Quantum correlations -- without entanglement

Few people doubt the "quantumness" of entanglement. Quantifying the quantum correlation of entanglement is something that is relatively regular right now. However, things change a bit when it comes to quantum correlations other than entanglement. However, there is a growing interest in the use of non-entanglement quantum correlations in a number of possible future applications.
“A few years ago, scientists proposed quantum discord as a quantum correlation measure that goes beyond the entanglement paradigm,” Roberto Serra tells PhysOrg.com. “Quantum discord may be present, even in separate, non-entangled states. However, some doubt was being shed on the quantum qualities of non-entangled states because of the difficulty in quantifying the correlations.”
In order to remedy the difficulty in “seeing” the correlations in a laboratory setting, Serra, a scientist at the Federal University of ABC in São Paulo, Brazil, worked with a group to create a technique that makes it possible to recognize nonclassical correlations in quantum discord. Serra worked with a team from different Brazilian institutions of higher learning, including the Brazilian Agricultural Research Corporation's and the Brazilian Center for Physics Research both in Rio de Janeiro, and the Physics Institute of São Carlos, in São Paulo State. The results of the work can be seen in Physical Review Letters: Experimentally Witnessing the Quantumness of Correlations.”
“Nuclear Magnetic Resonance systems at room temperature were used to test principles of quantum computation with a good level of success,” Serra explains. “The quantum nature of these demonstrations was questioned because there is no entanglement in such a system. In our experiment we reveled directly the quantum nature of this system at room temperature. We used a sample of chloroform molecules, since it’s the simplest two-qubit system. We folded a qubit in the carbon nucleus and another one in the hydrogen nucleus.”
Next the Brazilian scientists were able to manipulate the system. Even though they used hot quantum bits, the system actually works as a quantum mechanical one. “We displaced the system from the thermal equilibrium by a very tiny deviation, and the phase coherence present there could encode quantum correlations as the measured by the quantum discord,” Serra says.
“Our methods can be applied to another system, such as an optical system. This can enable us to say if a given system is purely classical in nature, or if it has truly quantum correlations,” he continues. Serra thinks that using this test, which is relatively simple to perform in a laboratory setting, could help lay to rest the debate over whether or not these other types of correlations are truly quantum.
“We test the quantumness of discord at room temperature, and this very robust quantumness can be used to get an advantage in quantum protocols,” Serra insists. He believes that this method can already be used for metrology. “We are involved now in a test of principles in quantum metrology using this type of system, and exploiting this very tiny nonclassical correlation. We are testing those right now, to see about advantages over classical protocols, and we hope to have new results in the next few months.”
“We hope to develop future applications, and advance our comprehension about the rule played by this kind of quantumness in tasks as, for example, quantum communications,” Serra continues. “We are building collaborations between theoretical and experimental researchers, and we hope that we can do more to show the usefulness of other quantum correlations beyond entanglement.”

13 year old researcher finds tree inspired solar collection more efficient

Aidan Dwyer, a 13 year old Junior High School student from New York state, noticed that the phyllotaxy of the leaves on trees he was observing while hiking through the Catskill Mountains, did so in the form of a Fibonacci sequence. Wondering if there was a reason for it, he deduced that it might be because such an arrangement provides the most efficient means of solar power collection for the trees. To find out if this was the case, he built a small solar tree from PVC pipe and small solar panels, then built another in a normal flat panel array, attached voltage readers to both, and lo and behold, discovered the tree model array was indeed more efficient, at least during times of low or indirect sunlight. Dwyer won a Young Naturist Award for his efforts after writing and submitting his essay, The Secret of the Fibonacci Sequence in Trees.
The Young Naturist Awards are given (by the American Museum of Natural History) to two students from each grade, K-12, every year for young scientists who have investigated questions they have in the areas of biology, Earth science, ecology, and astronomy. Dwyer’s entry, took the known, that tree leaves grow in a Fibonacci sequence (where each number is the sum of the previous two) and applied it in a novel way that advanced the study of solar energy.

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The spiral on trees showing the Fibonacci Sequence

With trees, The Fibonacci pattern shows up in the way that limbs spiral around the trunk, specifically in the fraction that arises when computing the number of limbs it takes to complete a run all the way around the tree. Dwyer gives the example of the Oak tree which takes five branches to spiral two times around the trunk, giving the fraction 2/5. This is important because it was the basis of the model he built to replicate the tree structure. Interestingly, to find this fraction, and those of other types of trees, he fashioned his own measuring device out of a clear plastic tube with circle protractors on it. Branch angles were measured by inserting them into the tube.

Next, he built a small model tree (mimicking the Oak’s Fibonacci series as closely as possible) out of various sizes of PVC pipe to which he affixed small solar panels. After that, he put together a traditional flat panel solar array comprised of the same size solar panels. Then, after hooking up both to a data logger connected to a voltage meter, he then let them sit.

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The two models collecting sunlight

After analyzing his data, he found that the tree design appeared to be far more efficient than the traditional flat-panel structure during so-called off peak times, such as when the sun was low, and that the model appeared to be close to 50% more efficient overall during the winter. Not bad for someone who’s still a kid.

Dwyer theorizes that tree branches have evolved such an arrangement because it’s likely the most efficient pattern available, i.e. the one that best takes into account the shading created by branches or leaves hiding one another from direct sunlight.

Quantum memory works at room temperature

Quantum memory works at room temperature

How to store a photon at room temperature


A quantum memory for photons that works at room temperature has been created by physicists in the UK. The breakthrough could help researchers to develop a quantum repeater device that allow quantum information to be transmitted over long distances.

Quantum bits (or qubits) of information can be transmitted using photons and put to use in a number of applications, including cryptography. These schemes rely on the fact that photons can travel relatively long distances without interacting with their environment. This means that photon qubits are able, for example, to remain in entangled states with other qubits – something that is crucial for many quantum-information schemes.

However, the quantum state of a photon will be gradually changed (or degraded) due to scattering as it travels hundreds of kilometres in a medium such as air or an optical fibre. As a result, researchers are keen on developing quantum repeaters, which take in the degraded signal, store it briefly, and then re-emit a fresh signal. This way, says Ian Walmsley of the University of Oxford, "you can build up entanglement over much longer distances".
Difficult to repair

A quantum memory, which stores and re-emits photons, is the critical component of a quantum repeater. Those made so far in laboratories must be maintained at extremely cold temperatures or under vacuum conditions. They also only tend to work over very narrow wavelength ranges of light and store the qubit for very short periods of time. Walmsley and his colleagues argue that it isn't feasible to use such finicky systems in intercontinental quantum communication – these links will need to cross oceans and other remote areas, where it's difficult to send a repair person to fix a broken cryogenic or vacuum system.

Moreover, they should also absorb a broad range of frequencies of light and store data for periods much longer than the length of a signal pulse. Walmsley calls this combination a "key enabling step for building big networks". The broad range of frequencies means the memory can handle larger volumes of data, while a long storage time makes it easier to accumulate multiple photons with desired quantum states.

Working towards this goal, Walmsley and his team made a cloud of caesium atoms into a quantum memory that operates at an easy-to-achieve temperature of about 62 °C. Unlike previous quantum memories, the photons stored and re-emitted do not have to be tuned to a frequency that caesium electrons would like to absorb. Instead, a pulse from an infrared control laser converts the photon into a "spin wave", encoding it in the spins of the caesium electrons and nuclei.
Paint it black

Walmsley compares the cloud of caesium atoms to a pane of glass – transparent, so it allows the light through. The first laser paints the glass black in a sense, allowing it to absorb all the light that reaches it. However, instead of becoming dissipating as heat and as it would in the darkened glass, the light that passed into the caesium cloud is stored in the spin wave.

Up to 4 µs later, a second laser pulse converts the spin wave back into a photon and makes the caesium transparent to light again. The researchers say that the caesium's 30% efficiency in absorbing and re-emitting photons could increase with more energetic pulses from the control laser, while the storage time could be improved with better shielding from stray magnetic fields, which disturb the spins in the caesium atoms.

Even at 30% efficiency, Ben Buchler of the Australian National University in Canberra calls the device "a big deal" because it absorbs a wide band of photon frequencies. Due to Heisenberg's uncertainty principle, the ultra-short single-photon pulses from today's sources don't have well defined energies, so an immediately useful quantum memory must be able to absorb a wide range of frequencies – which Buchler says high-efficiency memories can't yet do.
Noise not a problem

Background noise, or extra photons generated in the caesium clouds that are unrelated to the signal photons, was a major concern for room-temperature memories. "People thought that if you started using room-temperature gases in storage mode, you'd just have a lot of noise," says Walmsley.

Temperatures near absolute zero suppress these extra photons other memories. But because the control and signal pulses in the Oxford team's set-up are far from caesium's favoured frequencies, the cloud was less susceptible to photon-producing excitations and the noise level remained small even at room temperature.

Hugues de Riedmatten of the Institute of Photonic Sciences in Barcelona, Spain, says that the researchers showed that the remaining noise is fundamental to the system, not caused by their set-up. If improvements cannot further reduce the noise, it will be challenging to maintain the integrity of the signal across a large, complex network, he explains.

Nevertheless, he says, "This approach is potentially very interesting because it may lead to a quantum memory for single photon qubits at room temperature, which would be a great achievement for quantum-information science."

The work is described in a paper to be published in Physical Review Letters and a preprint on arXiv.

The 'quantum magnet': Physicists expand prospects for engineering unusual materials

The physicists prepared a chain of single-atom magnets (red spheres with black arrows indicating north-south orientation) that repel one another, and aligned them (back row) with an external field. By reducing the field, they were able to observe reorientation (front row) caused by the magnetic repulsion (yellow helix) and minute quantum fluctuations. The background shows an image of the individual magnets, each comprising a single atom, as observed in the experiment.

  Harvard physicists have expanded the possibilities for quantum engineering of novel materials such as high-temperature superconductors by coaxing ultracold atoms trapped in an optical lattice -- a light crystal -- to self-organize into a magnet, using only the minute disturbances resulting from quantum mechanics. The research, published in the journal Nature, is the first demonstration of such a “quantum magnet” in an optical lattice.

As modern technology depends more and more on materials with exotic quantum mechanical properties, researchers are coming up against a natural barrier.
“The problem is that what makes these materials useful often makes them extremely difficult to design,”said senior author Markus Greiner, an associate professor in Harvard’s Department of Physics. “They can become entangled, existing in multiple configurations at the same time. This hallmark of quantum mechanics is difficult for normal computers to represent, so we had to take another approach.”
That approach is using a so-called “quantum simulator” — the properties of a quantum material are simulated with an artificial quantum system that can behave similarly, but that is easier to manipulate and observe.
The physicists found that when they applied a force to a crystal formed by ultracold atoms trapped in an optical lattice, a Mott insulator, the atoms behaved like a chain of little magnets that repelled one another, in the presence of an external magnetic field that sought to align them.
“When the external magnetic field was strong, all of the magnets aligned to it, forming a paramagnet,” said co-author Jonathan Simon, a postdoctoral fellow in physics. “When we reduced the magnetic field, the magnets spontaneously anti-aligned to their neighbors, producing an antiferromagnet.”
While such self-organization is common in everyday materials, it typically depends on temperature to jostle the system into the new order, like shaking a Boggle game to help the dice settle, the researchers say. “But the temperature was so low that thermal fluctuations were absent,” explained Simon. “Our fluctuations arose from quantum mechanics.”
When quantum mechanics takes over, things get bizarre. “Quantum fluctuations can make the magnets point in multiple directions simultaneously,” Greiner said. “This ‘quantum weirdness’ gives rise to many of the fascinating properties of quantum magnets.”
Greiner and his colleagues used a quantum gas microscope to observe individual magnets at temperatures of one billionth of a degree above absolute zero (-273 Celsius). They were able to watch as quantum fluctuations flipped the magnets around, turning a paramagnet into an antiferromagnet and back again.
“Observing quantum magnetism in a cold gas is a crucial first step toward quantum simulation of real magnetic materials,” Greiner said. “There remain many exciting questions to answer, and we have only just scratched the surface. By studying the bizarre and wonderful ways that quantum mechanics works, we open new perspectives not only for developing novel high-tech materials, but also for quantum information processing and computation.”

A team of physicists experimentally produces quantum-superpositions, simply using a mirror.

Standing in front of a mirror, we can easily tell apart ourselves from our mirror image. The mirror does not affect our motion in any way. For quantum particles, this is much more complicated. In a spectacular experiment in the labs of the Heidelberg University, a group of physicists from Heidelberg Unversity, together with colleagues at TU Munich and TU Vienna extended a gedankenexperiment by Einstein and managed to blur the distinction between a particle and its mirror image. The results of this experiment have now been published in the journal Nature Physics.
Emitted Light, Recoiling Atom
When an atom emits light (i.e. a photon) into a particular direction, it recoils in the opposite direction. If the photon is measured, the motion of the atom is known too. The scientists placed atoms very closely to a mirror. In this case, there are two possible paths for any photon travelling to the observer: it could have been emitted directly into the direction of the observer, or it could have travelled into the opposite direction and then been reflected in the mirror. If there is no way of distinguishing between these two scenarios, the motion of the atom is not determined, the atom moves in a superposition of both paths.
“If the distance between the atom and the mirror is very small, it is physically impossible to distinguish between these two paths,” Jiri Tomkovic, PhD student at Heidelberg explains. The particle and its mirror image cannot be clearly separated any more. The atom moves towards the mirror and away from the mirror at the same time. This may sound paradoxical and it is certainly impossible in classical phyiscs for macroscopic objects, but in quantum physics, such superpositions are a well-known phenomenon. “This uncertainty about the state of the atom does not mean that the measurement lacks precision”, Jörg Schmiedmayer (TU Vienna) emphasizes. “It is a fundamental property of quantum physics: The particle is in both of the two possible states simultaneousely, it is in a superposition.” In the experiment the two motional states of the atom – one moving towards the mirror and the other moving away from the mirror – are then combined using Bragg diffraction from a grating made of laser light. Observing interference it can be directly shown that the atom has indeed been traveling both paths at once.