Wednesday, April 23, 2014

Researchers Bolster Development of Programmable Quantum Computers

Researchers bolster development of programmable quantum computers

University of Chicago researchers and their colleagues at University College London have performed a proof-of-concept experiment that will aid the future development of programmable quantum computers.

Many complex problems are difficult and slow to solve using conventional computers, and over the last several years, research has grown steadily toward developing quantum computation. In particular, optimization problems such as the "traveling salesman" problem, which calculates the shortest possible route needed to visit a set of towns, become intractable as the number of towns grows.

[caption id="attachment_296" align="alignleft" width="400"]Researchers bolster development of programmable quantum computers The spheres in this image represent the atoms of a quantum magnetic material, while the arrows denote their spin (magnetic orientation). The green bars indicate coupling between atoms, and the adjacent ghosted images illustrate quantum tunneling between different spin directions, a prominent characteristic of quantum mechanical systems. This particular spin configuration displays a quantum solution to a challenging computational problem.[/caption]

A quantum computer would exploit effects on the atomic and molecular scales to solve such problems dramatically faster than conventional computers. Recently a first generation of specialized computers has become available—with a new architecture that exploits quantum mechanics to help solve problems akin to the traveling salesman problem, with up to a few hundred towns.

In a study published in the Proceedings of the National Academy of Sciences, a team from the James Franck Institute at UChicago and the London Centre for Nanotechnology at University College London describes an experiment that was performed on a crystal containing trillions, rather than hundreds, of quantum mechanical spins, which replicates some of the features of the current generation of much smaller, specialized computers.

The lead author is Michael Schmidt, PhD'12, now a research scientist with Intel in Portland. His co-authors are Daniel Silevitch, research scientist in the James Franck Institute; Thomas Rosenbaum, the John T. Wilson Distinguished Service Professor in Physics; and Prof. Gabriel Aeppli of University College London.

The crystalline quantum magnet used to perform this experiment contains atoms whose spins (magnetic orientation) oscillate. Thermal annealing and quantum annealing are the processes by which the researchers manipulated the magnetic spins in this experimental magnetic crystal. Many types of magnetic materials can orient spins in any direction, but this special crystal limits the orientation to either up or down.

Quantum annealing relates to quantum tunneling, a phenomenon that allows particles to pass through barriers via interactions that Newtonian physics cannot predict. "If you run the system in a regime where quantum tunneling is completely turned off, then you end up with one solution to your problem, and a different solution when quantum tunneling is turned on," Silevitch said.

In this magnetic crystal at temperatures near absolute zero (minus 459.67 degrees Fahrenheit), the speed and strength of thermal annealing can be controlled by rods of sapphire attached to a refrigerator via more or less contact with the crystal. At the same time, the rate of quantum annealing can be controlled by means of a magnetic field, which sets the rate of quantum tunneling in the magnetic sample.

Thermal annealing can only be turned down by cooling the system, but it cannot be turned off. But if the system runs in a mode where thermal annealing is turned down and quantum annealing is turned up, the result is a different state of magnetic spins, which represents a different solution to the computational problem.

The special purpose computer solves problems such as the traveling salesman problem in a semi-abstract landscape where the heights and depths of features represent the total distance traveled. The best solution corresponds to the deepest valley.

Finding the deepest valley can be visualized as a pool of water moving between valleys, either via a wave splashing over the intermediate saddle points and then descending, or via quantum tunneling between valleys.

The first approach represents thermal annealing, which is comparable to conventional computing methods. The second corresponds to quantum annealing, a characteristic of potentially more capable quantum computing.

Thermal annealing reaches a final state, or problem solution, by hopping over the energy barriers, then gradually restricting the size of the barrier that it can overcome via lowering the temperature. Quantum annealing, by contrast, reaches the final state via quantum tunneling through the barriers, then gradually clamping down (and ultimately turning off) the tunneling rate.

In thermal annealing, the "waves" slosh back and forth, and if they reach a sufficient height, they will splash over the hill and then drain into an adjacent valley.

High-temperature thermal annealing corresponds to violently sloshing water, which means that it can surmount high barriers. As the researchers slowly drop the strength of the waves, the water can only top middle-sized hills. With further cooling of the system, the waves can only wash over molehills.

A problem arises, however, for thermal annealing when a bowl-like valley sits next to a deeper, narrower well. In this situation, most of the sloshing water will end up at the bottom of the valley. Water naturally seeks its lowest level, but as the temperature drops and the wave heights become reduced, the entrance of the well becomes inaccessible.

Quantum annealing allows the water to pass through the hill via the quantum tunneling process.

"If you have this bowl, and then right next to it there's this really deep well, the odds of getting out of the bowl into the well through thermal annealing is very, very low," Silevitch explained. "You have to wait for a randomly big wave to come sloshing over. But with quantum annealing, you can go right through the hill and you can find that deep well, which is where you prefer to be."

The experiments found that when the system reached its final valley via thermal annealing alone, it was dramatically different from the state reached when the thermal annealing was weakened and quantum annealing was turned on.

After the application of quantum annealing, certain regions of the crystal were in "quantum superposition states," which can simultaneously exist in two different states according to the counter-intuitive rules of quantum physics. Other regions have the characteristics typical of the physics that predominates at macroscopic scales. Thermal annealing in these experiments leaves behind regions exclusively of the latter variety.

Applied to practical and programmable quantum optimization computers, the results imply that quantum optimizers could obtain different solutions to problems such as the traveling salesman problem, when compared with conventional techniques. The research team concluded that these findings would affect both the design and use of quantum optimization systems.


Citation: "Using thermal boundary conditions to engineer the quantum state of a bulk magnet," by M.A. Schmidt, D.M. Silevitch, G. Aeppli, and T.F. Rosenbaum," Proceedings of the National Academy of Sciences. First published online Feb. 24, 2014, then in the print March 11, 2014, print edition.

Funding: U.S. Department of Energy and the United Kingdom's Engineering and Physical Sciences Research Council.

News Release Source :  Researchers bolster development of programmable quantum computers

Tuesday, April 22, 2014

New 'switch' could help in the development of powerful quantum computing systems

New 'switch' could power quantum computing

A light lattice that traps atoms may help scientists build networks of quantum information transmitters

Using a laser to place individual rubidium atoms near the surface of a lattice of light, scientists at MIT and Harvard University have developed a new method for connecting particles — one that could help in the development of powerful quantum computing systems.

The new technique, described in a paper published today in the journal Nature, allows researchers to couple a lone atom of rubidium, a metal, with a single photon, or light particle. This allows both the atom and photon to switch the quantum state of the other particle, providing a mechanism through which quantum-level computing operations could take place.

[caption id="attachment_291" align="aligncenter" width="500"]New 'switch' could help in the development of powerful quantum computing systems New 'switch' could help in the development of powerful                                   quantum computing systems[/caption]

Moreover, the scientists believe their technique will allow them to increase the number of useful interactions occurring within a small space, thus scaling up the amount of quantum computing processing available.

"This is a major advance of this system," says Vladan Vuletic, a professor in MIT's Department of Physics and Research Laboratory for Electronics (RLE), and a co-author of the paper. "We have demonstrated basically an atom can switch the phase of a photon. And the photon can switch the phase of an atom."

That is, photons can have two polarization states, and interaction with the atom can change the photon from one state to another; conversely, interaction with the photon can change an atom's energy level from its "ground" state to its "excited" state. In this way the atom-photon coupling can serve as a quantum switch to transmit information — the equivalent of a transistor in a classical computing system. And by placing many atoms within the same field of light, the researchers may be able to build networks that can process quantum information more effectively.

"You can now imagine having several atoms placed there, to make several of these devices — which are only a few hundred nanometers thick, 1,000 times thinner than a human hair — and couple them together to make them exchange information," Vuletic adds.

Using a photonic cavity

Quantum computing could enable the rapid performance of calculations by taking advantage of the distinctive quantum-level properties of particles. Some particles can be in a condition of superposition, appearing to exist in two places at the same time. Particles in superposition, known as qubits, could thus contain more information than particles at classical scales, and allow for faster computing.

However, researchers are in the early stages of determining which materials best allow for quantum-scale computing. The MIT and Harvard researchers have been examining photons as a candidate material, since photons rarely interact with other particles. For this reason, an optical quantum computing system, using photons, could be harder to knock out of its delicate alignment. But since photons rarely interact with other bits of matter, they are difficult to manipulate in the first place.

In this case, the researchers used a laser to place a rubidium atom very close to the surface of a photonic crystal cavity, a structure of light. The atoms were placed no more than 100 or 200 nanometers — less than a wavelength of light — from the edge of the cavity. At such small distances, there is a strong attractive force between the atom and the surface of the light field, which the researchers used to trap the atom in place.

Other methods of producing a similar outcome have been considered before — such as, in effect, dropping atoms into the light and then finding and trapping them. But the researchers found that they had greater control over the particles this way.

"In some sense, it was a big surprise how simple this solution was compared to the different techniques you might envision of getting the atoms there," Vuletic says.

The result is what he calls a "hybrid quantum system," where individual atoms are coupled to microscopic fabricated devices, and in which atoms and photons can be controlled in productive ways. The researchers also found that the new device serves as a kind of router separating photons from each other.

"The idea is to combine different things that have different strengths and weaknesses in such a way to generate something new," Vuletic says, adding: "This is an advance in technology. Of course, whether this will be the technology remains to be seen."

'Still amazing' to hold onto one atom

The paper, "Nanophotonic quantum phase switch with a single atom," is co-authored by Vuletic; Tobias Tiecke, a postdoc affiliated with both RLE and Harvard; Harvard professor of physics Mikhail Lukin; Harvard postdoc Nathalie de Leon; and Harvard graduate students Jeff Thompson and Bo Liu.

The collaboration between the MIT and Harvard researchers is one of two advances in the field described in the current issue of Nature. Researchers at the Max Planck Institute of Quantum Optics in Germany have concurrently developed a new method of producing atom-photon interactions using mirrors, forming quantum gates, which change the direction of motion or polarization of photons.

If the research techniques seem a bit futuristic, Vuletic says that even as an experienced researcher in the field, he remains slightly awed by the tools at his disposal.

"For me what is still amazing, after working in this for 20 years," Vuletic reflects, "is that we can hold onto a single atom, we can see it, we can move it around, we can prepare quantum superpositions of atoms, we can detect them one by one."


Written by Peter Dizikes, MIT News Office

Funding for the research was provided in part by the National Science Foundation, the MIT-Harvard Center for Ultracold Atoms, the Natural Sciences and Engineering Research Council of Canada, the Air Force Office of Scientific Research, and the Packard Foundation.

News Release Source :  New 'switch' could power quantum computing

Image Credit : Christine Daniloff/MIT

Monday, April 21, 2014

Opens The Door to Multi-Party Quantum Communication

Experiment opens the door to multi-party quantum communication

In the world of quantum science, Alice and Bob have been talking to one another for years. Charlie joined the conversation a few years ago, but now with spacelike separation, scientists have measured that their communication occurs faster than the speed of light.

[caption id="attachment_283" align="aligncenter" width="440"]Opens The Door to Multi-Party Quantum Communication Opens The Door to Multi-Party Quantum Communication[/caption]

For the first time, physicists at the Institute for Quantum Computing (IQC) at the University of Waterloo have demonstrated the distribution of three entangled photons at three different locations (Alice, Bob and Charlie) several hundreds of metres apart, proving quantum nonlocality for more than two entangled photons.

The findings of the experiment, Experimental Three-Particle Quantum Nonlocality under Strict Locality Conditions, are published in Nature Photonics today.

Once described by Einstein as "spooky action at a distance", this three-photon entanglement leads to interesting possibilities for multi-party quantum communication.

Nonlocality describes the ability of particles to instantaneously know about each other's state, even when separated by large distances. In the quantum world, this means it might be possible to transfer information instantaneously – faster than the speed of light. This contravenes what Einstein called the "principle of local action," the rule that distant objects cannot have direct influence on one another, and that an object is directly influenced only by its immediate surroundings.

To truly test that the hidden local variables are not responsible for the correlation between the three photons, IQC scientists needed the experiment to close what is known as the locality loophole. They achieved this separation of the entangled photons in a way that did not allow for a signal to coordinate the behaviour of the photons, but beaming the entangled photons to trailers parked in fields several hundred meters from their lab.

"Correlations measured from quantum systems can tell us a lot about nature at the most fundamental level," said co-author Professor Kevin Resch, Canada Research Chair in Optical Quantum Technologies and recent winner of the E.W.R. Steacie Fellowship from the Natural Sciences and Engineering Research Council of Canada (NSERC). "Three-particle entanglement is more complex than that of pairs. We can exploit the complex behaviour to rule out certain descriptions of nature or as a resource for new quantum technologies.

The project team studied the correlations of three photons in a Greenberger-Horne-Zeilinger (GHZ) state – a type of entangled quantum state involving at least three particles.

First, photon triplets were generated in Resch's lab – the Alice in the experiment. Then, the first photon was delayed in a 580m optical fibre in the lab while the two other photons travelled up 85m of optical fibre to the rooftop where they were sent through two telescopes. Both photons were then sent to two trailers, Bob and Charlie, about 700m away from the source and from each other.

To maintain the spacelike separate in the experiment, a fourth party, Randy, located in a third trailer randomly selected each of the measurements that Alice was to perform on her photons in the lab.

Each trailer contained detectors, time-tagging devices developed by IQC spin off company Universal Quantum Devices (UQD), and quantum random number generators. To ensure the locality loophole was closed, the random number generators determined how the photon at each trailer would be measured independently. The UQD time tagging devices also ensured the measurements happened in a very small time window (three nanoseconds), meaning that no information could possibly be transmitted from one location to the other during the measurement period ¬– a critical condition to prove the non-locality of entanglement.

"The idea of entangling three photons has been around for a long time," said Professor Thomas Jennewein, a co-author of the paper. "It took the right people with the right knowledge to come together to make the experiment happen in the short time it did. IQC had the right mix at the right time."

The experiment demonstrated the distribution of three entangled particles, which can eventually be used to do more than pairwise communication where only one party can communicate with another. It opens the possibility for multipartite quantum communication protocols, including Quantum Key Distribution (QKD), third man cryptography and quantum secret sharing.

"The interesting result is that we now have the ability to do more than paired quantum communication," said the paper's lead author Chris Erven, a former IQC PhD student who is now a research assistant at the University of Bristol. "QKD, so far, has been a pairwise system – meaning that it works best and with less assumptions when you're only talking with one other person. This is the first experiment where you can now imagine a network of people connected in different ways using the correlations between three or more photons."

The team from the Institute for Quantum Computing and the Department of Physics and Astronomy in the Faculty of Science at the University of Waterloo included students Chris Erven, Evan Meyer-Scott, Kent Fisher, Jonathan Lavoie, Christopher Pugh, Jean-Paul Bourgoin, Laura Richards, Nickolay Gigov, postdoctoral fellow Brendon Higgins, Professor Jennewein, Professor Resch and Raymond Laflamme, executive director of IQC.

The project team also included former IQC postdoctoral fellows Robert Prevedel, now at the Max F. Perutz Laboratories (MFPL) and the Institute for Molecular Pathology (IMP); Zhizhong Yan, now at Macquarie University; Krister Shalm now at the National Institute of Standards and Technology (NIST); and former faculty member Gregor Weihs, now at the Institut fur Experimentalphysik at the University of Innsbruck.

News Release Source :  Experiment opens the door to multi-party quantum communication

Tuesday, April 1, 2014

Ultracold molecules promising for quantum computing

'Ultracold' molecules promising for quantum computing, simulation

WEST LAFAYETTE, Ind. – Researchers have created a new type of "ultracold" molecule, using lasers to cool atoms nearly to absolute zero and then gluing them together, a technology that might be applied to quantum computing, precise sensors and advanced simulations.

[caption id="attachment_275" align="aligncenter" width="500"]Ultracold molecules promising for quantum computing Ultracold molecules promising for quantum computing[/caption]

"It sounds counterintuitive, but you can use lasers to take away the kinetic energy, resulting in radical cooling," said Yong P. Chen, an associate professor of physics and electrical and computer engineering at Purdue University.

Physicists are using lasers to achieve such extreme cooling, reducing the temperature to nearly absolute zero, or minus 273 degrees Celsius (minus 459 degrees Fahrenheit) - the lowest temperature possible in the universe.

At these temperatures atoms are brought to a near standstill, making possible new kinds of chemical interactions that are predominantly quantum mechanical in nature. The process is performed inside of an apparatus called a magneto-optical trap, a system that uses a vacuum chamber, magnetic coils and a series of lasers to cool and trap the atoms.

"This is our test tube," said Daniel S. Elliott, a professor of electrical and computer engineering and physics. "In ultracold chemistry, molecules are really moving slowly so they have a long time to interact with each other."

Other researchers have used the method to create cold molecules out of atoms of other alkali metals, which are relatively easy to turn into ultracold molecules. The Purdue researchers are the first to achieve the milestone with the alkali metals lithium and rubidium, in work led by Chen and Elliott.

Findings are detailed in a research paper that appeared as a "Rapid Communication" in the February issue of the journal Physical Review A, a publication of the American Physical Society. The paper was authored by former Purdue physics doctoral student Sourav Dutta, who has graduated; graduate students John Lorenz and Adeel Altaf; Elliott and Chen. The paper is available online at

The method is called photoassociation: two atoms are merged using lasers to induce a chemical bond between them, forming a molecule. These molecules may contain two of the same types of atoms - making them homonuclear - or they can contain two different types of atoms, heteronuclear, such as the case with the lithium-rubidium molecules created by the team.

If the molecules are heteronuclear there is a difference in electric charge between these two atoms and the molecule is said to be polar. This difference in charge is called a dipole moment, which enables interaction between molecules. The greater the dipole moment, the stronger the interaction.

The lithium-rubidium molecule is potentially ideal for various applications, including quantum computing, because it has a significant dipole moment, which can enable these molecules to be used as "quantum bits."

Quantum computers would take advantage of a phenomenon described by quantum theory called "entanglement." Instead of only the states of one and zero used in conventional computer processing, there are many possible "entangled quantum states" in between one and zero, dramatically increasing the capacity to process information.

"In quantum computing the larger the dipole moment the stronger the interaction would be between molecules, and you need that interaction," Elliott said. "They need to interact with each other in order to affect each other, the key to entanglement."

Another potential advantage for the lithium-rubidium molecule is that it can be produced in large quantities.

"The rate of production is much greater for lithium-rubidium than for other bi-alkali-metal molecules," Chen said. "That was a pleasant surprise. It was already known that it has the third- largest dipole moment among bi-alkali-metal molecules, but nobody expected it would be made so efficiently."

Ultracold means temperatures less than about one thousandth of degree above absolute zero. Achieving such frigid extremes requires reducing the kinetic energy of molecules as well as their "internal excitation energies," which are stored in three ways: the rotation of the molecule itself, the vibrations of the atomic nuclei, and the movement of electrons in "shells" surrounding the nuclei. The combined energy of the trio is called rovibronic, a shortened version of rotational, vibrational and electronic.

"We are reporting a highly efficient production of ultracold lithium-rubidium molecules by photoassociation," Dutta said. "This provides the first step towards the production of such ultracold lithium-rubidium molecules in their ground, polar state."

Molecules in their "ground state" have the lowest possible rovibronic energy, which would make them more stable and easier to control.

A related research paper was also published by the team in January in the journal Europhysics Letters, a publication of the European Physical Society. That paper is available online at

"Lithium rubidium is one of the last bi-alkali molecules to be made cold, and we are the first to do this," Chen said. "People knew virtually nothing about these molecules."

Ultimately, researchers are seeking more efficient methods for the production of ultracold molecules.

The research has been funded by Purdue's Bilsland Dissertation Fellowship, the National Science Foundation, Army Research Office, and more recently by a research incentive grant from Purdue's Office of Vice President for Research.

The research falls within a field called AMO, for atomic, molecular, and optical physics, an area under expansion at Purdue.

"AMO physics is an exciting area in the landscape of experimental and theoretical physics," Elliott said. "Seven years ago we had one person working in this area."

Since then, the department has added three faculty members working in AMO and is in the process of adding more.

"Purdue is positioned to become a leader in AMO physics," Chen said.


Writer: Emil Venere, 765-494-4709,

Sources: Yong Chen, 765-494-0947,
Daniel S. Elliott, 765-494-3442,

Related websites:
Yong Chen:
Daniel S. Elliott:


Photoassociation of ultracold LiRb∗ molecules: Observation of high efficiency and unitarity-limited rate saturation Sourav Dutta,1 , * John Lorenz,1 Adeel Altaf,1 , D. S. Elliott,1, 2 and Yong P. Chen 1, 2
1 Department of Physics, Purdue University
2 School of Electrical and Computer Engineering, Purdue University

We report the production of ultracold heteronuclear 7 Li85 Rb molecules in excited electronic states by photoassociation (PA) of ultracold 7 Li and 85 Rb atoms. PA is performed in a dual-species 7 Li-85 Rb magneto-optical trap (MOT) and the PA resonances are detected using trap loss spectroscopy. We identify several strong PA resonances below the Li (2s 2 S1/2 ) + Rb (5p 2 P3/2 ) asymptote and xperimentally determine the long range C6 dispersion coefficients. We find a molecule formation rate (PLiRb ) of 3.5 × 10 7 s−1 and a PA rate coefficient (KPA ) of 1.3 × 10−10 cm3 /s, the highest among heteronuclear bi-alkali-metal molecules. At large PA laser intensity we observe the saturation of the PA rate coefficient (KPA ) close to the theoretical value at the unitarity limit.

News Release Source :    'Ultracold' molecules promising for quantum computing, simulation

Seeking quantum-ness: D-Wave chip passes rigorous tests

Seeking quantum-ness: D-Wave chip passes rigorous tests

With cutting-edge technology, sometimes the first step scientists face is just making sure it actually works as intended.

The USC Viterbi School of Engineering is home to the USC-Lockheed Martin Quantum Computing Center (QCC), a super-cooled, magnetically shielded facility specially built to house the first commercially available quantum computing processors – devices so advanced that there are only two in use outside the Canadian lab where they were built: The first one went to USC and Lockheed Martin, and the second to NASA and Google.

[caption id="attachment_271" align="aligncenter" width="500"]Seeking quantum-ness: D-Wave chip passes rigorous tests Seeking quantum-ness: D-Wave chip passes rigorous tests[/caption]

Since USC's facility opened in October 2011, a key task for researchers has been to determine whether D-Wave processors operate as hoped – using the special laws of quantum mechanics to offer potentially higher-speed processing, instead of operating in a classical, traditional way.

An international collaboration of scientists has now published several papers rejecting classical models of the first-generation D-Wave One processor housed at USC, including one on an elaborate test of all 108 of the chip's functional quantum bits ("qubits"). The test demonstrates that the D-Wave One behaved in a way that agrees with a model called "quantum Monte Carlo," yet disagreed with two candidate classical models that could have described the processor in the absence of quantum effects.

The research was published on Feb. 28 by Nature Physics.

"The challenge is that the tests we can perform on the USC-based D-Wave processor can't directly 'prove' that the D-Wave processor is quantum – we can only disprove candidate classical models one at a time," said QCC Director Prof. Daniel Lidar. "But so far we find that the D-Wave processor is always consistent with our quantum models. Our tests continually get more rigorous and complex."

Add this to recent work involving USC Information Sciences Institute researcher Federico Spedalieri demonstrating entanglement in a chip at the company's headquarters in Burnaby BC as well as previous testing of a smaller group of qubits by Spedalieri, Lidar and their collaborators, and the evidence is mounting that quantum effects are at play in the D-Wave processors.

Quantum processors encode data in qubits, which have the capability of representing the two digits of one and zero at the same time – as opposed to traditional bits, which can encode distinctly either a one or a zero. This property, called superposition, along with the ability of quantum states to "interfere" (cancel or reinforce each other like waves in a pond) and "tunnel" through energy barriers, is what may one day allow quantum processors ultimately perform optimization calculations much faster than traditional processors.

Optimization problems can take many forms, and quantum processors have been theorized to be useful for a variety of big data problems like stock portfolio optimization, image recognition and classification, and detecting anomalies, such as rooting out bugs in complex software.

The first quantum chip housed at the QCC was a 128-qubit D-Wave One, which was replaced about a year ago with the 512-qubit D-Wave Two. Though every chip is unique, the repeated validation of the older chip bodes well for its successor, which shares the same architecture.

"Our work is part of a large scale effort by the research community aimed at validating the potential of quantum information processing, which we all hope might one day surpass its classical counterparts," Lidar said.

This research was funded by the Swiss National Science Foundation, the Army Research Office, the Lockheed Martin Corporation, DARPA, and the National Science Foundation.

News Release Source :  Seeking quantum-ness: D-Wave chip passes rigorous tests