Quantum split: Particle this way, properties that way - physics-math - 23 July 2014 - New Scientist

Quantum split: Particle this way, properties that way - physics-math - 23 July 2014 - New Scientist: In Grenoble, the Vienna team used a feeble magnetic field and a weakly interacting neutron absorber to make the weak measurements. They found that when they put the absorber in one path of the interferometer (say left), there was a discernible effect at the output. But when they put it in the right path, it had no such effect. The neutrons were travelling in one path only.

Next, the experimenters introduced a weak magnetic field near each arm of the interferometer, to interact with the spin of the neutrons. When they did this in the left path, there was no change in the interferometer's output. If they introduced the magnetic field in the right path, though, there was a change: the magnetic field had interacted with the spin. In other words, they had confirmed that the spin had chosen the path not taken by the parent neutron...

Quantum Method Closes in on Gravitational Constant - Scientific American

Quantum Method Closes in on Gravitational Constant - Scientific American; Researchers have been unable to identify the source of errors causing the disagreement in the conventional measurements. The set-up of the latest measurement is unlikely to contain the same errors as the torque method...

In the experiment described by Tino’s team, pulses of laser light tickle a cloud of rubidium atoms cooled to nearly absolute zero, driving the atoms to rise and fall like a fountain under the influence of gravity. The pulses split the 'matter wave' associated with each atom into a superposition of two energy states, each of which has a different velocity and reaches a different height — 60 or 90 centimeters — before falling back. The matter wave that rises farthest has a greater separation from the tungsten cylinders, and thus senses a slightly different gravitational pull. The difference in force imparts a measurable shift in the final state of the two matter waves when they recombine, creating an interference pattern.

Quantum biology: Algae evolved to switch quantum coherence on and off

Quantum biology: Algae evolved to switch quantum coherence on and off: "Most cryptophytes have a light-harvesting system where quantum coherence is present. But we have found a class of cryptophytes where it is switched off because of a genetic mutation that alters the shape of a light-harvesting protein.
"This is a very exciting find. It means we will be able to uncover the role of quantum coherence in photosynthesis by comparing organisms with the two different types of proteins."

New ‘switch’ could power quantum computing | MIT News Office

New ‘switch’ could power quantum computing | MIT News Office:  “We have demonstrated basically an atom can switch the phase of a photon. And the photon can switch the phase of an atom...”

In this case, the researchers used a laser to place a rubidium atom very close to the surface of a photonic crystal cavity, a structure of light. The atoms were placed no more than 100 or 200 nanometers — less than a wavelength of light — from the edge of the cavity. At such small distances, there is a strong attractive force between the atom and the surface of the light field, which the researchers used to trap the atom in place...

“In some sense, it was a big surprise how simple this solution was compared to the different techniques you might envision of getting the atoms there,” Vuletić says.

The Astounding Link Between the P≠NP Problem and the Quantum Nature of Universe — The Physics arXiv Blog — Medium

The Astounding Link Between the P≠NP Problem and the Quantum Nature of Universe — The Physics arXiv Blog — Medium: ...He says the key is to think of Schrodinger’s cat as a problem of computational complexity theory...

...He says there is an implicit assumption when physicists say that Schrödinger’s equation can describe macroscopic systems. This assumption is that the equations can be solved in a reasonable amount of time to produce an answer...

If P ≠ NP and there is no efficient algorithm for solving Schrödinger’s equation, then there is only one way of finding a solution, which is a brute force search...

So the number of elementary operations needed to exactly solve this equation would be equal to 2^10^24...

...this time scale is considerably shorter than the Planck timescale, which is roughly equal to 10^-43 seconds.

New material for quantum computing discovered out of the blue

New material for quantum computing discovered out of the blue: The pigment, copper phthalocyanine (CuPc), which is similar to the light harvesting section of the chlorophyll molecule, is a low-cost organic semiconductor that is found in many household products...

...the electrons in CuPc can remain in 'superposition' ...  for surprisingly long times...

CuPc possesses many other attributes that could exploit the spin of electrons, rather than their charge, to store and process information which are highly desirable in a more conventional quantum technology. For example, the pigment strongly absorbs visible light and is easy to modify chemically and physically, so its magnetic and electrical properties can be controlled.

World's First Quantum Metamaterial Unveiled | MIT Technology Review

World's First Quantum Metamaterial Unveiled | MIT Technology Review: the split-ring resonators introduced losses because of their internal resistance...

a solution to this problem: use superconducting resonators...

The problem in the past is that physicists had arranged the circuits in series so that the combined state must be a superposition of the states of all the circuits. So if a single circuit was out of kilter, the entire experiment failed.

Macha and co got around this by embedding the quantum circuits inside a microwave resonator–a chamber about a wavelength long in which the microwaves become trapped.

To interact with a photon, each quantum circuit need only couple with the resonator itself and its nearest neighbours.

The Quantum Zeno Effect actually does stop the world

The Quantum Zeno Effect actually does stop the world: Let's say an atom is very likely to have decayed after three seconds, but very unlikely to have decayed after one. Check on it after three seconds, and it probably will have decayed. But, Misra and Sudarshan argue, check on it three times in one second intervals, and it will most likely not have decayed. Every time you check on it, it will revert to its "original" measured state, and the clock will start over. Amazingly, this actually does happen. Researchers observing sodium atoms observed that, "Depending on the frequency of measurements we observe a decay that is suppressed or enhanced as compared to the unperturbed system." The "enhanced" decay is the result of the Quantum Anti-Zeno Effect. Time your measurements just right and you can actually push a system to decay faster than it would if it were unobserved.

What We Can Learn From the Quantum Calculations of Birds and Bacteria - Wired Science

What We Can Learn From the Quantum Calculations of Birds and Bacteria - Wired Science: We can now show that a single electronic excitation acting as a probability amplitude wave can simultaneously sample the various molecular paths connecting the antenna cells to the reaction center. The excitation effectively “picks” the most efficient route from leaf surface to sugar conversion site from a quantum menu of possible paths. This requires that all possible states of the traveling particle be superposed in a single, coherent quantum state for tens of femtoseconds.

We have seen this remarkable phenomenon in the green sulphur bacteria, but humans have not yet figured out how it is that nature can stabilize a coherent electronic quantum state in such complex systems for such long periods of time...

Remarkably, it seems that these photosynthesizing bacteria can actually use decoherence to speed up the transfer of electronic information by accessing vibrational energies in the protein bath surrounding the biological-quantum wire without losing the integrity of the information...

It seems that quantum mechanical processes in the avian eye send signals to the brain that are sensitively dependent on the angle of change in magnetic field inclination, thereby allowing the bird to map routes. The hypothesis is that pairs of light-absorbing molecules in the bird retina produce quantum mechanically entangled electrons whose quantum mechanical state depends on the angular inclination of the field and which catalyze chemical reactions that send differently valued signals to the brain depending upon the degree of inclination.

Quantum effects get a weirdness scale

Quantum effects get a weirdness scale: This makes it possible to compare theoretical and real experiments, and creates a list of chart-toppers. Attempts at neutron superposition in the 1960s score around 5 or 6, while modern experiments involving nearly 500 atoms hit 12. It is a logarithmic scale, so this is roughly a million-fold improvement, but it pales next to Schrödinger's cat. Implementing this thought experiment with existing quantum technology would max out at 24. A version in which an actual cat simultaneously sits in two spots that are 10 centimetres apart, would score 57...

Using laser beams, scientists generate quantum matter with novel, crystal-like properties

Using laser beams, scientists generate quantum matter with novel, crystal-like properties:  As the scientists chose Rydberg-states which give rise to repulsive van der Waals forces the excited atoms have to keep a minimum distance of several micrometers from each other. This mutual blockade leads to spatial correlations between the atoms such that, depending on the number of Rydberg-atoms, states with different geometrical configurations can emerge (see fig. 1). "However, we have to be aware that in our excited quantum system all geometrical orders are present at the same time. To be precise, all the excitation states form a coherent superposition, " Dr. Marc Cheneau says, a scientist at the experiment. "This new state of matter is a very fragile, crystal-like formation; it exists as long as the excitation is sustained, and fades away once the beam is switched off."

A one-way street for spinning atoms

A one-way street for spinning atoms: In previous experiments, the RLE researchers created a superfluid — a completely frictionless gas — of lithium atoms. In their new experiment, the researchers used laser beams to trap a cloud of lithium atoms about 50 micrometers in diameter. The atoms were cooled to just a few billionths of a degree above absolute zero...

The researchers illuminated the gas with a pair of laser beams, sorting the atoms into two lanes, each of which consists of atoms with the same spin moving in the same direction. For the first time in an atomic system, this correlation of atoms’ spins with their velocities was directly measured.

“The combined system of ultracold atoms and the light we shine on them forms a material with unique properties,” says Lawrence Cheuk, lead author of the paper and a graduate student in MIT’s physics department. “The gas acts as a quantum diode, a device that regulates the flow of spin currents.”

Blog - Quantum PageRank Algorithm Outperforms Classical Version

Blog - Quantum PageRank Algorithm Outperforms Classical Version: The way they think about this is to imagine a quantum page crawler wandering around the network along paths that connect one quantum node to the next. While the quantum paths remain in a quantum superposition, the importance of a page is the probability of finding the crawler on that page at any instant.

Paparo and Martín-Delgado then outline a quantum algorithm that will produce a ranking of pages at any given instant using these quantum probabilities. They go on to simulate its performance on two relatively small networks: a tree-like network and a directed graph network with no specific structure.

One clock with two times: When quantum mechanics meets general relativity

One clock with two times: When quantum mechanics meets general relativity: The team at the University of Vienna considers a single clock (any particle with evolving internal degrees of freedom such as spin) which is brought in a superposition of two locations – one closer and one further away from the surface of the Earth. According to general relativity, the clock ticks at different rates in the two locations, in the same way as the two twins would age differently. But since the time measured by the clock reveals the information on where the clock was located, the interference and the wave-nature of the clock is lost. "It is the twin paradox for a quantum 'only child', and it requires general relativity as well as quantum mechanics. Such an interplay between the two theories has never been probed in experiments yet..."

Quantum simulation with light: Frustrations between photon pairs

Quantum simulation with light: Frustrations between photon pairs: Currently, many international groups are focusing their research on frustrated quantum systems, which have been conjectured to explain high-temperature superconductivity. A quantum system is frustrated if competing requirements cannot be satisfied simultaneously. The Viennese research group realized for the first time an experimental quantum simulation, where the frustration regarding the "pairing" of correlations was closely investigated.
Using two pairs of entangled photons, a frustrated quantum system could be simulated that consists of four particles. "Just the recent development of our quantum technology allows us to not only rebuild other quantum systems, but also to simulate its dynamics"

Moved By Light - Science News

Moved By Light - Science News: While other scientists built stuff that shook thousands or millions of times a second, he created a ceramic wafer 30 micro-meters long that expanded and contracted 6 billion times per second. The faster an object’s natural quiver, the easier it is to remove energy, meaning less cooling needed to reach the ground state. Using a state-of-the-art liquid-helium refrigerator capable of achieving millikelvin temperatures, Cleland’s team put the wafer in its ground state 93 percent of the time.

By measuring the electric fields produced by this object, Cleland and his colleagues showed that they could nudge the wafer into a state of superposition — both moving and still at the same time.

Researchers Succeed in Quantum Teleportation of Light Waves | Popular Science

Researchers Succeed in Quantum Teleportation of Light Waves | Popular Science: This is a major advance, as previous teleportation experiments were either very slow or caused some information to be lost.
The team employed a mind-boggling set of quantum manipulation techniques to achieve this, including squeezing, photon subtraction, entanglement and homodyne detection. The photo above depicts their device, nicknamed the Teleporter, in the lab of Akira Furusawa at the University of Tokyo.

Atom and its quantum mirror image

Atom and its quantum mirror image: 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...
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...
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.

Quantum Mechanics Braces for the Ultimate Test

Quantum Mechanics Braces for the Ultimate Test: The now-celebrated Aspect experiment, along with similar ones, helped to write nonlocality into physics textbooks. But there is another loophole that those experiments did not close. The trouble is that photons are slippery customers: small, fast, and notoriously hard to detect. Typically, if five photons are hurled at a detector, it will register only one. That means that physicists can trust that Bell's bound has been violated only if they assume that the photons caught provide a fair representation of how all the photons in the experiment behaved—much the way exit polls at voting booths predict election results.

Most physicists accept that the fair-sampling assumption is a good one. “It's unlikely that nature is so malicious that it conspires with the apparatus to hold back particular photons just to fool us into thinking that quantum mechanics works,” Gisin says.
Nonetheless, physicists hate loose ends, so the chase to find a perfect, loophole-free test has continued over the past decade. “Until the test is done, we can't honestly say that hidden variables have been ruled out—even if the consensus is they don't make sense—because we haven't proved it,” says Harald Weinfurter of the Ludwig Maximilian University in Munich, Germany...
Building on Wineland's experiment, Weinfurter's group is attempting to tie up both loopholes at once, by weaving photons together with atoms to reap the benefits of both. The idea is to start with two initially unentangled atoms in separate laboratories—ideally more than 100 meters apart, so that the atoms cannot influence each other over the course of the test. Each atom emits a photon; the two photons are captured and transmitted along optical fibers to a third location, where they are entangled. “The magic is that as soon as the photons are entangled, their parent atoms automatically become entangled, too...”

Ethereal quantum state stored in solid crystal - physics-math - 12 January 2011 - New Scientist

Ethereal quantum state stored in solid crystal: "Tittel's team started by channelling one photon of an entangled pair into a crystal of lithium niobate doped with ions of thulium. This sends the crystal into a quantum superposition, in which many thulium ions absorb the photon at once and vibrate at different frequencies. The frequencies aren't random, however. Beforehand, the team strategically removed some thulium ions, leaving only those that absorb a particular sequence of frequencies."