Superfast Switching of Quantum Light Sources

Sep. 27, 2013 — Usually, an elementary light source -- such as an excited atom or molecule -- emits light of a particular color at an unpredictable instance in time. Recently, however, scientists from the MESA+ Institute for Nanotechnology of the UT, FOM and the Institute for Nanoscience and Cryogenics (CEA/INAC) in France have shown that a light source can be coaxed to emit light at a desired moment in time, within an ultrashort burst. The superfast switching of a light source has applications in fast stroboscopes without laser speckle, in the precise control of quantum systems and for ultrasecure communication using quantum cryptography.

Cartoon of the superfast emission of a light source. The light source is embedded in an optical resonator where it spontaneously emits a photon. During the emission of the photon the favored color of the resonator is quickly switched – symbolized by a hammer to match the color of the light source. During this short interval the light source is triggered to emit an ultrashort burst of photons within a desired moment in time. (Credit: Image courtesy of University of Twente)

The theoretical results were published in Optics Express.
Spontaneous emission of light from excited sources, such as atoms, molecules or quantum dots, is a fundamental process with many applications in modern technology, such as LEDs and lasers. As the term 'spontaneous emission' indicates, the emission is random in nature and it is therefore impossible to predict the exact emission time of a photon. However, for several applications it is desirable to receive single photons exactly when they are needed with as little uncertainty as possible. This property is crucial for ultra-secure communication using quantum cryptography and in quantum computers. Therefore, the important goal is to fabricate a quantum light source such that it emits a single photon exactly at a desired moment in time.
Switching light emission
The average emission time of quantum light sources can be reduced by locating them in various nanostructures, like optical resonators or waveguides. But the distribution of emission times is always exponential in time in a usual stationary environment. In addition, the smallest uncertainty in the emission time is limited by both the maximum intensity in the resonator and the variations in the preparation time of the emitter. The Dutch-French team proposes to overcome these limitations by quickly switching the resonator length, in which the light source is located. The time duration of the switch should be much shorter than the average emission time. The result is that the favored color of the resonator only matches the emission color of the light source within a short time interval. Only within this short time frame are the photons emitted by the light source into the resonator.
Ultrafast light source
The researchers propose to use quantum dot light sources, which can easily be integrated in semiconductor optical resonators with lengths on the order of microns. The switching of the resonator will be achieved by shining an ultrashort laser pulse at the micropillar resonator during the emission time of the quantum dots. This quickly changes the refractive in the resonator and thereby the effective resonator length. The switching time can be directly controlled by the arrival time of the short laser pulse and by the lifetime of the excited electrons. These controlled light switches have great prospects for creating incoherent ultrafast light sources for fast stroboscopes without laser speckle, in quantum cryptography, in quantum information and for studying ultrafast cavity Quantum electrodynamics.

 

Human Robot Getting Closer: iCub Robot Must Learn from Its Experiences

Sep. 27, 2013 — A robot that feels, sees and, in particular, thinks and learns like us. It still seems like science fiction, but if it's up to University of Twente (UT) researcher Frank van der Velde, it won't be. In his work he wants to implement the cognitive process of the human brain in robots. The research should lead to the arrival of the latest version of the iCub robot in Twente. This human robot (humanoid) blurs the boundaries between robot and human.

A robot that feels, sees and, in particular, thinks and learns like us. It still seems like science fiction, but if it's up to UT researcher Frank van der Velde, it won't be. In his work he wants to implement the cognitive process of the human brain in robots. The research should lead to the arrival of the latest version of the iCub robot in Twente. This human robot (humanoid) blurs the boundaries between robot and human. (Credit: Image courtesy of University of Twente)

Decades of scientific research into cognitive psychology and the brain have given us knowledge about language, memory, motor skills and perception. We can now use that knowledge in robots, but Frank van der Velde's research goes even further. "The application of cognition in technical systems should also mean that the robot learns from its experiences and the actions it performs. A simple example: a robot that spills too much when pouring a cup of coffee can then learn how it should be done."
Possible first iCub in the Netherlands
The arrival of the iCub robot at the University of Twente should signify the next step in this research. Van der Velde submitted an application together with other UT researchers Stefano Stramigioli, Vanessa Evers, Dirk Heylen and Richard van Wezel, all active in the robotics and cognitive research. At the moment, twenty European laboratories have an iCub, which was developed in Italy (thanks to a European FP7 grant for the IIT). The Netherlands is still missing from the list. Moreover, a newer version is currently being developed, with for example haptic sensors. In February it will be announced whether the robotics club will actually bring the latest iCub to the UT. The robot costs a quarter of a million Euros and NWO (Netherlands Organisation for Scientific Research) will reimburse 75% of the costs. Then the TNO (Netherlands Organisation for Applied Scientific Research) and the universities of Groningen, Nijmegen, Delft and Eindhoven can also make use of it. Within the UT, the iCub can be deployed in different laboratories thanks to a special transport system.

Robot guide dog
The possibilities are endless, according to Van der Velde. "The new iCub has a skin and fingers that have a much better sense of touch and can feel strength. That makes interaction with humans much more natural. We want to ensure that this robot continues to learn and understands how people function. This research ensures, for example, that robots actually gather knowledge by focusing on certain objects or persons. In areas of application like healthcare and nursing, such robots can play an important role. A good example would be that in ten years' time you see a blind person walking with a robot guide dog."
Nano-neural circuits
A recent line of research that is in line with this profile is the development of electronic circuits that resemble a web of neurons in the human brain. Contacts have already been made to start this research in Twente. In the iCub robot, this can for example be used for the robot's visual perception. This requires a lot of relatively simple operations that must all be performed in parallel. This takes a lot of time and energy in the current systems. With electronic circuits in the form of a web of nerve cells this is much easier.
"These connections are only possible at the nanoscale, that is to say the scale at which the material is only a few atoms thick. In combination with the iCub robot, it can be investigated how the experiences of the robot are recorded in such materials and how the robot is controlled by nano-neural circuitry. The bottleneck of the existing technical systems is often the energy consumption and the size. The limits of Moore's Law, the proposition that the number of transistors in a circuit doubles every two years through technological advances, are reached. In this area we are therefore also on the verge of many new applications."
 

Do Black Holes Have 'Hair'? New Hypothesis Challenges 'Clean' Model

Sep. 30, 2013 — A black hole. A simple and clear concept, at least according to the hypothesis by Roy Kerr, who in 1963 proposed a "clean" black hole model, which is the current theoretical paradigm. From theory to reality things may be quite different. According to a new research carried out by a group of scientists that includes Thomas Sotiriou, a physicist of the International School for Advanced Studies (SISSA) of Trieste, black holes may be much "dirtier" than what Kerr believed.

Artist's illustration of a black hole. (Credit: iStockphoto)

According to the traditional model, black holes are defined by only two quantities: mass and angular momentum (a black hole rotation velocity). Once their progenitor has collapsed (a high mass star, for instance, that at the end of its life cycle implodes inwards) its memory is lost forever. All that is left is a quiescent black hole, with almost no distinctive features: all black holes, mass and angular momentum aside, look almost the same.
According to Sotiriou, things may not have occurred this way. "Black holes, according to our calculations, may have hair," explains Sotiriou, referring to a well-known statement by physicist John Wheeler, who claimed that "black holes have no hair." Wheeler meant that mass and angular momentum are all one needs to describe them.
"Although Kerr's 'bald' model is consistent with General Relativity, it might not be consistent with some well-known extensions of Einstein's theory, called tensor-scalar theories," adds Sotiriou. "This is why we have carried out a series of new calculations that enabled us to focus on the matter that normally surrounds realistic black holes, those observed by astrophysicists. This matter forces the pure and simple black hole hypothesized by Kerr to develop a new 'charge' (the hair, as we call it) which anchors it to the surrounding matter, and probably to the entire Universe."
The experimental confirmation of this new hypothesis may come from the observations carried out with the interferometers, instruments capable of recording the gravitational waves. "According to our calculations, the growth of the black hole's hair," concludes Sotiriou "is accompanied by the emission of distinctive gravitational waves. In the future, the recordings by the instrument may challenge Kerr's model and broaden our knowledge of the origins of gravity."