Simplifying the process of detecting genuine multiparticle entanglement

Enlarge

(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.”

---

To thank our25,000fans in the Facebook community, this story was posted on thePhysorg.com FB pagea few hours before going live on the main site

---

“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.”

Source

Apparent roadblock in the development of quantum lithography

Enlarge

(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.

Source

D-Wave researchers demonstrate progress in quantum computing

Enlarge

(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.”

Source

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.

Source

Electron ping pong in the nano-world

Enlarge

(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.

Enlarge

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.”

Source

Sensitivity of precision measurements enhanced by the environment

Enlarge

(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.”

Source

Single atom stores quantum information

Enlarge

(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".

Source