An efficient optomechanical system utilizing ‘squeezed’ states of light has been designed by a RIKEN-led research team ("Squeezed Optomechanics with Phase-Matched Amplification and Dissipation"). The scheme makes it possible to implement quantum processes based on single light particles, or photons, using currently available optomechanical technology, with potential applications in quantum computing and the study of exotic quantum effects. 
A vibrating mirror interacts with light inside a mirror-confined cavity. Adding a nonlinear optical crystal to the cavity creates a squeezed state of light that is coupled to the cavity modes and induces strong optomechanical coupling with the vibrating mirror with single-photon control. Interactions involving single photons display many quantum physical phenomena. One of the more interesting effects is the coupling between photons and mechanical systems. Light carries a very small quantity of momentum, which can become noticeable when light interacts with micro- and nanoscale objects. Researchers studying these optomechanical effects utilize the quantum physical effects arising from the interaction of tiny mirrors with light trapped in mirror-confined cavities. In such cavities, light bounces resonantly back and forth, which intensifies the normally weak interactions between light and the mechanical vibration of the mirrors. “Yet due to the intrinsically weak coupling between photons and the mechanical vibrations in current systems,” explains head researcher Franco Nori, “many of the expected effects have not yet been observed experimentally.” The solution proposed by Nori and his colleagues from the RIKEN Center for Emergent Matter Science and RIKEN Interdisciplinary Theoretical Science Research Group, in collaboration with the Huazhong University of Science and Technology in China, involves the use of ‘squeezed’ states of light to strengthen optomechanical coupling. Squeezed states are those in which the energy and momentum of a photon are known with the maximum accuracy possible within the limits imposed by quantum physics. The researchers have demonstrated theoretically that such states can be generated by placing a nonlinear optical crystal into the mirror cavity and applying an external amplification scheme (see image above). In this proposed system, there is a direct link between the number of photons remaining in the original light state of the cavity and the number in the squeezed state—the presence of fewer photons in the squeezed state means that more photons remain in the cavity mode state to interact with the mirror vibrations. Tuning the photon numbers in the squeezed state to the single-photon limit induces a strong coupling to the mechanical vibrations and provides control over the strength of the optomechanical interaction. This coupling, says Nori, is expected to open the door to studying complex nonlinear quantum effects. “Our scheme offers an alternative approach for controlling optomechanical systems by using a squeezed cavity mode, and is implementable using currently available optomechanical technology.”
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A vibrating mirror interacts with light inside a mirror-confined cavity. Adding a nonlinear optical crystal to the cavity creates a squeezed state of light that is coupled to the cavity modes and induces strong optomechanical coupling with the vibrating mirror with single-photon control. Interactions involving single photons display many quantum physical phenomena. One of the more interesting effects is the coupling between photons and mechanical systems. Light carries a very small quantity of momentum, which can become noticeable when light interacts with micro- and nanoscale objects. Researchers studying these optomechanical effects utilize the quantum physical effects arising from the interaction of tiny mirrors with light trapped in mirror-confined cavities. In such cavities, light bounces resonantly back and forth, which intensifies the normally weak interactions between light and the mechanical vibration of the mirrors. “Yet due to the intrinsically weak coupling between photons and the mechanical vibrations in current systems,” explains head researcher Franco Nori, “many of the expected effects have not yet been observed experimentally.” The solution proposed by Nori and his colleagues from the RIKEN Center for Emergent Matter Science and RIKEN Interdisciplinary Theoretical Science Research Group, in collaboration with the Huazhong University of Science and Technology in China, involves the use of ‘squeezed’ states of light to strengthen optomechanical coupling. Squeezed states are those in which the energy and momentum of a photon are known with the maximum accuracy possible within the limits imposed by quantum physics. The researchers have demonstrated theoretically that such states can be generated by placing a nonlinear optical crystal into the mirror cavity and applying an external amplification scheme (see image above). In this proposed system, there is a direct link between the number of photons remaining in the original light state of the cavity and the number in the squeezed state—the presence of fewer photons in the squeezed state means that more photons remain in the cavity mode state to interact with the mirror vibrations. Tuning the photon numbers in the squeezed state to the single-photon limit induces a strong coupling to the mechanical vibrations and provides control over the strength of the optomechanical interaction. This coupling, says Nori, is expected to open the door to studying complex nonlinear quantum effects. “Our scheme offers an alternative approach for controlling optomechanical systems by using a squeezed cavity mode, and is implementable using currently available optomechanical technology.”
Studying atomic scale friction, teams from Penn and UC Merced helped slow experiments and fast simulations meet in the middle. (Image: University of Pennsylvania) A phenomenon known as "stick-slip friction" is very often involved in sliding at both the macro and the atomic scales. The resistance associated with friction is the product of atomic points of contact between two objects being temporarily stuck together, where they remain until the applied force provides enough elastic energy for those points to break apart. These points then slip and slide until they get stuck again. At the atomic scale, sticking points occur for every repeating set of atoms along the sliding direction. Studying the atomic interactions that underlie stick-slip friction is inherently difficult as the points of contact are obscured by being flush against one another. To get around this problem, friction researchers often use the tip of an atomic force microscope, or AFM, an ultra-sensitive instrument capable of measuring nanonewton forces, as one of the points of contact. Since an AFM tip works much like a record needle, researchers can measure the friction the tip experiences while it is dragged over surface. Friction researchers also use simulations, which can model the dynamics of all of the individual atoms. 
Instead of moving the AFM tip, Penn researchers moved the sample being scanned, maintaining high resolution at faster speeds. "A powerful approach is to combine experiment with simulations," Liu said, "But the major problem doing this in the past has been that the sliding speeds at which the experiments and the simulations are performed don't match up." The quality of the measurements in an AFM experiment depends on isolating the tip from any stray vibrations, so traditionally researchers drag the tip very slowly, moving about one micrometer in a second at the fastest. To match this experiment in a simulation, the individual atoms of the tip and the surface are modeled on a computer, and the virtual tip is dragged the same distance as the real AFM tip. This presents a problem, however, because, to capture the impact individual atoms have, each frame in the simulations must be calculated in femtosecond steps. A computer processing a million steps a second would need about 30 years to simulate the real AFM experiment's micrometer-per-second speed. "That means to get the same distance in a shorter period of time, we need to move the model tip much, much faster," said Martini. With the sliding speed of the virtual tips starting a million times faster than the physical ones, the researchers resolved to meet in the middle. The UC Merced contingent worked on slowing the tip in their simulations, while their counterparts at Penn developed ways to speed up their physical experiments. As traditional motors can't move AFM tips with the nanoscopic precision necessary for their experiments, the tip and the cantilever it is mounted on is driven by a piezoelectric plate. The top layer of this type of the plate shifts laterally away from the bottom layer when a certain voltage is applied, pushing the cantilever and tip across a sample surface. "For the resolution required for our atomic friction study, the scanner inside a commercial AFM can only reach a few hundred nanometers per second," Carpick said. "That's an intrinsic limitation of the instrument; if you go over that top speed, you get large oscillations in your signal. Our solution was to make a very compact shear piezo plate and use it to move the sample instead of the tip." By moving the sample, a thin film of gold coated on a silicon die, instead of the tip that is driven by a much heavier scanner, the Penn team was able radically increase the experiment's overall speed. With lower mass, the smaller plate can move faster without causing noisy oscillations. "The relative motion is the same," Liu said, "but this means we can go a thousand times faster than before while maintaining the resolution we need. We had to add entirely new electronics for capturing the data as well since no one has had to record it so fast before." While the Penn team was speeding up their systems, the UC Merced team was slowing them down. The researchers there took advantage of the relatively long periods of inactivity where the tip was stuck, waiting for enough energy to slip forward. Some of this energy is provided by the relative motion of the sample against the tip, but the random vibrations of the atoms involved, resulting from thermal energy, can make the slipping transition occur faster or slower. "Recognizing that," Martini said, "gives us the ability to use a suite of simulation tools for what are called 'infrequent event systems.' These are tools for making these infrequent events happen more quickly while still preserving the underlying physics." Using a technique known as "parallel replica dynamics," Martini's group used the fact that the probability of one of these infrequent events occurring is the same whether one simulation was run for a thousand femtoseconds or a thousand simulations were run for one femtosecond each. Running identical simulations on as many processors as possible, the researchers would stop them all as soon as one virtual tip slipped, then synchronize the simulations at that point and start them all again. "This allows us to effectively increase the duration of the simulation by parallelizing it in time," said Martini. "You're increasing the simulation time and therefore decreasing the model tip speed by a factor of how many processors you have." By matching the tip speeds in the physical and virtual experiments, the researchers were able to demonstrate a heretofore-theoretical difference between macroscale and atomic slip-stick friction. Velocity typically doesn't factor into the amount of friction macroscale objects encounter, but on the atomic scale the vibration of individual atoms due to thermal energy could play a role. The researchers showed that these vibrations do counteract friction by helping the tip slip forward but only to a point. At fast enough speeds, the tip is not stuck long enough to receive a "boost" from thermal energy. "Investigating and understanding the effect of friction at the speeds in our experiment are important," Liu said," as they're much closer to what our current and future engineering applications, such as micro- and nanomechanical devices, will experience than what we can normally do with an atomic force microscope." "This study," Carpick said, "now opens up many possibilities for using the full atomic insights available in atomistic simulations to reliably interpret the results of experimental studies. We're optimistic this will eventually lead to general and practical insights to understand, control and reduce friction and wear."
Authors of the Science paper: A. Knoll, E. Koren, C. Rawlings, E. Lörtscher (U. Duerig is missing). The paper details how, for the first time, IBM scientists can mechanically measure the tension and friction of two sliding sheets of graphite. Since this was previously poorly understood and just based on theory, the team also had to invent a mathematical expression to illustrate what they were seeing with their atomic force microscope (AFM), which turned out to be in excellent agreement with theoretical models. The answer the team discovered is that the friction is randomly determined in nature and directly based on the interaction between the proportionate lattices of the material, in this case graphite. As reported in the paper: The results suggest that the friction force originates from a genuine interaction between the rotationally misaligned graphite lattices at the sliding interface. This is remarkable because it is a well known empirical fact and also predicted theoretically that fractional scaling is an extremely fragile interface property which can only occur if the lattice interaction is not perturbed by defects or contaminations. 
SEM image showing several bearing devices pointing to different directions. Adhesion and friction are critical to understand with MEMS. At this scale, energy dissipation and wear have a huge influence on how the devices are designed, particularly with what materials they are designed with, due to the large surface to volume ratio. IBM scientists have been motivated by this challenge particularly around the use of carbon based materials like graphite, which are very promising for MEMS applications. A general phenomenon associated with 2D layered materials like graphite is the strong suppression of sliding friction and striation forces or superlubricity. Using an AFM, the team sheared the surface of the graphite and then took measurements which revealed the mechanisms of friction and adhesion. IBM scientists are interested in using superlubricity in the possible design of a new transistor, commonly referred to as the "next switch." The lack of friction could make a transistor that generates less heat and therefore uses less energy. The team also hopes that these findings will someday help other scientists design energy efficient MEMS devices.