Quantum Memory for Communication Networks of the Future

Quantum networks will be able to protect the security of information better than the current conventional communication networks. The cornerstone of quantum communication is a phenomenon called entanglement between two quantum systems, for example, two light beams. Entanglement means that the two light beams are connected to each other, so that they have well defined common characteristics, a kind of common knowledge. A quantum state can -- according to the laws of quantum mechanics, not be copied and can therefore be used to transfer data in a secure way.

In professor Eugene Polzik's research group Quantop at the Niels Bohr Institute researchers have now been able to store the two entangled light beams in two quantum memories. The research is conducted in a laboratory where a forest of mirrors and optical elements such as wave plates, beam splitters, lenses etc. are set up on a large table, sending the light around on a more than 10 meter long labyrinthine journey. Using the optical elements, the researchers control the light and regulate the size and intensity to get just the right wavelength and polarisation the light needs to have for the experiment.

The two entangled light beams are created by sending a single blue light beam through a crystal where the blue light beam is split up into two red light beams. The two red light beams are entangled, so they have a common quantum state. The quantum state itself is information.

The two light beams are sent on through the labyrinth of mirrors and optical elements and reach the two memories, which in the experiment are two glass containers filled with a gas of caesium atoms. The atoms' quantum state contains information in the form of a so-called spin, which can be either 'up' or 'down'. It can be compared with computer data, which consists of the digits 0 and 1. When the light beams pass the atoms, the quantum state is transferred from the two light beams to the two memories. The information has thus been stored as the new quantum state in the atoms.

"For the first time such a memory has been demonstrated with a very high degree of reliability. In fact, it is so good that it is impossible to obtain with conventional memory for light that is used in, for example, internet communication. This result means that a quantum network is one step closer to being a reality," explains professor Eugene Polzik.

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Quantum Computers Easier to Build: Can Tolerate Faulty or Missing Components, Researchers Say

This surprising discovery brings scientists one step closer to designing and building real-life quantum computing systems -- devices that could have enormous potential across a wide range of fields, from drug design, electronics, and even code-breaking.

Scientists have long been fascinated with building computers that work at a quantum level -- so small that the parts are made of just single atoms or electrons. Instead of 'bits', the building blocks normally used to store electronic information, quantum systems use quantum bits or 'qubits', made up of an arrangement of entangled atoms.

Materials behave very differently at this tiny scale compared to what we are used to in our everyday lives -- quantum particles, for example, can exist in two places at the same time."Quantum computers can exploit this weirdness to perform powerful calculations, and in theory, they could be designed to break public key encryption or simulate complex systems much faster than conventional computers," said Dr Sean Barrett, the lead author of the study, who is a Royal Society University Research Fellow in the Department of Physics at Imperial College London.

The machines have been notoriously hard to build, however, and were thought to be very fragile to errors. In spite of considerable buzz in the field in the last 20 years, useful quantum computers remain elusive.

Barrett and his colleague Dr. Thomas Stace, from the University of Queensland in Brisbane, Australia, have now found a way to correct for a particular sort of error, in which the qubits are lost from the computer altogether. They used a system of 'error-correcting' code, which involved looking at the context provided by the remaining qubits to decipher the missing information correctly.

"Just as you can often tell what a word says when there are a few missing letters, or you can get the gist of a conversation on a badly-connected phone line, we used this idea in our design for a quantum computer," said Dr Barrett. They discovered that the computers have a much higher threshold for error than previously thought -- up to a quarter of the qubits can be lost -- but the computer can still be made to work."It's surprising, because you wouldn't expect that if you lost a quarter of the beads from an abacus that it would still be useful," he added.

The findings indicate that quantum computers may be much easier to build than previously thought, but as the results are still based on theoretical calculations, the next step is to actually demonstrate these ideas in the lab. Scientists will need to devise a way for scaling the computers to a sufficiently large number of qubits to be viable, says Barrett. At the moment the biggest quantum computers scientists have built are limited to just two or three qubits.

"We are still some way off from knowing what the true potential of a quantum computer might be, says Barrett."At the moment quantum computers are good at particular tasks, but we have no idea what these systems could be used for in the future," he said."They may not necessarily be better for everything, but we just don't know. They may be better for very specific things that we find impossible now."

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All-Optical Transistor: Controling the Flow of Light With a Novel Optical Transistor

Controlling and modulating the flow of light is essential in today's telecommunications-based society. Professor Tobias Kippenberg and his team in EPFL's Laboratory of Photonics and Quantum Measurements have discovered a novel way to couple light and vibrations. Using this discovery, they built a device in which a beam of light traveling through an optical microresonator could be controlled by a second, stronger light beam. The device thus acts like an optical transistor, in which one light beam influences the intensity of another.

Their optical microresonator has two characteristics: first, it traps light in a tiny glass structure, guiding the beam into a circular pattern. Second, the structure vibrates, like a wine glass, at well-defined frequencies. Because the structure is so tiny (a fraction of the diameter of a human hair), these frequencies are 10,000 times higher than a wineglass vibration. When light is injected into the device, the photons exert a force called radiation pressure, which is greatly enhanced by the resonator. The increasing pressure deforms the cavity, coupling the light to the mechanical vibrations. If two light beams are used, the interaction of the two lasers with the mechanical vibrations results in a kind of optical"switch": the strong"control" laser can turn on or off a weaker"probe" laser just as in a electronic transistor.

"We have known for more than two years that this effect was theoretically possible," explains Max-Planck Institute scientist Albert Schliesser, but pinning it down proved difficult."Once we knew where to look, it was right there," recalls EPFL PhD student Stefan Weis, one of the lead authors of the paper. Senior EPFL scientist Samuel Deléglise notes that"the agreement between theory and experiment is really striking."

Applications of this novel effect, baptised"OMIT" (optomechanically-induced transparency), could provide entirely new functionality to photonics. Radiation-to-vibration conversions are already widely used; in mobile phones, for example, a receiver converts electromagnetic radiation to mechanical vibration, enabling the signal to be filtered efficiently. But it has been impossible to do this kind of conversion with light. With an OMIT-based device, an optical light field could for the first time be converted into a mechanical vibration. This could open up a huge range of possibilities in telecommunications. For example, novel optical buffers could be designed that could store optical information for up to several seconds.

On a more fundamental level, researchers around the world have been trying to find ways to control optomechanical systems at the quantum level: the switchable coupling demonstrated by the EPFL-Max Planck team could help the community clear this hurdle, by serving as an important interface in hybrid quantum systems.

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A New Electromagnetism Can Be Simulated Through a Quantum Simulator

There are two fundamental aspects that make these devices attractive for scientists. On the one hand, quantum simulators will play a leading role in clarifying some important, but yet unsolved, puzzles of theoretical physics.. On the other hand, such deeper understanding of a given phenomenon will certainly give rise to useful technological applications.

One of the best quantum simulators consists of a gas of extremely cold atoms loaded in an artificial crystal made of light: an optical lattice. Experimental physicists have developed efficient techniques to control the quantum properties of this system, to such extent, that it serves as an ideal quantum simulator of different phenomena.

So far, efforts have been focused on condensed-matter systems, where many open and interesting problems remain to be solved.

In a recent work published inPhysical Review Lettersby a collaboration of international teams (Universidad Complutense de Madrid: A. Bermudez and M.A. Martin-Delgado; ICFO Barcelona: M. Lewenstein; Max-Planck Institute Garching: L. Mazza, M. Rizzi; Universite de Brussels: N. Goldman), this platform has also been shown to be a potential quantum simulator of high-energy physics.

The authors have proposed a clean and controllable setup where a variety of exotic, but still unobserved, phenomena arise. They describe how to build a quantum simulator of Axion Electrodynamics (high-energy physics), and 3D Topological Insulators (condensed matter). In particular, these results pave the way to the fabrication of an Axion, a long sought-after missing particle in the standard model of elementary particles. They show that their atomic setup constitutes an axion medium, where an underlying topological order gives rise to a non-vanishing axion field.

Besides, they show how the value of the axion can attain arbitrary values, and how its dynamics and space-dependence can be experimentally controlled. Accordingly, their optical-lattice simulator offers a unique possibility to observe diverse effects, such as the Wiiten effect, the Wormhole effect, or a fractionally charged capacitor, in atomic-physics laboratories.

This work has an interdisciplinary character, which brings together physicists specializing in lattice gauge theories, atomic molecular and optical physics, and condensed matter physics.

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Physicists Demonstrate a Four-Fold Quantum Memory

Their work, described in the November 18 issue of the journalNature,also demonstrated a quantum interface between the atomic memories -- which represent something akin to a computer"hard drive" for entanglement -- and four beams of light, thereby enabling the four-fold entanglement to be distributed by photons across quantum networks. The research represents an important achievement in quantum information science by extending the coherent control of entanglement from two to multiple (four) spatially separated physical systems of matter and light.

The proof-of-principle experiment, led by William L. Valentine Professor and professor of physics H. Jeff Kimble, helps to pave the way toward quantum networks. Similar to the Internet in our daily life, a quantum network is a quantum"web" composed of many interconnected quantum nodes, each of which is capable of rudimentary quantum logic operations (similar to the"AND" and"OR" gates in computers) utilizing"quantum transistors" and of storing the resulting quantum states in quantum memories. The quantum nodes are"wired" together by quantum channels that carry, for example, beams of photons to deliver quantum information from node to node. Such an interconnected quantum system could function as a quantum computer, or, as proposed by the late Caltech physicist Richard Feynman in the 1980s, as a"quantum simulator" for studying complex problems in physics.

Quantum entanglement is a quintessential feature of the quantum realm and involves correlations among components of the overall physical system that cannot be described by classical physics. Strangely, for an entangled quantum system, there exists no objective physical reality for the system's properties. Instead, an entangled system contains simultaneously multiple possibilities for its properties. Such an entangled system has been created and stored by the Caltech researchers.

Previously, Kimble's group entangled a pair of atomic quantum memories and coherently transferred the entangled photons into and out of the quantum memories. For such two-component -- or bipartite -- entanglement, the subsystems are either entangled or not. But for multi-component entanglement with more than two subsystems -- or multipartite entanglement -- there are many possible ways to entangle the subsystems. For example, with four subsystems, all of the possible pair combinations could be bipartite entangled but not be entangled over all four components; alternatively, they could share a"global" quadripartite (four-part) entanglement.

Hence, multipartite entanglement is accompanied by increased complexity in the system. While this makes the creation and characterization of these quantum states substantially more difficult, it also makes the entangled states more valuable for tasks in quantum information science.

To achieve multipartite entanglement, the Caltech team used lasers to cool four collections (or ensembles) of about one million Cesium atoms, separated by 1 millimeter and trapped in a magnetic field, to within a few hundred millionths of a degree above absolute zero. Each ensemble can have atoms with internal spins that are"up" or"down" (analogous to spinning tops) and that are collectively described by a"spin wave" for the respective ensemble. It is these spin waves that the Caltech researchers succeeded in entangling among the four atomic ensembles.

The technique employed by the Caltech team for creating quadripartite entanglement is an extension of the theoretical work of Luming Duan, Mikhail Lukin, Ignacio Cirac, and Peter Zoller in 2001 for the generation of bipartite entanglement by the act of quantum measurement. This kind of"measurement-induced" entanglement for two atomic ensembles was first achieved by the Caltech group in 2005.

In the current experiment, entanglement was"stored" in the four atomic ensembles for a variable time, and then"read out" -- essentially, transferred -- to four beams of light. To do this, the researchers shot four"read" lasers into the four, now-entangled, ensembles. The coherent arrangement of excitation amplitudes for the atoms in the ensembles, described by spin waves, enhances the matter-light interaction through a phenomenon known as superradiant emission.

"The emitted light from each atom in an ensemble constructively interferes with the light from other atoms in the forward direction, allowing us to transfer the spin wave excitations of the ensembles to single photons," says Akihisa Goban, a Caltech graduate student and coauthor of the paper. The researchers were therefore able to coherently move the quantum information from the individual sets of multipartite entangled atoms to four entangled beams of light, forming the bridge between matter and light that is necessary for quantum networks.

The Caltech team investigated the dynamics by which the multipartite entanglement decayed while stored in the atomic memories."In the zoology of entangled states, our experiment illustrates how multipartite entangled spin waves can evolve into various subsets of the entangled systems over time, and sheds light on the intricacy and fragility of quantum entanglement in open quantum systems," says Caltech graduate student Kyung Soo Choi, the lead author of the Nature paper. The researchers suggest that the theoretical tools developed for their studies of the dynamics of entanglement decay could be applied for studying the entangled spin waves in quantum magnets.

Further possibilities of their experiment include the expansion of multipartite entanglement across quantum networks and quantum metrology."Our work introduces new sets of experimental capabilities to generate, store, and transfer multipartite entanglement from matter to light in quantum networks," Choi explains."It signifies the ever-increasing degree of exquisite quantum control to study and manipulate entangled states of matter and light."

In addition to Kimble, Choi, and Goban, the other authors of the paper are Scott Papp, a former postdoctoral scholar in the Caltech Center for the Physics of Information now at the National Institute of Standards and Technology in Boulder, Colorado, and Steven van Enk, a theoretical collaborator and professor of physics at the University of Oregon, and an associate of the Institute for Quantum Information at Caltech.

This research was funded by the National Science Foundation, the National Security Science and Engineering Faculty Fellowship program at the U.S. Department of Defense (DOD), the Northrop Grumman Corporation, and the Intelligence Advanced Research Projects Activity.

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