Not one, not two, not three, but four clones: First quantum cloning machine to produce four copies

ScienceDaily (Nov. 6, 2011) — Xi-Jun Ren and Yang Xiang from Henan Universities in China, in collaboration with Heng Fan at the Institute of Physics of the Chinese Academy of Sciences, have produced a theory for a quantum cloning machine able to produce several copies of the state of a particle at atomic or sub-atomic scale, or quantum state, in an article about to be published in The European Physical Journal D. The advance could have implications for quantum information processing methods used, for example, in message encryption systems.

Quantum cloning is difficult because quantum mechanics laws only allow for an approximate copy—not an exact copy—of an original quantum state to be made, as measuring such a state prior to its cloning would alter it.

In this study, researchers have demonstrated that it is theoretically possible to create four approximate copies of an initial quantum state, in a process called asymmetric cloning. The authors have extended previous work that was limited to quantum cloning providing only two or three copies of the original state. One key challenge was that the quality of the approximate copy decreases as the number of copies increases.

The authors were able to optimise the quality of the cloned copies, thus yielding four good approximations of the initial quantum state. They have also demonstrated that their quantum cloning machine has the advantage of being universal and therefore is able to work with any quantum state, ranging from a photon to an atom.

Assymetric quantum cloning has applications in analysing the security of messages encryption systems, based on shared secret quantum keys. Two people will know whether their communication is secure by analysing the quality of each copy of their secret key. Any third party trying to gain knowledge of that key would be detected as measuring it would disturb the state of that key.

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Journal Reference:

X. J. Ren, Y. Xiang, H. Fan. Optimal asymmetric 1 ? 4 quantum cloning in arbitrary dimension. The European Physical Journal D, 2011; DOI: 10.1140/epjd/e2011-20370-2

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Physicists identify room temperature quantum bits in widely used semiconductor

ScienceDaily (Nov. 2, 2011) — A discovery by physicists at UC Santa Barbara may earn silicon carbide -- a semiconductor commonly used by the electronics industry -- a role at the center of a new generation of information technologies designed to exploit quantum physics for tasks such as ultrafast computing and nanoscale sensing.

The research team discovered that silicon carbide contains crystal imperfections that can be controlled at a quantum mechanical level. The finding is published this week in the journal Nature.

The research group of David Awschalom, senior author, made the finding. Awschalom is director of UCSB's Center for Spintronics & Quantum Computation, professor of physics, electrical and computer engineering, and the Peter J. Clarke Director of the California NanoSystems Institute.

In conventional semiconductor-based electronic devices, crystal defects are often deemed undesirable because of their tendency to immobilize electrons by "trapping" them at a particular crystal location. However, the UCSB team discovered that electrons that become trapped by certain imperfections in silicon carbide do so in a way that allows their quantum states to be initialized, precisely manipulated, and measured using a combination of light and microwave radiation. This means that each of these defects meets the requirements for use as a quantum bit, or "qubit," which is often described as the quantum mechanical analog of a transistor, since it is the basic unit of a quantum computer.

"We are looking for the beauty and utility in imperfection, rather than struggling to bring about perfect order," said Awschalom, "and to use these defects as the basis for a future quantum technology."

Most crystal imperfections do not possess these properties, which are intimately tied to the atomic structure of a defect and the electronic characteristics of its semiconductor host, explained Awschalom. In fact, before this research, the only system known to possess these same characteristics was a flaw in diamond known as the nitrogen-vacancy center.

The diamond nitrogen-vacancy center is renowned for its ability to function as a qubit at room temperature, while many other quantum states of matter require an extremely cold temperature, near absolute zero. However, this center exists in a material that is difficult to grow and challenging to manufacture into integrated circuits.

In contrast, high-quality crystals of silicon carbide, multiple inches in diameter, are commonly produced for commercial purposes. They can be readily fashioned into a multitude of intricate electronic, optoelectronic, and electromechanical devices. In addition, the defects studied by Awschalom and his group are addressed using infrared light that is close in energy to the light used widely throughout modern telecommunications networks. And while several distinct defect types were studied at a range of temperatures, two of them were capable of room temperature operation, just like the diamond nitrogen-vacancy center.

The combination of these features makes silicon carbide, with its defects, an attractive candidate for future work seeking to integrate quantum mechanical objects with sophisticated electronic and optical circuitry, according to the researchers. This research fits within a wider effort at UCSB to engineer quantum devices by fostering collaboration between the fields of materials science and quantum physics.

While defects in silicon carbide may offer many technologically attractive qualities, an immense number of defects in other semiconductors are still left to be explored.

"Our dream is to make quantum mechanics fully engineerable," said William Koehl, lead author and a graduate student in the Awschalom lab. "Much like a civil engineer is able to design a bridge based on factors such as load capacity and length span, we'd like to see a day when there are quantum engineers who can design a quantum electronic device based on specifications such as degree of quantum entanglement and quality of interaction with the surrounding environment."

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The above story is reprinted from materials provided by University of California - Santa Barbara.

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Journal Reference:

William F. Koehl, Bob B. Buckley, F. Joseph Heremans, Greg Calusine, David D. Awschalom. Room temperature coherent control of defect spin qubits in silicon carbide. Nature, 2011; 479 (7371): 84 DOI: 10.1038/nature10562

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Laser light used to cool object to quantum ground state

ScienceDaily (Oct. 9, 2011) — For the first time, researchers at the California Institute of Technology (Caltech), in collaboration with a team from the University of Vienna, have managed to cool a miniature mechanical object to its lowest possible energy state using laser light. The achievement paves the way for the development of exquisitely sensitive detectors as well as for quantum experiments that scientists have long dreamed of conducting.

"We've taken a solid mechanical system -- one made up of billions of atoms -- and used optical light to put it into a state in which it behaves according to the laws of quantum mechanics. In the past, this has only been achieved with trapped single atoms or ions," says Oskar Painter, professor of applied physics and executive officer for applied physics and materials science at Caltech and the principal investigator on a paper describing the work that appears in the October 6 issue of the journal Nature.

As described in the paper, Painter and his colleagues have engineered a nanoscale object -- a tiny mechanical silicon beam -- such that laser light of a carefully selected frequency can enter the system and, once reflected, can carry thermal energy away, cooling the system.

By carefully designing each element of the beam as well as a patterned silicon shield that isolates it from the environment, Painter and colleagues were able to use the laser cooling technique to bring the system down to the quantum ground state, where mechanical vibrations are at an absolute minimum. Such a cold mechanical object could help detect very small forces or masses, whose presence would normally be masked by the noisy thermal vibrations of the sensor.

"In many ways, the experiment we've done provides a starting point for the really interesting quantum-mechanical experiments one wants to do," Painter says. For example, scientists would like to show that a mechanical system could be coaxed into a quantum superposition -- a bizarre quantum state in which a physical system can exist in more than one position at once. But they need a system at the quantum ground state to begin such experiments.

To reach the ground state, Painter's group had to cool its mechanical beam to a temperature below 100 millikelvin (-273.15°C). That's because the beam is designed to vibrate at gigahertz frequencies (corresponding to a billion cycles per second) -- a range where a large number of phonons are present at room temperature. Phonons are the most basic units of vibration just as the most basic units or packets of light are called photons. All of the phonons in a system have to be removed to cool it to the ground state.

Conventional means of cryogenically cooling to such temperatures exist but require expensive and, in some cases, impractical equipment. There's also the problem of figuring out how to measure such a cold mechanical system. To solve both problems, the Caltech team used a different cooling strategy.

"What we've done is used the photons -- the light field -- to extract phonons from the system," says Jasper Chan, lead author of the new paper and a graduate student in Painter's group. To do so, the researchers drilled tiny holes at precise locations in their mechanical beam so that when they directed laser light of a particular frequency down the length of the beam, the holes acted as mirrors, trapping the light in a cavity and causing it to interact strongly with the mechanical vibrations of the beam.

Because a shift in the frequency of the light is directly related to the thermal motion of the mechanical object, the light -- when it eventually escapes from the cavity -- also carries with it information about the mechanical system, such as the motion and temperature of the beam. Thus, the researchers have created an efficient optical interface to a mechanical element -- or an optomechanical transducer -- that can convert information from the mechanical system into photons of light.

Importantly, since optical light, unlike microwaves or electrons, can be transmitted over large, kilometer-length distances without attenuation, such an optomechanical transducer could be useful for linking different quantum systems -- a microwave system with an optical system, for example. While Painter's system involves an optical interface to a mechanical element, other teams have been developing systems that link a microwave interface to a mechanical element. What if those two mechanical elements were the same? "Then," says Painter, "I could imagine connecting the microwave world to the optical world via this mechanical conduit one photon at a time."

The Caltech team isn't the first to cool a nanomechanical object to the quantum ground state; a group led by former Caltech postdoctoral scholar Andrew Cleland, now at the University of California, Santa Barbara, accomplished this in 2010 using more conventional refrigeration techniques, and, earlier this year, a group from the National Institute of Standards and Technology in Boulder, Colorado, cooled an object to the ground state using microwave radiation. The new work, however, is the first in which a nanomechanical object has been put into the ground state using optical light.

"This is an exciting development because there are so many established techniques for manipulating and measuring the quantum properties of systems using optics," Painter says.

The other cooling techniques used starting temperatures of approximately 20 millikelvin -- more than a factor of 10,000 times cooler than room temperature. Ideally, to simplify designs, scientists would like to initiate these experiments at room temperature. Using laser cooling, Painter and his colleagues were able to perform their experiment at a much higher temperature -- only about 10 times lower than room temperature.

The work was supported by Caltech's Kavli Nanoscience Institute; the Defense Advanced Research Projects Agency's Microsystems Technology Office through a grant from the Air Force Office of Scientific Research; the European Commission; the European Research Council; and the Austrian Science Fund.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by California Institute of Technology, via EurekAlert!, a service of AAAS.

Journal Reference:

Jasper Chan, T. P. Mayer Alegre, Amir H. Safavi-Naeini, Jeff T. Hill, Alex Krause, Simon Gröblacher, Markus Aspelmeyer, Oskar Painter. Laser cooling of a nanomechanical oscillator into its quantum ground state. Nature, 2011; 478 (7367): 89 DOI: 10.1038/nature10461

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Quantum Scheme Could Allow Submarines to Communicate Securely

Quantum Scheme Could Allow Submarines to Communicate Securely | Popular Science@import "/files/css/a1c433465f8fe485195cb11d70c36108.css";@import "/files/css/33f6b7ecb4513ed2fe6c670880a27187.css"; home Login/Register Newsletter Subscribe RSS GadgetsComputersCamerasSmartphonesVideo GamesCarsConceptsHybridsElectric CarsScienceFuture of the EnvironmentEnergyHealthPopSci Eco TourTechnologyMilitaryAviationSpaceRobotsEngineeringDIYProjectsHacksToolsAuto DIYMore From Our Partner: Toolmonger GalleriesVideosColumnsThe GrouseSex FilesGreen Dream Innovation ChallengesHow It WorksFeatures Tweet Digg Quantum Scheme Could Allow Submarines to Communicate Securely By Clay Dillow Posted 11.01.2011 at 2:41 pm 5 Comments
A Virginia Class Attack Sub Concept Artist's rendering. U.S. Navy via Wikimedia

Submarines are excellent at avoiding detection. When submerged they are so far off the grid, in fact, that it’s difficult for them to stay in contact with the naval bases that supply them with orders and information, or for them to beam information back to base. But a new quantum communications solution could change all this, allowing submerged submarines to communicate via laser pulses by exchanging encryption keys and messages over satellites.

Currently, submarines use randomized codes or “keys” to encrypt messages. These are established ahead of time between a sub and its communications base, and each code is only used one time so that an enemy cannot crack a code and use it to decipher future communications.

Related ArticlesNASA Puts Its Money on Quantum Communications, Plasma Propulsion, and Other Future TechWith New Quantum Encryption Scheme, Messages Can Only Be Read in Designated Geographical LocationScientists Reproduce Quantum Entanglement, Einstein's “Spooky Action” TagsTechnology, Clay Dillow, military, quantum communication, quantum entanglement, quantum mechanics, spooky action, submarinesBut this is problematic. For one, it is logistically cumbersome. And a submarine that’s going on a long-duration mission has to pack lots and lots of keys, which could fall into enemy hands if the sub is overtaken. Moreover, even with plenty of secure keys, the communications between sub and base are extremely slow. In order to penetrate water, transmitters have to use very low frequency radio waves. These only allow for a few characters to be transmitted each second. While the rest of the world (and the battle space) communicates at high speed, subs are stuck with dial-up. In order to send or receive information in bulk or at speed subs have to surface, and that leaves them visible and vulnerable.

But researchers at defense firm ITT have an idea. Using quantum key distribution, subs could encode a key into photons (using the photons’ polarization to represent ones and zeros) making for a virtually un-hackable key--any attempt to intercept the photons would disturb the quantum system in a measurable way, alerting the sender and receiver that a third party is listening in (this is generally achieved via quantum entanglement).

With a secure key established, submarines could then--theoretically at least--stay a few hundred feet below the surface and transmit photons via lasers to satellites, where they could be bounced back to a base on the ground. The researchers’ simulations apparently demonstrate a system that could send and receive data at rates of 170 megabytes per second while a submarine is below the water line. That’s video-worthy data streaming.

Of course, first you would have to figure out just how well photons will hold their quantum states as they travel through water, as any deviation that disrupts the quantum system would render the system insecure (and pointless). And then you need a satellite that can receive and relay quantum-encrypted signaling--again, without disrupting the quantum states of the photons. In other words, this technology is far from battle-ready. But it could someday take high-speed, secure data transfer to depths where it currently doesn’t exist.

[New Scientist]

Previous Article: Chinese Officials Deny Hacking U.S. Environment-Monitoring Satellites 5 Comments Link to this comment Grunt 11/01/11 at 3:25 pm

First, it is easily possible to float to the surface UHF or VHF antenna and communicate with a satellite, while the submarine is several hundred feet below water.

Now add to this line of underwater conduit a floating laser and the problem is solved. Radio waves to guide the laser towards the satellite and laser communication to make it harder to listen in too.

Link to this comment scientific anomaly 11/01/11 at 3:39 pm

i think they should make a drone that can be launched from the sub while it is still hundreds of feet underwater that goes to the surface, communicates whatever it is the people are trying to convey, then self destructs so the enemies cant get it and hack it or something.

-Knock knock
-Who's there?
-The Doctor.
-Doctor Who?
-Yes

Link to this comment Vector13 11/01/11 at 7:16 pm

@scientific anomaly

Well, the problem is with someone intercepting the transmission itself, not necessarily at the transmission point. Just as well, there would need to be a drone for every time someone wanted to transmit or receive a message, and that would be pretty cumbersome :p

Link to this comment TheZomb 11/02/11 at 1:14 am

This article is incorrect in several points. First, the codes submarines use to communicate aka one time pad ciphers are impossible to "hack" or break individually, no matter of time or computing power will ever break one. The only way to break a one time pad cipher is to use two encrypted with the same key to break each other. This is the real reason the codes can be used once. The sentence "These are established ahead of time between a sub and its communications base, and each code is only used one time so that an enemy cannot crack a code and use it to decipher future communications." is incorrect in that respect.

Second, Quantum encryption has nothing to do with entanglement and bits are not encoded by polarity. Its called quantum encryption because it relies on a superposition of states in the light particles being sent (like schrodingers cat) and the Heisenberg uncertainty principle.

Each particle has a polarity, determined by the sender and unknown by the receiver. The receiver has to guess what polarity to use to decode a particle. If you guess wrong the data may not be encoded to bits correctly. You only get on chance to read a particle since reading it permanently changes its polarity. The sender and receiver then pick some at random and send them unencrypted, if they are the wrong polarity someone has tried to eavesdrop. The receiver then tells the sender all of his polarity guesses and then they use the bits from the correctly guessed polarities to form a one time pad cipher just like before.

Link to this comment gizmowiz 11/02/11 at 2:35 am

Wouldn't that in theory allow for an enemy to use detection systems to identify Photon signals being sent to a submerged sub and thereby LOCATE the sub EXCACTLY and blast it out of the water?

It seems plausible this could nullify the subs ability to play hide and seek.

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November 2011: Data Is Power

This month, we examine all the ways information is driving our future, from dating to crime to how we see the world.

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Quantum computer components 'coalesce' to 'converse'

ScienceDaily (Oct. 26, 2011) — If quantum computers are ever to be realized, they likely will be made of different types of parts that will need to share information with one another, just like the memory and logic circuits in today's computers do. However, prospects for achieving this kind of communication seemed distant -- until now. A team of physicists working at the National Institute of Standards and Technology (NIST) has shown for the first time how these parts might communicate effectively.

The goal to develop quantum computers -- a long-awaited type of computer that could solve otherwise intractable problems, such as breaking complex encryption codes -- has inspired scientists the world over to invent new devices that could become the brains and memory of these machines. Many of these tiny devices use particles of light, or photons, to carry the bits of information that a quantum computer will use.

But while each of these pieces of hardware can do some jobs well, none are likely to accomplish all of the functions necessary to build a quantum computer. This implies that several different types of quantum devices will need to work together for the computer or network to function. The trouble is that these tiny devices frequently create photons of such different character that they cannot transfer the quantum bits of information between one another. Transmuting two vastly different photons into two similar ones would be a first step toward permitting quantum information components to communicate with one another over large distances, but until now this goal has remained elusive.

However, the team has demonstrated that it is possible to take photons from two disparate sources and render these particles partially indistinguishable. That photons can be made to "coalesce" and become indistinguishable without losing their essential quantum properties suggests in principle that they can connect various types of hardware devices into a single quantum information network. The team's achievement also demonstrates for the first time that a "hybrid" quantum computer might be assembled from different hardware types.

The team connected single photons from a "quantum dot," which could be useful in logic circuits, with a second single-photon source that uses "parametric down conversion," which might be used to connect different parts of the computer. These two sources typically produce photons that differ so dramatically in spectrum that they would be unusable in a quantum network. But with a deft choice of filters and other devices that alter the photons' spectral shapes and other properties, the team was able to make the photons virtually identical.

"We manipulate the photons to be as indistinguishable as possible in terms of spectra, location and polarization -- the details you need to describe a photon. We attribute the remaining distinguishability to properties of the quantum dot," says Glenn Solomon, of NIST's Quantum Measurement Division. "No conceivable measurement can tell indistinguishable photons apart. The results prove in principle that a hybrid quantum network is possible and can be scaled up for use in a quantum network."

The research team includes scientists from the NIST/University of Maryland Joint Quantum Institute (JQI) and Georgetown University. The NSF Physics Frontier Center at JQI provided partial funding.

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Sergey Polyakov, Andreas Muller, Edward Flagg, Alex Ling, Natalia Borjemscaia, Edward Van Keuren, Alan Migdall, Glenn Solomon. Coalescence of Single Photons Emitted by Disparate Single-Photon Sources: The Example of InAs Quantum Dots and Parametric Down-Conversion Sources. Physical Review Letters, 2011; 107 (15) DOI: 10.1103/PhysRevLett.107.157402

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New hybrid technology could bring 'quantum information systems'

ScienceDaily (Oct. 28, 2011) — The merging of two technologies under development -- plasmonics and nanophotonics -- is promising the emergence of new "quantum information systems" far more powerful than today's computers.

The technology hinges on using single photons -- the tiny particles that make up light -- for switching and routing in future computers that might harness the exotic principles of quantum mechanics.

The quantum information processing technology would use structures called "metamaterials," artificial nanostructured media with exotic properties.

The metamaterials, when combined with tiny "optical emitters," could make possible a new hybrid technology that uses "quantum light" in future computers, said Vladimir Shalaev, scientific director of nanophotonics at Purdue University's Birck Nanotechnology Center and a distinguished professor of electrical and computer engineering.

The concept is described in an article published on October 28 in the journal Science. The article appeared in the magazine's Perspectives section and was written by Shalaev and Zubin Jacob, an assistant professor of electrical and computer engineering at the University of Alberta, Canada.

"A seamless interface between plasmonics and nanophotonics could guarantee the use of light to overcome limitations in the operational speed of conventional integrated circuits," Shalaev said.

Researchers are proposing the use of "plasmon-mediated interactions," or devices that manipulate individual photons and quasiparticles called plasmons that combine electrons and photons.

One of the approaches, pioneered at Harvard University, is a tiny nanowire that couples individual photons and plasmons. Another approach is to use hyperbolic metamaterials, suggested by Jacob; Igor Smolyaninov, a visiting research scientist at the University of Maryland; and Evgenii Narimanov, an associate professor of electrical and computer engineering at Purdue. Quantum-device applications using building blocks for such hyperbolic metamaterials have been demonstrated in Shalaev's group.

"We would like to record and read information with single photons, but we need a very efficient source of single photons," Shalaev said. "The challenge here is to increase the efficiency of generation of single photons in a broad spectrum, and that is where plasmonics and metamaterials come in."

Today's computers work by representing information as a series of ones and zeros, or binary digits called "bits."

Computers based on quantum physics would have quantum bits, or "qubits," that exist in both the on and off states simultaneously, dramatically increasing the computer's power and memory. Quantum computers would take advantage of a strange phenomenon described by quantum theory called "entanglement." Instead of only the states of one and zero, there are many possible "entangled quantum states" in between one and zero.

An obstacle in developing quantum information systems is finding a way to preserve the quantum information long enough to read and record it. One possible solution might be to use diamond with "nitrogen vacancies," defects that often occur naturally in the crystal lattice of diamonds but can also be produced by exposure to high-energy particles and heat.

"The nitrogen vacancy in diamond operates in a very broad spectral range and at room temperature, which is very important," Shalaev said.

The work is part of a new research field, called diamond photonics. Hyperbolic metamaterials integrated with nitrogen vacancies in diamond are expected to work as efficient "guns" of single photons generated in a broad spectral range, which could bring quantum information systems, he said.

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Journal Reference:

Z. Jacob, V. M. Shalaev. Plasmonics Goes Quantum. Science, 2011; 334 (6055): 463 DOI: 10.1126/science.1211736

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Laser light used to cool object to quantum ground state

ScienceDaily (Oct. 9, 2011) — For the first time, researchers at the California Institute of Technology (Caltech), in collaboration with a team from the University of Vienna, have managed to cool a miniature mechanical object to its lowest possible energy state using laser light. The achievement paves the way for the development of exquisitely sensitive detectors as well as for quantum experiments that scientists have long dreamed of conducting.

"We've taken a solid mechanical system -- one made up of billions of atoms -- and used optical light to put it into a state in which it behaves according to the laws of quantum mechanics. In the past, this has only been achieved with trapped single atoms or ions," says Oskar Painter, professor of applied physics and executive officer for applied physics and materials science at Caltech and the principal investigator on a paper describing the work that appears in the October 6 issue of the journal Nature.

As described in the paper, Painter and his colleagues have engineered a nanoscale object -- a tiny mechanical silicon beam -- such that laser light of a carefully selected frequency can enter the system and, once reflected, can carry thermal energy away, cooling the system.

By carefully designing each element of the beam as well as a patterned silicon shield that isolates it from the environment, Painter and colleagues were able to use the laser cooling technique to bring the system down to the quantum ground state, where mechanical vibrations are at an absolute minimum. Such a cold mechanical object could help detect very small forces or masses, whose presence would normally be masked by the noisy thermal vibrations of the sensor.

"In many ways, the experiment we've done provides a starting point for the really interesting quantum-mechanical experiments one wants to do," Painter says. For example, scientists would like to show that a mechanical system could be coaxed into a quantum superposition -- a bizarre quantum state in which a physical system can exist in more than one position at once. But they need a system at the quantum ground state to begin such experiments.

To reach the ground state, Painter's group had to cool its mechanical beam to a temperature below 100 millikelvin (-273.15°C). That's because the beam is designed to vibrate at gigahertz frequencies (corresponding to a billion cycles per second) -- a range where a large number of phonons are present at room temperature. Phonons are the most basic units of vibration just as the most basic units or packets of light are called photons. All of the phonons in a system have to be removed to cool it to the ground state.

Conventional means of cryogenically cooling to such temperatures exist but require expensive and, in some cases, impractical equipment. There's also the problem of figuring out how to measure such a cold mechanical system. To solve both problems, the Caltech team used a different cooling strategy.

"What we've done is used the photons -- the light field -- to extract phonons from the system," says Jasper Chan, lead author of the new paper and a graduate student in Painter's group. To do so, the researchers drilled tiny holes at precise locations in their mechanical beam so that when they directed laser light of a particular frequency down the length of the beam, the holes acted as mirrors, trapping the light in a cavity and causing it to interact strongly with the mechanical vibrations of the beam.

Because a shift in the frequency of the light is directly related to the thermal motion of the mechanical object, the light -- when it eventually escapes from the cavity -- also carries with it information about the mechanical system, such as the motion and temperature of the beam. Thus, the researchers have created an efficient optical interface to a mechanical element -- or an optomechanical transducer -- that can convert information from the mechanical system into photons of light.

Importantly, since optical light, unlike microwaves or electrons, can be transmitted over large, kilometer-length distances without attenuation, such an optomechanical transducer could be useful for linking different quantum systems -- a microwave system with an optical system, for example. While Painter's system involves an optical interface to a mechanical element, other teams have been developing systems that link a microwave interface to a mechanical element. What if those two mechanical elements were the same? "Then," says Painter, "I could imagine connecting the microwave world to the optical world via this mechanical conduit one photon at a time."

The Caltech team isn't the first to cool a nanomechanical object to the quantum ground state; a group led by former Caltech postdoctoral scholar Andrew Cleland, now at the University of California, Santa Barbara, accomplished this in 2010 using more conventional refrigeration techniques, and, earlier this year, a group from the National Institute of Standards and Technology in Boulder, Colorado, cooled an object to the ground state using microwave radiation. The new work, however, is the first in which a nanomechanical object has been put into the ground state using optical light.

"This is an exciting development because there are so many established techniques for manipulating and measuring the quantum properties of systems using optics," Painter says.

The other cooling techniques used starting temperatures of approximately 20 millikelvin -- more than a factor of 10,000 times cooler than room temperature. Ideally, to simplify designs, scientists would like to initiate these experiments at room temperature. Using laser cooling, Painter and his colleagues were able to perform their experiment at a much higher temperature -- only about 10 times lower than room temperature.

The work was supported by Caltech's Kavli Nanoscience Institute; the Defense Advanced Research Projects Agency's Microsystems Technology Office through a grant from the Air Force Office of Scientific Research; the European Commission; the European Research Council; and the Austrian Science Fund.

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Jasper Chan, T. P. Mayer Alegre, Amir H. Safavi-Naeini, Jeff T. Hill, Alex Krause, Simon Gröblacher, Markus Aspelmeyer, Oskar Painter. Laser cooling of a nanomechanical oscillator into its quantum ground state. Nature, 2011; 478 (7367): 89 DOI: 10.1038/nature10461

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Point defects in super-chilled diamonds may offer stable candidates for quantum computing bits

ScienceDaily (Oct. 13, 2011) — Diamond, nature's hardest known substance, is essential for our modern mechanical world -- drills, cutters, and grinding wheels exploit the durability of diamonds to power a variety of industries. But diamonds have properties that may also make them excellent materials to enable the next generation of solid-state quantum computers and electrical and magnetic sensors.

To further explore diamonds' quantum computing potential, researchers from the University of Science and Technology of China tested the properties of a common defect found in diamond: the nitrogen-vacancy (NV) center.

Consisting of a nitrogen atom impurity paired with a 'hole' where a carbon atom is absent from the matrix structure, the NV center has the potential to store information because of the predictable way in which electrons confined in the center interact with electromagnetic waves. The research team probed the energy level properties of the trapped electrons by cooling the diamonds to an extremely chilly 5.6 degrees Kelvin and then measuring the magnetic resonance and fluorescent emission spectra. The team also measured the same spectra at gradually warmer increments, up to 295 degrees Kelvin.

The results, as reported in the AIP's journal Applied Physics Letters, show that at temperatures below 100 Kelvin the electrons' transition energies, or the energies required to get from one energy level to the next, were stable. Shifting transition energies could make quantum mechanical manipulations tricky, so cooler temperatures may aid the study and development of diamonds for quantum computation and ultra-sensitive detectors, the authors write.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by American Institute of Physics, via EurekAlert!, a service of AAAS.

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X.-D. Chen, C.-H. Dong, F.-W. Sun, C.-L. Zou, J.-M. Cui, Z.-F. Han, and G.-C Guo. Temperature dependent energy level shifts of nitrogen-vacancy centers in diamond. Applied Physics Letters, 2011

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One clock with two times: When quantum mechanics meets general relativity

ScienceDaily (Oct. 20, 2011) — The unification of quantum mechanics and Einstein's general relativity is one of the most exciting and still open questions in modern physics. General relativity, the joint theory of gravity, space and time gives predictions that become clearly evident on a cosmic scale of stars and galaxies. Quantum effects, on the other hand, are fragile and are typically observed on small scales, e.g. when considering single particles and atoms. That is why it is very hard to test the interplay between quantum mechanics and general relativity.

Now theoretical physicists led by Caslav Brukner at the University of Vienna propose a novel experiment which can probe the overlap of the two theories. The focus of the work is to measure the general relativistic notion of time on a quantum scale.

The findings were recently published in Nature Communications.

Time in general relativity

One of the counterintuitive predictions of Einstein's general relativity is that gravity distorts the flow of time. The theory predicts that clocks tick slower near a massive body and tick faster the further they are away from the mass. This effect results in a so-called "twin paradox": if one twin moves out to live at a higher altitude, he will age faster than the other twin who remains on the ground. This effect has been precisely verified in classical experiments, but not in conjunction with quantum effects, which is the aim of the newly proposed experiment.

Quantum interference and complementarity

The Viennese group of researchers wants to exploit the extraordinary possibility that a single quantum particle can lose the classical property of having a well-defined position, or as phrased in quantum mechanical terms: it can be in a "superposition." This allows for wave-like effects, called interference, with a single particle. However, if the position of the particle is measured, or even if it can in principle be known, this effect is lost. In other words, it is not possible to observe interference and simultaneously know the position of the particle. Such a connection between information and interference is an example of quantum complementarity -- a principle proposed by Niels Bohr. The experimental proposal now published in "Nature Communications" combines this principle with the "twin paradox" of general relativity.

Einstein's "twin paradox" for a quantum "only child"

The team at the University of Vienna considers a single clock (any particle with evolving internal degrees of freedom such as spin) which is brought in a superposition of two locations -- one closer and one further away from the surface of Earth. According to general relativity, the clock ticks at different rates in the two locations, in the same way as the two twins would age differently. But since the time measured by the clock reveals the information on where the clock was located, the interference and the wave-nature of the clock is lost. "It is the twin paradox for a quantum 'only child', and it requires general relativity as well as quantum mechanics. Such an interplay between the two theories has never been probed in experiments yet" -- says Magdalena Zych, the lead author of the paper and member of the Vienna Doctoral Program CoQuS. It is therefore the first proposal for an experiment that allows testing the genuine general relativistic notion of time in conjunction with quantum complementarity.

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Magdalena Zych, Fabio Costa, Igor Pikovski, Caslav Brukner. Quantum interferometric visibility as a witness of general relativistic proper time. Nature Communications, 2011; 2: 505 DOI: 10.1038/ncomms1498

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Progress in quantum computing: Researchers control rate of photon emission from luminescent imperfections in diamond

ScienceDaily (Oct. 12, 2011) — Engineers and physicists at Harvard have managed to capture light in tiny diamond pillars embedded in silver, releasing a stream of single photons at a controllable rate.

The advance represents a milestone on the road to quantum networks in which information can be encoded in spins of electrons and carried through a network via light, one photon at a time.

The finding was published in Nature Photonics, appearing online Oct. 9.

"We can make the emission of photons faster, which will allow us to do more processing per second -- for example, more computations -- in the future quantum network," explains principal investigator Marko Loncar, Associate Professor of Electrical Engineering at the Harvard School of Engineering and Applied Sciences (SEAS).

The device Loncar's research team has built consists of parallel rows of tiny, nanofabricated diamond posts, embedded in a layer of silver, that can each act as a single photon source.

By removing the silver wrapping from their nanostructures, the team was also able to achieve a slower release of photons, which is of interest for probing the dynamics of the quantum system.

The breakthrough takes advantage of imperfections in the diamond's crystal lattice, where carbon atoms are replaced by other elements. To the naked eye, these imperfections can appear as discolorations in the diamond, turning it yellow in the case of nitrogen. Occasionally, there is also a vacancy (missing carbon atom) next to the nitrogen atom.

Each nitrogen-vacancy imperfection can serve as a nearly perfect quantum emitter, capable of emitting red photons one by one, even at room temperature. The technology is a promising candidate for realization of scalable, on-chip quantum networks.

"The color centers in diamond are very interesting as qubits for quantum information processing, where they can be used as memory to store information," says Loncar. "More importantly, they can be interrogated -- they can be written into and read out -- with light."

Loncar's team fabricates diamond posts that contain negatively charged nitrogen vacancy centers, which can absorb light and hold its energy for a given amount of time, finally releasing it in the form of photons.

"The rate at which photons are emitted can be controlled by carefully nano-engineering the center's surrounding," says co-author Irfan Bulu, a research associate in the Loncar group. Attaining fine control of that release, however, has been difficult.

"One of the main challenges has been the efficiency with which you can write information into the spin of these color centers, as well as the efficiency with which you can collect photons emitted from the color centers," explains co-author Jennifer Choy, a graduate student in Loncar's lab at SEAS. "The other challenge has been the rate -- how quickly you can perform these processes."

Previous work from Loncar's group solved the collection efficiency problem by using diamond nanowires to channel and direct the flow of photons. The new research manipulates the radius of diamond pillars and adds the silver coating. The diamond-silver construction acts as an optical nanoresonator, creating a strong electromagnetic field around the emitter and offering a new level of control over the rate of emission.

Moreover, the device functions at room temperature -- an essential requirement for practical computing applications -- and the nanostructured chips are fully scalable.

"We've designed everything in parallel in a massive system, which allows us to make thousands or millions of devices with more or less the same properties, and we use conventional microfabrication and nanofabrication techniques, unlike what has been done in this field before," says Birgit Hausmann, a graduate student in Loncar's lab at SEAS and one of the co-authors.

In addition to Loncar, Choy, Hausmann, and Bulu, co-authors included Tom Babinec, a graduate student at SEAS; Mughees Khan, a staff scientist at the Wyss Institute for Biologically Inspired Engineering at Harvard; Patrick Maletinsky, a fellow of the Department of Physics at Harvard; and Amir Jacoby, Professor of Physics in the Harvard Faculty of Arts and Sciences.

The work was supported by grants and fellowships from the U.S. Department of Defense, the Defense Advanced Research Projects Agency (DARPA) QuEST program, the National Science Foundation (NSF), the King Abdullah University of Science and Technology (KAUST), the Sloan Foundation, and the NSF-supported Nanoscale Science and Engineering Center (NSEC) at Harvard. Fabrication took place at the NSF-supported Center for Nanoscale Systems (CNS) at Harvard.

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Jennifer T. Choy, Birgit J. M. Hausmann, Thomas M. Babinec, Irfan Bulu, Mughees Khan, Patrick Maletinsky, Amir Yacoby, Marko Loncar. Enhanced single-photon emission from a diamond–silver aperture. Nature Photonics, 2011; DOI: 10.1038/nphoton.2011.249

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New knowledge about 'flawed' diamonds could speed the development of diamond-based quantum computers

ScienceDaily (Oct. 12, 2011) — A University at Buffalo-led research team has established the presence of a dynamic Jahn-Teller effect in defective diamonds, a finding that will help advance the development of diamond-based systems in applications such as quantum information processing.

"We normally want things to be perfect, but defects are actually very important in terms of electronic applications," said Peihong Zhang, the UB associate professor of physics who led the study. "There are many proposals for the application of defective diamonds, ranging from quantum computing to biological imaging, and our research is one step toward a better understanding of these defect systems."

The research was published online Sept. 30 in Physical Review Letters.

The findings deal with diamonds whose crystal structure contains a particular defect: a nitrogen atom that sits alongside a vacant space in an otherwise perfect lattice made only of carbon.

At the point of the imperfection -- the so-called "nitrogen-vacancy center" -- a single electron can jump between different energy states. (The electron rises to a higher, "excited" energy state when it absorbs a photon and falls back to a lower energy state when it emits a photon).

Understanding how the diamond system behaves when the electron rises to an excited state called a "3E" state is critical to the success of such proposed applications as quantum computing.

The problem is that at the nitrogen-vacancy center, the 3E state has two orbital components with exactly the same energy -- a configuration that is inherently unstable.

In response, the lattice "stabilizes" by rearranging itself. Atoms near the nitrogen-vacancy center move slightly, resulting in a new geometry that has a lower energy and is more stable.

This morphing is known as the Jahn-Teller effect, and until recently, the effect's precise parameters in defective diamonds remained unknown.

Zhang and colleagues from the Rensselaer Polytechnic Institute in Troy, N.Y., are the first to crack that mystery. Using UB's supercomputing facility, the Center for Computational Research, the team conducted calculations that reveal how, exactly, the diamond lattice distorts.

Their findings align with experimental results from other research studies, and shed light on important topics such as how long an excited electron at the nitrogen-vacancy center will stay coherently at a higher energy state.

The UB-Rensselaer study was funded by the Department of Energy.

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Tesfaye Abtew, Y. Sun, Bi-Ching Shih, Pratibha Dev, S. Zhang, Peihong Zhang. Dynamic Jahn-Teller Effect in the NV- Center in Diamond. Physical Review Letters, 2011; 107 (14) DOI: 10.1103/PhysRevLett.107.146403

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New scheme for photonic quantum computing

ScienceDaily (Oct. 13, 2011) — The concepts of quantum technology promise to achieve more powerful information processing than is possible with even the best possible classical computers. To actually build efficient quantum computers remains a significant challenge in practice. A new scheme, called "coherent photon conversion," could potentially overcome all of the currently unresolved problems for optical implementations of quantum computing.

The international team of scientists led by researchers from the Vienna Center for Quantum Science and Technology (VCQ) at the University of Vienna (group of Anton Zeilinger) introduces this new scheme this week in Nature.

A new way for photons

Quantum technology derives its potential by exploiting uniquely quantum features such as superposition and entanglement. Single photons are excellent quantum information carriers, because they are naturally almost perfectly isolated from their environment. Also, quantum computers based on photons promise to be extremely fast. But current schemes for preparing, processing and measuring photons are inefficient.

The new scheme provides a method of coherent conversion between different photon states and is based on enhancing the nonlinearity of a medium by a strong laser field. The method paves a road to solving all open challenges for optical quantum computation. For example, deterministically doubling single photons solves the preparation and measuring problems, and a novel type of photon-photon interaction gives efficient quantum gates. This new quantum optics toolbox opened up by "coherent photon conversion" promises to lead to a nonlinear optical quantum computer.

First experiments

In a first series of experiments the group uses photons and highly non-linear glass fibers for a proof-of-principle demonstration of a process suitable for implementing the scheme. While deterministic operation has yet to be achieved, the authors' results suggest a line for development how this might be possible with sophisticated optical technologies, such as using highly nonlinear glasses and stronger lasers. Interestingly, the general idea of "coherent photon conversion" can also be applied to various other physical systems like atoms or nanomechanical devices.

This work was carried out as a joint effort by researchers of the Vienna Center for Quantum Science and Technology (VCQ) of the University of Vienna, the Institute for Quantum Optics and Quantum Information Vienna (IQOQI) of the Austrian Academy of Sciences, the National Institute of Informatics and the NTT Basic Research Laboratories (NTT Corporation) in Japan, as well as the Centre for Engineered Quantum Systems at the University of Queensland in Australia. It was supported by the European Research Council (Advanced Grant QIT4QAD), the Austrian Science Fund (grant F4007, an Erwin Schroedinger Fellowship, the Vienna Doctoral Program CoQuS FWF W121), the European Commission (QU-ESSENCE and QAP), the John Templeton Foundation and in part by the Japanese FIRST programme and the Ontario Ministry of Research and Innovation.

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N. K. Langford, S. Ramelow, R. Prevedel, W. J. Munro, G. J. Milburn, A. Zeilinger. Efficient quantum computing using coherent photon conversion. Nature, 2011; DOI: 10.1038/nature10463

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A hidden order unraveled: Microscopic views on quantum fluctuations

ScienceDaily (Oct. 14, 2011) — Fluctuations are fundamental to many physical phenomena in our everyday life, such as the phase transitions from a liquid into a gas or from a solid into a liquid. But even at absolute zero temperature, where all motion in the classical world is frozen out, special quantum mechanical fluctuations prevail that can drive the transition between two quantum phases.

Now a team around Immanuel Bloch and Stefan Kuhr at Ludwig-Maximilians University (LMU) and the Max Planck Institute of Quantum Optics (MPQ) has succeeded in directly observing such quantum fluctuations. Using a high resolution microscope, they were able to image quantum-correlated particle-hole pairs in a gas of ultracold atoms. This allowed the physicists to unravel a hidden order in the crystal and to characterize the different phases of the quantum gas. The work was performed together with scientists from the Theory Division at the MPQ and ETH Zurich. These measurements open new ways to characterize novel quantum phases of matter.

The scientists start by cooling a small cloud of rubidium atoms down to a temperature near absolute zero, about minus 273 degree Celsius. The ensemble is then subjected to a light field that severely restricts the motion of the particles along one-dimensional tubes of light aligned in parallel. An additional standing laser wave along the tubes creates a one-dimensional optical lattice that holds the atoms in a periodic array of bright and dark regions of light.

The atoms move in the periodic light field like electrons in solids. As these can be electric conductors or insulators, also the one-dimensional quantum gases can behave like a superfluid or like an insulator at low temperatures. In particular, the height of the optical lattice potential plays an important role: it determines whether the atom is fixed on a particular lattice site or whether is able to move to a neighbouring site. At very large lattice depths, each lattice site is occupied by exactly one atom. This highly ordered state is called a "Mott insulator," after the British physicist and Nobel laureate Sir Neville Mott. When the lattice depth is decreased slightly, the atoms have enough energy to reach a neighbouring site by quantum mechanical tunneling. In this way, pairs of empty and doubly occupied sites emerge, so-called particle-hole pairs. Intriguingly, these quantum fluctuations also occur at absolute zero temperature, when all movement in the classical world is frozen out. The position of the quantum-correlated particle-hole pairs in the crystal is completely undetermined and is fixed only by the measurement process.

In recent experiments, the physicists around Stefan Kuhr and Immanuel Bloch had already developed a method, which allowed to image single atoms lattice site by lattice site. The atoms are cooled using laser beams, and the fluorescence photons emitted in this process are used to observe the atoms with a high resolution microscope. Holes naturally show up as dark spots, but so do doubly occupied sites as the two particles kick each other out of the lattice in the experiment. Therefore particle-hole pairs appear as two neighbouring dark lattice sites. "With our technique, we can directly observe this fundamental quantum phenomenon for the first time," describes doctoral student Manuel Endres enthusiastically.

The physicists measure the number of neighbouring particle-hole pairs through a correlation function. With increasing kinetic energy, more and more particles tunnel to neighbouring sites and the pair correlations increase. However, when the number of particle-hole pairs is very large, it becomes difficult to unambiguously identify them. Hence the correlation function takes on smaller values. Finally, the ordered state of a Mott insulator vanishes completely und the quantum gas becomes a superfluid again. Here fluctuations of holes and particles occur independently. The correlation function measured in the experiment is very well reproduced by model calculations, which were performed by scientists from the Theory Division at the MPQ and the ETH Zurich. Interestingly, the same investigations on two-dimensional quantum-gases clearly showed that quantum fluctuations are not as prominent as in one-dimensional systems.

The scientists extended their analysis to correlations between several lattice sites along a string. Such non-local correlation functions contain important information about the underlying many-body system and can be used as an order parameter to characterize different quantum phases. In the experiment described here, such non-local order parameters have been measured for the first time. In the future, the scientists plan to use these measurements for the detection of topological quantum phases. These can be useful for robust quantum computers and could help to understand superconductivity at high temperatures. (MPQ)

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Ludwig-Maximilians-Universität München.

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M. Endres, M. Cheneau, T. Fukuhara, C. Weitenberg, P. Schauss, C. Gross, L. Mazza, M. C. Banuls, L. Pollet, I. Bloch, S. Kuhr. Observation of Correlated Particle-Hole Pairs and String Order in Low-Dimensional Mott Insulators. Science, 2011; 334 (6053): 200 DOI: 10.1126/science.1209284

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New scheme for photonic quantum computing

ScienceDaily (Oct. 13, 2011) — The concepts of quantum technology promise to achieve more powerful information processing than is possible with even the best possible classical computers. To actually build efficient quantum computers remains a significant challenge in practice. A new scheme, called "coherent photon conversion," could potentially overcome all of the currently unresolved problems for optical implementations of quantum computing.

The international team of scientists led by researchers from the Vienna Center for Quantum Science and Technology (VCQ) at the University of Vienna (group of Anton Zeilinger) introduces this new scheme this week in Nature.

A new way for photons

Quantum technology derives its potential by exploiting uniquely quantum features such as superposition and entanglement. Single photons are excellent quantum information carriers, because they are naturally almost perfectly isolated from their environment. Also, quantum computers based on photons promise to be extremely fast. But current schemes for preparing, processing and measuring photons are inefficient.

The new scheme provides a method of coherent conversion between different photon states and is based on enhancing the nonlinearity of a medium by a strong laser field. The method paves a road to solving all open challenges for optical quantum computation. For example, deterministically doubling single photons solves the preparation and measuring problems, and a novel type of photon-photon interaction gives efficient quantum gates. This new quantum optics toolbox opened up by "coherent photon conversion" promises to lead to a nonlinear optical quantum computer.

First experiments

In a first series of experiments the group uses photons and highly non-linear glass fibers for a proof-of-principle demonstration of a process suitable for implementing the scheme. While deterministic operation has yet to be achieved, the authors' results suggest a line for development how this might be possible with sophisticated optical technologies, such as using highly nonlinear glasses and stronger lasers. Interestingly, the general idea of "coherent photon conversion" can also be applied to various other physical systems like atoms or nanomechanical devices.

This work was carried out as a joint effort by researchers of the Vienna Center for Quantum Science and Technology (VCQ) of the University of Vienna, the Institute for Quantum Optics and Quantum Information Vienna (IQOQI) of the Austrian Academy of Sciences, the National Institute of Informatics and the NTT Basic Research Laboratories (NTT Corporation) in Japan, as well as the Centre for Engineered Quantum Systems at the University of Queensland in Australia. It was supported by the European Research Council (Advanced Grant QIT4QAD), the Austrian Science Fund (grant F4007, an Erwin Schroedinger Fellowship, the Vienna Doctoral Program CoQuS FWF W121), the European Commission (QU-ESSENCE and QAP), the John Templeton Foundation and in part by the Japanese FIRST programme and the Ontario Ministry of Research and Innovation.

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N. K. Langford, S. Ramelow, R. Prevedel, W. J. Munro, G. J. Milburn, A. Zeilinger. Efficient quantum computing using coherent photon conversion. Nature, 2011; DOI: 10.1038/nature10463

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Progress in quantum computing: Researchers control rate of photon emission from luminescent imperfections in diamond

ScienceDaily (Oct. 12, 2011) — Engineers and physicists at Harvard have managed to capture light in tiny diamond pillars embedded in silver, releasing a stream of single photons at a controllable rate.

The advance represents a milestone on the road to quantum networks in which information can be encoded in spins of electrons and carried through a network via light, one photon at a time.

The finding was published in Nature Photonics, appearing online Oct. 9.

"We can make the emission of photons faster, which will allow us to do more processing per second -- for example, more computations -- in the future quantum network," explains principal investigator Marko Loncar, Associate Professor of Electrical Engineering at the Harvard School of Engineering and Applied Sciences (SEAS).

The device Loncar's research team has built consists of parallel rows of tiny, nanofabricated diamond posts, embedded in a layer of silver, that can each act as a single photon source.

By removing the silver wrapping from their nanostructures, the team was also able to achieve a slower release of photons, which is of interest for probing the dynamics of the quantum system.

The breakthrough takes advantage of imperfections in the diamond's crystal lattice, where carbon atoms are replaced by other elements. To the naked eye, these imperfections can appear as discolorations in the diamond, turning it yellow in the case of nitrogen. Occasionally, there is also a vacancy (missing carbon atom) next to the nitrogen atom.

Each nitrogen-vacancy imperfection can serve as a nearly perfect quantum emitter, capable of emitting red photons one by one, even at room temperature. The technology is a promising candidate for realization of scalable, on-chip quantum networks.

"The color centers in diamond are very interesting as qubits for quantum information processing, where they can be used as memory to store information," says Loncar. "More importantly, they can be interrogated -- they can be written into and read out -- with light."

Loncar's team fabricates diamond posts that contain negatively charged nitrogen vacancy centers, which can absorb light and hold its energy for a given amount of time, finally releasing it in the form of photons.

"The rate at which photons are emitted can be controlled by carefully nano-engineering the center's surrounding," says co-author Irfan Bulu, a research associate in the Loncar group. Attaining fine control of that release, however, has been difficult.

"One of the main challenges has been the efficiency with which you can write information into the spin of these color centers, as well as the efficiency with which you can collect photons emitted from the color centers," explains co-author Jennifer Choy, a graduate student in Loncar's lab at SEAS. "The other challenge has been the rate -- how quickly you can perform these processes."

Previous work from Loncar's group solved the collection efficiency problem by using diamond nanowires to channel and direct the flow of photons. The new research manipulates the radius of diamond pillars and adds the silver coating. The diamond-silver construction acts as an optical nanoresonator, creating a strong electromagnetic field around the emitter and offering a new level of control over the rate of emission.

Moreover, the device functions at room temperature -- an essential requirement for practical computing applications -- and the nanostructured chips are fully scalable.

"We've designed everything in parallel in a massive system, which allows us to make thousands or millions of devices with more or less the same properties, and we use conventional microfabrication and nanofabrication techniques, unlike what has been done in this field before," says Birgit Hausmann, a graduate student in Loncar's lab at SEAS and one of the co-authors.

In addition to Loncar, Choy, Hausmann, and Bulu, co-authors included Tom Babinec, a graduate student at SEAS; Mughees Khan, a staff scientist at the Wyss Institute for Biologically Inspired Engineering at Harvard; Patrick Maletinsky, a fellow of the Department of Physics at Harvard; and Amir Jacoby, Professor of Physics in the Harvard Faculty of Arts and Sciences.

The work was supported by grants and fellowships from the U.S. Department of Defense, the Defense Advanced Research Projects Agency (DARPA) QuEST program, the National Science Foundation (NSF), the King Abdullah University of Science and Technology (KAUST), the Sloan Foundation, and the NSF-supported Nanoscale Science and Engineering Center (NSEC) at Harvard. Fabrication took place at the NSF-supported Center for Nanoscale Systems (CNS) at Harvard.

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Jennifer T. Choy, Birgit J. M. Hausmann, Thomas M. Babinec, Irfan Bulu, Mughees Khan, Patrick Maletinsky, Amir Yacoby, Marko Loncar. Enhanced single-photon emission from a diamond–silver aperture. Nature Photonics, 2011; DOI: 10.1038/nphoton.2011.249

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Point defects in super-chilled diamonds may offer stable candidates for quantum computing bits

ScienceDaily (Oct. 13, 2011) — Diamond, nature's hardest known substance, is essential for our modern mechanical world -- drills, cutters, and grinding wheels exploit the durability of diamonds to power a variety of industries. But diamonds have properties that may also make them excellent materials to enable the next generation of solid-state quantum computers and electrical and magnetic sensors.

To further explore diamonds' quantum computing potential, researchers from the University of Science and Technology of China tested the properties of a common defect found in diamond: the nitrogen-vacancy (NV) center.

Consisting of a nitrogen atom impurity paired with a 'hole' where a carbon atom is absent from the matrix structure, the NV center has the potential to store information because of the predictable way in which electrons confined in the center interact with electromagnetic waves. The research team probed the energy level properties of the trapped electrons by cooling the diamonds to an extremely chilly 5.6 degrees Kelvin and then measuring the magnetic resonance and fluorescent emission spectra. The team also measured the same spectra at gradually warmer increments, up to 295 degrees Kelvin.

The results, as reported in the AIP's journal Applied Physics Letters, show that at temperatures below 100 Kelvin the electrons' transition energies, or the energies required to get from one energy level to the next, were stable. Shifting transition energies could make quantum mechanical manipulations tricky, so cooler temperatures may aid the study and development of diamonds for quantum computation and ultra-sensitive detectors, the authors write.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by American Institute of Physics, via EurekAlert!, a service of AAAS.

Journal Reference:

X.-D. Chen, C.-H. Dong, F.-W. Sun, C.-L. Zou, J.-M. Cui, Z.-F. Han, and G.-C Guo. Temperature dependent energy level shifts of nitrogen-vacancy centers in diamond. Applied Physics Letters, 2011

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A hidden order unraveled: Microscopic views on quantum fluctuations

ScienceDaily (Oct. 14, 2011) — Fluctuations are fundamental to many physical phenomena in our everyday life, such as the phase transitions from a liquid into a gas or from a solid into a liquid. But even at absolute zero temperature, where all motion in the classical world is frozen out, special quantum mechanical fluctuations prevail that can drive the transition between two quantum phases.

Now a team around Immanuel Bloch and Stefan Kuhr at Ludwig-Maximilians University (LMU) and the Max Planck Institute of Quantum Optics (MPQ) has succeeded in directly observing such quantum fluctuations. Using a high resolution microscope, they were able to image quantum-correlated particle-hole pairs in a gas of ultracold atoms. This allowed the physicists to unravel a hidden order in the crystal and to characterize the different phases of the quantum gas. The work was performed together with scientists from the Theory Division at the MPQ and ETH Zurich. These measurements open new ways to characterize novel quantum phases of matter.

The scientists start by cooling a small cloud of rubidium atoms down to a temperature near absolute zero, about minus 273 degree Celsius. The ensemble is then subjected to a light field that severely restricts the motion of the particles along one-dimensional tubes of light aligned in parallel. An additional standing laser wave along the tubes creates a one-dimensional optical lattice that holds the atoms in a periodic array of bright and dark regions of light.

The atoms move in the periodic light field like electrons in solids. As these can be electric conductors or insulators, also the one-dimensional quantum gases can behave like a superfluid or like an insulator at low temperatures. In particular, the height of the optical lattice potential plays an important role: it determines whether the atom is fixed on a particular lattice site or whether is able to move to a neighbouring site. At very large lattice depths, each lattice site is occupied by exactly one atom. This highly ordered state is called a "Mott insulator," after the British physicist and Nobel laureate Sir Neville Mott. When the lattice depth is decreased slightly, the atoms have enough energy to reach a neighbouring site by quantum mechanical tunneling. In this way, pairs of empty and doubly occupied sites emerge, so-called particle-hole pairs. Intriguingly, these quantum fluctuations also occur at absolute zero temperature, when all movement in the classical world is frozen out. The position of the quantum-correlated particle-hole pairs in the crystal is completely undetermined and is fixed only by the measurement process.

In recent experiments, the physicists around Stefan Kuhr and Immanuel Bloch had already developed a method, which allowed to image single atoms lattice site by lattice site. The atoms are cooled using laser beams, and the fluorescence photons emitted in this process are used to observe the atoms with a high resolution microscope. Holes naturally show up as dark spots, but so do doubly occupied sites as the two particles kick each other out of the lattice in the experiment. Therefore particle-hole pairs appear as two neighbouring dark lattice sites. "With our technique, we can directly observe this fundamental quantum phenomenon for the first time," describes doctoral student Manuel Endres enthusiastically.

The physicists measure the number of neighbouring particle-hole pairs through a correlation function. With increasing kinetic energy, more and more particles tunnel to neighbouring sites and the pair correlations increase. However, when the number of particle-hole pairs is very large, it becomes difficult to unambiguously identify them. Hence the correlation function takes on smaller values. Finally, the ordered state of a Mott insulator vanishes completely und the quantum gas becomes a superfluid again. Here fluctuations of holes and particles occur independently. The correlation function measured in the experiment is very well reproduced by model calculations, which were performed by scientists from the Theory Division at the MPQ and the ETH Zurich. Interestingly, the same investigations on two-dimensional quantum-gases clearly showed that quantum fluctuations are not as prominent as in one-dimensional systems.

The scientists extended their analysis to correlations between several lattice sites along a string. Such non-local correlation functions contain important information about the underlying many-body system and can be used as an order parameter to characterize different quantum phases. In the experiment described here, such non-local order parameters have been measured for the first time. In the future, the scientists plan to use these measurements for the detection of topological quantum phases. These can be useful for robust quantum computers and could help to understand superconductivity at high temperatures. (MPQ)

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by Ludwig-Maximilians-Universität München.

Journal Reference:

M. Endres, M. Cheneau, T. Fukuhara, C. Weitenberg, P. Schauss, C. Gross, L. Mazza, M. C. Banuls, L. Pollet, I. Bloch, S. Kuhr. Observation of Correlated Particle-Hole Pairs and String Order in Low-Dimensional Mott Insulators. Science, 2011; 334 (6053): 200 DOI: 10.1126/science.1209284

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Subatomic quantum memory in diamond demonstrated

ScienceDaily (June 28, 2011) — Physicists working at the University of California, Santa Barbara and the University of Konstanz in Germany have developed a breakthrough in the use of diamond in quantum physics, marking an important step toward quantum computing. The results are reported in this week's online edition of Nature Physics.

The physicists were able to coax the fragile quantum information contained within a single electron in diamond to move into an adjacent single nitrogen nucleus, and then back again using on-chip wiring.

"This ability is potentially useful to create an atomic-scale memory element in a quantum computer based on diamond, since the subatomic nuclear states are more isolated from destructive interactions with the outside world," said David Awschalom, senior author. Awschalom is director of UCSB's Center for Spintronics & Quantum Computation, professor of physics, electrical and computer engineering, and the Peter J. Clarke director of the California NanoSystems Institute.

Awschalom said the discovery shows the high-fidelity operation of a quantum mechanical gate at the atomic level, enabling the transfer of full quantum information to and from one electron spin and a single nuclear spin at room temperature. The process is scalable, and opens the door to new solid-state quantum device development.

Scientists have recently shown that it is possible to synthesize thousands of these single electron states with beams of nitrogen atoms, intentionally creating defects to trap the single electrons. "What makes this demonstration particularly exciting is that a nitrogen atom is a part of the defect itself, meaning that these sub-atomic memory elements automatically scale with the number of logical bits in the quantum computer," said lead author Greg Fuchs, a postdoctoral fellow at UCSB.

Rather than using logical elements like transistors to manipulate digital states like "0" or "1," a quantum computer needs logical elements capable of manipulating quantum states that may be "0" and "1" at the same time. Even at ambient temperature, these defects in diamond can do exactly that, and have recently become a leading candidate to form a quantum version of a transistor.

However, there are still major challenges to building a diamond-based quantum computer. One of these is finding a method to store quantum information in a scalable way. Unlike a conventional computer, where the memory and the processor are in two different physical locations, in this case they are integrated together, bit-for-bit.

"We knew that the nitrogen nuclear spin would be a good choice for a scalable quantum memory -- it was already there," said Fuchs. "The hard part was to transfer the state quickly, before it is lost to decoherence."

Awschalom explained: "A key breakthrough was to use a unique property of quantum physics -- that two quantum objects can, under special conditions, become mixed to form a new composite object." By mixing the quantum spin state of the electrons in the defect with the spin state of the nitrogen nucleus for a brief time -- less than 100 billionths of a second -- information that was originally encoded in the electrons is passed to the nucleus.

"The result is an extremely fast transfer of the quantum information to the long-lived nuclear spin, which could further enhance our capabilities to correct for errors during a quantum computation," said co-author Guido Burkard, a theoretical physicist at the University of Konstanz, who developed a model to understand the storage process.

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The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of California - Santa Barbara, via EurekAlert!, a service of AAAS.

Journal Reference:

G. D. Fuchs, G. Burkard, P. V. Klimov, D. D. Awschalom. A quantum memory intrinsic to single nitrogen–vacancy centres in diamond. Nature Physics, 2011; DOI: 10.1038/nphys2026

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