Quantum hot potato: Researchers entice two atoms to swap smallest energy units

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Physicists at the National Institute of Standards and Technology (NIST) have for the first time coaxed two atoms in separate locations to take turns jiggling back and forth while swapping the smallest measurable units of energy. By directly linking the motions of two physically separated atoms, the technique has the potential to simplify information processing in future quantum computers and simulations.

Described in a paper published Feb. 23 by Nature,* the NIST experiments enticed twoberylliumions(electrically charged atoms) to take turns vibrating in an electromagnetic trap, exchanging units of energy, or quanta, that are a hallmark ofquantum mechanics. As little as one quantum was traded back and forth in these exchanges, signifying that the ions are"coupled"or linked together. These ions also behave like objects in the larger, everyday world in that they are"harmonic oscillators"similar to pendulums and tuning forks, making repetitive, back-and-forth motions.

"First one ion is jiggling a little and the other is not moving at all; then the jiggling motion switches to the other ion. The smallest amount of energy you could possibly see is moving between the ions,"explains first author Kenton Brown, a NIST post-doctoral researcher."We can also tune the coupling, which affects how fast they exchange energy and to what degree. We can turn the interaction on and off."

The experiments were made possible by a novel, one-layerion trapcooled to minus 269 C (minus 452 F) with aliquid heliumbath. The ions, 40 micrometers apart, float above the surface of the trap. In contrast to a conventional two-layer trap, the surface trap features smallerelectrodesand can position ions closer together, enabling stronger coupling. Chilling to cryogenic temperatures suppresses unwanted heat that can distort ion behavior.

The energy swapping demonstrations begin by cooling both ions with a laser to slow their motion. Then one ion is cooled further to a motionless state with two opposing ultraviolet laser beams. Next the coupling interaction is turned on by tuning the voltages of the trap electrodes. In separate experiments reported in Nature, NIST researchers measured the ions swapping energy at levels of several quanta every 155 microseconds and at the single quantum level somewhat less frequently, every 218 microseconds. Theoretically, the ions could swap energy indefinitely until the process is disrupted by heating. NIST scientists observed two round-trip exchanges at the single quantum level.

To detect and measure the ions' activity, NIST scientists apply an oscillating pulse to the trap at different frequencies while illuminating both ions with an ultraviolet laser and analyzing the scattered light. Each ion has its own characteristic vibration frequency; when excited, the motion reduces the amount of laser light absorbed. Dimming of the scattered light tells scientists an ion is vibrating at a particular pulse frequency.

To turn on the coupling interaction, scientists use electrode voltages to tune the frequencies of the two ions, nudging them closer together. The coupling is strongest when the frequencies are closest. The motions become linked due to the electrostatic interactions of the positively charged ions, which tend to repel each other. Coupling associates each ion with both characteristic frequencies.

The new experiments are similar to the same NIST research group's 2009 demonstration of entanglement—a quantum phenomenon linking properties of separated particles—in a mechanical system of two separated pairs of vibrating ions (seehttp://www.physorg.com/news163253992.html). However, the new experiments coupled the oscillators' motions more directly than before and, therefore, may simplifyinformation processing. In this case the researchers observed quantum behavior but did not verify entanglement.

The new technique could be useful in a future quantum computer, which would use quantum systems such as ions to solve problems that are intractable today. For example, quantum computers could break today's most widely used data encryption codes. Direct coupling of ions in separate locations could simplify logic operations and help correct processing errors. The technique is also a feature of proposals for quantum simulations, which may help explain the mechanisms of complex quantum systems such as high-temperature superconductors.

In addition, the demonstration also suggests that similar interactions could be used to connect different types of quantum systems, such as a trapped ion and a particle of light (photon), to transfer information in a future quantum network. For example, a trapped ion could act as a"quantum transformer"between a superconducting quantum bit (qubit) and a qubit made of photons.

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Experimental evidence adds to the likelihood of the existence of supersolids, an exotic phase of matter

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Supersolids and superfluids rank among the most exotic of quantum mechanical phenomena. Superfluids can flow without any viscosity, and experience no friction as they flow along the walls of a container, because their atoms 'condense' into a highly coherent state of matter. Supersolids are also characterized by coherent effects, but between vacancies in a crystal lattice rather than between the solid’s atoms themselves.

The reduction in the rotational inertia of a bar of solid helium-4 as it was cooled to very low temperatures provided the first experimental evidence for supersolids. Physicists interpreted the reduction to mean that some amount of supersolid helium had formed and decoupled from the remainder of the bar, affecting its rotational inertia and frequency. Others argued that the reduction in inertia resulted from a change in the helium’s viscosity and elasticity with temperature, rather than from the onset of supersolidity.

Kimitoshi Kono from the RIKEN Advanced Science Institute in Wako, Japan, Eunseong Kim from KAIST in Korea, and their colleagues from these institutes have now disproved the alternative interpretation by simultaneously measuring the shear modulus (a measure ofviscosityand elasticity) and the rotational inertia of a solid helium-4 cell as its temperature dropped from 1 kelvin to 15 thousandths of a kelvin. The cell was made to rotate clockwise and then counterclockwise periodically, as well as to rotate clockwise or counterclockwise continuously (Fig. 1). The continuous rotation affected the inertial mass of the helium but its shear modulus, allowing these quantities to be monitored independently.

Under continuous rotation, the degree of change in the rotational inertia had a clear dependence on rotation velocity, while the shear modulus did not. In addition, the energy dissipated by the rotation increased at high speeds. Both of these observations contrast to what would be expected if viscoelastic effects were at play, rather than supersolidity. The researchers also found that periodic rotation and continuous rotation affected the rotation differently, raising new questions about the experimental system.

The data support the interpretation that changes in the rotational inertia of helium-4 at low temperature result from supersolidity. This is important because of the novel and surprising nature of the phenomenon itself, says Kono.“Superfluidity in a solid is a very radical concept which, if proven, is certainly a good candidate for the Nobel Prize” he adds.“Therefore the first priority is to determine whether it can be proven in a fashion that will convince the low-temperature physics community.”

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Scientists say it's high 'NOON' for microwave photons

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An important milestone toward the realization of a large-scale quantum computer, and further demonstration of a new level of the quantum control of light, were accomplished by a team of scientists at UC Santa Barbara and in China and Japan.

The study, published in the Feb. 7 issue of the journalPhysical Review Letters, involved scientists from Zhejiang University, China, and NEC Corporation, Japan. The experimental effort was pursued in the research groups of UC Santa Barbara physics professors Andrew Cleland and John Martinis.

The team described how they used a superconducting quantum integrated circuit to generate unique quantum states of light known as"NOON"states. These states, generated from microwave frequency photons, the quantum unit of light, were created and stored in two physically-separated microwave storage cavities, explained first author Haohua Wang, postdoctoral fellow in physics at UC Santa Barbara. The quantum NOON states were created using one, two, or three photons, with all the photons in one cavity, leaving the other cavity empty. This was simultaneous with the first cavity being empty, with all the photons stored in the second cavity.

"This seemingly impossible situation, allowed byquantum mechanics, led to interesting results when we looked inside the cavities,"said second author Matteo Mariantoni, postdoctoral fellow in physics at UC Santa Barbara."There was a 50 percent chance of seeing all the photons in one cavity, and a 50 percent chance of not finding any— in which case all the photons could always be found in the other cavity."

However, if one of the cavities was gently probed before looking inside, thus changing the quantum state, the effect of the probing could be seen, even if that cavity was subsequently found to be empty, he added.

"It's kind of like the states are ghostly twins or triplets,"said Wang."They are always together, but somehow you never know where they are. They also have a mysterious way of communicating, so they always seem to know what is going to happen."Indeed, these types of states display what Einstein famously termed,"spooky action at a distance,"where prodding or measuring aquantum statein one location affects its behavior elsewhere.

The quantum integrated circuit, which includes superconducting quantum bits in addition to the microwave storage cavities, forms part of what eventually may become a quantum computational architecture.

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Quantum quirk: Scientists pack atoms together to prevent collisions in atomic clock

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In a paradox typical of the quantum world, JILA scientists have eliminated collisions between atoms in an atomic clock by packing the atoms closer together. The surprising discovery, described in the Feb. 3 issue of<i>Science Express</i>, can boost the performance of experimental atomic clocks made of thousands or tens of thousands of neutral atoms trapped by intersecting laser beams.

JILA is jointly operated by the National Institute of Standards and Technology and the University of Colorado Boulder.

JILA scientists demonstrated the new approach using their experimental clock made of about 4,000 strontium atoms. Instead of loading the atoms into a stack of pancake-shaped optical traps as in their previous work, scientists packed the atoms into thousands of horizontal optical tubes. The result was a more than tenfold improvement in clock performance because the atoms interacted so strongly that, against all odds, they stopped hitting each other. The atoms, which normally like to hang out separately and relaxed, get so perturbed from being forced close together that the ensemble is effectively frozen in place.

"The atoms used to have the whole dance floor to move around on and now they are confined in alleys, so the interaction energy goes way up,"says NIST/JILA Fellow Jun Ye, leader of the experimental team.

How exactly does high interaction energy—the degree to which an atom's behavior is modified by the presence of others—prevent collisions? The results make full sense in the quantum world. Strontium atoms are a class of particles known as fermions. If they are in identical energy states, they cannot occupy the same place at the same time—that is, they cannot collide. Normally thelaser beamused to operate the clock interacts with the atoms unevenly, leaving the atoms dissimilar enough to collide. But the interaction energy of atoms packed in optical tubes is now higher than any energy shifts that might be caused by the laser, preventing the atoms from differentiating enough to collide.

The idea was proposed by JILA theorist Ana Maria Rey and demonstrated in the lab by Ye's group.

Given the new knowledge, Ye believes his clock and others based on neutral atoms will become competitive in terms of accuracy with world-leading experimental clocks based on single ions (electrically charged atoms). The JILA strontium clock is currently the best performing experimental clock based onneutral atomsand, along with several NIST ion and neutral atom clocks, a possible candidate for a future international time standard. The devices provide highly accurate time by measuring oscillations (which serve as"ticks") between the energy levels in the atoms.

In addition to preventing collisions, the finding also means that the more atoms in the clock, the better."As atom numbers increase, both measurement precision and accuracy increase accordingly,"Ye says.

To trap the atoms in optical tubes, scientists first use blue and red lasers to coolstrontiumatomsto about 2 microKelvin in a trap that uses light and magnetic fields. A vertical lattice of light waves is created using an infrared laser beam that spans and traps the atom cloud. Then a horizontal infraredlaser beamis turned on, creating optical tube traps at the intersection with the vertical laser.

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Quantum robins lead the way

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(PhysOrg.com) -- Did you know that the humble robin uses quantum physics?

Researchers have been investigating the mechanism which enables birds to detect the Earth's magnetic field to help them navigate over vast distances. This ability, known as magnetoreception, has been linked to chemical reactions inside birds' eyes.

Now a team from Oxford University and Singapore believe that this 'compass' is making use of something calledquantum coherence.

In a forthcoming article inPhysical Review Lettersthe team report how they anaylsed data from anexperimentby Oxford and Frankfurt scientists on robins.

The experiment showed that themagnetic compassused by robins could be distrupted by extremely small levels of magnetic 'noise'. When this noise, a tiny oscillating magnetic field, was introduced it completely disabled the Robins' compass sense which then returned to normal once the noise was removed - good news for robins which have to navigate on the long migration route to Scandinavia and Africa and back every year.

In their analysis the Oxford/Singapore team show that only a system with components operating at aquantum levelwould be this sensitive to such a small amount of noise.

'Quantum information technology is a field of physics aimed at harnessing some of the deepest phenomena in physics to create wholly new forms of technology, such as computers and communication systems,' said Erik Gauger of Oxford University's Department of Materials, an author of the paper.

'Progress in this area is proving to be very difficult because the phenomena that must be harnessed are extremely delicate. It would normally be thought almost inconceivable that a living organism could have evolved similar capabilities.'

Co-author Simon Benjamin from Singapore explained: 'Coherent quantum states decay very rapidly, so that the challenge is to hold on to them for as long as possible. Themolecular structuresin the bird's compass can evidently keep these states alive for at least 100 microseconds, probably much longer.'

'While this sounds like a short time, the best comparable artificial molecules can only manage 80 microseconds at room temperature. And that's in ideal laboratory conditions.'

Erik and Simon now hope that further research into how birds harness these quantum states could enable researchers to mimic them and help in the development of practical quantum technologies.

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Research uses quantum mechanics to melt glass at absolute zero

Quantum mechanics, developed in the 1920s, has had an enormous impact in explaining how matter works. The elementary particles that make up different forms of matter -- such as electrons, protons, neutrons and photons -- are well understood within the model quantum physics provides. Even now, some 90 years later, new scientific principles in quantum physics are being described. The most recent gives the world a glimpse into the seemingly impossible.

Prof. Eran Rabani of Tel Aviv University's School of Chemistry and his colleagues at Columbia University have discovered a new quantum mechanical effect with glass-formingliquids. They've determined that it's possible to melt glass— not by heating it, but by cooling it to a temperature near Absolute Zero.

This new basic science research, to be published inNature Physics, has limited practical application so far, says Prof. Rabani. But knowing why materials behave as they do paves the way for breakthroughs of the future."The interesting story here,"says Prof. Rabani,"is that by quantum effect, we can melt glass by cooling it. Normally, we melt glasses with heat."

Turning the thermometer upside-down

Classical physics allowed researchers to be certain about the qualities of physical objects. But at the atomic/molecular level, as a result of the duality principle which describes small objects as waves, it's impossible to determine exact molecular position and speed at any given moment— a fact known as the"Heisenberg Principle."Based on this principle, Prof. Rabani and his colleagues were able to demonstrate their surprising natural phenomenon with glass.

Many different materials on earth, like the silica used in windows, can become a glass–– at least in theory— if they are cooled fast enough. But the new research by Prof. Rabani and his colleagues demonstrates that under very special conditions, a few degrees above Absolute Zero (−459.67° Fahrenheit), a glass might melt.

It all has to do with how molecules in materials are ordered, Prof. Rabani explains. At some point in the cooling phase, a material can become glass and then liquid if the right conditions exist.

"We hope that future laboratory experiments will prove our predictions,"he says, looking forward to this new basic science paving the way for continued research.

Classical glass

The research was inspired by Nobel Prize winner Philip W. Anderson, who wrote that the understanding of classical glasses was one of the biggest unsolved problems in condensed matter physics. After the challenge was presented, research teams around the world rose to it.

Until now, structural quantum glasses had never been explored— that is, what happens when you mix the unique properties in glass and add quantum effects. Prof. Rabani was challenged to ask: if we looked at the quantum level, would we still see the hallmarks of a classical glass?

What the researchers unearthed is a new and unique hallmark, showing that quantum glasses have a unique signature. Many materials he says can form aglassif they're cooled fast enough. Even though their theory is not practical for daily use: few individuals own freezers that dip down nearly 500 degrees below zero.

Prof. Rabani is currently on sabbatical at the University of California, Berkeley, as a Miller Visiting Professor.

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Physicists describe method to observe timelike entanglement

(PhysOrg.com) -- In"ordinary"quantum entanglement, two particles possess properties that are inherently linked with each other, even though the particles may be spatially separated by a large distance. Now, physicists S. Jay Olson and Timothy C. Ralph from the University of Queensland have shown that it's possible to create entanglement between regions of spacetime that are separated in time but not in space, and then to convert the timelike entanglement into normal spacelike entanglement. They also discuss the possibility of using this timelike entanglement from the quantum vacuum for a process they call"teleportation in time."

"To me, the exciting aspect of this result (that entanglement exists between the future and past) is that it is quite a general property of nature and opens the door to new creativity, since we know that entanglement can be viewed as a resource for quantum technology,"Olson toldPhysOrg.com.“The greatest significance of our result is almost certainly in some application that is yet to be imagined.”

Olson and Ralph’s paper, which is posted at arXiv.org, describes how timelike entanglement can be converted into spacelike entanglement using two detectors.

“Essentially, a detector in the past is able to‘capture’ some information on the state of the quantum field in the past, and carry it forward in time to the future -- this is information that would ordinarily escape to a distant region ofspacetimeat the speed of light,” Olson said.“When another detector then captures information on the state of the field in the future at the same spatial location, the two detectors can then be compared side-by-side to see if their state has become entangled in the usual sense that people are familiar with -- and we find that indeed they should be entangled. This process thus takes a seemingly exotic, new concept (timelike entanglement in the field) and converts it into a familiar one (standard entanglement of two detectors at a given time in the future).”

In their study, the scientists also proposed a thought experiment in which they move a quantum state into the future using timelike entanglement as the resource. They call the process“teleportation in time.”

In the thought experiment, the physicists described two qubit detectors, one of which is coupled to the field in the past and one to the field in the future. First, the detector coupled to the past operates on a qubit and generates information about how the qubit can be detected. The qubit is then teleported into the future, essentially skipping over a middle period of time. Then the first detector is removed and the second, future-coupled detector is placed in the first detector’s spatial location, so that the detectors are separated in time but not in space. After a certain amount of time, the second detector receives the information from the first detector, which it uses to reconstruct the teleported qubit.

The physicists emphasized that there is an important symmetric time correlation that must be followed in order for the procedure to work. If the qubit is teleported at t=0, then the first detector must have operated the same amount of time before t=0 as the second detector operated after t=0. For example, if t=0 is 12:00, and the first detector operated at 11:45, then the second detector must wait to operate at exactly 12:15 in order to achieve entanglement. The scientists also noted that between 12:00 and 12:15, it’s impossible to recover the teleported qubit.

According to the physicists’ previous work, such timelike entanglement should generate a new thermal effect arising from the quantum vacuum (the quantum vacuum is thought to exhibit several thermal effects, including Hawking radiation from black holes, though none of these thermal effects have been observed). The physicists predict that the new thermal effect may be easier to observe than other thermal effects using current technology. If such a procedure for extracting and converting timelike entanglement can be realized, then it could provide a way for scientists to directly observe thequantum entanglementinherent in the space-time vacuum for the firsttime.

“Entanglement is observed every day,” Olson said.“However, direct observation of entanglement in the vacuum state would be new, and being able to observe it would potentially enable us to use this entanglement as a resource for quantum technology. Since the vacuum state is the closest thing we have to‘nothing’ in physics (it is the state with zero ordinaryparticlesaround), observing and using theentanglementinherent in the vacuum as a technological resource would potentially give us a way to build quantum devices with just empty space as the most fundamental ingredient.”

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