Friday, April 25, 2014

Superconducting qubit array points the way to quantum computers

Superconducting qubit array points the way to quantum computers


A new 5-qubit array from UCSB's Martinis Group is on the threshold of making a quantum computer technologically feasible to build

A fully functional quantum computer is one of the holy grails of physics. Unlike conventional computers, the quantum version uses qubits (quantum bits), which make direct use of the multiple states of quantum phenomena. When realized, a quantum computer will be millions of times more powerful at certain computations than today's supercomputers.
Superconducting qubit array points the way to quantum computers
These are control signals for all five qubits.

A group of UC Santa Barbara physicists has moved one step closer to making a quantum computer a reality by demonstrating a new level of reliability in a five-qubit array. Their findings appear Thursday in the journal Nature.

Quantum computing is anything but simple. It relies on aspects of quantum mechanics such as superposition. This notion holds that any physical object, such as an atom or electron — what quantum computers use to store information — can exist in all of its theoretical states simultaneously. This could take parallel computing to new heights.

"Quantum hardware is very, very unreliable compared to classical hardware," says Austin Fowler, a staff scientist in the physics department, whose theoretical work inspired the experiments of the Martinis Group. "Even the best state-of-the-art hardware is unreliable. Our paper shows that for the first time reliability has been reached."

While the Martinis Group has shown logic operations at the threshold, the array must operate below the threshold to provide an acceptable margin of error. "Qubits are faulty, so error correction is necessary," said graduate student and co-lead author Julian Kelly who worked on the five-qubit array.

"We need to improve and we would like to scale up to larger systems," said lead author Rami Barends, a postdoctoral fellow with the group. "The intrinsic physics of control and coupling won't have to change but the engineering around it is going to be a big challenge."
The unique configuration of the group's array results from the flexibility of geometry at the superconductive level, which allowed the scientists to create cross-shaped qubits they named Xmons. Superconductivity results when certain materials are cooled to a critical level that removes electrical resistance and eliminates magnetic fields. The team chose to place five Xmons in a single row, with each qubit talking to its nearest neighbor, a simple but effective arrangement.

"Motivated by theoretical work, we started really thinking seriously about what we had to do to move forward," said John Martinis, a professor in UCSB's Department of Physics. "It took us a while to figure out how simple it was, and simple, in the end, was really the best."

"If you want to build a quantum computer, you need a two-dimensional array of such qubits, and the error rate should be below 1 percent," said Fowler. "If we can get one order of magnitude lower — in the area of 10-3 or 1 in 1,000 for all our gates — our qubits could become commercially viable. But there are more issues that need to be solved. There are more frequencies to worry about and it's certainly true that it's more complex. However, the physics is no different."

According to Martinis, it was Fowler's surface code that pointed the way, providing an architecture to put the qubits together in a certain way. "All of a sudden, we knew exactly what it was we wanted to build because of the surface code," Martinis said. "It took a lot of hard work to figure out how to piece the qubits together and control them properly. The amazing thing is that all of our hopes of how well it would work came true."

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.
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

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.


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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

Monday, April 14, 2014

Quantum manipulation: Filling the gap between quantum and classicalworld

Quantum manipulation: Filling the gap between quantum and classical world


Quantum superposition is a fundamental and also intriguing property of the quantum world. Because of superposition, a quantum system can be in two different states simultaneously, like a cat that can be both "dead" and "alive" at the same time. However, this anti-intuitive phenomenon cannot be observed directly, because whenever a classical measuring tool touches a quantum system, it immediately collapse into a classical state. On the other hand, quantum superposition is also the core of quantum computer's enormous computational power. A quantum computer can easily break the widely used RSA (Rivest, Shamir and Adleman) security system with Shor's algorithm. But for now, quantum computation still suffers from the decoherence induced by environment. Obviously, the key to manipulate a quantum system is to make it stay coherent as long as possible, to achieve this, one need to isolate the system from its environment. "For ground-breaking experimental methods that enable measuring and manipulation of individual quantum systems", Serge Haroche and David Wineland won the 2012 Nobel Prize in Physics.
Quantum manipulation - Filling the gap  between quantum and classical world
                                          Quantum manipulation - Filling the gap                                        
between quantum and classical world

This review begins by introducing the interesting property of quantum superposition, explaining its physical meaning, potential applications and main obstacles ahead. Then the author goes on to introduce the work of the two 2012 Nobel Prize Laureates – Serge Haroche and David Wineland. Instead of manipulating a neutral atom or a photon, Wineland and his team focused on controlling a charged atom, the ion, in an electromagnetic well. In order to break the limit of Doppler cooling, a new cooling technique – Side-Band cooling was used to reach extreme low temperature. The well cooled ions made an ideal platform for building optical clock and quantum computer. Since 2001, Wineland and his team had realized several optical clocks with very high precision. They had also realized basic quantum logic gate in ion trap and demonstrated the scalability of ion system, proving their system is promising for practical quantum computation. This article covers the above topics and gives detailed review.

In the fourth section, the author introduces the work of Haroche and his collaborators. Haroche et al managed to build a high-Q microwave cavity with superconducting materials and cooled it down to superconducting phase. According to Meissner effect, photons in the cavity cannot penetrate the superconducting mirror and will be trapped inside, thus isolate the photons from its environment. Since the cavity has extremely high-Q, the Rydberg atoms inside the cavity are strongly correlated to the photon field, which makes a perfect platform for testing the fundamental principles of quantum mechanics. With the aid of quantum non-demolition measurement, quantum processes can be observed without destroying the state. Using this platform, Haroche et al had directly observed decoherence, quantum jump and several other quantum information processes.

Finally, the review introduces recent developments and further applications of quantum manipulation, and then ends with a discussion of the relationship between quantum and classical world. With advanced quantum manipulation techniques, people are able to investigate fundamental quantum mechanics. In return, a better understanding of quantum mechanics makes it possible to develop new technologies that will change our classical world.


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Publication:

A new epoch of quantum manipulation. Yongjian Han, Zhen Wang, and Guang-Can Guo Natl Sci Rev (March 2014) 1 (1): 91-100 DOI:10.1093/nsr/nwt024


Wednesday, April 9, 2014

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

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."


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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.


Image Credit : Christine Daniloff/MIT