Benefits of single atoms acting as catalysts in hydrogen-related reactions

Hydrogenation – the addition of hydrogen atoms to an organic compound – is critical to the food, petrochemical and pharmaceutical industries. Hydrogenation requires the presence of a catalyst, usually a metal or an alloy of both precious and common metals, that allows the hydrogen atoms to bind with other molecules. It is difficult to produce alloys that are selective hydrogenation catalysts, able to attach the hydrogen atoms to specific sites of another molecule.

Tufts chemists and chemical engineers reported that when single atoms of palladium, an expensive precious metal, were added to copper, which is much cheaper and readily available, the resulting "single atom alloy" became active and selective for hydrogenation reactions.

This is the first published research to directly relate the arrangement of individual atoms in a metal alloy to their ability to catalyze hydrogenation reactions, according to E. Charles H. Sykes, associate professor of chemistry at Tufts and senior author on the paper. Sykes focuses much of his research on single molecule chemistry.

Industrial processes typically use small clumps of precious metal five to 10 nanometers wide on supports to make a catalyst. The Tufts scientists scattered single atoms of palladium less than half a nanometer wide onto a copper support. With palladium costing about $650 per ounce, the single atom alloy approach offers big cost savings.

"The chemical reactions we're looking at with smaller amounts of palladium use less energy and yield less chemical byproduct waste, hence they're better for the environment," said Sykes. "These reactions are also more cost-effective because we're working with single atoms of precious metals, which is therefore much cheaper than big clusters of the material. Given that hydrogenation reactions are carried out on a scale of millions of tons per year, there is great potential for this new and less expensive type of catalytic surface."

For this research the Tufts team heated very small amounts of palladium to almost 1,000°C. At that temperature individual atoms evaporated and embedded themselves on the copper surface about three inches away.

A scanning tunneling microscope, which records images of objects at the atomic level, enabled the team to see how these single atoms dispersed in the copper and how molecular hydrogen could then dissociate at individual, isolated palladium sites and spill over onto the copper surface layer.

"This is the first time there has been a definitive microscopic picture of the arrangement of atoms that promote a catalytic hydrogenation reaction. This picture is important because the catalytic hydrogenations that we're studying are vital to many industrial processes," added Sykes. "For example, in petroleum refining, catalytic hydrogenations are performed to make light and hydrogen-rich products like gasoline."

Georgios Kyriakou, research assistant professor of chemistry in the School of Arts and Sciences and first author of the paper; Maria Flytzani-Stephanopoulos, the Robert and Marcy Haber Endowed Professor in Energy Sustainability in the School of Engineering; and a joint Ph.D. student, Matthew Boucher, led testing that determined that the single atom alloy was more effective in catalyzing hydrogenation than denser mixtures of palladium and copper. Mass spectrometry showed that the new alloy catalyzed the hydrogenation of both styrene and acetylene with greater than 95% selectivity.

"With the rising cost of precious metals and the increasing scarcity of these metals, learning more about these reactions is encouraging in the search for sustainable global solutions," said Flytzani-Stephanopoulos. "We are looking at how these single-atom alloy catalysts could eventually be used as low-cost alternatives in hydrogenation and dehydrogenation processes for the production of 'green' agricultural chemicals, foods and pharmaceuticals," said Flytzani-Stephanopoulos.

More information: Kyriakou, G., Boucher, M., Jewell, A., Lewis, E., Lawton, T., Baber, A., Tierney, H., Flytzani-Stephanopoulos, M., Sykes, E.C. "Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations." Science. Published online and print March 9, 2012. DOI: 10.1126/Science.1215864

Provided by Tufts University

World's smallest radio stations: Two molecules communicate via single photons

 We know since the dawn of modern physics that although events in our everyday life can be described by classical physics, the interaction of light and matter is down deep governed by the laws of quantum mechanics. Despite this century-old wisdom, accessing truly quantum mechanical situations remains nontrivial, fascinating and noteworthy even in the laboratory. Recently, interest in this area has been boosted beyond academic curiosity because of the potential for more efficient and novel forms of information processing.

In one of the most basic proposals, a single atom or molecule acts as a quantum bit that processes signals that have been delivered via single photons. In the past twenty years scientists have shown that single molecules can be detected and single photons can be generated. However, excitation of a molecule with a photon had remained elusive because the probability that a molecule sees and absorbs a photon is very small. As a result, billions of photons per second are usually impinged on a molecule to obtain a signal from it.

One common way to get around this difficulty in atomic physics has been to build a cavity around the atom so that a photon remains trapped for long enough times to yield a favorable interaction probability. Scientists at ETH Zurich and Max Planck Institute for the Science of Light in Erlangen have now shown that one can even interact a flying photon with a single molecule. Among many challenges in the way of performing such an experiment is the realization of a suitable source of single photons, which have the proper frequency and bandwidth. Although one can purchase lasers at different colors and specifications, sources of single photons are not available on the market.

So a team of scientists led by Professor Vahid Sandoghdar made its own. To do this, they took advantage of the fact that when an atom or molecule absorbs a photon it makes a transition to a so-called excited state. After a few nanoseconds (one thousand millionth of a second) this state decays to its initial ground state and emits exactly one photon. In their experiment, the group used two samples containing fluorescent molecules embedded in organic crystals and cooled them to about 1.5 K (-272 °C). Single molecules in each sample were detected by a combination of spectral and spatial selection.

To generate single photons, a single molecule was excited in the “source” sample. When the excited state of the molecule decayed the emitted photons were collected and tightly focused onto the “target” sample at a distance of a few meters. To ensure that a molecule in that sample “sees” the incoming photons, the team had to make sure that they have the same frequency. Furthermore, the precious single photons had to interact with the target molecule in an efficient manner. A molecule is about one nanometer is size (100000 times smaller than the diameter of a human hair) but the focus of a light beam cannot be smaller than a few hundred nanometers.

This usually means that most of the incoming light goes around the molecule, i.e. without them seeing each other. However, if the incoming photons are resonant with the quantum mechanical transition of the molecule, the latter acts as a disk that is comparable to the area of the focused light. In this process the molecule acts as an antenna that grabs the light waves in its vicinity. The results of the study published in Physical Review Letters provide the first example of long-distance communication between two quantum optical antennas in analogy to the 19th century experiments of Hertz and Marconi with radio antennas. In those early efforts, dipolar oscillators were used as transmitting and receiving antennas.

In the current experiment, two single molecules mimic that scenario at optical frequencies and via a nonclassical optical channel, namely a single-photon stream. This opens many doors for further exciting experiments in which single photons act as carriers of quantum information to be processed by single emitters.

The experimental work was performed at ETH Zurich before the group of Prof. Sandoghdar moved to the newly founded Max Planck Institute for the Science of Light in Erlangen in 2011.

Share this story on Facebook, Twitter, and Google:

Other social bookmarking and sharing tools:

Story Source:

The above story is reprinted from materials provided by ETH Zürich, via AlphaGalileo.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

Y. L. A. Rezus, S. G. Walt, R. Lettow, A. Renn, G. Zumofen, S. Götzinger, and V. Sandoghdar. Single-Photon Spectroscopy of a Single Molecule. Physical Review Letters, 108, 093601; Feb 27, 2012

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.

Lab mimics Jupiter's Trojan asteroids inside a single atom

 Rice University physicists have gone to extremes to prove that Isaac Newton's classical laws of motion can apply in the atomic world: They've built an accurate model of part of the solar system inside a single atom of potassium.

In a new paper published this week in Physical Review Letters, Rice's team and collaborators at the Oak Ridge National Laboratory and the Vienna University of Technology showed they could cause an electron in an atom to orbit the nucleus in precisely the same way that Jupiter's Trojan asteroids orbit the sun.

The findings uphold a prediction made in 1920 by famed Danish physicist Niels Bohr about the relationship between the then-new science of quantum mechanics and Newton's tried-and-true laws of motion.

"Bohr predicted that quantum mechanical descriptions of the physical world would, for systems of sufficient size, match the classical descriptions provided by Newtonian mechanics," said lead researcher Barry Dunning, Rice's Sam and Helen Worden Professor of Physics and chair of the Department of Physics and Astronomy. "Bohr also described the conditions under which this correspondence could be observed. In particular, he said it should be seen in atoms with very high principal quantum numbers, which are exactly what we study in our laboratory."

Bohr was a pioneer of quantum physics. His 1913 atomic model, which is still widely invoked today, postulated a small nucleus surrounded by electrons moving in well-defined orbits and shells. The word "quantum" in quantum mechanics derives from the fact that these orbits can have only certain well-defined energies. Jumps between these orbits lead to absorption or emission of specific amounts of energy termed quanta. As an electron gains energy, its quantum number increases, and it jumps to higher orbits that circle ever farther from the nucleus.

In the new experiments, Rice graduate students Brendan Wyker and Shuzhen Ye began by using an ultraviolet laser to create a Rydberg atom. Rydberg atoms contain a highly excited electron with a very large quantum number. In the Rice experiments, potassium atoms with quantum numbers between 300 and 600 were studied.

"In such excited states, the potassium atoms become hundreds of thousands of times larger than normal and approach the size of a period at the end of a sentence," Dunning said. "Thus, they are good candidates to test Bohr's prediction."

He said comparing the classical and quantum descriptions of the electron orbits is complicated, in part because electrons exist as both particles and waves. To "locate" an electron, physicists calculate the likelihood of finding the electron at different locations at a given time. These predictions are combined to create a "wave function" that describes all the places where the electron might be found. Normally, an electron's wave function looks like a diffuse cloud that surrounds the atomic nucleus, because the electron might be found on any side of the nucleus at a given time.

Dunning and co-workers previously used a tailored sequence of electric field pulses to collapse the wave function of an electron in a Rydberg atom; this limited where it might be found to a localized, comma-shaped area called a "wave packet." This localized wave packet orbited the nucleus of the atom much like a planet orbits the sun. But the effect lasted only for a brief period.

"We wanted to see if we could develop a way to use radio frequency waves to capture this localized electron and make it orbit the nucleus indefinitely without spreading out," Ye said.

They succeeded by applying a radio frequency field that rotated around the nucleus itself. This field ensnared the localized electron and forced it to rotate in lockstep around the nucleus.

A further electric field pulse was used to measure the final result by taking a snapshot of the wave packet and destroying the delicate Rydberg atom in the process. After the experiment had been run tens of thousands of times, all the snapshots were combined to show that Bohr's prediction was correct: The classical and quantum descriptions of the orbiting electron wave packets matched. In fact, the classical description of the wave packet trapped by the rotating field parallels the classical physics that explains the behavior of Jupiter's Trojan asteroids.

Jupiter's 4,000-plus Trojan asteroids -- so called because each is named for a hero of the Trojan wars -- have the same orbit as Jupiter and are contained in comma-shaped clouds that look remarkably similar to the localized wave packets created in the Rice experiments. And just as the wave packet in the atom is trapped by the combined electric field from the nucleus and the rotating wave, the Trojans are trapped by the combined gravitational field of the sun and orbiting Jupiter.

The researchers are now working on their next experiment: They're attempting to localize two electrons and have them orbit the nucleus like two planets in different orbits.

"The level of control that we're able to achieve in these atoms would have been unthinkable just a few years ago and has potential applications in, for example, quantum computing and in controlling chemical reactions using ultrafast lasers," Dunning said.

The research was funded by the National Science Foundation, the Robert A. Welch Foundation, the Austrian Science Fund and the Department of Energy. Paper co-authors include S. Yoshida of the Vienna University of Technology; C.O. Reinhold of Oak Ridge National Laboratory and the University of Tennessee; and J. Burgdörfer of Vienna University of Technology and the University of Tennessee.

Story Source:

The above story is reprinted from materials provided by Rice University.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

B. Wyker, S. Ye, F. Dunning, S. Yoshida, C. Reinhold, J. Burgdörfer. Creating and Transporting Trojan Wave Packets. Physical Review Letters, 2012; 108 (4) DOI: 10.1103/PhysRevLett.108.043001

Relay race with single atoms: New ways of manipulating matter

 Thanks to a collaboration between scientists in San Sebastian and Japan, a relay reaction of hydrogen atoms at a single-molecule level has been observed in real-space. This way of manipulating matter could open up new ways to exchange information between novel molecular devices in future electronics. Dr. Thomas Frederiksen, presently working in the Donostia International Physics Center (DIPC) is one of the scientists that has participated in this research project. The results have been published in the journal Nature Materials.

An athletic relay race is a competition where each member of a team sprints a short distance with the baton before passing it onwards to the next team member. This collective way of transporting something rapidly along a well-defined track is not only a human activity and invention -- a similar relay mechanism, often refered to as structural diffusion, exists at the atomic scale that facilitate transport of hydrogen atoms and protons in hydrogen bonded networks, such as liquid water, biological systems, functional compounds, etc. However, direct visualization of this important transfer process in these situations is extremely difficult because of the highly complex environments.

Scientists in San Sebastian and Japan discovered that the relay reaction also occurs in well-defined molecular chains assembled on a metal surface. This new setup allowed the researchers to gain insight into the relay reactions at the level of single atoms and visualize the process using a scanning tunneling microscope (STM). By sending a pulse of electrons through a water molecule at one end of the chain, hydrogen atoms propagate one by one along the chain like dominoes in motion.

The result is that a hydrogen atom has been transferred from one end to the other via the relay mechanism.  The demonstrated control of the atom transfer along these molecular chains not only sheds new insight on a fundamental problem. It could also open up new ways to exchange information between novel molecular devices in future electronics by passing around hydrogen atoms.

Story Source:

The above story is reprinted from materials provided by Elhuyar Fundazioa, via AlphaGalileo.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

T. Kumagai, A. Shiotari, H. Okuyama, S. Hatta, T. Aruga, I. Hamada, T. Frederiksen, H. Ueba. H-atom relay reactions in real space. Nature Materials, 2011; DOI: 10.1038/NMAT3176

Note: If no author is given, the source is cited instead.

Researchers suspend, image single DNA molecules

Researchers in the lab of Harold Craighead, the Charles W. Lake Professor of Engineering, used advanced nanofabrication techniques to make it easier to see how single molecules of DNA subtly change during a chemical process called methylation. A better understanding of this process could lead to further study into genetic machinery of numerous diseases, including Alzheimer's, Parkinson's, diabetes and cancer.

This work was published online Oct. 7 in the journal Analytical Chemistry.

DNA is normally packed tightly into the nucleus of a cell and housed into chromosomes. Within chromosomes lie many chemical perturbations that modify how genes are expressed, without anything to do with the underlying DNA sequence, explained Aline Cerf, a postdoctoral associate who led the study.

Studying these chemical modifications is a relatively new field called epigenetics. The researchers focused on a particular process in which a section of the DNA, called 5-cytosine, becomes methylated -- its chemical structure changes by the addition of a methyl group (CH3).

Cerf and colleagues first took genetic material extracted from cells and suspended it in solution. Using a technique called soft lithography, they made a stamp, consisting of micrometer-sized wells, out of polydimethylsiloxane (PDMS). The solution of coiled DNA molecules was deposited onto the PDMS stamp, which was then moved at a controlled speed across a glass surface. The molecules became trapped by capillary forces in the wells, and stretched.

The result was an ordered array of elongated molecules that were transferred by contact onto a support to be imaged and studied.

In related work, the researchers also published a paper in the journal Nano Letters, Sept. 16, which details how their molecular arrays could be transferred to and imaged on substrates of graphene -- single-layer sheets of carbon atoms. This let them get even better pictures of the molecules by allowing the use of transmission electron microscopy for imaging of the added tags and the underlying base sequence in the DNA.

"The DNA molecule arrays can be tagged, separated and lined up to quickly read off the genetic and epigenetic information," Craighead said.

Researchers have in the past studied these same molecules while still contained in their nuclei, he said, but this new technique simplifies that process.

"We're no longer dealing with bundles of molecules that are hindering the information of another one," Craighead said. "We are taking the equivalent of the DNA contained in one cell and spreading it out on a surface, so you can look at all of these things and search for and study things of interest that will no longer be lost in this disordered world we were in before. Aline's results have brought us out of that world."

Provided by Cornell University (news : web)

World's smallest electric motor made from a single molecule

 The smallest electrical motor on the planet, at least according to Guinness World Records, is 200 nanometers. Granted, that's a pretty small motor -- after all, a single strand of human hair is 60,000 nanometers wide -- but that tiny mark is about to be shattered in a big way.

Chemists at Tufts University's School of Arts and Sciences have developed the world's first single molecule electric motor, a development that may potentially create a new class of devices that could be used in applications ranging from medicine to engineering.

In research published online Sept. 4 in Nature Nanotechnology, the Tufts team reports an electric motor that measures a mere 1 nanometer across, groundbreaking work considering that the current world record is a 200 nanometer motor. A single strand of human hair is about 60,000 nanometers wide.

According to E. Charles H. Sykes, Ph.D., associate professor of chemistry at Tufts and senior author on the paper, the team plans to submit the Tufts-built electric motor to Guinness World Records.

"There has been significant progress in the construction of molecular motors powered by light and by chemical reactions, but this is the first time that electrically-driven molecular motors have been demonstrated, despite a few theoretical proposals," says Sykes. "We have been able to show that you can provide electricity to a single molecule and get it to do something that is not just random."

Sykes and his colleagues were able to control a molecular motor with electricity by using a state of the art, low-temperature scanning tunneling microscope (LT-STM), one of about only 100 in the United States. The LT-STM uses electrons instead of light to "see" molecules.

The team used the metal tip on the microscope to provide an electrical charge to a butyl methyl sulfide molecule that had been placed on a conductive copper surface. This sulfur-containing molecule had carbon and hydrogen atoms radiating off to form what looked like two arms, with four carbons on one side and one on the other. These carbon chains were free to rotate around the sulfur-copper bond.

The team determined that by controlling the temperature of the molecule they could directly impact the rotation of the molecule. Temperatures around 5 Kelvin (K), or about minus 450 degrees Fahrenheit (oF), proved to be the ideal to track the motor's motion. At this temperature, the Tufts researchers were able to track all of the rotations of the motor and analyze the data.

While there are foreseeable practical applications with this electric motor, breakthroughs would need to be made in the temperatures at which electric molecular motors operate. The motor spins much faster at higher temperatures, making it difficult to measure and control the rotation of the motor.

"Once we have a better grasp on the temperatures necessary to make these motors function, there could be real-world application in some sensing and medical devices which involve tiny pipes. Friction of the fluid against the pipe walls increases at these small scales, and covering the wall with motors could help drive fluids along," said Sykes. "Coupling molecular motion with electrical signals could also create miniature gears in nanoscale electrical circuits; these gears could be used in miniature delay lines, which are used in devices like cell phones."

The Changing Face of Chemistry

Students from the high school to the doctoral level played an integral role in the complex task of collecting and analyzing the movement of the tiny molecular motors.

"Involvement in this type of research can be an enlightening, and in some cases life changing, experience for students," said Sykes. "If we can get people interested in the sciences earlier, through projects like this, there is a greater chance we can impact the career they choose later in life."

As proof that gaining a scientific footing early can matter, one of the high school students involved in the research, Nikolai Klebanov, went on to enroll at Tufts; he is now a sophomore majoring in chemical engineering.

This work was supported by the National Science Foundation, the Beckman Foundation and the Research Corporation for Scientific Advancement.

Tufts University, located on three Massachusetts campuses in Boston, Medford/Somerville, and Grafton, and in Talloires, France, is recognized among the premier research universities in the United States. Tufts enjoys a global reputation for academic excellence and for the preparation of students as leaders in a wide range of professions. A growing number of innovative teaching and research initiatives span all campuses, and collaboration among the faculty and students in the undergraduate, graduate and professional programs across the university is widely encouraged.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Tufts University.

Journal Reference:

Heather L. Tierney, Colin J. Murphy, April D. Jewell, Ashleigh E. Baber, Erin V. Iski, Harout Y. Khodaverdian, Allister F. McGuire, Nikolai Klebanov, E. Charles H. Sykes. Experimental demonstration of a single-molecule electric motor. Nature Nanotechnology, 2011; DOI: 10.1038/NNANO.2011.142

Squeezed light from single atoms

MPQ scientists have generated amplitude-squeezed light fields using single atoms trapped inside optical cavities.

In classical optics light is usually described as a wave, but at the most fundamental quantum level this wave consists of discrete particles called photons. Over the time, physicists developed many tools to manipulate both the wave-like and the particle-like quantum properties of the light. For instance, they created single photon sources with single atoms, using their ability to absorb and emit photons one by one. A team around Professor Gerhard Rempe, Director at the Max Planck Institute of Quantum Optics (Garching near Munich) and head of the Quantum Dynamics Division, has now observed that the light emitted by a single atom may exhibit much richer dynamics. Strongly interacting with light inside a cavity, the atom modifies the wave-like properties of the light field, reducing its amplitude or phase fluctuations below the level allowed for classical electromagnetic radiation. This is the very first observation of "squeezed" light produced by a single atom.

The "graininess" of the photons in a light wave causes small fluctuations of the wave's amplitude and phase. For classical beams, the minimal amount of amplitude and phase fluctuations is equal. However, by creating interactions between the photons, one can "squeeze" the fluctuations of the amplitude below this so-called "shot noise" level at the expense of increasing the fluctuations of the phase, and vice-versa. Unfortunately, the photonic interactions inside standard optical media are very weak, and require bright light beams to be observed. Single atoms are promising candidates to enable such interactions at a few-photon level. Their ability to generate squeezed light has been predicted 30 years ago, but the amount of light they emit is very tiny and so far all attempts to set this idea into realization have failed. In the Quantum Dynamics Division at MPQ sophisticated methods for cooling, isolating and manipulating single atoms have been developed over many years, and made this observation possible.

A single rubidium atom is trapped inside a cavity made of two very reflective mirrors in a distance of about a tenth of a millimetre from each other. When weak laser light is injected into this cavity, the atom can interact with one photon many times, and forms a kind of artificial molecule with the photons of the light field. As a consequence, two photons can enter the system at the same time and become correlated. "According to the model of Bohr, a single atom emits exactly one single energy quantum, i.e., one photon. That means that the number of photons is exactly known, but the phase of the light is not defined," Professor Gerhard Rempe explains. "But the two photons that are emitted by this strongly coupled atom are indistinguishable and oscillate together. Therefore this time the wave-like properties of the propagating light field are modified."

When the physicists use a laser beam which is resonant with the excitation frequency of the atom, the measurements show a suppression of the phase fluctuations. If the laser light is resonant with the cavity, they observe a squeezing of the amplitude instead.

The latter situation is illustrated in the image: The atom in the cavity turns a laser beam into light which has less amplitude and more phase fluctuations than the shot-noise limit. "Our experiment shows that the light emitted by single atoms is much more complex than in the simple view of Albert Einstein concerning photo-emission," Dr. Karim Murr emphasizes. "The squeezing that we observe is due to the coherent interaction between the two photons emitted from the system. Our measurement is in excellent agreement with the predictions of quantum electrodynamics in the strong-coupling regime." And Dr. Alexei Ourjoumtsev, who has been working on the experiment as a post doc, adds: "Usually single quantum objects are used to manipulate the particle-like properties of light. It is interesting to see that they can also modify its wave-like properties, and create observable squeezing with excitations beams containing only two photons on average."

So far squeezed light has only been generated with systems containing many atoms, such as crystals, using very high intensity beams, i.e. many photons. For the first time now physicists have succeeded in generating this kind of non-classical radiation with single atoms and extremely weak light fields. The ability of a single atom to induce strong coherent interactions between propagating photons opens up new perspectives for photonic quantum logic with single emitters.

Story Source:

The above story is reprinted (with editorial adaptations ) from materials provided by Max Planck Institute of Quantum Optics.

Journal Reference:

A. Ourjoumtsev, A. Kubanek, M. Koch, C. Sames, P. W. H. Pinkse, G. Rempe, K. Murr. Observation of squeezed light from one atom excited with two photons. Nature, 2011; 474 (7353): 623 DOI: 10.1038/nature10170

Single atom stores quantum information

 A data memory can hardly be any smaller: researchers working with Gerhard Rempe at the Max Planck Institute of Quantum Optics in Garching have stored quantum information in a single atom. The researchers wrote the quantum state of single photons, i.e. particles of light, into a rubidium atom and read it out again after a certain storage time. This technique can be used in principle to design powerful quantum computers and to network them with each other across large distances.

Quantum computers will one day be able to cope with computational tasks in no time where current computers would take years. They will take their enormous computing power from their ability to simultaneously process the diverse pieces of information which are stored in the quantum state of microscopic physical systems, such as single atoms and photons. In order to be able to operate, the quantum computers must exchange these pieces of information between their individual components. Photons are particularly suitable for this, as no matter needs to be transported with them. Particles of matter however will be used for the information storage and processing. Researchers are therefore looking for methods whereby quantum information can be exchanged between photons and matter. Although this has already been done with ensembles of many thousands of atoms, physicists at the Max Planck Institute of Quantum Optics in Garching have now proved that quantum information can also be exchanged between single atoms and photons in a controlled way.

Using a single atom as a storage unit has several advantages -- the extreme miniaturization being only one, says Holger Specht from the Garching-based Max Planck Institute, who was involved in the experiment. The stored information can be processed by direct manipulation on the atom, which is important for the execution of logical operations in a quantum computer. "In addition, it offers the chance to check whether the quantum information stored in the photon has been successfully written into the atom without destroying the quantum state," says Specht. It is thus possible to ascertain at an early stage that a computing process must be repeated because of a storage error.

The fact that no one had succeeded until very recently in exchanging quantum information between photons and single atoms was because the interaction between the particles of light and the atoms is very weak. Atom and photon do not take much notice of each other, as it were, like two party guests who hardly talk to each other, and can therefore exchange only a little information. The researchers in Garching have enhanced the interaction with a trick. They placed a rubidium atom between the mirrors of an optical resonator, and then used very weak laser pulses to introduce single photons into the resonator. The mirrors of the resonator reflected the photons to and fro several times, which strongly enhanced the interaction between photons and atom. Figuratively speaking, the party guests thus meet more often and the chance that they talk to each other increases.

The photons carried the quantum information in the form of their polarization. This can be left-handed (the direction of rotation of the electric field is anti-clockwise) or right-handed (clock-wise). The quantum state of the photon can contain both polarizations simultaneously as a so-called superposition state. In the interaction with the photon the rubidium atom is usually excited and then loses the excitation again by means of the probabilistic emission of a further photon. The Garching-based researchers did not want this to happen. On the contrary, the absorption of the photon was to bring the rubidium atom into a definite, stable quantum state. The researchers achieved this with the aid of a further laser beam, the so-called control laser, which they directed onto the rubidium atom at the same time as it interacted with the photon.

The spin orientation of the atom contributes decisively to the stable quantum state generated by control laser and photon. Spin gives the atom a magnetic moment. The stable quantum state, which the researchers use for the storage, is thus determined by the orientation of the magnetic moment. The state is characterized by the fact that it reflects the photon's polarization state: the direction of the magnetic moment corresponds to the rotational direction of the photon's polarization, a mixture of both rotational directions being stored by a corresponding mixture of the magnetic moments.

This state is read out by the reverse process: irradiating the rubidium atom with the control laser again causes it to re-emit the photon which was originally incident. In the vast majority of cases, the quantum information in the read-out photon agrees with the information originally stored, as the physicists in Garching discovered. The quantity that describes this relationship, the so-called fidelity, was more than 90 percent. This is significantly higher than the 67 percent fidelity that can be achieved with classical methods, i.e. those not based on quantum effects. The method developed in Garching is therefore a real quantum memory.

The physicists measured the storage time, i.e. the time the quantum information in the rubidium can be retained, as around 180 microseconds. "This is comparable with the storage times of all previous quantum memories based on ensembles of atoms," says Stephan Ritter, another researcher involved in the experiment. Nevertheless, a significantly longer storage time is necessary for the method to be used in a quantum computer or a quantum network. There is also a further quality characteristic of the single-atom quantum memory from Garching which could be improved: the so-called efficiency. It is a measure of how many of the irradiated photons are stored and then read out again. This was just under 10 percent.

The storage time is mainly limited by magnetic field fluctuations from the laboratory surroundings, says Ritter. "It can therefore be increased by storing the quantum information in quantum states of the atoms which are insensitive to magnetic fields." The efficiency is limited by the fact that the atom does not sit still in the centre of the resonator, but moves. This causes the strength of the interaction between atom and photon to decrease. The researchers can thus also improve the efficiency: by greater cooling of the atom, i.e. by further reducing its kinetic energy.

The researchers at the Max Planck Institute in Garching now want to work on these two improvements. "If this is successful, the prospects for the single-atom quantum memory would be excellent," says Stephan Ritter. The interface between light and individual atoms would make it possible to network more atoms in a quantum computer with each other than would be possible without such an interface; a fact that would make such a computer more powerful. Moreover, the exchange of photons would make it possible to quantum mechanically entangle atoms across large distances. The entanglement is a kind of quantum mechanical link between particles which is necessary to transport quantum information across large distances. The technique now being developed at the Max Planck Institute of Quantum Optics could some day thus become an essential component of a future "quantum Internet."

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Max-Planck-Gesellschaft.

Journal Reference:

Holger P. Specht, Christian Nölleke, Andreas Reiserer, Manuel Uphoff, Eden Figueroa, Stephan Ritter, Gerhard Rempe. A single-atom quantum memory. Nature, 2011; DOI: 10.1038/nature09997

Single molecule controlled at room temperature: Tiny magnetic switch discovered

 A Kiel research group headed by the chemist, Professor Rainer Herges, has succeeded for the first time in directly controlling the magnetic state of a single molecule at room temperature. The paper was recently published in the journal Science. The switchable molecule, which is the result of a sub-project of the Collaborative Research Centre 677 "Function by Switching," could be used both in the construction of tiny electromagnetic storage units and in the medical imaging.

The scientists at the Kiel University developed a molecular machine constructed in a similar way to a record player. The molecule consists of a nickel ion surrounded by a pigment ring (porphyrin), and a nitrogen atom which hovers above the ring like the tone arm on a record player. "When we irradiate this molecule with blue-green light, the nitrogen atom is placed exactly vertically to the nickel ion like a needle," Rainer Herges explains. "This causes the nickel ion to become magnetic, because the pairing of two electrons is cancelled out," says the chemistry professor. The counter effect is blue-violet light: The nitrogen atom is raised, the electrons form a pair and the nickel ion is no longer magnetic. "We can repeat this switching of the magnetic state over 10,000 times by varied irradiation with the two different wavelengths of light, without wearing out the molecular machine or encountering side reactions," Herges enthuses.

The switch which has been discovered, with its diameter of only 1.2 nanometres, could be used as a tiny magnetic reservoir in molecular electronics. Most of all, hard disk manufacturers may be interested in this, as a higher storage capacity can be achieved by reducing the size of the magnetic particles on the surface of the disks. Professor Herges also believes the use of the magnetic switch in the medical field is feasible: "The record player molecule can be used intravenously as a contrast agent in MRT (magnetic resonance tomography) in order to search for tumours or constricted blood vessels. Initial tests in the University Medical Center Schleswig-Holstein's neuroradiology department were successful."

As the signal-to-noise ratio is improved by the switching process, a smaller amount of the contrast agent is required than for the magnetic salts currently being used. In addition, according to Herges, the molecular machine could also serve as a basis for developing new contrast agents to depict such features as temperature, pH value or even certain biochemical markers in the body in a three-dimensional form. Rainer Herges lists the possible fields of application: "Using contrast agents such as these, it could be possible to localise centres of inflammation, detect tumours and visualise many metabolic processes."

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Christian-Albrechts-Universitaet zu Kiel, via AlphaGalileo.

Journal Reference:

S. Venkataramani, U. Jana, M. Dommaschk, F. D. Sonnichsen, F. Tuczek, R. Herges. Magnetic Bistability of Molecules in Homogeneous Solution at Room Temperature. Science, 2011; 331 (6016): 445 DOI: 10.1126/science.1201180

Note: If no author is given, the source is cited instead.

Disclaimer: Views expressed in this article do not necessarily reflect those of ScienceDaily or its staff.