Attosecond physics: Molecules brilliantly illuminated

A new high-power laser system generates ultrashort pulses of light covering a large share of the mid-infrared spectrum.

Molecules are the building blocks of life. Like all other organisms, we are made of them. They control our biorhythm, and they can also reflect our state of health. Researchers led by Ferenc Krausz at the Laboratory for Attosecond Physics (LAP) -- a joint venture between Ludwig-Maximilians-Universität (LMU) and the Max Planck Institute for Quantum Optics (MPQ) in Munich -- want to use brilliant infrared light to study molecular disease markers in much greater detail, for example to facilitate early stage cancer diagnosis. The team has developed a powerful femtosecond light source which emits at wavelengths between 1.6 and 10.2 micrometers. This instrument should make it possible to detect organic molecules present in extremely low concentrations in blood or aspirated air.

Myriads of molecules react in highly specific ways to light of certain wavelengths in the mid-infrared region. By absorbing particular wavelengths, each type of molecule in a sample imprints a specific signature on the transmitted beam, which serves as a molecular fingerprint. With a source of broadband mid-infrared light one detects the fingerprints of many molecular structures at once -- in a sample of blood or aspirated air, for example. If the sample contains marker molecules that are associated with specific disease states, these too will reveal their presence in the spectrum of the transmitted infrared light.

LAP physicists have now constructed such a light source, which covers the wavelengths between 1.6 and 10.2 microns. The laser system exhibits watt-level average output power, and is well focusable which results in a highly brilliant infrared light source. This feature enhances the ability to detect molecules present in extremely low concentrations. In addition, the laser can produce trains of femtosecond pulses [a femtosecond is a millionth of a billionth of a second (10-15 sec)], which makes it possible to carry out time-resolved as well as low-noise and highly- precise measurements.

At present, infrared spectroscopy is often based on the use of incoherent light, which provides coverage of the whole mid-infrared region. However, the relatively low brilliance of the beam produced by incoherent sources markedly reduces the ability to detect very weak molecular fingerprints. Synchrotron radiation produced in particle accelerators can alternatively be used, but such facilities are in short supply and are extremely expensive. However, laser-based methods can generate even brighter beams than synchrotrons do. The physicists at LAP have now succeeded in building a coherent light source which produces brilliant laser light over a broad spectral region in the infrared range. That used to be the major drawback of laser sources Moreover, the new system has a much smaller footprint (and is far less costly) than a synchrotron: it fits on a large table.

"Of course, there is still a long way to go until we can diagnose cancer at much early stage than at present. We need a better understanding of disease markers and we have to design an efficient way to quantify them, for example," says Marcus Seidel, one of the researchers involved in the project. "But now having significantly improved light sources available, we can begin to tackle these issues." Moreover, the new laser system will find applications in areas beyond the biosciences. After all, the precise observation of molecules and their transformations is at the core of both chemistry and physics too.

Yale's latest work expanding the reach of quantum information science is actually a game of quantum pitch and catch

In a new study published April 23 in the journal Nature Physics, Yale researchers "pitch" a qubit -- a tiny bit of quantum data -- from one physical point in a microwave cavity to a separate point in a different cavity. It is the first time an end-to-end quantum transmission has been done on demand and represents the first of two Yale experiments involving "pitch-and-catch" technologies that will be published this year.

Quantum computing offers the possibility of computation speeds that are orders of magnitude faster than today's supercomputers. Yale researchers are at the forefront of efforts to develop the first fully useful quantum computers, and have done pioneering work in quantum computing with superconducting circuits.

But in order for a quantum computer to run more complex algorithms, it will need more processing power, just as a classical computer does. To do that, qubits must be interfaced with each other -- which is why a "pitch and catch" capability would come in handy.

"Our approach is to use a quantum network to connect many qubits together in independent modules," said Christopher Axline, a Yale graduate student and co-lead author of the new study. "The strategy is similar to clustering computers together on a local area network."

Axline works in the Yale lab of Robert Schoelkopf, the study's principal investigator. The other co-lead authors of the study are Yale graduate student Luke Burkhart and former Yale postdoctoral associate Wolfgang Pfaff, who is now at Microsoft.

Previous work by the researchers enabled them to pitch a qubit, while preserving its information. Now they're able to catch the information, as well.

"You might think catching our flying qubit would be a straightforward extension of our other work, but it actually requires some careful treatment," Burkhart said. "It meant varying how quickly, and at what frequency, the information is released. If we open the floodgates and let energy flow out as quickly as possible, it will overwhelm the catcher."

Instead, the researchers carefully shape their pitch-and-catch over time, so that both ends of the transaction are in sync.

Another first for the experiment is the use of the cavities -- in addition to the qubit itself -- as the memory for the system. "Much of the research in our lab and at the Yale Quantum Institute focuses on how to take advantage of cavity modes for quantum information processing," Axline said. "Superconducting cavities are the most secure places we can store quantum information, and even more important, cavities are flexible as to the form of the stored information."

This quantum game of pitch and catch also includes quantum entanglement, a key concept in quantum physics and a requirement in any quantum algorithm. In this instance, it means the pitcher is pitching and not pitching, simultaneously.

"We entangle the states between the pitcher and the catcher," Burkhart said. "This remote entanglement will be crucial in quantum networks."

The 'missing link' in conducting molecules, butadiene -- solved Researchers have solved a long-standing puzzle in the dynamics of the smallest linear polyene, butadiene

Linear polyenes are hydrocarbon chains with unusual optical and electrical properties. They have become a paradigm for studying photoisomerization -- when molecular structures rearrange from absorbing light -- because of their straightforward molecular structure, potential for electrical conductivity, and role in vision. Understanding how these molecules simultaneously rearrange through photoisomerization could advance materials science research by enabling artificial vision and producing wires from plastic, and new photovoltaic technologies.

Trans 1,3-butadiene, the smallest polyene, has challenged researchers over the past 40 years because of its complex excited-state electronic structure and its ultrafast (femtosecond, 10-15 s) dynamics. Butadiene remains the "missing link" between ethylene (C2H4,), which has only one double bond, and longer linear polyenes with three or more double bonds.

Now, an experimental team headed by Albert Stolow at the University of Ottawa and the National Research Council of Canada has solved trans 1,3-butadiene's electronic-structural dynamics. The researchers recently reported their findings in The Journal of Chemical Physics, from AIP Publishing.

Stolow's group developed an ultrafast laser spectroscopy called time-resolved photoelectron-photoion coincidence spectroscopy (TRPEPICO) to conduct this research. The method involves a femtosecond pump-probe process wherein an emitted photoelectron is measured as a function of time. The photoelectron spectrum and angular distribution is sensitive to the electronic and structural dynamics of molecules. Over the past 20 years, Stolow has applied his method to a broad range of problems, including the ultraviolet stability of DNA bases and intramolecular proton transfer.

"We've shown over many years that our approach works and have provided lots of examples," Stolow said. He previously studied under John C. Polanyi and Yuan T. Lee, two Nobel Prize winners who researched molecular collision dynamics.

"Many of us thought that if we could understand ethylene, the basic building block, we would be able to understand the longer linear polyenes," Stolow said. "But butadiene is the 'missing link.' It didn't seem to behave like either case."

Stolow's team discovered that trans 1,3-butadiene behaves, simultaneously, like both ethylene and longer polyenes. Specifically, there is an ultrafast competition between ethylenelike dynamics and polyenelike dynamics.

The research team's experimental results were independently modeled and confirmed computationally by Todd J. Martínez's research team. Martinez is a researcher and professor of chemistry at Stanford University, who specializes in molecular quantum dynamics. Michael S. Schuurman of the NRC, a theorist specializing in quantum dynamics, also helped confirm this work.

"This collaboration is key. We each independently came up with the same results," Stolow said. "Dramatic technical advances in both experiment and theory have allowed us to finally solve the long-standing puzzle of electronic dynamics in butadiene, the 'missing link' of polyene photophysics."

The elusive quantum mechanical phenomenon of entanglement has now been made a reality in objects almost macroscopic in size

This is an illustration of the 15-micrometre-wide drumheads prepared on silicon chips used in the experiment. The drumheads vibrate at a high ultrasound frequency, and the peculiar quantum state predicted by Einstein was created from the vibrations.

Perhaps the strangest prediction of quantum theory is entanglement, a phenomenon whereby two distant objects become intertwined in a manner that defies both classical physics and a "common-sense" understanding of reality. In 1935, Albert Einstein expressed his concern over this concept, referring to it as "spooky action at a distance."

Nowadays, entanglement is considered a cornerstone of quantum mechanics, and it is the key resource for a host of potentially transformative quantum technologies. Entanglement is, however, extremely fragile, and it has previously been observed only in microscopic systems such as light or atoms, and recently in superconducting electric circuits.

In work recently published in Nature, a team led by Prof. Mika Sillanpää at Aalto University in Finland has shown that entanglement of massive objects can be generated and detected.

The researchers managed to bring the motions of two individual vibrating drumheads -- fabricated from metallic aluminium on a silicon chip -- into an entangled quantum state. The objects in the experiment are truly massive and macroscopic compared to the atomic scale: the circular drumheads have a diametre similar to the width of a thin human hair.

The team also included scientists from the University of New South Wales Canberra in Australia, the University of Chicago, and the University of Jyväskylä in Finland. The approach taken in the experiment was based on a theoretical innovation developed by Dr. Matt Woolley at UNSW and Prof. Aashish Clerk, now at the University of Chicago.

'The vibrating bodies are made to interact via a superconducting microwave circuit. The electromagnetic fields in the circuit are used to absorb all thermal disturbances and to leave behind only the quantum mechanical vibrations,' says Mika Sillanpää, describing the experimental setup.

Eliminating all forms of noise is crucial for the experiments, which is why they have to be conducted at extremely low temperatures near absolute zero, at -273 °C. Remarkably, the experimental approach allows the unusual state of entanglement to persist for long periods of time, in this case up to half an hour.

'These measurements are challenging but extremely fascinating. In the future, we will attempt to teleport the mechanical vibrations. In quantum teleportation, properties of physical bodies can be transmitted across arbitrary distances using the channel of "spooky action at a distance",' explains Dr. Caspar Ockeloen-Korppi, the lead author on the work, who also performed the measurements.

The results demonstrate that it is now possible to have control over large mechanical objects in which exotic quantum states can be generated and stabilized. Not only does this achievement open doors for new kinds of quantum technologies and sensors, it can also enable studies of fundamental physics in, for example, the poorly understood interplay of gravity and quantum mechanics.

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A new material created by Oregon State University researchers is a key step toward the next generation of supercomputers.

Those "quantum computers" will be able to solve problems well beyond the reach of existing computers while working much faster and consuming vastly less energy.

Researchers in OSU's College of Science have developed an inorganic compound that adopts a crystal structure capable of sustaining a new state of matter known as quantum spin liquid, an important advance toward quantum computing.

In the new compound, lithium osmium oxide, osmium atoms form a honeycomb-like lattice, enforcing a phenomenon called "magnetic frustration" that could lead to quantum spin liquid as predicted by condensed matter physics theorists.

Corresponding author Mas Subramanian, Milton Harris Professor of Materials Science at OSU, explains that in a permanent magnet like a compass needle, the electrons spin in an aligned manner -- that is, they all rotate in the same direction.

"But in a frustrated magnet, the atomic arrangement is such that the electron spins cannot achieve an ordered alignment and instead are in a constantly fluctuating state, analogous to how ions would appear in a liquid," Subramanian said.

The lithium osmium oxide discovered at OSU shows no evidence for magnetic order even when frozen to nearly absolute zero, which suggests an underlying quantum spin liquid state is possible for the compound, he said.

"We are excited about this new development as it widens the search area for new quantum spin liquid materials that could revolutionize the way we process and store data," Subramanian said. "The quantum spin liquid phenomenon has so far been detected in very few inorganic materials, some containing iridium. Osmium is right next to iridium in the periodic table and has all the right characteristics to form compounds that can sustain the quantum spin liquid state."

Arthur Ramirez, condensed matter physicist at the University of California, Santa Cruz, one of the co-authors in the paper, noted that this compound is the first honeycomb-structured material to contain osmium and expects more to follow.

Ramirez also noted that this study demonstrates the importance of multidisciplinary collaboration involving materials chemists and condensed matter physicists engaged in synthesis, theory and measurements to tackle emerging science like quantum spin liquid.

The next step for Subramanian's team is exploring the chemistry needed to create various perfectly ordered crystal structures with osmium.

The National Science Foundation is funding the research through its DMREF program: Designing Materials to Revolutionize and Engineer our Future. Findings were published today in Scientific Reports.

The concept of quantum computing is based on the ability of subatomic particles to exist in more than one state at any time.

Classical computing relies on bits -- pieces of information that exist in one of two states, a 0 or a 1. In quantum computing, information is translated to quantum bits, or qubits, that can store much more information than a 0 or 1 because they can be in any "superposition" of those values.

Think of bits and qubits by visualizing a sphere. A bit can only be at either of the two poles on the sphere, whereas a qubit can be anywhere on the sphere. What that means is much more information storage potential and much less energy consumption.

A new material created by Oregon State University researchers is a key step toward the next generation of supercomputers.

A new source of intense terahertz (THz) radiation, which could offer a less harmful alternative to x-rays and has strong potential for use in industry, is being developed by scientists at the University of Strathclyde and Capital Normal University in Beijing.

Unlike visible light, THz radiation penetrates materials such as plastic, cardboard, wood and composite materials, making it an excellent replacement for harmful X-rays used in imaging, and security.

Although it is well known that THz electromagnetic waves can carry ultra-high bandwidth communications, far exceeding those of Wi-Fi, it is less well-known that it is a highly useful probe for detecting molecules and analysing semiconductors.

A research team led by Professor Dino Jaroszynski, of Strathclyde's Department of Physics, has shown experimentally that unprecedentedly high-charge bunches of relativistic electrons can be produced by a laser wakefield accelerator (LWFA). These are produced in addition to the usual high-energy, low-charge beams that are emitted.

The team showed that when an intense ultra-short laser pulse is focussed into helium gas, a plasma bubble moving at the nearly the speed of light is formed. These high-charge beams of electrons are distinct from the usual low-charge (picocoloumb), high-energy (100s MeV to GeV), femtosecond duration electron bunches that are commonly observed from the LWFA.

The research has been published in the New Journal of Physics.

Professor Jaroszynski, Director of the Scottish Centre for the Application Plasma-based Accelerators (SCAPA), who initiated the project, said: "This is an unprecedented efficiency at these THz energies. The increasing availability of intense THz sources will lead to completely new avenues in science and technology.

"New tools for scientists lead to new advances. The interaction of intense THz radiation with matter allows access to nonlinear processes, which enables the identification of normally hidden phenomena, and also unique control of matter, such as aligning molecules using high THz fields or distorting band structure in semiconductors.

"SCAPA provides an ideal environment for investigating these phenomena, which should lead to new advances in science. Our theoretical studies are the first steps in this exciting new direction."

Dr Enrico Brunetti, of Strathclyde's Department of Physics, carried out most of the simulations in the research. He said: "Since the charge of wide-angle beams increases linearly with laser intensity and plasma density, the energy of THz radiation will scale to milijoule-levels, which would make an intense source of THz radiation with peak powers in excess of GW, which is comparable with that of a far-infrared free-electron laser. An optical to terahertz conversion efficiency of the order of 1% can be reached."

Dr Xue Yang, a researcher in the project from Capital Normal University, said: "When electrons cross an interface between two media of different dielectric constant, transition radiation is emitted over a wide range of frequencies.

"Simulations show that wide-angle electron beams emitted by laser-wakefield accelerators can produce coherent terahertz radiation with 10s ?J to 100s ?J energy when passed through a thin metal foil or at the plasma-vacuum boundary of the accelerator."

THz radiation is far-infrared electromagnetic radiation that has a frequency between 0.1 THz and 10 THz (1 THz = 10^12 Hz), which fits between the mid-infrared and microwave spectra. The vibrational and rotational spectral fingerprints of large molecules coincide with the THz band, which makes THz spectroscopy a powerful tool for identifying hazardous substances, such as drugs and explosives. Moreover, THz radiation is important for biology and medicine because many biological macromolecules, such as DNA and proteins, have their collective motion at THz frequencies.

THz radiation can also be used to uncover the intricacies of semiconductors and nanostructures, and therefore are important tools for developing new electro-mechanical devices and solar cells.

Many different methods of generating THz radiation exist, including driving photocurrents in semiconductor antennas, excitation of quantum wells and optical rectification in electro-optic crystals. However, their maximum power is restricted because of damage to the optical materials at high powers. Plasma, in contrast, has no such limitation, as it is already broken

The new research shows that these high-charge -- nanocoloumb-, and relatively low energy (MeV), sub-picosecond duration electron bunches are emitted in a hollow cone with an opening angle of nearly 45 degrees to the laser beam axis. The researchers show that laser energy can be efficiently transferred to a very intense pulse of THz radiation.

The study was funded by the Engineering and Physical Sciences Research Council.