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Could graphene turn light to electricity? Scientists have shown that graphene can convert a single photon into multiple electrons, showing much promise for future photovoltaic devices.

In the last decade or so, invisibility cloaks have captured the imagination of researchers and the public alike. The excitement is based on two advances.

The first is the idea of "transformation optics," or the ability to bend light around a region of space to make it look as if it weren't there. The second is the creation of metamaterials — synthetic substances with optical properties unknown in nature that can be designed to achieve this goal.

One of the goals in this area is to create a Harry Potter-style cloak capable of hiding a human at all optical frequencies in all directions. A bonus would be the ability to make this device as big or small as required so that it can hide objects of any size — even ones as large as orbiting satellites, according to U.S. researchers.

Achieving all these features in a single gadget is currently impossible. The first cloak worked only at a single microwave frequency. More recent cloaks operate over the entire a range of optical frequencies but can only hide tiny objects over a limited viewing angle.

In a paper published on June 5, John Howell at the University of Rochester in New York and Benjamin Howell show how to make simple cloaks that hide huge objects over the entire optical spectrum, albeit with a significant compromise. One of their devices is big enough to cloak a person.

Their approach is head-slappingly simple. Instead of using complex metamaterials to steer light, the Howells do the same job with conventional lenses and mirrors. They simply create an array of lenses or mirrors that steer light around the region of space they want to hide. That means their cloaks are simple to build and easy to scale. Mirrors, in particular, can be made almost any size.

The results are impressive. Their images clearly show how they can cloak objects the size of children's toys across the entire optical range.

And the cloak they have built using mirrors is big enough to hide a human. The image here shows how they've used it to cloak a chair with a trash can behind it. "This volume is sufficient to cloak a human, albeit with not as much convenience as Harry Potter's cloak," the Howells explain.

The mirror cloak is not actually a new design and has been used for years by magicians, a point the Howells readily acknowledge. "The point we wish to emphasize is not the novelty but the ease of scaling to nearly arbitrary size," they say.

These cloaks aren't perfect by any means. In fact there is one very significant caveat in their design — they only work in one direction. View these cloaks from anything other than this direction and the ruse is quickly revealed.

Nevertheless, there are important applications for large, unidirectional cloaks. "The devices may have value, for example, in cloaking satellites in mid- to high-Earth orbit," the Howells point out.

That's a simple idea with impressive results. And one that might end up being used sooner rather than later. It's not hard to think of organizations that might want to cloak satellites as they orbit the Earth. But if that happens, how will we ever know?

Marc Millis, former head of NASA's Breakthrough Propulsion Physics Project, explains how physicists approach the intriguing possibility of faster-than-light travel.

Petters' research is focused on the development of mathematical theory of gravitational lensing. He is the leading author of the book, /Singularity Theory and Gravitational Lensing/.

JQI researchers under the direction of Chris Monroe have produced quantum entanglement between a single atom's motion and its spin state thousands of times faster than previously reported, demonstrating unprecedented control of atomic motion.

This experiment focuses on using highly energetic laser pulses to perform qubit operations. Previously, they set a record for the fastest spin flip in these systems: a mere 50 picoseconds. Here they continue their work by blasting the ion so strongly that the qubit quickly becomes linked to its motion. Such speedy operations are more typically associated with solid state systems such as electrons in semiconductors or superconductors. Here the speed of operations combined with the pristine quantum environment of atoms provide the best of both worlds.

Though subdued, trapped ions are not entirely tame. Like caged tigers pacing back and forth, they exhibit regular motion, oscillating inside an electrostatic trap at a particular frequency. This motion is used as a medium for entangling multiple ions. The motion is also the enemy, introducing noise because logic operations and/or experiments are typically completed after many trap oscillation cycles have passed. In quantum computing, speed is critical because such noise can destroy the quantum-ness of the system.

Providing a solution, this team has achieved spin-motion entanglement on an ultrafast timescale of about 3 nanoseconds, nearly a thousand of times faster than the trap oscillation time. For reference, a beam of light travels only a foot in one nanosecond. Blinking your eye is about 100 million times slower than this entangling process.

To create this spin-dependent kick, laser pulses arrive from opposite directions, overlapping to form a diffraction grating. The ion qubit interacts with the light grating by absorbing and emitting a photon. Photons carry momentum, and so the absorption/emission cycle kicks the ion qubit. For a fixed color of light, the kick strength depends primarily on the laser power.

The light spectrum also contains a frequency that induces spin flips. Thus the ion qubit "feels" this kick based on its associated spin: opposite spin states are brushed in opposite directions. This is analogous when atoms are diffracted by an optical lattice; but here the resulting diffraction pattern depends on the qubit's spin state (up or down).

In the ion trap, the arrival of the first set of overlapping pulses creates many discrete momentum states, each tangled with an alternating spin.

Subsequent pulses serve to further accumulate momentum states. The goal is to apply the grating kicks in a way that builds up a particular diffraction pattern corresponding to just two momentum states, each entangled with an opposing spin. The process of building up amplitude in an interference pattern can be thought of in terms of the not-so-quantum trampoline. Say that my daughter is jumping at a rate of 0.5 bounce per second. Some of her friends can build up amplitude by simultaneously jumping at the same time, or "in phase." Some kids can also jump at a rate that is an integer (1X, 2X, 3X…) multiple of the other kids, which will create a kind of interference pattern of jumps. The kids will go the highest at the time when the most kids are simultaneously jumping. Similarly, if the team wants to accumulate more and more momentum, they need to apply the grating (kick) at a particular time with respect to the ion trap oscillations.

At the other extreme, the kids can all jump at random times and their bounces will cancel each other leading to reduced or even zero jumping amplitude over all. You could imagine that perfect cancellation could take place at special combinations of jumps as well. Likewise, the team can even gain back the original spin superposition, with the motion entirely removed. If they flash on the grating at the precise moment the motion packets overlap, this kick neatly disentangles the spin from the motion.

This exquisite control over the spin-motion entanglement by adjusting the delay between kicks is equivalent to an interferometer using pairs of spin dependent kicks. Interferometers have numerous applications, ranging from radio astronomy to cellular imaging. When the same type of ultrafast interferometer applied to multiple atoms, their internal spin states can become entangled, which is a fundamental building block of a quantum computer.

Penn physicists have recently shown that simply changing particle shape can eliminate the ring-shaped stain that is left behind when drops of certain liquids dry. In this video microscopy footage from their experiments, watch as spherical particles get swept to the edges, while oblong particles are distributed consistently.