Simplifying the process of detecting genuine multiparticle entanglement

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(PhysOrg.com) -- The ability to entangle particles is considered essential for a number of experiments and applications. While we have seen evidence for quantum entanglement, it is still difficult to detect unambiguously. Multiparticle quantum correlations are especially important for work with optical lattices, superconducting qubits and quantum information processing."Entanglement in large qubit systems is becoming more important,"Bastian Jungnitsch tells<i>PhysOrg.com</i>."Unfortunately, the characterization of multiparticle entanglement is difficult."

Jungnitsch, a Ph.D. student at the Institute forQuantum OpticsandQuantum Informationat the Austrian Academy of Sciences in Innsbruck, Austria, has been working on a way to simplify the way that multiparticleentanglementis characterized. Along with Tobias Moroder and Otfried Gühne from the University of Siegen in Germany, Jungnitsch created criteria that can be easily implemented in experiments. Their work is published inPhysical Review Letters:“Taming Multiparticle Entanglement.”

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“We developed a criterion that can easily be implemented so that others can use it,” Jungnitsch says.“Experimentalists can download it from the Internet and use it to measure entanglement. It provides results that others can understand, helping to let them know that theseparticlesare really entangled.”

The team from Austria and Germany used what they call“suitable relaxations” to develop their criteria. The method involves first considering a quantum state involving three particles. If the state can be separated out using different partitions, it is called biseparable. When in a biseparable state, the system is not considered to be entangled. If, however, the quantum state is not biseparable, it can be thought of as genuinely multiparticle-entangled, meaning that all particles are entangled and not only some of them.

“There are many states that are entangled in some sense,” Jungnitsch points out,“but it’s not easy to see if they are genuinely entangled. We relaxed the definition a bit, and it still works pretty well. Our method won’t work for allentangled states, but you can still catch the phenomena of entanglement pretty well.”

“The point is to make sure that your particles are in a genuinely entangled state,” Jungnitsch continues.“In many cases, you want to know if all of the items prepared are entangled.” Otherwise, for some operations, you might not get the desired results.“When particles are genuinely entangled, they can be useful for a number of operations. Our criteria can help experimentalists verify that the state really is entangled. It’s something useful and practical.”

Jungnitsch says that they have already provided the code for download.“It can be used in experiments now,” he says.“We wrote the code and put it on the Internet. Experimentalists can download and then run the code on their own computers as part of their efforts to see whether or not their multiparticle systems are entangled.”

Some refining is still possible, however.“We have been working on applying the code to specific states– important states.” Because there are some applications that require certain quantum states, being able to detect whether or not particles have reached those states is important. With some adjustments to the code, it should be possible to characterize such entangled states even more accurately.

“Of course, the long view is always quantum computers,” Jungnitsch acknowledges.“But there are other applications forquantum correlationsas well. The first step to many of these applications, and to improvingquantum information processing, is to verify entangled states. Our efforts can help with that.”

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Apparent roadblock in the development of quantum lithography

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(PhysOrg.com) -- Just when it began to appear that scientists had found a viable way around the problem of the blurring that occurs when using masks to create smaller and smaller silicon wafers for computer chips, a previous study on beam splitting optics showed that the new approach would not work, at least as it has thus far been proposed. A group of researchers explain why in a paper in<i>New Journal of Physics.</i>

Currently, thesilicon wafersthat make upcomputer chipsare made by the process oflithography, whereby optics are used to create an image on a piece of wafer. To create the channels that make up thetransistors,masksare used to prevent some of thephotonsdirected towards a wafer from arriving. When the wafer is then immersed in special chemicals, the parts that were struck react differently than those that weren’t, creating the channels. The problem is in the clarity of the image produced on the other side due to the use of optic lenses to focus the photons, as some degree of blurring will always occur due to the nature of lenses. As researchers try to make smaller transistors, the blurring eventually becomes a roadblock, which is why some are looking for alternatives.

One such approach is to take advantage of the unique properties of entangled photons; those wily quantum particles that for some inexplicable reason, tend to mimic the behavior of one another, without any apparent means of communication, and at a rate faster than the speed of light. Because they are correlated, the thinking went, they’d always arrive at the same place at the same time (in this case a sensor) creating a near perfect image; so if say a mask were made, in this case a simple one with just two slits in it; it would make sense that the pair of entangled photons would interfere with one another as they tend to do, as they pass through the slit, then arrive together on the other side at exactly the same place and time, which is just what you’d need if you wanted to impact the material on the other side to create your wafer the way you intended.

Unfortunately, things haven’t worked out quite that way, because as it turns out, while you can expect a pair of entangled photons to do their thing simultaneously, you can’t rely on them to arrive at the same target, or again in this case, the same sensor, while they are doing so; which of course is a big problem if you’re trying to make awaferwhere the photons have to hit their target not only at exactly the same time, but in exactly the right place or you’ve got nothing to show for your efforts.

Even so, researchers hoped that enough photons would arrive in the same place at the same time by chance to allow for the process to work; but this meant adding in an exposure time (waiting for enough of the photons to arrive at the same place) which as it turned out rose too rapidly as the feature size requirements went up, making the process unfeasible.

While it appears the original idea for usingentangled photonsfor the development of quantum lithography won’t work, researchers aren’t giving up hope just yet; the stakes are too high. The hope now is that some other new imaginative way can be thought of to get around the problems encountered, allowing for the creation of almost unimaginably small chips.

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D-Wave researchers demonstrate progress in quantum computing

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(PhysOrg.com) -- Taking another step toward demonstrating quantum behavior in a quantum computer, researchers from the Vancouver-based company D-Wave Systems, Inc., have performed a technique called quantum annealing, which could provide the computational model for a quantum processor. They have published a study describing the demonstration in a recent issue of<i>Nature</i>.

"This is the first time we’ve been able to open up the black box and show how {D-Wave’s devices} are harnessing quantum mechanics in solving problems,"D-Wave’s chief technology officer Geordie Rose said in a recent news article atphysicsworld.com.

D-Wave, which is a spin-out company from the University of British Columbia, made headlines in 2007 when it boldly announced to have built theworld’s first commercially viable quantum computer. Due to the difficulty in demonstrating that the computer does in fact exhibitquantum behavior, many people have been skeptical of the claim.

Nevertheless, D-Wave has continued to work toward the challenging goal of harnessing the power ofquantum computing. In their study, they show that quantum annealing can be used to find the ground state of eight superconducting flux qubits that aren’t corrupted by heat or noise. Since many complex problems can be reduced to finding the ground state of a system of interacting spins, quantum annealing has been predicted to provide better methods for solving certain types of complex problems.

To demonstrate quantum annealing, the researchers first adjusted the eight qubits to resemble a 1D chain of magnets, where each qubit wants to point in the same direction (up or down) as its two neighbors. The researchers then set the qubits on the ends of the chain in opposite directions, and allowed the six qubits in the middle to orient their spins with their neighbors. Since this set-up forces two neighboring qubits to have opposing spins, the process resulted in a“frustrated” ferromagnetic arrangement. Then, by tilting the qubits in the same direction and raising the energy barrier, the researchers caused the system to move toward one specific arrangement of frustrated spins, which is the ground state.

Qubits can flip spins in two ways: through a quantum mechanical mechanism (tunneling) and a classical mechanism (thermal activation). Since thermal activation destroys the quantum nature of the qubit, the researchers had to show that the qubits were flipping spins due solely to quantum tunneling. They did this by applying a current to the system until both tunneling and heat-driven transitions stopped, and the qubit“froze.” By repeating this process at different temperatures, the researchers could determine that annealing occurred by tunneling alone. In other words, the results cannot be explained by classical physics.

As the researchers explain, increasing the number of spins could enable the system to provide a practical physical way to implement a quantum algorithm. The researchers are currently working on this challenge, and plan to apply the process to areas such as machine learning and artificial intelligence.

According to Rose, the demonstration in this paper is the first of several results to be announced in the near future, including one that he describes as“mind-blowing.”

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Australian-led research in nanotechnology a huge breakthrough

(PhysOrg.com) -- Groundbreaking research in quantum light source led by the University of Sydney will result in information speeds many times faster and data that is almost impossible to hack.

The breakthrough, which uses silicon photonic crystals to slow downlight, is a collaboration between Centre of Excellence for Ultrahigh Bandwidth Devices forOptical Systems(CUDOS) nodes at the University of Sydney and Macquarie University, along with colleagues at the University of Bristol and the University of St Andrews (UK), and the Ecole Centrale de Lyon in France.

CUDOS researchers have generated individual pairs of photons in the smallest device ever by slowing light down using silicon photonic crystals. At 100 microns long (approximately the thickness of a human hair) CUDOS's quantum photon device is 100 times smaller than the one-centimetre devices used by other groups.

Dr Chunle Xiong of the University of Sydney, a co-author and project leader for the CUDOS program in Quantum Integrated Photonics, says the device's nano-scale means that potentially hundreds of these photon devices can be incorporated into a single chip. This is a key step to building practicalquantum technologiesthat will make communications much more secure and computations many times faster.

"We are able to do this by slowing light down through the use of siliconphotonic crystals, which means the ultrashort device behaves as a much longer device, so that the photons are generated in only 100 microns,"says Dr Xiong.

Macquarie University's Associate Professor Michael Steel, co-author and CUDOS Chief Investigator, says:"Current systems use classical light to carry information, which hackers can easily tap into and use to their advantage. But you cannot copy the information encoded in quantum states without being noticed by the system. Single photon devices will ensure communication and information systems are secure from hackers, guaranteeing peace of mind for the users."

This pioneering technology will ensure the next generation of information systems is secure and faster, says the University of Sydney's Professor Ben Eggleton, co-author and director of ARC Centre of Excellence CUDOS. The experiment is outlined in a groundbreaking paper to be presented at a prestigious international conference in Baltimore, USA next week for the world's leading researchers in laser and quantum electronics.

Professor Eggleton says this breakthrough is taking CUDOS 'Mark II' into a new and exciting direction. Federal Minister for Innovation, Industry, Science and Research, Senator Kim Carr, officially launched CUDOS II only three weeks ago.

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Electron ping pong in the nano-world

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(PhysOrg.com) -- An international team of researchers succeeded at the Max Planck Institute of Quantum Optics to control and monitor strongly accelerated electrons from nano-spheres with extremely short and intense laser pulses. (<i>Nature Physics</i>, 24. April 2011).

When intense laser light interacts with electrons innanoparticlesthat consist of many million individual atoms, these electrons can be released and strongly accelerated.

Such an effect in nano-spheres of silica was recently observed by an international team of researchers in the Laboratory for Attosecond Physics (LAP) at the Max Planck Institute ofQuantum Optics. The researchers report how strong electrical fields (near-fields) build up in the vicinity of the nanoparticles and release electrons. Driven by the near-fields and collective interactions of the charges resulting fromionizationby the laser light, the released electrons are accelerated, such that they can by far exceed the limits in acceleration that were observed so far for single atoms. The exact movement of the electrons can be precisely controlled via the electric field of the laser light. The new insights into this light-controlled process can help to generate energetic extreme ultraviolet (XUV) radiation. The experiments and their theoretical modeling, which are described by the scientists in the journalNature Physics, open up new perspectives for the development of ultrafast, light-controlled nano-electronics, which could potentially operate up to one million times faster than current electronics.
 
Electron acceleration in a laser field is similar to a short rally in a ping pongmatch:  a serve, a return and a smash  securing the point. A similar scenario occurs when electrons in nanoparticles are hit by light pulses. An international team, led by three German groups including Prof. Matthias Kling from the Laboratory for Attosecond Physics (LAP) at the Max Planck Institute of Quantum Optics in Garching and the Ludwig-Maximilian University Munich, Prof. Eckart Rühl from the Free University of Berlin and Prof. Thomas Fennel from the University of Rostock, was now successful in observing the mechanisms and aftermath of such a ping pong play of electrons in nanoparticles interacting with strong laser light-fields.

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Figure 2: Amplified near-fields at the poles of a silica nanosphere. The local field on the polar axis is plotted as function of time, where time within the few-cycle wave runs from the lower right to the upper left. The fields show a pronounced asymmetry along the polarization axis of the laser (i.e. along the rims and valleys of the wave). This asymmetry leads to higher energies gained by electrons on one side of the nanoparticle as compared to the other side. For the given example the most energetic electrons are emitted from the backside, where the highest peak field is reached. The energies of the electrons and their emission directions are determined from the experiment. Credit: Christian Hackenberger/LMU

The researchers illuminatedsilicananoparticles with a size of around 100 nm with very intense light pulses, lasting around five femtoseconds (one femtosecond is a millionth of a billionth of a second). Such shortlaser pulsesconsist of only a few wave cycles. The nanoparticles contained around 50 million atoms each. The electrons are ionized within a fraction of a femtosecond and accelerated by the electric field of the remaining laser pulse. After travelling less than one nanometer away from the surface of the nano-spheres, some of the electrons can be returned to the surface by the laser field to the surface, where they were smashed right back (such as the ping pong ball being hit by the paddle). The resulting energy gain of the electrons can reach very high values. In the experiment electron energies of ca. 60 times the energy of a 700 nm wavelength laser photon (in the red spectral region of light) have been found.

For the first time, the researchers could observe and record the direct elastic recollision phenomenon from a nanosystem in detail. The scientists used polarized light for their experiments. With polarized light, the light waves are oscillating only along one axis and not, as with normal light, in all directions.“Intense radiation pulses can deform or destroy nanoparticles. We have thus prepared the nanoparticles in a beam, such that fresh nanoparticles were used for every laser pulse. This was of paramount importance for the observation of the highly energetic electrons.”, explains Eckart Rühl.

The accelerated electrons left the atoms with different directions and different energies. The flight trajectories were recorded by the scientists in a three-dimensional picture, which they used to determine the energies and emission directions of the electrons.“The electrons were not only accelerated by the laser-induced near-field, which by itself was already stronger than the laser field, but also by the interactions with other electrons, which were released from the nanoparticles,” describes Matthias Kling about the experiment. Finally, the positive charging of the nanoparticle-surface also plays a role. Since all contributions add up, the energy of the electrons can be very high.“The process is complex, but shows that there is much to explore in the interaction of nanoparticles with strong laser fields,” adds Kling.

The electron movements can also produce pulses of extreme ultraviolet light when electrons that hit the surface do not bounce back, but are absorbed releasing photons with wavelengths in the XUV. XUV light is of particular interest for biological and medical research.“According to our findings, the recombination of electrons on the nanoparticles can lead to energies of the generated photons, which are up to seven-times higher than the limit that was so far observed for single atoms,” explains Thomas Fennel. The evidence of collective acceleration ofelectronswith nanoparticles offers great potential. Matthias Kling believes that“From this may arise new, promising applications in future, light-controlled ultrafast electronics, which may work up to one million times faster than conventional electronics.”

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Sensitivity of precision measurements enhanced by the environment

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(PhysOrg.com) -- When it comes to quantum measurements, interaction with the environment usually limits sensitivity, since it causes decoherence. But in a new study, scientists have shown that the environment can be advantageous. They have designed a method to increase the sensitivity of quantum precision measurements by using the environment to enhance a quantum sensor’s response to weak perturbations in an external field.

The researchers, Garry Goldstein from Harvard University, along with coauthors from Harvard, MIT, Copenhagen University, and the California Institute of Technology, have published their study called“Environment-Assisted Precision Measurement” in a recent issue ofPhysical Review Letters. In their study, the scientists first describe an idealized case, and then demonstrate that it works in two different cases: quantum clocks with trapped ions and spin-based magnetometry.

“We realized that part of the environment can be used to increase sensitivity,” coauthor Paola Cappellaro of MIT toldPhysOrg.com.“We found that entangled states, other than the ones usually proposed for metrology (GHZ states, squeezed states) can improve the sensitivity while being more robust to decoherence.”

As the scientists explain, a quantum sensor can be constructed with a central spin coupled to a bath of dark spins, which are part of the environment. All of these spins act as qubits, each having a state of 0, 1, or a superposition of both. While the central spin can be controlled and read out, the dark spins can only be collectively controlled and not directly detected. Also, the central spin and the dark spins can be coupled, and this coupling can be effectively turned on and off at will.

The central spin can indirectly measure the external field, such as a magnetic field, by sensing the dynamics of the surrounding dark spins, which are in turn affected by the external field. To do this, the researchers first entangled the central spin to the dark spins, and then used this entangled state to sense the external field. As the entangled dark spins evolve, they acquire a phase that depends on the state of the central spin. Then, the researchers could flip the central spin and read out its signal. By reading this signal, the researchers could measure the phase difference between the states of the dark spins, which provides a measurement of the external field.

Importantly, the additional phase difference due to the dark spins amplifies the signal of the central spin and allows it to read out a smaller field than before; the smaller the field that a sensor can read out during a given time, the higher its quantum sensitivity. While the signal is enhanced, the background noise stays the same.

“Here we assume that part of the environment (the‘dark spins’) can be controlled, although it cannot be directly measured,” Cappellaro said.“In this scenario, there are two possible strategies: manipulate the environment dark spins to decouple them from the sensor or exploit them by creating an entangled state with the sensor spin. We found that this second strategy is viable and yields better sensitivity.”

Overall, the method achieves precision that approaches the Heisenberg limit. This limit results from the Heisenberg uncertainty principle and marks the maximum sensitivity that any measurement can achieve.

When comparing this method to another measurement precision procedure based on a spin-echo, the researchers found that the new method has greater sensitivity due to the central spin’s signal amplification. Both methods have about the same coherence times, since, for both methods, decoherence arises from interactions among dark spins, not the rest of the environment.

As the simulations demonstrated, the new method could have applications in improving clocksensitivityusing trapped ions and magnetic sensing based on electronic spins in diamond. The scientists also predict that this method could be applied more generally to a wide variety of systems.

“Extremely sensitive clocks are very important, for example, for global positioning,” Cappellaro said.“Magnetic sensors could find applications in a broad range of areas, from materials science to bio-imaging.”

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Single atom stores quantum information

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(PhysOrg.com) -- A data memory can hardly be any smaller: researchers working with Gerhard Rempe at the Max Planck Institute of Quantum Optics in Garching have stored quantum information in a single atom. The researchers wrote the quantum state of single photons, i.e. particles of light, into a rubidium atom and read it out again after a certain storage time. This technique can be used in principle to design powerful quantum computers and to network them with each other across large distances.

Quantum computers will one day be able to cope with computational tasks in no time where current computers would take years. They will take their enormous computing power from their ability to simultaneously process the diverse pieces of information which are stored in the quantum state of microscopic physical systems, such as single atoms and photons. In order to be able to operate, the quantum computers must exchange these pieces of information between their individual components. Photons are particularly suitable for this, as no matter needs to be transported with them. Particles of matter however will be used for the information storage and processing. Researchers are therefore looking for methods whereby quantum information can be exchanged between photons and matter. Although this has already been done with ensembles of many thousands of atoms, physicists at the Max Planck Institute of Quantum Optics in Garching have now proved that quantum information can also be exchanged between single atoms and photons in a controlled way.

Using a single atom as a storage unit has several advantages - the extreme miniaturization being only one, says Holger Specht from the Garching-based Max Planck Institute, who was involved in the experiment. The stored information can be processed by direct manipulation on the atom, which is important for the execution of logical operations in a quantum computer."In addition, it offers the chance to check whether the quantum information stored in the photon has been successfully written into the atom without destroying the quantum state,"says Specht. It is thus possible to ascertain at an early stage that a computing process must be repeated because of a storage error.

The fact that no one had succeeded until very recently in exchanging quantum information between photons and single atoms was because the interaction between the particles of light and the atoms is very weak. Atom and photon do not take much notice of each other, as it were, like two party guests who hardly talk to each other, and can therefore exchange only a little information. The researchers in Garching have enhanced the interaction with a trick. They placed a rubidium atom between the mirrors of an optical resonator, and then used very weak laser pulses to introduce single photons into the resonator. The mirrors of the resonator reflected the photons to and fro several times, which strongly enhanced the interaction between photons and atom. Figuratively speaking, the party guests thus meet more often and the chance that they talk to each other increases.

The photons carried the quantum information in the form of their polarization. This can be left-handed (the direction of rotation of the electric field is anti-clockwise) or right-handed (clock-wise). The quantum state of the photon can contain both polarizations simultaneously as a so-called superposition state. In the interaction with the photon the rubidium atom is usually excited and then loses the excitation again by means of the probabilistic emission of a further photon. The Garching-based researchers did not want this to happen. On the contrary, the absorption of the photon was to bring the rubidium atom into a definite, stable quantum state. The researchers achieved this with the aid of a further laser beam, the so-called control laser, which they directed onto the rubidium atom at the same time as it interacted with the photon.

The spin orientation of the atom contributes decisively to the stable quantum state generated by control laser and photon. Spin gives the atom a magnetic moment. The stable quantum state, which the researchers use for the storage, is thus determined by the orientation of the magnetic moment. The state is characterized by the fact that it reflects the photon's polarization state: the direction of the magnetic moment corresponds to the rotational direction of the photon's polarization, a mixture of both rotational directions being stored by a corresponding mixture of the magnetic moments.

This state is read out by the reverse process: irradiating the rubidium atom with the control laser again causes it to re-emit the photon which was originally incident. In the vast majority of cases, the quantum information in the read-out photon agrees with the information originally stored, as the physicists in Garching discovered. The quantity that describes this relationship, the so-called fidelity, was more than 90 percent. This is significantly higher than the 67 percent fidelity that can be achieved with classical methods, i.e. those not based on quantum effects. The method developed in Garching is therefore a real quantum memory.

The physicists measured the storage time, i.e. the time the quantum information in the rubidium can be retained, as around 180 microseconds."This is comparable with the storage times of all previous quantum memories based on ensembles of atoms,"says Stephan Ritter, another researcher involved in the experiment. Nevertheless, a significantly longer storage time is necessary for the method to be used in a quantum computer or a quantum network. There is also a further quality characteristic of the single-atom quantum memory from Garching which could be improved: the so-called efficiency. It is a measure of how many of the irradiated photons are stored and then read out again. This was just under 10 percent.

The storage time is mainly limited by magnetic field fluctuations from the laboratory surroundings, says Ritter."It can therefore be increased by storing the quantum information in quantum states of the atoms which are insensitive to magnetic fields."The efficiency is limited by the fact that the atom does not sit still in the centre of the resonator, but moves. This causes the strength of the interaction between atom and photon to decrease. The researchers can thus also improve the efficiency: by greater cooling of the atom, i.e. by further reducing its kinetic energy.

The researchers at the Max Planck Institute in Garching now want to work on these two improvements."If this is successful, the prospects for the single-atom quantum memory would be excellent,"says Stephan Ritter. The interface between light and individual atoms would make it possible to network more atoms in a quantum computer with each other than would be possible without such an interface; a fact that would make such a computer more powerful. Moreover, the exchange of photons would make it possible to quantum mechanically entangle atoms across large distances. The entanglement is a kind of quantum mechanical link between particles which is necessary to transport quantum information across large distances. The technique now being developed at the Max Planck Institute of Quantum Optics could some day thus become an essential component of a future"quantum Internet".

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Quantum simulation with light: Frustrations between photon pairs

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Researchers of the University of Vienna used a quantum mechanical system in the laboratory to simulate complex many-body systems. This experiment promises future quantum simulators enormous potential insights into unknown quantum phenomena.

Researchers from the Vienna Center for Quantum Science and Technology at the University of Vienna and the Institute ofQuantum OpticsandQuantum Informationat the Austrian Academy of Sciences used a quantummechanical systemin the laboratory to simulate complex many-body systems. This experiment, which is published inNature Physics, promises future quantum simulators enormous potential insights into unknownquantum phenomena.

Already the behavior of relatively small quantum systems cannot be calculated because quantum states contain much more information than their classical counter-parts. However, if another quantum system is used to simulate the quantum system of interest, then answers about the properties of the complex quantum system can be obtained.

When is a quantum system frustrated?

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"says Philip Walther (University of Vienna)."Now we can prepare quantum states of individual photons to gain insights into otherquantum systems", explains Xiao-song Ma (Austrian Academy of Sciences).Therefore, two in polarization entangled photons exhibit in many ways the same quantum physical properties as for example electrons in matter.

Conflict over partnerships

The research team of international scientists from China, Serbia, New Zeeland and Austria prepared single photons that were facing the conflict over partnerships between each other. Each photon can establish a single bond to only one partner exclusively, but wants to get correlated with several partners– obviously this leads to frustration. As a result, the quantum system uses"tricks"that allow quantum fluctuations that different pairings can coexist as superposition.

The work of the Viennese group underlines that quantum simulations are a very good tool for calculating quantum states of matter and are thus opening the path for the investigation of more complex systems.

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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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10 billion bits of entanglement achieved in silicon

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(PhysOrg.com) -- Scientists from Oxford University have made a significant step towards an ultrafast quantum computer by successfully generating 10 billion bits of quantum entanglement in silicon for the first time– entanglement is the key ingredient that promises to make quantum computers far more powerful than conventional computing devices.

The researchers used high magnetic fields and low temperatures to produce entanglement between the electron and the nucleus of an atom of phosphorous embedded in a highly purifiedsiliconcrystal. The electron and the nucleus behave as a tiny magnet, or 'spin', each of which can represent a bit of quantum information. Suitably controlled, these spins can interact with each other to be coaxed into an entangled state– the most basic state that cannot be mimicked by a conventional computer.

An international team from the UK, Japan, Canada and Germany, report their achievement in this week's Nature.

‘The key to generating entanglement was to first align all the spins by using high magnetic fields and low temperatures,’ said Stephanie Simmons of Oxford University’s Department of Materials, first author of the report.‘Once this has been achieved, the spins can be made to interact with each other using carefully timed microwave and radiofrequency pulses in order to create the entanglement, and then prove that it has been made.’

The work has important implications for integration with existing technology as it uses dopant atoms in silicon, the foundation of the modern computer chip. The procedure was applied in parallel to a vast number of phosphorous atoms.

‘Creating 10 billion entangled pairs in silicon with high fidelity is an important step forward for us,’ said co-author Dr John Morton of Oxford University’s Department of Materials who led the team.‘We now need to deal with the challenge of coupling these pairs together to build a scalable quantum computer in silicon.’

In recent yearsquantum entanglementhas been recognised as a key ingredient in building new technologies that harness quantum properties. Famously described by Einstein as“spooky action at distance”– when two objects are entangled it is impossible to describe one without also describing the other and the measurement of one object will reveal information about the other object even if they are separated by thousands of miles.

Creating true entanglement involves crossing the barrier between the ordinary uncertainty encountered in our everyday lives and the strange uncertainties of the quantum world. For example, flipping a coin there is a 50% chance that it comes up heads and 50% tails, but we would never imagine the coin could land with both heads and tails facing upwards simultaneously: a quantum object such as the electron spin can do just that.

Dr Morton said:‘At high temperatures there is simply a 50/50 mixture of spins pointing in different directions but, under the right conditions, all the spins can be made to point in two opposing directions at the same time. Achieving this was critical to the generation of spinentanglement.’

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Quantum quirk contained: Discovery moves quantum networks closer to reality

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Researchers at the University of Calgary, in Canada, collaborating with the University of Paderborn, in Germany, are working on a way to make quantum networks a reality and have published their findings in the journal<i>Nature</i>. A similar finding by a group at the University of Geneva, in Switzerland is reported in the same issue.

“We have demonstrated, for the first time, that a crystal can storeinformationencoded into entangled quantum states of photons,” says paper co-author Dr. Wolfgang Tittel of the University of Calgary’s Institute for Quantum Information Science.“This discovery constitutes an important milestone on the path toward quantum networks, and will hopefully enable building quantum networks in a few years.”

In current communication networks, information is sent through pulses of light moving through optical fibre. The information can be stored on computer hard disks for future use.

Quantum networks operate differently than the networks we use daily.

“What we have is similar but it does not use pulses of light,” says Tittel, who is a professor in the Department of Physics and Astronomy at the University of Calgary.“In quantum communication, we also have to store and retrieve information. But in our case, the information is encoded into entangled states of photons.”

In this state, photons are“entangled,” and remain so even when they fly apart. In a way, they communicate with each other even when they are very far apart. The difficulty is getting them to stay put without breaking this fragile quantum link.

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Wolfgang Tittel of the University of Calgary is researching ways of integrating quantum memory with current telecommunication technology. Credit: Riley Brandt/University of Calgary

To achieve this task, the researchers used a crystal doped with rare-earth ions and cooled it to -270 Celsius. At these temperatures, material properties change and allowed the researchers to store and retrieve these photons without measurable degradation.

An important feature is that this memory device uses almost entirely standard fabrication technologies.“The resulting robustness, and the possibility to integrate the memory with current technology such as fibre-optic cables is important when moving the currently fundamental research towards applications.”

Quantum networkswill allow sending information without being afraid of somebody listening in.

“The results show that entanglement, a quantum physical property that has puzzled philosophers and physicists since almost hundred years, is not as fragile as is generally believed,” says Tittel.

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