New method of quantum entanglement vastly increases how much information can be carried in a photon

Led by UCLA researchers, research could have applications in finance, health care, government and military communications

A team of researchers led by UCLA electrical engineers has demonstrated a new way to harness light particles, or photons, that are connected to each other and act in unison no matter how far apart they are — a phenomenon known as quantum entanglement.

In previous studies, photons have typically been entangled by one dimension of their quantum properties — usually the direction of their polarization.

In the new study, researchers demonstrated that they could slice up and entangle each photon pair into multiple dimensions using quantum properties such as the photons’ energy and spin. This method, called hyperentanglement, allows each photon pair to carry much more data than was possible with previous methods.

Quantum entanglement could allow users to send data through a network and know immediately whether that data had made it to its destination without being intercepted or altered. With hyperentanglement, users could send much denser packets of information using the same networks.

The research, published today in Nature Photonics, was led by Zhenda Xie, a research scientist in the lab of Chee Wei Wong, a UCLA associate professor of electrical engineering who was the research project’s principal investigator. Researchers from MIT, Columbia University, the University of Maryland and the National Institute of Standards and Technology were also part of the team.

Albert Einstein famously described quantum entanglement as “spooky action at a distance” because it seems so improbable that what happens to one particle in an entangled pair also happens instantly to the other particle, even over great distances. The phenomenon exceeds the speed of light.

In the new study, researchers sent hyperentangled photons in a shape known as a biphoton frequency comb, essentially breaking up entangled photons into smaller parts.

In secure data transfer, photons sent over fiber optic networks can be encrypted through entanglement. With each dimension of entanglement, the amount of information carried on a photon pair is doubled, so a photon pair entangled by five dimensions can carry 32 times as much data as a pair entangled by only one. The result greatly extends from wavelength multiplexing, the method for carrying many videos over a single optical fiber.

“We show that an optical frequency comb can be generated at single photon level,” Xie said. “Essentially, we’re leveraging wavelength division multiplexing concepts at the quantum level.”

Potential applications for the research include secure communication and information processing, in particular for high-capacity data transfer with minimal error. This could be useful for medical servers, government data communications, financial markets and military communication channels, as well as quantum cloud communications and distributed quantum computing.

“We are fortunate to verify a decades-old theoretical prediction by Professor Jeff Shapiro of MIT, that quantum entanglement can be observed in a comb-like state,” Wong said. “With the help of state-of-the-art high-speed single photon detectors at NIST and support from Dr. Franco Wong, Dr. Xie was able to verify the high-dimensional and multi-degrees-of-freedom entanglement of photons. These observations demonstrate a new fundamentally secure approach for dense information processing and communications.”

Co-authors on the paper are Sajan Shrestha, XinAn Xu and Junlin Liang, prior students and postdoctoral scientists at Columbia with Wong; Tian Zhong, professors Jeffrey Shapiro and Franco N.C. Wong of MIT; Yan-Xiao Gong of Southeast University in Nanjing, China; and Joshua Bienfang and Alessandro Restelli, affiliated with both the University of Maryland and the NIST.

The work was funded by the Defense Advanced Research Projects Agency.

http://www.nanotechnologyworld.org/#!New-method-of-quantum-entanglement-vastly-increases-how-much-information-can-be-carried-in-a-photon/c89r/5592af200cf2585ebcda12a4 

Even steps to quantum computation

A rare class of quantum state that could be useful in information processes is observed in a two-dimensional oxide material system

 

Electrons are normally free to move through a solid in all three dimensions. Restricting their motion to a two-dimensional surface can, however, radically alter the properties of the material. A RIKEN-led team has now created a two-dimensional system that displays an exotic physical effect that could be useful for quantum computing.

Applying an electric potential between the two sides of a two-dimensional sheet of a semiconducting material under a magnetic field can cause charge carriers to flow sideways along the sheet. This is known as the Hall effect, and such materials display electrical resistance both in the direction of the applied voltage and perpendicular to it. The quantum Hall effect, a signature of two-dimensional systems, becomes evident when the magnetic field is increased and the perpendicular Hall resistance increases in discrete steps. 

Each of these steps corresponds to an electrical conductance equal to a fundamental constant multiplied by a fraction in which both the numerator and denominator are integers.

A team of researchers from the RIKEN Center for Emergent Matter Science, University of Tokyo, the Max Planck Institute for Solid State Research in Germany and other Japanese institutions has now observed the fractional quantum Hall effect in a two-dimensional system formed at the interface between zinc oxide and magnesium zinc oxide.

Fundamental to the team’s success in observing such an exotic quantum effect was the fabrication of high-quality material systems. The researchers created their ZnO-based structure using a method called molecular beam epitaxy, which is known for its ability to produce materials with high crystalline quality. They then attached eight electrical contacts to their sample and performed magnetoresistance measurements at ultralow temperatures.

The researchers observed a series of levels corresponding to fractional states, or filling factors, between 4/3 and 9/2. Most notably, even-denominator states were observed at 3/2 and 7/2, with some evidence for 9/2. Such a series has not been observed in any other material system.

These states are believed to arise because of the existence of quasiparticles made of pairs of electrons (Fig. 1). Such particle pairs are expected to be useful in quantum computers. “These quasiparticles are said to be topologically protected and are robust against weak perturbations,” says the study’s lead author Joseph Falson. “This is in contrast to quantum bits in say, silicon, which are very sensitive to slight changes in temperature or electric field. We now plan to probe the details of the states this work has unveiled.”

http://www.nanotechnologyworld.org/#!Even-steps-to-quantum-computation/c89r/558c7aec0cf2ef0f928ca7e5

Stanford researchers stretch a thin crystal to get better solar cells

Colorized image, enlarged 100,000 times, shows an ultrathin layer of molybdenum disulfide stretched over the peaks and valleys of part of an electronic device. Just 3 atoms thick, this semiconductor material is stretched in ways to enhance its electronic potential to catch solar energy.

Crystalline semiconductors like silicon can catch photons and convert their energy into electron flows. New research shows a little stretching could give one of silicon's lesser-known cousins its own place in the sun.

Nature loves crystals. Salt, snowflakes and quartz are three examples of crystals – materials characterized by the lattice-like arrangement of their atoms and molecules.

Industry loves crystals, too. Electronics are based on a special family of crystals known as semiconductors, most famously silicon.

To make semiconductors useful, engineers must tweak their crystalline lattice in subtle ways to start and stop the flow of electrons.

Semiconductor engineers must know precisely how much energy it takes to move electrons in a crystal lattice.

This energy measure is the band gap. Semiconductor materials like silicon, gallium arsenide and germanium each have a band gap unique to their crystalline lattice. This energy measure helps determine which material is best for which electronic task.

Now an interdisciplinary team at Stanford has made a semiconductor crystal with a variable band gap. Among other potential uses, this variable semiconductor could lead to solar cells that absorb more energy from the sun by being sensitive to a broader spectrum of light.

The material itself is not new. Molybdenum disulfide, or MoS2, is a rocky crystal, like quartz, that is refined for use as a catalyst and a lubricant.

But in Nature Communications, Stanford mechanical engineer Xiaolin Zheng and physicist Hari Manoharan proved that MoS2 has some useful and unique electronic properties that derive from how this crystal forms its lattice.

Molybdenum disulfide is what scientists call a monolayer: A molybdenum atom links to two sulfurs in a triangular lattice that repeats sideways like a sheet of paper. The rock found in nature consists of many such monolayers stacked like a ream of paper. Each MoS2 monolayer has semiconductor potential.

"From a mechanical engineering standpoint, monolayer MoS2 is fascinating because its lattice can be greatly stretched without breaking," Zheng said.

By stretching the lattice, the Stanford researchers were able to shift the atoms in the monolayer. Those shifts changed the energy required to move electrons. Stretching the monolayer made MoS2 something new to science and potentially useful in electronics: an artificial crystal with a variable band gap.

"With a single, atomically thin semiconductor material we can get a wide range of band gaps," Manoharan said. "We think this will have broad ramifications in sensing, solar power and other electronics."

Scientists have been fascinated with monolayers since the Nobel Prize-winning discovery of graphene, a lattice made from a single layer of carbon atoms laid flat like a sheet of paper.

In 2012, nuclear and materials scientists at MIT devised a theory that involved the semiconductor potential of monolayer MoS2. With any semiconductor, engineers must tweak its lattice in some way to switch electron flows on and off. With silicon, the tweak involves introducing slight chemical impurities into the lattice.

In their simulation, the MIT researchers tweaked MoS2 by stretching its lattice. Using virtual pins, they poked a monolayer to create nanoscopic funnels, stretching the lattice and, theoretically, altering MoS2's band gap.

Band gap measures how much energy it takes to move an electron. The simulation suggested the funnel would strain the lattice the most at the point of the pin, creating a variety of band gaps from the bottom to the top of the monolayer.

The MIT researchers theorized that the funnel would be a great solar energy collector, capturing more sunlight across a wide swath of energy frequencies.

When Stanford postdoctoral scholar Hong Li joined the mechanical engineering department in 2013, he brought this idea to Zheng. She led the Stanford team that ended up proving all of this by literally standing the MIT theory on its head.

Instead of poking down with imaginary pins, the Stanford team stretched the MoS2 lattice by thrusting up from below. They did this – for real rather than in simulation – by creating an artificial landscape of hills and valleys underneath the monolayer.

They created this artificial landscape on a silicon chip, a material they chose not for its electronic properties, but because engineers know how to sculpt it in exquisite detail. They etched hills and valleys onto the silicon. Then they bathed their nanoscape with an industrial fluid and laid a monolayer of MoS2 on top.

Evaporation did the rest, pulling the semiconductor lattice down into the valleys and stretching it over the hills.

Alex Contryman, a PhD student in applied physics in Manoharan's lab, used scanning tunneling microscopy to determine the positions of the atoms in this artificial crystal. He also measured the variable band gap that resulted from straining the lattice this way.

The MIT theorists and specialists from Rice University and Texas A&M University contributed to the Nature Communications paper.

Team members believe this experiment sets the stage for further innovation on artificial crystals.

"One of the most exciting things about our process is that is scalable," Zheng said. "From an industrial standpoint, MoS2 is cheap to make."

Added Manoharan: "It will be interesting to see where the community takes this."

http://www.nanotechnologyworld.org/#!Stanford-researchers-stretch-a-thin-crystal-to-get-better-solar-cells/c89r/558d6a480cf2f97c80ec59d0

What’s on the surface of a black hole?

Are black holes the ruthless killers we’ve made them out to be? Samir Mathur says no.

According to the professor of physics at The Ohio State University, the recently proposed idea that black holes have “firewalls” that destroy all they touch has a loophole.

In a paper posted online to the arXiv preprint server, Mathur takes issue with the firewall theory, and proves mathematically that black holes are not necessarily arbiters of doom.

In fact, he says the world could be captured by a black hole, and we wouldn’t even notice.

More than a decade ago, Mathur used the principles of string theory to show that black holes are actually tangled-up balls of cosmic strings. His “fuzzball theory” helped resolve certain contradictions in how physicists think of black holes.

But when a group of researchers recently tried to build on Mathur’s theory, they concluded that the surface of the fuzzball was actually a firewall.

According to the firewall theory, the surface of the fuzzball is deadly. In fact, the idea is called the firewall theory because it suggests that a very literal fiery death awaits anything that touches it.

Mathur and his team have been expanding on their fuzzball theory, too, and they’ve come to a completely different conclusion. They see black holes not as killers, but rather as benign copy machines of a sort.

They believe that when material touches the surface of a black hole, it becomes a hologram, a near-perfect copy of itself that continues to exist just as before.

“Near-perfect” is the point of contention. There is a hypothesis in physics called complementarity, which was first proposed by Stanford University physicist Leonard Susskind in 1993. Complementarity requires that any such hologram created by a black hole be a perfect copy of the original.

Mathematically, physicists on both sides of this new fuzzball-firewall debate have concluded that strict complementarity is not possible; that is to say, a perfect hologram can’t form on the surface of a black hole.

Mathur and his colleagues are comfortable with the idea, because they have since developed a modified model of complementarity, in which they assume that an imperfect hologram forms. That work was done with former Ohio State postdoctoral researcher David Turton, who is now at the Institute of Theoretical Physics at theCEA-Saclay research center in France.

Proponents of the firewall theory take an all-or-nothing approach to complementarity. Without perfection, they say, there can only be fiery death.

With his latest paper, Mathur counters that he and his colleagues have now proven mathematically that modified complementarity is possible.

It’s not that the firewall proponents made some kind of math error, he added. The two sides based their calculations on different assumptions, so they got different answers. One group rejects the idea of imperfection in this particular case, and the other does not.

Imperfection is a common topic in cosmology. Physicist Stephen Hawking has famously said that the universe was imperfect from the very first moments of its existence. Without an imperfect scattering of the material created in the Big Bang, gravity would not have been able to draw together the atoms that make up galaxies, stars, the planets—and us.

This new dispute about firewalls and fuzzballs hinges on whether physicists can accept that black holes are imperfect, just like the rest of the universe.

“There’s no such thing as a perfect black hole, because every black hole is different,” Mathur explained.

His comment refers to the resolution of the “ information paradox,” a long-running physics debate in which Hawking eventually conceded that the material that falls into a black hole isn’t destroyed, but rather becomes part of the black hole.

The black hole is permanently changed by the new addition. It’s as if, metaphorically speaking, a new gene sequence has been spliced into its DNA. That means every black hole is a unique product of the material that happens to come across it.

The information paradox was resolved in part due to Mathur’s development of the fuzzball theory in 2003. The idea, which he published in the journal Nuclear Physics B in 2004, was solidified through the work of other scientists including Oleg Lunin of University at Albany, Stefano Giusto of theUniversity of Padova, Iosif Bena of CEA-Saclay, and Nick Warner of the University of Southern California. Mathur’s co-authors included then-students Borun Chowdhury (now a postdoctoral researcher at Arizona State University), and Steven Avery (now a postdoctoral researcher at Brown University).

Their model was radical at the time, since it suggested that black holes had a defined—albeit “fuzzy”—surface. That means material doesn’t actually fall into black holes so much as it falls onto them.

The implications of the fuzzball-firewall issue are profound. One of the tenets of string theory is that our three-dimensional existence—four-dimensional if you count time—might actually be a hologram on a surface that exists in many more dimensions.

“If the surface of a black hole is a firewall, then the idea of the universe as a hologram has to be wrong,” Mathur said.

The very nature of the universe is at stake, but don’t expect rival physicists to come to blows about it.

“It’s not that kind of disagreement,” Mathur laughed. “It’s a simple question, really. Do you accept the idea of imperfection, or do you not?”

Source: http://www.nanotechnologyworld.org/#!What’s-on-the-surface-of-a-black-hole/c89r/5582da250cf23681a55f1ccb