Professor Howard Wiseman, Director of Griffith University's Centre for Quantum Dynamics. Credit: Griffith University

An experiment devised in Griffith University's Centre for Quantum Dynamics has for the first time demonstrated Albert Einstein's original conception of "spooky action at a distance" using a single particle.

In a paper published in the journal Nature Communications, CQD Director Professor Howard Wiseman and his experimental collaborators at the University of Tokyo report their use of homodyne measurements to show what Einstein did not believe to be real, namely the non-local collapse of a particle's wave function.

According to quantum mechanics, a single particle can be described by a wave function that spreads over arbitrarily large distances, but is never detected in two or more places.

This phenomenon is explained in quantum theory by what Einstein disparaged in 1927 as "spooky action at a distance", or the instantaneous non-local collapse of the wave function to wherever the particle is detected.

Almost 90 years later, by splitting a single photon between two laboratories, scientists have used homodyne detectors—which measure wave-like properties—to show the collapse of the wave function is a real effect.

This phenomenon is the strongest yet proof of the entanglement of a single particle, an unusual form of quantum entanglement that is being increasingly explored for quantum communication and computation.

"Einstein never accepted orthodox quantum mechanics and the original basis of his contention was this single-particle argument. This is why it is important to demonstrate non-local wave function collapse with a single particle," says Professor Wiseman.

"Einstein's view was that the detection of the particle only ever at one point could be much better explained by the hypothesis that the particle is only ever at one point, without invoking the instantaneous collapse of the wave function to nothing at all other points.

"However, rather than simply detecting the presence or absence of the particle, we used homodyne measurements enabling one party to make different measurements and the other, using quantum tomography, to test the effect of those choices."

"Through these different measurements, you see the wave function collapse in different ways, thus proving its existence and showing that Einstein was wrong."

This artist’s rendering, based on computer simulations, depicts a phonon heating solid material. Atoms of the material, shown in orange, are joined with flexible atomic bonds, shown as springs. The phonon imparts heat by colliding with the center atom, creating a vibration in the springs. The trail of the passing phonon is marked with increased magnetic field intensity, shown in green. The figure in the lower right shows the direction of the applied magnetic field. Credit: The Ohio State University

Researchers at The Ohio State University have discovered how to control heat with a magnetic field.

In the March 23 issue of the journal Nature Materials, they describe how a magnetic field roughly the size of a medical MRI reduced the amount of heat flowing through a semiconductor by 12 percent.

The study is the first ever to prove that acoustic phonons—the elemental particles that transmit both heat and sound—have magnetic properties.

"This adds a new dimension to our understanding of acoustic waves," said Joseph Heremans, Ohio Eminent Scholar in Nanotechnology and professor of mechanical engineering at Ohio State. "We've shown that we can steer heat magnetically. With a strong enough magnetic field, we should be able to steer sound waves, too."

People might be surprised enough to learn that heat and sound have anything to do with each other, much less that either can be controlled by magnets, Heremans acknowledged. But both are expressions of the same form of energy, quantum mechanically speaking. So any force that controls one should control the other.

"Essentially, heat is the vibration of atoms," he explained. "Heat is conducted through materials by vibrations. The hotter a material is, the faster the atoms vibrate.

"Sound is the vibration of atoms, too," he continued. "It's through vibrations that I talk to you, because my vocal chords compress the air and create vibrations that travel to you, and you pick them up in your ears as sound."

The name "phonon" sounds a lot like "photon." That's because researchers consider them to be cousins: Photons are particles of light, and phonons are particles of heat and sound. But researchers have studied photons intensely for a hundred years—ever since Einstein discovered the photoelectric effect. Phonons haven't received as much attention, and so not as much is known about them beyond their properties of heat and sound.

Researchers at The Ohio State University have discovered that heat can be controlled with a magnetic field. Here, study leader Joseph Heremans, Ohio Eminent Scholar in Nanotechnology, holds the material used in the experiment: a piece of indium antimonide semiconductor shaped into a lopsided tuning fork. The wider arm of the fork (left) measures 4 mm wide, and the narrower one (right) measures 1 mm. The researchers were able to slow the movement of heat through the wider arm of the fork using a magnetic field. Credit: Photo by Kevin Fitzsimons, courtesy of The Ohio State University.

This study shows that phonons have magnetic properties, too.

"We believe that these general properties are present in any solid," said Hyungyu Jin, Ohio State postdoctoral researcher and lead author of the study.

The implication: In materials such as glass, stone, plastic—materials that are not conventionally magnetic—heat can be controlled magnetically, if you have a powerful enough magnet. The effect would go unnoticed in metals, which transmit so much heat via electrons that any heat carried by phonons is negligible by comparison.

There won't be any practical applications of this discovery any time soon: 7-tesla magnets like the one used in the study don't exist outside of hospitals and laboratories, and the semiconductor had to be chilled to -450 degrees Fahrenheit (-268 degrees Celsius)—very close to absolute zero—to make the atoms in the material slow down enough for the phonons' movements to be detectible.

That's why the experiment was so difficult, Jin said. Taking a thermal measurement at such a low temperature was tricky. His solution was to take a piece of the semiconductor indium antimonide and shape it into a lopsided tuning fork. One arm of the fork was 4 mm wide and the other 1 mm wide. He planted heaters at the base of the arms.

The design worked because of a quirk in the behavior of the semiconductor at low temperatures. Normally, a material's ability to transfer heat would depend solely on the kind of atoms of which it is made. But at very low temperatures, such as the ones used in this experiment, another factor comes into play: the size of the sample being tested. Under those conditions, a larger sample can transfer heat faster than a smaller sample of the same material. That means that the larger arm of the tuning fork could transfer more heat than the smaller arm.

Heremans explained why.

"Imagine that the tuning fork is a track, and the phonons flowing up from the base are runners on the track. The runners who take the narrow side of the fork barely have enough room to squeeze through, and they keep bumping into the walls of the track, which slows them down. The runners who take the wider track can run faster, because they have lots of room.

"All of them end up passing through the material—the question is how fast," he continued. "The more collisions they undergo, the slower they go."

In the experiment, Jin measured the temperature change in both arms of the tuning fork and subtracted one from the other, both with and without a 7-tesla magnetic field turned on.

In the absence of the magnetic field, the larger arm on the tuning fork transferred more heat than the smaller arm, just as the researchers expected. But in the presence of the magnetic field, heat flow through the larger arm slowed down by 12 percent.

So what changed? Heremans said that the magnetic field caused some of the phonons passing through the material to vibrate out of sync so that they bumped into one another, an effect identified and quantified through computer simulations performed by Nikolas Antolin, Oscar Restrepo and Wolfgang Windl, all of Ohio State's Department of Materials Science and Engineering.

In the larger arm, the freedom of movement worked against the phonons—they experienced more collisions. More phonons were knocked off course, and fewer—12 percent fewer—passed through the material unscathed.

The phonons reacted to the magnetic field, so the particles must be sensitive to magnetism, the researchers concluded. Next, they plan to test whether they can deflect sound waves sideways with magnetic fields.

Fiber-shaped polymer light-emitting electrochemical cells with tunable colors are twisted together to generate colorful lights. Credit: Zhitao Zhang

A large team of researchers in China has developed a type of light emitting electrochemical cell (LEC) that can be woven into fabric material. As the team notes in their paper published in the journal Nature Photonics, their cells can be used to create wearable electronics. Henk Bolink and Enrique Ortí with the University of Valencia in Spain, offer a News & Views piece on the work done by the team in the same journal issue.

Ever since the development of OLEDs, researchers have been hot on the idea of using them to create wearable electronics, such as clothes that light up like an LED screen. But OLEDs proved too difficult to weave into fiber, which led researchers to LECs, which are essentially OLEDs with salt added to overcome some of the limitations of OLEDs. In this new effort the researchers in China have found a way to create LECs that are both strong enough and flexible enough to allow for weaving into textile fabrics.

To make the LECs, the researchers started with a tiny bare wire, which they coated with zinc oxide nanoparticles; that was followed by applying an electroluminescent polymer and than a transparent layer of carbon nanotubes—the result is a cell that is long, flexible and thin allowing for weaving into fabric. Currently, fabrics created with the cells emit just blue and yellow light (when subjected to just a few volts of electricity) but the team reports it will be a simple matter to add many more colors. The team also reports that the process for making the cells can be ramped up easily, which means the cells, and clothes with them, could be available for sale in the very near future.

There is still one down side, however, the light generated by the cells only persists for a few hours—after that they grow less and less bright. But that problem may be temporary as well, as other ongoing research with LECs suggests that much longer lasting cells may soon be made. If such wearable electronics do become available it could mark a rapid change in clothing, from body suits that show mood by color, to human billboards, to clothes that light up in artistic ways, sort of like glowing tattoos.

Prototype of the light-emitting fibre. Credit: Huisheng Peng

1 cm2 monolithic perovskite-silicon tandem solar cell. Credit: Rongrong Cheacharoen/Stanford University

Silicon solar cells dominate 90 percent of the global photovoltaic market today, yet the record power conversion efficiency of silicon photovoltaics has progressed merely from 25 percent to 25.6 percent during the past 15 years—meaning the industry is keen to explore alternatives.

A collaboration between the Massachusetts Institute of Technology (MIT) and Stanford University may be poised to shake things up in the solar energy world. By exploring ways to create solar cells using low-cost manufacturing methods, the team has developed a novel prototype device that combines perovskite with traditional silicon solar cells into a two-terminal "tandem" device.

As the team reports in the journal Applied Physics Letters, their new tandem cells have the potential to achieve significantly higher energy conversion efficiencies than standard single-junction silicon solar cells.

Perovskite is an inexpensive crystalline material that can easily be produced in labs and, as it turns out, stacking it atop a conventional silicon solar cell forms a tandem that has the potential to improve the cell's overall efficiency, a measure of the amount of sunlight the cell can convert into electricity.

The team focused on tandem solar cells because there was big room for improvement in their cost and market penetration. Tandem solar cells have only garnered a worldwide market share of 0.25 percent compared to silicon solar cells' 90 percent. "Despite having higher efficiency, tandems are traditionally made using expensive processes—making it difficult for them to compete economically," said Colin Bailie, a Ph.D. student at Stanford and an author on the new paper.

Designing low-cost perovskite-silicon tandem solar cells

The team's tandem approach focuses on keeping costs low and "integrates perovskite solar cells monolithically—building them sequentially in layers—onto a silicon solar cell, without significant optical or electrical losses, by using commonly available semiconductor materials and deposition methods," explained Jonathan P. Mailoa, a graduate student in MIT's Photovoltaic Research Laboratory and another co-author on the paper.

Before creating the tandems, the researchers first needed to design an interlayer to facilitate electronic charge carrier recombination without significant energy losses. "Fortunately, the physical concepts already exist for other types of multijunction solar cells, so we simply needed to find the best interlayer material combination for the perovskite-silicon pair," Mailoa said.

To form a connecting layer, known technically as the semiconductor "tunnel junction," between the two sub-cells, the team used degenerately doped p-type and n-type silicon, which facilitates the recombination of positive charge carriers (holes) from the silicon solar cell and negative charge carriers (electrons) from the perovskite solar cell.

Because the two silicon layers are highly doped, "the energy barrier between them is thin enough so that electrons and holes in the semiconductor easily pass through using quantum mechanical tunneling," Mailoa added.

While electrons from a perovskite solar cell won't normally enter this tunnel junction layer, a titanium-dioxide (TiO2) layer commonly used in perovskite solar cells works as an electron-selective contact for silicon. This allows electrons to flow from the perovskite solar cell through the TiO2 layer, eventually passing into the silicon tunnel junction, where they recombine with the holes from the silicon solar cell.

How do perovskite-silicon tandem solar cells work?

Once the tandem is set up, it relies on its multiple absorber layers to absorb different portions of the solar spectrum. "The perovskite absorbs all of the visible photons [higher in energy], for example, while the silicon absorbs the infrared photons [lower in energy]," Bailie said.

Splitting the solar spectrum allows these specialized absorbing layers to convert their range of the spectrum into electrical power much more efficiently than a single absorber can convert the entire solar spectrum on its own.

"This minimizes an undesirable process in solar cells called "thermalization," in which the energy of an absorbed photon is released as heat until it reaches the energy of the absorber's bandgap," Bailie explained. "Using a high-bandgap absorber on top of the low-bandgap absorber recovers some of this energy in the high-energy photons that would otherwise be 'thermalized' if absorbed in the low-bandgap absorber."

Another key part of the tandem's design is that it uses a serial connection, which means that the two solar cells are connected in a manner so that the same amount of current passes through each of the solar cells. In other words, the same amount of light is absorbed in each solar cell and their voltage is added together.

Efficiency evolution ahead

The team's tandem "demonstrated an open-circuit voltage of 1.65 V, which is essentially the sum of top and bottom cells, with very little voltage loss," said Tonio Buonassisi, an associate professor of mechanical engineering at MIT who led the research.

An open-circuit voltage of 1.65 V was the highest best-case scenario the team had predicted, which indicates that their tunnel junction performs very well.

But an efficiency evolution is on the horizon. The efficiency record for single-junction perovskite cells ranges from 16 to 20 percent, depending on the formula used. "By contrast, the perovskite in our tandem is based on a technology that achieves only 13 percent in our lab as a single-junction device," said Bailie. "Improving the quality of our perovskite layer will lead to better tandem devices."

Another area for improving the tandem's efficiency is by "reducing parasitic optical losses in other layers of the multijunction solar cell devices and predicting their efficiency potentials through simulation to determine whether or not this approach is truly cost effective," added Mailoa.

The team also plans "to make improvements to the silicon bottom cell," said Buonassisi. "The back contact isn't well passivated, so we lose power at longer wavelengths. But the photovoltaic industry has developed several solutions for this problem...we just need to incorporate one best practice into our next-generation devices."

While their work is still far from becoming commercially available, it frees other researchers to start to focus their efforts on important aspects of the multijunction device to help further improve both its stability and efficiencies in the future.

"This is the first step in the evolution of a technology that has the potential to disrupt the photovoltaic industry," noted Bailie.