Tuesday, August 19, 2014

The Quantum Mechanical Behavior of an Individual Electron

Molecular engineers record an electron’s quantum behavior

A University of Chicago-led team of researchers has developed a technique to record the quantum mechanical behavior of an individual electron contained within a nanoscale defect in diamond. Their technique uses ultrafast pulses of laser light, both to control the defect’s entire quantum state and observe how that single electron state changes over time. The work appears in this week’s online Science Express and will be published in print later this month in Science.

[caption id="attachment_415" align="aligncenter" width="500"]The Quantum Mechanical Behavior of an Individual Electron www.quantumcomputingtechnologyaustralia.com-061 The Quantum Mechanical Behavior of an Individual Electron - Scientists at the University of Chicago and the University of California, Santa Barbara, used this optical apparatus to direct ultrafast pulses of light to manipulate the quantum state of a single electron spin in diamond.[/caption]

This research contributes to the emerging science of quantum information processing, which demands that science leave behind the unambiguous universe of traditional binary logic—0 or 1—and embrace the counterintuitive quantum world, in which electrons can be in many states at once.

The team researched a quantum mechanical property of the electron known as spin. Much like conventional computers use the charge state of electrons to constitute bits of information, a quantum computer uses the spin state of an electron as its quantum bit, or qubit. The work could accelerate development of quantum computing devices, and the extra computing power that would come with them because it will be easier to identify materials that have appropriate quantum properties.

The spin system studied is known as the nitrogen-vacancy (NV) center, an atom-sized defect that occurs naturally in diamond, consisting of a nitrogen atom next to a vacant spot in the crystal lattice. “These defects have garnered great interest over the past decade, providing a test-bed system for developing semiconductor quantum bits as well as nanoscale sensors,” said team leader David Awschalom, the Liew Family Professor of Molecular Engineering at UChicago. “Here, we were able to harness light to completely control the quantum state of this defect at extremely high speeds.”

Quantum snapshots

In this new technique, the researchers locate a single NV center and then illuminate it with a pair of extremely short pulses of laser light. Each pulse lasts less than a picosecond (or a millionth of a millionth of a second). The first pulse excites the quantum states of the defect-bound electron, which then change or evolve in characteristic ways. The second pulse stops that evolution, capturing a picture of the quantum state at that elapsed time.

By progressively extending the elapsed time between the two pulses, the team creates a sequence of quantum-state snapshots—a movie of how the quantum state changes in time. The elapsed time can be as short as femtoseconds (a billionth of a millionth of a second) or as long as nanoseconds (a thousandth of a millionth of a second). On the human scale, this range of time is like the difference between an hour and a century.

Having this vast range of timescales makes the technique especially valuable. The electron is susceptible and interacts with its complex local environment in many different ways, each with a characteristic timescale. Being able to test a wide range of these timescales gives a far more complete picture of the dynamics of the NV center than has been obtained previously.

“Our goal was to push the limits of quantum control in these remarkable defect systems,” explained Lee Bassett, co-lead author of the paper and an assistant professor of electrical and systems engineering at the University of Pennsylvania, “but the technique also provides an exciting new measurement tool. By using pulses of light to direct the defect’s quantum dynamics on super-short timescales, we can extract a wealth of information about the defect and its environment.”

“It’s quite a versatile technique, providing a full picture of the excited state of the quantum defect,” said F. Joseph Heremans, a UChicago postdoctoral scholar, the other co-lead author on the paper. “Previous work on the nitrogen-vacancy center has hinted at some of these processes, but here, simply through the application of these ultrafast pulses, we get a much richer understanding of this quantum beast.”

Spin control

It’s not just a matter of observation, though. “This technique also provide a means of control of the spin state—an important precursor for any quantum information system,” said Evelyn Hu, a professor of applied physics and electrical engineering at Harvard University, who is not connected with the new work.

In addition, the method is not limited to investigating this particular defect. It could be applied to quantum states of matter in a host of materials and technologies, including many semiconductor materials. “You only have to be able to use light to transfer an electron between a ground state and an excited state,” said Awschalom.

Prof. Guido Burkard, theoretical physicist at the University of Konstanz and a co-author of the paper, remarked, “This technique offers a path toward understanding and controlling new materials at the atomic level.”

Hu agrees that the technique opens many new avenues. “Each new system will pose new challenges to understanding the energy levels, local environments and other properties, but the general approach should provide an enormous step forward for the field,” said Hu.

In addition to researchers from UChicago’s Institute for Molecular Engineering, the team included collaborators at the University of California, Santa Barbara (co-lead author Lee Bassett is now at the University of Pennsylvania), and the University of Konstanz, Germany.

Citation: “Ultrafast Optical Control of Orbital and Spin Dynamics in a Solid-State Defect,” by Lee C. Bassett, F. Joseph Heremans, David J. Christle, Christopher G. Yale, Guido Burkard, Bob B. Buckley and David D. Awschalom,Science Express, Aug. 14, 2014.

News Release Source :  Molecular engineers record an electron’s quantum behavior

Image Credit  : Christopher Yale and Joseph Heremans/University of Chicago

Tuesday, August 12, 2014

NIST Ion Duet Could be Used to Perform Logic Operations in Quantum Computers

NIST ion duet offers tunable module for quantum simulator

BOULDER, Colo -- Physicists at the National Institute of Standards and Technology (NIST) have demonstrated a pas de deux of atomic ions that combines the fine choreography of dance with precise individual control.

NIST Ion Duet Could be Used to Perform Logic Operations in Quantum Computers www.quantumcomputingtechnologyaustralia.com-060
NIST Ion Duet Could be Used to Perform                                                                                    Logic Operations in Quantum Computers

NIST's ion duet, described in the August 7 issue of Nature, is a component for a flexible quantum simulator that could be scaled up in size and configured to model quantum systems of a complexity that overwhelms traditional computer simulations. Beyond simulation, the duet might also be used to perform logic operations in future quantum computers, or as a quantum-enhanced precision measurement tool.

In the experiments, researchers coaxed two beryllium ions located in separate zones of an electric-field trap (a storage device) into an "entangled" state. An important resource for quantum technologies, entanglement involves an intimate connection between the particles such that a measurement of one ordains the state of the other. This is the first time ions in separate zones have been entangled by manipulating their electric interactions, an important feature that could be used in quantum simulation and computing.

The work demonstrates a high level of quantum control with microfabricated trap technology well suited to the scaling-up needed to make powerful quantum information processors. Having separate trapping zones enabled the research team to tune the ions' interactions from weak to strong—a feature expected to be useful for simulating the behavior of complex quantum materials.

"Even though the ions are confined apart from one another, we can now entangle them," NIST physicist Andrew Wilson says. "We plan to use this for quantum simulation and computing, but when I explain to my family what we're doing, the remote entanglement sounds kind of romantic."

"We focus on the idea that everything needs to be scalable," Wilson notes. "To do useful simulations we'll need versatile traps with more than two ions, and making traps using the same technology used to make computer chips gives us this capability. NIST pioneered this approach and we're fortunate to have great facilities for doing this sort of work."

Inducing the ions to perform a number of intricate quantum dances, the researchers first coaxed the ions to exchange a single quantum of vibrational energy (the smallest amount that nature allows). They then used lasers and microwaves to entangle the ions' "spins." Analogous to tiny bar magnets, the spins of the entangled ions pointed in the same direction, but were also in a "superposition" of pointing in the opposite direction at the same time. Superposition is another strange but useful feature of the quantum world.

The researchers say that extending the new module to make a two-dimensional network of a few tens of ions would be enough to perform useful simulations of phenomena that are extremely difficult to model even on the most powerful traditional computers. An example is the high-temperature superconductitivity—electron flow without resistance—observed in certain ceramics. Despite more than 20 years of study, the underlying mechanism remains a mystery. A quantum simulator might provide deeper insights.

The ion duet also could be used to perform logic operations in quantum computers, which would have a wider range of applications than quantum simulators. And NIST researchers also envision the ion duet as a sensor, in which one well-controlled ion is used to investigate a second ion with interesting features. For instance, a beryllium ion might be used to probe a charged anti-matter particle in another trap zone, Wilson says.


This research was funded by the Office of the Director of National Intelligence, the Intelligence Advanced Research Projects Activity and the Office of Naval Research. A.C. Wilson, Y. Colombe, K.R. Brown, E. Knill, D. Leibfried and D.J. Wineland. Entangling spin-spin interactions of ions in individually controlled potential wells. Nature. August 7. DOI 10.1038/nature13565.

Image Credit :  NIST

News Release Source : NIST ion duet offers tunable module for quantum simulator

'NV Centers' at Specific Spots within a Diamond Lattice could Advance Quantum Computing

Diamond Defect Interior Design

Planting imperfections called 'NV centers' at specific spots within a diamond lattice could advance quantum computing and atomic-scale measurement

WASHINGTON, D.C., August 5, 2014 – By carefully controlling the position of an atomic-scale diamond defect within a volume smaller than what some viruses would fill, researchers have cleared a path toward better quantum computers and nanoscale sensors. They describe their technique in a paper published in the journal Applied Physics Letters, from AIP Publishing.

[caption id="attachment_402" align="alignleft" width="400"]'NV Centers' at Specific Spots within a Diamond Lattice could Advance Quantum Computing www.quantumcomputingtechnologyaustralia.com-059 'NV Centers' at Specific Spots within a Diamond Lattice could Advance Quantum Computing[/caption]

David Awschalom, a physicist at the Institute for Molecular Engineering at the University of Chicago, and his colleagues study a technologically useful diamond defect called a nitrogen vacancy (NV) center. NV centers consist of a nitrogen atom adjacent to a vacant spot that replaces two carbon atoms in the diamond crystal, leaving an unpaired electron. Researchers can use a property of the unpaired electron known as its spin to store and transmit quantum information at room temperature.

Qubits and Quantum Sensors

NV centers are attractive candidates for qubits, the quantum equivalent of a classical computing bit. A single NV center can also be used for completely different applications, such as measuring temperature, as well as to image electric and magnetic fields on the nanometer-scale by placing it at the tip of a diamond-based scanning probe.

A primary obstacle to further exploiting NV centers for practical quantum computing and nanoscale sensing devices lies in the difficulty of placing the centers within what Awschalom calls the functional "sweet spots" of the devices. Another challenge is increasing the NV center density without sacrificing their spin lifetimes, which must remain long in order to extract the most useful information from the system.

Awschalom and his colleagues have developed a new way to create NV centers that could help overcome both these challenges.

That's the Spot

The key to the team's new approach is to create the nitrogen and vacancy defects separately, Awschalom said. First, the team grew a layer of nitrogen-doped crystal within a diamond film. The researchers kept the nitrogen layer extremely thin by reducing the growth rate of the film to approximately 8 nanometers/hour. The nanometer-scale nitrogen-doped layer constrains the possible location of the NV centers in the depth direction.

Secondly, the researchers created a mask to cover the film, leaving only pinprick holes. They blasted carbon ions through the holes to create vacancies and heated the diamond to make the vacancies mobile within the crystal. NV centers could form in the nitrogen-doped layer below where the holes were placed.

Using this approach the team successfully localized NV centers within a cavity approximately 180 nanometers across -- a volume small enough to be compatible with many diamond-based nanostructures used in sensing devices and experimental quantum information systems.

The localized NV centers could also hold a specific spin for longer than 300 microseconds. This so-called spin coherence time was an order of magnitude better than that achieved by other 3-D localization methods. The longer spin lifetime means the NV centers can detect smaller magnetic signals and hold quantum information for longer.

One of the team's goals for using their new technique is to measure the nuclear spins of hydrogen atoms – one of the tiniest magnetic signals – within a biological molecule. The research could reveal new insights into how important biological functions like photosynthesis work. "Our research impacts diverse fields of science and technology," Awschalom said. "Technological advancements always open new avenues of scientific research."

The article, "Three-dimensional localization of spins in diamond using 12C implantation," is authored by Kenichi Ohno, F. Joseph Heremans, Charles F. de las Casas, Bryan A. Myers, Benjamín J. Alemán, Ania C. Bleszynski Jayich, and David D. Awschalom. It will be published in the journal Applied Physics Letters on August 5, 2014 (DOI: 10.1063/1.4890613). After that date, it can be accessed at: http://scitation.aip.org/content/aip/journal/apl/105/5/10.1063/1.4890613

The authors of this paper are affiliated with the University of California, Santa Barbara and the University of Chicago.


Applied Physics Letters features concise, rapid reports on significant new findings in applied physics. The journal covers new experimental and theoretical research on applications of physics phenomena related to all branches of science, engineering, and modern technology. See: http://apl.aip.org

Image Credit: F.J. Heremans and D. Awschalom/U. Chicago and K. Ohno/UCSB

News Release SourceDiamond Defect Interior Design

Sunday, August 10, 2014

Quantum Cheshire Cat - Scientists Separate a Particle from its Properties

Scientists separate a particle from its properties

Researchers from the Vienna University of Technology have performed the first separation of a particle from one of its properties. The study, carried out at the Institute Laue-Langevin (ILL) and published in Nature Communications, showed that in an interferometer a neutron's magnetic moment could be measured independently of the neutron itself, thereby marking the first experimental observation of a new quantum paradox known as the 'Cheshire Cat'. The new technique, which can be applied to any property of any quantum object, could be used to remove disturbance and improve the resolution of high precision measurements.

[caption id="attachment_398" align="aligncenter" width="400"]Diagram of Cheshire Cat Experiment www.quantumcomputingtechnologyaustralia.com-058 Diagram of Cheshire Cat Experiment[/caption]

The idea of a Quantum Cheshire Cat was proposed theoretically last year. It is based on the well known character from Alice in Wonderland who can vanish leaving his smile behind. In quantum physics, the term refers to an object whose properties can be separated from its physical location so that the two can be measured at different places. While this is clearly not possible in our everyday experience, where objects are spatially linked to their properties, the laws of Quantum Mechanics allow it to be achieved.

Quantum mechanics already tells us that particles can be in different physical states at the same time, a phenomenon known as superposition. For example if a neutron beam is divided in two using a crystal, individual neutrons do not have to decide which of the two paths to take. Instead, they can travel along both paths at the same time in a quantum superposition.

"This experimental technique is called neutron interferometry", says Professor Yuji Hasegawa from the Vienna University of Technology. "It was invented here at the Atominstitut in the 1970s, and it has turned out to be the perfect tool to investigate the foundations of quantum mechanics."

To see if the same technique could separate the properties of a particle from the particle itself, Yuji Hasegawa brought together a team including colleagues Tobis Denkmayr, Hermann Geppert and Stephan Sponar from Vienna, together with Alexandre Matzkin from CNRS in France, Professor Jeff Tollaksen from Chapman University in California, and Hartmut Lemmel from the Institut Laue-Langevin to develop a brand new quantum experiment.

Their aim was to get neutrons at the ILL to travel along a different path from its magnetic moment - a property describing the particle's coupling strength to an external magnetic field. The neutron's magnetic moment has a directional preference, a property called spin. In the experiment the neutron beam was split into two paths with different spin directions. The upper beam path had a spin parallel to the neutrons' direction of flight whilst the spin of the lower beam pointed in the opposite direction.

After the two beams were recombined the experimental detector was set up so that only neutrons with spin parallel to the direction of motion - implying that those travelling along the upper path - are detected. "This is called postselection", says Hermann Geppert. "The beam contains neutrons of both spin directions, but we only detect a selection of the neutrons."

Things get tricky, when the location of the neutron spin is measured: the spin can be slightly changed using a magnetic field. When the two beams are recombined appropriately, they can amplify or cancel each other. This is exactly what can be seen in the measurement, if the magnetic field is applied at the lower beam – but that is the path, which the neutrons are actually never supposed to take. A magnetic field applied to the upper beam, on the other hand, does not have any effect.

"By preparing the neutrons in a special initial state and then postselecting them, we can achieve a situation in which both possible paths in the interferometer are important for the experiment, but in very different ways", says Tobias Denkmayr. "Along one of the paths, only an interaction with the particles themselves has an effect, but the other path is only sensitive to a magnetic spin coupling. The system behaves as if the particles were spatially separated from their properties."

The success of this unique type of quantum experiment was dependent on making so called 'weak measurements' to avoid the collapse of the superposition in accordance with the laws of quantum mechanics.

"These weak measurements give you less information," explains Hartmut Lemmel, instrument leader on S18, the ILL's crystal thermal neutron interferometer on which the Cheshire Cat was observed. "As a result you need to do lots of observations to achieve any sort of certainty that you have seen what you think you have seen. This was only possible due the strength of the neutron source available at the ILL which can uniquely provide the numbers of neutrons required to run these repeat experiments."

With their landmark observation suitably vindicated, questions turn to the potential impact of their fundamental discovery. One application might high precision measurements of quantum systems which are often affected by disturbance.

"Consider a quantum system that has two properties: you want to measure the first one very precisely but the second makes the system prone to perturbations. The two can be separated using a Quantum Cheshire Cat, and possibly the perturbation can be minimized", says Stephan Sponar.

News Release Source :  Scientists separate a particle from its properties