profit research organization that mainly supports research at its own institutes.

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Quantum computers may be much easier to build than previously thought: study

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Quantum computers should be much easier to build than previously thought, because they can still work with a large number of faulty or even missing components, according to a study published today in<i>Physical Review Letters</i>. This surprising discovery brings scientists one step closer to designing and building real-life quantum computing systems– devices that could have enormous potential across a wide range of fields, from drug design, electronics, and even code-breaking.

Scientists have long been fascinated with building computers that work at a quantum level– so small that the parts are made of just single atoms or electrons. Instead of 'bits', the building blocks normally used to store electronic information, quantum systems use quantum bits or 'qubits', made up of an arrangement of entangled atoms.

Materials behave very differently at this tiny scale compared to what we are used to in our everyday lives– quantum particles, for example, can exist in two places at the same time."Quantum computers can exploit this weirdness to perform powerful calculations, and in theory, they could be designed to break public key encryption or simulate complex systems much faster than conventional computers,"said Dr Sean Barrett, the lead author of the study, who is a Royal Society University Research Fellow in the Department of Physics at Imperial College London.

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The machines have been notoriously hard to build, however, and were thought to be very fragile to errors. In spite of considerable buzz in the field in the last 20 years, useful quantum computers remain elusive.

Barrett and his colleague Dr. Thomas Stace, from the University of Queensland in Brisbane, Australia, have now found a way to correct for a particular sort of error, in which the qubits are lost from the computer altogether. They used a system of 'error-correcting' code, which involved looking at the context provided by the remaining qubits to decipher the missing information correctly.

"Just as you can often tell what a word says when there are a few missing letters, or you can get the gist of a conversation on a badly-connected phone line, we used this idea in our design for a quantum computer,"said Dr Barrett. They discovered that the computers have a much higher threshold for error than previously thought– up to a quarter of the qubits can be lost– but the computer can still be made to work."It's surprising, because you wouldn't expect that if you lost a quarter of the beads from an abacus that it would still be useful,"he added.

The findings indicate that quantum computers may be much easier to build than previously thought, but as the results are still based on theoretical calculations, the next step is to actually demonstrate these ideas in the lab. Scientists will need to devise a way for scaling the computers to a sufficiently large number of qubits to be viable, says Barrett. At the moment the biggest quantum computers scientists have built are limited to just two or three qubits.

"We are still some way off from knowing what the true potential of a quantum computer might be, says Barrett."At the moment quantum computers are good at particular tasks, but we have no idea what these systems could be used for in the future,"he said."They may not necessarily be better for everything, but we just don't know. They may be better for very specific things that we find impossible now."

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