Quantum computing research edges toward practicality in UCSB physics lab

(PhysOrg.com) -- An important step -- one that is essential to the ultimate construction of a quantum computer -- was taken for the first time by physicists at UC Santa Barbara. The discovery is published in the current issue of the journal<i>Nature</i>.

The research involves the entanglement of threequantum bitsof information, orqubits. Before now, entanglement research in the solid state has only been developed with two qubits. The UCSB finding comes from a collaboration of the research groups of physicists Andrew Cleland and John Martinis. Graduate student Matthew Neeley is the first author on theNaturepaper. Meanwhile, a research group at Yale reported the same result.

"These entangled states are interesting in their own right, but they are also very important from the perspective of the larger, long-term goal of creating a quantum computer with many qubits,"said Neeley.

The Cleland-Martinis group is studying superconducting quantum circuits and their potential uses inquantum computing. Quantum circuits are fabricated on microchips using techniques similar to those used in making conventional computers. When cooled to very low temperatures–– just a few hundredths of a degree above absolute zero–– they become superconducting and exhibit quantum effects. Essentially behaving like artificial atoms, they can be manipulated and measured using electrical signals. Unlike atoms, however, these circuits can be designed to have only the properties that the scientists desire for various experiments–– providing a tool for exploring many of the fundamental aspects of quantum mechanics.

The simplest type of quantum system is one with just two possible states, known as a quantum bits by analogy with the classical bits that are the fundamental elements of conventional computers. UCSB's team uses quantum circuits of a type known as phase qubits, designed to behave as two-levels quantum systems. In this most recent work, the team fabricated and operated a device with three coupled phase qubits, using them to produce entangled quantum states.

"Entanglement is one of the strangest and most counterintuitive features of quantum mechanics,"said Neeley."It is a property of certain kinds of quantum states in which different parts of the system are strongly correlated with each other. This is often discussed in the context of bipartite systems with just two components. However, when one considers tripartite or larger quantum systems, the physics of entanglement becomes even richer and more interesting."

In this work, the team produced entangled states of three qubits. Neeley explained that unlike the two-qubit case, three qubits can be entangled in two fundamentally different ways, exemplified by a state known as GHZ, and another state known as W. The GHZ state is highly entangled but fragile, and measuring just one of the qubits collapses the other two into an unentangled state.

"The W state is in a certain sense less entangled, but nevertheless more robustly so–– two thirds of the time, measuring one qubit will still leave the other two in an entangled state,"Neeley said."We produced both of these states with our phase qubits, and measured their fidelity compared to the theoretical ideal states. Experimentally, the fidelity is never perfect, but we showed that it is high enough to prove that the three qubits are entangled."

"Entanglement is a resource that gives quantum computers an advantage over classical computers, and so producing multipartiteentanglementis an important step for any system with which we might hope to construct a quantum computer,"said Neeley.

The same result was published simultaneously, based on similar research from the group of Rob Schoelkopf, a physics professor at Yale. Both results are the first work showing three coupled superconducting qubits. This is a significant step toward scaling to increasingly larger numbers of qubits.

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Debunking and closing quantum entanglement 'loopholes'

(PhysOrg.com) -- An international team of physicists, including a scientist based at The University of Queensland, has recently closed an additional 'loophole' in a test explaining one of science's strangest phenomena -- quantum entanglement.

Quantum entanglement is aphenomenonthat connects two particles (for example, photons) in such a way that changes to one of the particles are reflected instantly in the other, even if they are light-years apart.

“Despite the enormous success ofquantum mechanics, its completeness is experimentally still unproven after more than 75 years,” said Dr Alessandro Fedrizzi (now in UQ's School of Mathematics and Physics).

Dr Fedrizzi co-wrote the findings of the study together with a team from the Institute for Quantum Optics and Quantum Information, and the University of Vienna in Austria, led by Professor Anton Zeilinger.

In 1935, physicists Albert Einstein, Boris Podolsky and Nathan Rosen (EPR) argued in a now-famous paper that“(t)he quantum mechanical description of physical reality is incomplete”.

According to EPR,“hidden variables” must exist to explain the unintuitive results of experiments with entangled particles.

In 1964, John Bell developed his famousBell Inequalityas the basis to test for the existence of these hidden variables.

In an experiment, this inequality demonstrates that quantum correlations can be stronger than that explained by the local hidden variable theory earlier proposed by EPR.

In practice, this is achieved by performing measurements on two separated quantum particles.

Numerous Bell tests have concluded in favour of the principles of quantum mechanics, but some researchers still question the tests’ validity due to perceived“loopholes”, namely, the detection loophole (not all particles can be detected), the locality loophole (the outcomes or settings of one measurement could influence the outcomes of another measurement), and the freedom of choice loophole (the choice of the settings themselves could influence or be influenced by the hidden variables carried by the particle pair).

In their study, published online on November 1, 2010 in theProceedings of the National Academy of Sciences,the team conducted a Bell test that eliminated two of these loopholes: locality, and, for the first time, freedom of choice.

The researchers distributed entangled photons between two islands in the Atlantic Ocean.

To close both loopholes, they carefully located and timed the photon emission events, setting choices (which were generated by quantum number generators), and measurements (which were implemented by fast electro-optical switches).

In four 600-second long measurements carried out over a distance of 144km, the researchers conducted measurements on 19,917 photon pairs, which significantly violated Bell’s Inequality, in favour of quantum mechanics.

The authors concluded that the experiment represents the closest to a loophole-free Bell test to date.

“We are still chasing a loophole-free Bell experiment and we probably will be for a while,"Dr Fedrizzi said.

"Closing the freedom of choiceloopholehas however, narrowed down the potential classical theories explaining quantum mechanics and is an essential step towards closing this important chapter in science.”

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Could light and matter coupling lead to quantum computation?

(PhysOrg.com) -- In science, one of the issues of great interest is that of quantum computing, and creating a way to make it possible on a scalable level. This could be achieved by taking advantage of the strong interaction between light and matter, the so-called strong-coupling regime that can be found in ultra small optical cavities defined by photonic crystals.

“The first step is to ensure that such strongly coupled cavity is created within or at close proximity to a photonic channel for on-chip computation, which is what we demonstrated here,” Frederic Brossard tellsPhysOrg.com.

Brossard and colleagues at the Hitachi Cambridge Laboratory, together with fellow scientists at the university of Oxford and University of Sheffield, achieved this with a quantum dot and a cavity directly embedded in a photonic crystal waveguide, the photonic channel. Their work can be found inApplied Physics Letters:“Strongly coupled single quantum dot in a photonic crystal waveguide cavity.”

“From a purely optical point of view, the scalability of such structure, the resonant coupling between multiple cavities has already been demonstrated in silicon, Brossard says. (See M. Notomi, et al.,Nat. Photonics2, 741 2008.)“So it is now amatterof including quantum emitters such asquantum dotsinside these cavities fabricated in III-V materials.”

Such chain of strongly coupled cavities has been shown to be feasible for quantum operations by various groups, including colleagues of Brossard at the university of Cambridge. (See D. G. Angelakis*, et al.,Phys. Rev. A76, 031805R 2007.) However, some challenges must be overcome first:“Basically each dot has to be positioned at or very close to one of the field maxima of the nanoscale cavity,” Brossard explains.“The closer you are to a field maximum, the larger the interaction strength between the dot and the cavity mode.”

The team is encouraged by the single quantum dot that they were able to strongly couple with their cavity.“Thankfully, the cavity chosen for this study enables a relaxation in the conditions necessary for strong coupling when compared to those required in previous work by other researchers,” Brossard says.“This type of cavity makes it easier to align a quantum dot with a field maximum because of the larger volume occupied by the mode.”

Another advantage of the team’s work is the possibility that losses will be low.“It also has the potential of very low optical losses, which means that the coherence of the system can be maintained on a longer time scale,” he continues.

A chain of coupled systems will require some sort of alignment procedure between a chosen dot and the cavity. Brossard says that this is something that the group at the University of Oxford, led by Prof. Robert Taylor, ha developed over the last few years in collaboration with the group at Hitachi. (See K. H. Lee, et al.,Appl. Phys. Lett.88, 193106 2006.)

“Right now, the demonstration provides interesting insights into the ability to couplelightand matter by nanometer size modifications of the photonic crystal waveguide,” Brossard says. In the future, he thinks that optical quantum computation is a very real possibility:“After we scale up the system, we can probe it, and see what is possible. We want to try it with input and output to see an exchange of information that might indicate its fitness forquantum computing.”

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Physicists demonstrate a four-fold quantum memory

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Researchers at the California Institute of Technology (Caltech) have demonstrated quantum entanglement for a quantum state stored in four spatially distinct atomic memories.

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 entanglementis 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.

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In the Caltech experiment, these four ensembles of atoms serve as four spatially distinct quantum memories that globally store an entangled quantum state. The Caltech researchers demonstrate the coherent read out of the entangled state of the atomic memories to an entangled state for separate four beams of light. Credit: Akihisa Goban/Nature

Previously, Kimble's group entangled a pair of atomic quantum memories and coherently transferred the entangled photons into and out of the quantum memories (http://media.caltech.edu/press_releases/13115). 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 wasfirst 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."

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Physicists detect and control quantum states in diamond with light

Physicists at UC Santa Barbara have succeeded in combining laser light with trapped electrons to detect and control the electrons' fragile quantum state without erasing it. This is an important step toward using quantum physics to expand computing power and to communicate over long distances without the possibility of eavesdropping. The work appears online today at<i>Science Express</i>.

The research, led by David Awschalom, professor of physics, electrical and computer engineering, and director of UCSB's Center forSpintronicsand Quantum Computation, and graduate student Bob Buckley, exploits an unusual property of the microscopic quantum world: the ability to combine things that are very different.

Using electrons trapped in a single atom-sized defect within a thin crystal of diamond, combined with laser light of precisely the right color, the scientists showed that it was possible to briefly form a mixture of light and matter. After forming this light-matter mixture, they were able to use measurements of the light to determine the state of the electrons.

Likewise, by separately examining theelectrons, they showed that the electron configuration was not destroyed by the light. Instead, it was modified -- a dramatic demonstration of control over quantum states using light."Manipulating thequantum stateof a single electron in a semiconductor without destroying the information represents an extremely exciting scientific development with potential technological impact,"said Awschalom.

Preserving quantum states is a major obstacle in the nascent field of quantum computing. One benefit ofquantum informationis that it can never be copied, unlike information transferred between today's computers, providing a measure of security that is safeguarded by fundamental laws of nature. The ability to measure a quantum state without destroying it is an important step in the development of technologies that harness the advantages of the quantum world.

Buckley, putting this research in perspective, said:"Diamond may someday become for a quantum computer what silicon is for digital computers today -- the building blocks of logic, memory, and communication. Our experiment provides a new tool to make that happen."

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Heisenberg Uncertainty Principle sets limits on Einstein's 'spooky action at a distance,' new research finds

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Researchers have uncovered a fundamental link between the two defining properties of quantum physics. Stephanie Wehner of Singapore's Centre for Quantum Technologies and the National University of Singapore and Jonathan Oppenheim of the United Kingdom's University of Cambridge published their work today in the latest edition of the journal<i>Science</i>.

The result is being heralded as a dramatic breakthrough in our basic understanding ofquantum mechanicsand provides new clues to researchers seeking to understand the foundations ofquantum theory. The result addresses the question of why quantum behaviour is as weird as it is—but no weirder.

The strange behaviour of quantum particles, such as atoms, electrons and the photons that make up light, has perplexed scientists for nearly a century. Albert Einstein was among those who thought the quantum world was so strange that quantum theory must be wrong, but experiments have borne out the theory's predictions.

One of the weird aspects of quantum theory is that it is impossible to know certain things, such as a particle's momentum and position, simultaneously. Knowledge of one of these properties affects the accuracy with which you can learn the other. This is known as the"Heisenberg Uncertainty Principle".

Another weird aspect is the quantum phenomenon of non-locality, which arises from the better-known phenomenon of entanglement. When two quantum particles are entangled, they can perform actions that look as if they are coordinated with each other in ways that defy classical intuition about physically separated particles.

Previously, researchers have treated non-locality and uncertainty as two separate phenomena. Now Wehner and Oppenheim have shown that they are intricately linked. What's more, they show that this link is quantitative and have found an equation which shows that the"amount"of non-locality is determined by the uncertainty principle.

"It's a surprising and perhaps ironic twist,"said Oppenheim, a Royal Society University Research Fellow from the Department of Applied Mathematics&Theoretical Physics at the University of Cambridge. Einstein and his co-workers discovered non-locality while searching for a way to undermine the uncertainty principle."Now the uncertainty principle appears to be biting back."

Non-locality determines how well two distant parties can coordinate their actions without sending each other information. Physicists believe that even in quantum mechanics, information cannot travel faster than light. Nevertheless, it turns out that quantum mechanics allows two parties to coordinate much better than would be possible under the laws of classical physics. In fact, their actions can be coordinated in a way that almost seems as if they had been able to talk. Einstein famously referred to this phenomenon as"spooky action at a distance".

However, quantum non-locality could be even spookier than it actually is. It's possible to have theories which allow distant parties to coordinate their actions much better than nature allows, while still not allowing information to travel faster than light. Nature could be weirder, and yet it isn't– quantum theory appears to impose an additional limit on the weirdness.

"Quantum theory is pretty weird, but it isn't as weird as it could be. We really have to ask ourselves, why is quantum mechanics this limited? Why doesn't nature allow even stronger non-locality?"Oppenheim says.

The surprising result by Wehner and Oppenheim is that the uncertainty principle provides an answer. Two parties can only coordinate their actions better if they break the uncertainty principle, which imposes a strict bound on how strong non-locality can be.

"It would be great if we could better coordinate our actions over long distances, as it would enable us to solve many information processing tasks very efficiently,"Wehner says."However, physics would be fundamentally different. If we break the uncertainty principle, there is really no telling what our world would look like."

How did the researchers discover a connection that had gone unnoticed so long? Before entering academia, Wehner worked as a 'computer hacker for hire', and now works in quantum information theory, while Oppenheim is a physicist. Wehner thinks that applying techniques from computer science to the laws of theoretical physics was key to spotting the connection."I think one of the crucial ideas is to link the question to a coding problem,"Wehner says."Traditional ways of viewing non-locality and uncertainty obscured the close connection between the two concepts."

Wehner and Oppenheim recast the phenomena of quantum physics in terms that would be familiar to a computer hacker. They treat non-locality as the result of one party, Alice, creating and encoding information and a second party, Bob, retrieving information from the encoding. How well Alice and Bob can encode and retrieve information is determined by uncertainty relations. In some situations, they found that and a third property known as"steering"enters the picture.

Wehner and Oppenheim compare their discovery to uncovering what determines how easily two players can win a quantum board game: the board has only two squares, on which Alice, can place a counter of two possible colours: green or pink. She is told to place the same colour on both squares, or to place a different colour on each. Bob has to guess the colour that Alice put on square one or two. If his guess is correct, Alice and Bob win the game. Clearly, Alice and Bob could win the game if they could talk to each other: Alice would simply tell Bob what colours are on squares one and two. But Bob and Alice are situated so far apart from each other that light– and thus an information-carrying signal– does not have time to pass between them during the game.

If they can't talk, they won't always win, but by measuring on quantum particles, they can win the game more often than any strategy which doesn't rely on quantum theory. However, the uncertainty principle prevents them from doing any better, and even determines how often they lose the game.

The finding bears on the deep question of what principles underliequantum physics. Many attempts to understand the underpinnings of quantum mechanics have focused on non-locality. Wehner thinks there may be more to gain from examining the details of the uncertainty principle."However, we have barely scratched the surface of understanding uncertainty relations,"she says.

The breakthrough is future-proof, the researchers say. Scientists are still searching for a quantum theory of gravity and Wehner and Oppenheim's result concerning non-locality, uncertainty and steering applies to all possible theories– including any future replacement for quantum mechanics.

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Long distance, top secret messages

When the military needs to send the key to encrypted data across the world, it can't necessarily rely on today's communication lines, where the message could be covertly intercepted. But physicists at the Georgia Institute of Technology in Atlanta are developing a new, more secure way to send such information across far distances, using existing cables and the laws of quantum mechanics.

Alex Kuzmich and colleagues have built a critical component of a quantum repeater, a device that allowsquantum communications-- such as the encryption keys used to encode data transmitted over traditional lines -- to be relayed over larger distances. They will describe this device at the Optical Society's (OSA) 94th annual meeting, Frontiers in Optics (FiO) 2010, at the Rochester Riverside Convention Center in Rochester, N.Y., from Oct. 24-28.

Quantum cryptographyis an emerging technology currently used by both military and financial organizations to send information as entangled particles of light. In theory, anyone who tries to tap into this information changes it in a way that reveals their presence.

A quantum repeater is similar to a transformer on a traditional power line. Instead of converting electricity, it regenerates a communication signal to prevent it from degrading over distance. It contains two banks of memory, one to receive an entangled message and a second line to copy it.

Previously, the longest distance over which an encrypted key could be sent was approximately 100 kilometers. The new technology developed by the Georgia Tech team increases 30-fold the amount of time the memory can hold information, which means that series of these devices -- arrayed like Christmas lights on a string -- could reach distances in excess of 1,000 kilometers.

"This is another significant step toward improvingquantum informationsystems based onneutral atoms. For quantum repeaters, most of the basic steps have now been made, but achieving the final benchmarks required for an operating system will require intensive optical engineering efforts,"says Kuzmich.

Their device also converts the photons used in quantum devices from an infrared wavelength of 795 nm to a wavelength of 1,367 nm. This wavelength is used in traditional telecommunications lines, so the new device could someday plug into existing fiber optic cables.

"In order to preserve the quantum entanglement, we perform conversion at very high efficiency and with low noise,"says Alexander Radnaev, who also works on this project at Georgia Tech.

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