Physicists Propose New Method for Quantum Computing

(PhysOrg.com) -- The new system, which can compute faster and more efficiently than previous quantum computers, may bring the technology closer to reality.

Illustration from a poster by Susanne Yelin, Elena Kuznetsova, and Robin Côté

Quantum computers can solve in a matter of moments problems that would take ordinary computers years to work out. But thus far, these computers exist only as state-of-the-art experimental setups in a few physics laboratories.

Now, Elena Kuznetsova, a post-doctoral researcher in UConn’s Department of Physics, has proposed a new type of quantum computer that could bring the technology one step closer to becoming a reality.

“The main excitement about quantum computers,” says Kuznetsova, “ comes from their potential ability to solve certain problems exponentially faster compared to classical computers, such as factoring a large number into its primes, which would allow us to break cryptographic codes. These problems cannot be solved using a classical computer in the foreseeable future.”

Quantum processors take advantage of the principles of quantum mechanics, in which objects behave differently at very small scales than matter does at larger scales. Usually, their processors encode information into either individual atoms or molecules made up of two atoms. But Kuznetsova and her research group have proposed the first viable system that uses both atoms and molecules, taking advantage of the benefits of each. This system could be capable of computing faster and more efficiently than previous quantum processors.

Kuznetsova and her colleagues in physics, including graduate student Marko Gacesca and professors Susanne Yelin and Robin Côté, report their results in the March 2010 issue of Physical Review A.

As Yelin explains, there are several components to quantum computing. The first challenge is to create a system that you can control well enough to perform your computing, and another is to fashion a device that will report the results without damaging the system. The most advanced quantum computing to date is performed by neutral atoms, which, says Yelin, physicists have spent decades mastering and can now control to a very fine degree. These neutral atoms have no electric charge, and are therefore very difficult to get to interact with one another. This difficulty slows down the rate of computations.

In recent years, however, scientists discovered that polar molecules — which contain two atoms with equal and opposite charges — could lead to faster processing in quantum systems because the presence of these contrasting charges encourages the molecules to interact strongly with one other.

But this difference in molecular behavior is at once a great solution and a big problem, says Yelin. To be useful, these hyperactive molecules need to be cooled to only a few millionths of a degree above absolute zero, which slows them down and allows scientists to control them.

“Molecules in quantum states are very fragile,” Yelin says. “You heat them up, they’re gone. You bring them too close to each other, they’re gone. You look at them the wrong way, and they’re gone.”

Illustration from a poster by Susanne Yelin, Elena Kuznetsova, and Robin Côté.

The fragility of these molecules also poses another problem: when they’re used to report results from a quantum processor, scientists often lose control of them and the very data they’ve been trying to compute is destroyed. Until now, researchers hadn’t come up with a good way to read data out of these molecules.

In their recent paper, Kuznetsova’s group devised a way to separate the molecules into their component parts so that the processor’s results can be read from the more easily controllable individual atoms. By using lasers, says Kuznetsova, they were able to break down the molecules without compromising the data encoded in them.

“We let the molecule interact with a laser light with a very specific wavelength, or color,” she says. “This excites the molecule into another excited state, from which we can, with another laser light, break it down into two atoms. It’s a nondestructive, efficient way to keep the information and read it.”

Kuznetsova says that each portion of their concept is practically feasible using current experimental methods, but only for a generalized ensemble of qubits — as units of quantum information are known — all at once. The next step toward building a computer with polar molecules, says Yelin, is to create a system in which the qubits can be controlled individually.

Yelin admits that her work can sometimes seem supernatural to non-specialists, but she says she wouldn’t have it any other way.

“On first glance, these interactions of light and particles looks like magic,” she says. “These polar molecules are the pinnacle of quantum optics.”

Obtenido de: http://www.physorg.com/news195837003.html

Tirso Ramírez

CRF

'Quantum computer' a stage closer with silicon breakthrough

The remarkable ability of an electron to exist in two places at once has been controlled in the most common electronic material - silicon - for the first time. The research findings - published in Nature by a UK-Dutch team from the University of Surrey, UCL (University College) London, Heriot-Watt University in Edinburgh, and the FOM Institute for Plasma Physics near Utrecht - marks a significant step towards the making of an affordable "quantum computer".


The electron orbits a phosphorus atom embedded in the silicon lattice, shown in silver. The undisturbed electron density distribution, calculated from the quantum mechanical equations of motion is shown in yellow. A laser pulse can modify the electron’s state so that it has the density distribution shown in green. Our first laser pulse, arriving from the left, puts the electron into a superposition of both states, which we control with a second pulse, also from the left, to give a pulse which we detect, emerging to the right. The characteristics of this "echo" pulse tell us about the superposition we have made. Credit: UCL

According to the research paper in Nature the scientists have created a simple version of Schrodinger's cat - which is paradoxically simultaneously both dead and alive - in the cheap and simple material out of which ordinary computer chips are made.

"This is a real breakthrough for modern electronics and has huge potential for the future," explained Professor Ben Murdin, Photonics Group Leader at the University of Surrey. "Lasers have had an ever increasing impact on technology, especially for the transmission of processed information between computers, and this development illustrates their potential power for processing information inside the computer itself. In our case we used a far-infrared, very short, high intensity pulse from the Dutch FELIX laser to put an electron orbiting within silicon into two states at once - a so-called quantum superposition state. We then demonstrated that the superposition state could be controlled so that the electrons emit a burst of light at a well-defined time after the superposition was created. The burst of light is called a photon echo; and its observation proved we have full control over the quantum state of the atoms."

And the development of a silicon based "quantum computer" may be only just over the horizon. "Quantum computers can solve some problems much more efficiently than conventional computers - and they will be particularly useful for security because they can quickly crack existing codes and create un-crackable codes," Professor Murdin continued. "The next generation of devices must make use of these superpositions to do quantum computations. Crucially our work shows that some of the quantum engineering already demonstrated by atomic physicists in very sophisticated instruments called cold atom traps, can be implemented in the type of silicon chip used in making the much more common transistor."

Professor Gabriel Aeppli, Director of the London Centre for Nanotechnology added that the findings were highly significant to academia and business alike. "Next to iron and ice, silicon is the most important inorganic crystalline solid because of our tremendous ability to control electrical conduction via chemical and electrical means," he explained. "Our work adds control of quantum superpositions to the silicon toolbox."

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How 'spooky' quantum mechanical laws may affect everyday objects (Update)

(PhysOrg.com) -- In a study published in the July 1 issue of the journal Nature, Dartmouth researchers describe one example of the microscopic quantum world influencing--even dominating, they say--the behavior of something in the macroscopic classical world.



An optical micrograph of one of the samples measured by the research team is shown here. The electrical contacts are at the top and bottom. The gold gates used to form the QPC tunnel barrier are also labeled. Credit: Joel Stettenheim, Dartmouth College

"One major question in physics has to do with the connection between the microscopic and macroscopic worlds," said Alex Rimberg, associate professor of physics at Dartmouth College.

In the microscopic world, tiny sub-atomic particles such as photons and electrons, obey the sometimes bizarre laws of quantum mechanics. Meanwhile objects in the macroscopic world, generally anything visible with the naked eye, conform to the laws of classical physics discovered by Newton in the 17th century.

But a little more than 300 years after Newton, Einstein proved that light consists of tiny "packets" of energy, called quanta. This discovery marked the beginning of quantum theory, though it took decades of further work by several great scientific minds to finally settle on the modern theory of quantum mechanics.

One of the strangest laws of quantum mechanics is the Uncertainty Principle, first noted by German physicist and Nobel Laureate Werner Heisenberg in 1927. Heisenberg realized that when trying to locate a fast-moving particle, such as an electron, it was impossible to pin down both its position and its momentum at the same time.

"To do a measurement, an experiment has to interact with whatever is being measured," explained Rimberg. "But interaction means ultimately that you must exert a force on what you're measuring. If you're trying to measure the position of an object, any measurement will make the object move in an unpredictable and random way. This tendency to randomly affect what you are measuring is called "backaction."

Einstein could never accept this idea--that the act of measurement changes the object being measured--on philosophical grounds, and fought it until his dying breath. But the uncertainty principle is now known to be true for all quantum-level interactions.

What is not yet known is how the quantum and classical worlds relate. "What we don't understand, really, is how classical behavior emerges from quantum behavior as systems become larger and larger," Rimberg said. "We also don't really understand how large an influence quantum mechanics can have on the classical world we live in."

Making it real

Rimberg and colleague Miles Blencowe, both supported by grants from the National Science Foundation (NSF), have now led a team of researchers in demonstrating quantum mechanical events affecting the classical world.

The scientists didn't start out to accomplish any such thing, according to Rimberg. Instead, they were trying to measure fast changes in charge at nanometer scales.

To do this, they first created tiny semiconductor crystals, similar to a computer chip, each about 3 millimeters (about 1/10 of an inch) across. They deposited gold electrical gates running over the crystal, leaving a tiny break of only about a few hundred micrometers in the middle of the chip. This break is called a "quantum point contact," or QPC.

By hooking the chip up to an electrical circuit, electrons flow through metal contacts until they hit the QPC. And that's where they started to see one of quantum mechanics' quirks.

"You can think of the QPC as a tunnel barrier, sort of a wall for electrons," Rimberg explained. "When the wall is sufficiently high, the electrons do not have enough energy to go over it. If electrons were classical objects, that would be the end of the story. But since electrons obey the laws of quantum mechanics, instead of going over the barrier they can also "quantum tunnel" through it."

Thus, when a stream of electrons in an electrical current approaches the QPC, each electron in the stream randomly "chooses" to reflect backward off the barrier or go through it.

"This random process introduces noise into the electrical current, caused by random fluctuations in the number of electrons going through at any time," said Rimberg. "Because this noise is generated quantum mechanically, it is sometimes referred to as quantum noise."

Measuring quantum noise

For this experiment, the scientists used semiconductor crystals made of gallium arsenide, which happen to exhibit a property called piezoelectricity. The term "piezoelectric" means that an electrical current traveling through the crystal causes a mechanical or physical movement of the crystal itself, similar to the way a sponge expands when water hits it.

Piezoelectric crystals are sometimes called resonators, because they can resonate, or vibrate, in reaction to electrical signals. These resonators can move in different ways-stretching or bending, depending on the frequency of the signal and the shape of the crystal.

"The three-dimensional vibration of a resonator crystal is exactly like the vibration you get if you strike a tuning fork, or run a wet finger about the rim of a wine glass," Rimberg explained. "The glass (or tuning fork) starts to hum with a musical note; that's because there is a particular kind of vibrational pattern, determined by its geometry, that the atoms in the wine glass collectively take part in."

In the same way, the electrons bouncing off the QPC "wall" apply a random "backaction" force to the crystal, Rimberg said. In this case, the backaction force just happened to vibrate the crystal at one of its favorite frequencies. When the researchers measured the electrical current versus frequency and found strong peaks indicating that the backaction was creating a feedback loop, it took them by surprise.

"Neither I nor anyone else anticipated the spectral features that show the samples are vibrating," Rimberg said. "It took us quite a bit of time and effort to convince ourselves that it was a real effect, and yet more time and effort to figure out what it was."

Uncertainty in action

"In our case, the current running through the QPC gives information about the position of the semiconducting crystal that the QPC lives in," Blencowe said. "But because of the quantum noise in the current, at any given time there are random fluctuations in the number of electrons (on the order of 10,000 or so) on either side of the QPC."

And because these electrons have an electrical charge, they exert a piezoelectric force on the crystal, making it move. "The remarkable thing is that only 10,000 or so electrons are able to make all 1020 (100 quintillion) atoms in the crystal move at once," said Blencowe.

"The difference in size between the two parts of the system is really extreme," Blencowe explained. "To give a sense of perspective, imagine that the 10,000 electrons correspond to something small but macroscopic, like a flea. To complete the analogy, the crystal would have to be the size of Mt. Everest. If we imagine the flea jumping on Mt. Everest to make it move, then the resulting vibrations would be on the order of meters!"

"Our work is a direct example of the microscopic quantum world influencing, and even dominating the behavior of something in the macroscopic classical world," Rimberg said. "The motion of the semiconducting crystal is not dominated by something classical like thermal motion, but instead by the random quantum fluctuations in the number of tunneling electrons."

And in this case, Rimberg pointed out, the macroscopic world also influences the quantum world, because the crystal's vibrations cause the electrons to tunnel in large bunches.

In future research the team could take several possible directions. "First, we will actually use the QPC for charge detection, as we had intended to all along," Rimberg said. "Second, we'll continue to look at questions regarding the quantum-classical transition, but with resonators that are smaller than these crystals--things that are in the murky borderland between the much better understood quantum and classical regimes." This borderland is sometimes known as the "mesoscopic" scale.

"The study of these kinds of systems advances fundamental knowledge and also addresses some very practical questions, including: What are the fundamental limits of measurement? And what is the most sensitive measurement device that can be made?" said Daryl Hess, a program manager in NSF's Division of Materials Research.

"Questions of this kind become more pressing as our science and technology shrink to ever smaller scales with the vision of devices, electronic and mechanical, that are perhaps only a few atoms in one or more dimensions," added Hess. "At these scales, devices could show some aspects that appear squarely in the world of quantum mechanics and others that appear to be squarely in the world of classical mechanics."

Obtenido de: http://www.physorg.com/news197120339.html

Tirso Ramírez

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Novel ion trap with optical fiber could link atoms and light in quantum networks

Physicists at the National Institute of Standards and Technology have demonstrated an ion trap with a built-in optical fiber that collects light emitted by single ions (electrically charged atoms), allowing quantum information stored in the ions to be measured. The advance could simplify quantum computer design and serve as a step toward swapping information between matter and light in future quantum networks.

This is a diagram of a NIST ion trap that incorporates an optical fiber to collect light emitted by the ions (electrically charged atoms). Individual electrodes used to trap an ion 30 to 50 micrometers above the surface are shown in different colors surrounding a 50-micrometer-wide hole where light is collected and deposited in a fiber attached below. Credit: A. VanDevender/NIST


Described in a forthcoming issue of Physical Review Letters,* the new device is a 1-millimeter-square ion trap with a built-in optical fiber. The authors use ions as quantum bits (qubits) to store information in experimental quantum computing, which may someday solve certain problems that are intractable today. An ion can be adjustably positioned 80 to 100 micrometers from an optical fiber, which detects the ion's fluorescence signals indicating the qubit's information content.

"The design is helpful because of the tight coupling between the ion and the fiber, and also because it's small, so you can get a lot of fibers on a chip," says first author Aaron VanDevender, a NIST postdoctoral researcher.

NIST scientists demonstrated the new device using magnesium ions. Light emitted by an ion passes through a hole in an electrode and is collected in the fiber below the electrode surface (see image). By contrast, conventional ion traps use large external lenses typically located 5 centimeters away from the ions—about 500 times farther than the fiber—to collect the fluorescence light. Optical fibers may handle large numbers of ions more easily than the bulky optical systems, because multiple fibers may eventually be attached to a single ion trap.

The fiber method currently captures less light than the lens system but is adequate for detecting quantum information because ions are extremely bright, producing millions of photons (individual particles of light) per second, VanDevender says. The authors expect to boost efficiency by shaping the fiber tip and using anti-reflection coating on surfaces. The new trap design is intended as a prototype for eventually pairing single ions with single photons, to make an interface enabling matter qubits to swap information with photon qubits in a quantum computing and communications network.

Photons are used as qubits in quantum communications, the most secure method known for ensuring the privacy of a communications channel. In a quantum network, the information encoded in the "spins" of individual ions could be transferred to, for example, electric field orientations of individual photons for transport to other processing regions of the network.

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Tirso Ramírez
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Tunable quantum cascade laser

A team at the Central Research Laboratory at Hamamatsu Photonics K.K. in Japan has come up with, and tested, a tunable quantum cascade laser design that demonstrates broad optical gain. Their work is published in Applied Physics Letters: “High-performance, homogenous broad-gain quantum cascade lasers based on dual-upper-state design.”

“Usually, quantum cascade laser designs have only one upper state, except for superlattice active region,” Kazuue Fujita, the lead author on the paper, tells PhysOrg.com in an email interview. In this case, though, there is an additional upper state added to the design of the laser. “The additional upper state is created by a first quantum well adjacent to the injection barrier. This state corresponds to a lowest energy state in the first quantum well.”

Fujita goes on to explain that the regular upper state, seen in most conventional quantum cascade lasers, is designed to be nearly the same energy at operating condition as the state in the first quantum well. “Electrons are injected into the higher upper state via resonant tunneling from the previous injector. And then, electrons populated in the both upper states transit from both upper states to a lower state.” Both of these transitions contribute to optical gain.

This new laser design would have a number of advantages over current quantum cascade laser designs insists Fujita. This laser design allows for broadband tuning. On top of that, there is weak dependence on voltage by these lasers, and they are not sensitive to temperature change. “Slow efficiency at threshold is observed to be nearly constant over the wide range. Also I-L characteristics show super-linear behavior. These distinctive features have never been observed in a quantum cascade laser so far,” Fujita explains. “I think this design concept holds large potentialities.”

Fujita says that the design has actually been tested. “We have already fabricated and measured many lasers with the design. The lasers demonstrate very good performances. In addition, high temperature, continuous wave operation of the laser has also been achieved.”

In terms of application, Fujita sees a great deal of usefulness, especially in terms of spectroscopy. Trace gas sensing is considered one of the more likely applications, since wide tunability is desired. “The external cavity quantum cascade laser with this design may operate very stable due to its low dependences on voltage and temperature,” he says. Due to its tunability, this quantum cascade laser design could also increase the cost efficiency of some applications. “This design can lead to high-performance broadband tuning. Therefore, the laser allows a reduction in the number of lasers in a spectroscopic analysis system.”

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Tirso Ramírez
CRF

Quantum non-demolition measurement allows physicists to count photons without destroying them

(PhysOrg.com) -- In a way, the quantum world seems to know when it's being watched. When physicists make measurements on photons and other quantum-scale particles, the measurements always disturb the system in some way. Although an ideal disturbance should still enable physicists to make multiple measurements and get the same result twice, most real measurements cause a greater disturbance than this ideal minimum, and prohibit physicists from making repeated measurements. In a recent study, physicists have demonstrated a new way to make one of the ideal measurements - called quantum non-demolition (QND) measurements - allowing physicists to detect single particles repeatedly without destroying them.

The physicists performed a quantum non-demolition measurement, illustrated in this circuit schematic, that could detect single photons without destroying them. The technique allows repeated measurements to be made that give the same result. Image credit: B.R. Johnson, et al. ©2010 Macmillan Publishers Limited.


The concept of QND measurements has been around since the beginning of quantum mechanics, and physicists have demonstrated different QND measurement techniques since the ‘70s. In the latest technique, developed by a team of physicists from Yale University, Princeton University, and the University of Waterloo, the scientists have shown how to measure the number of photons inside a microwave cavity in a way that preserves the photon state 90% of the time; in other words, the method is 90% QND. The physicists explain that, unlike previously reported QND methods, the new technique is strongly selective to chosen photon number states, which could make it useful for applications such as monitoring the state of a photon-based memory in a quantum computer.

In their experiments, the physicists wanted to find out how many photons were in a microwave cavity. To do this without disturbing the system, they coupled a superconducting qubit to a cavity. This cavity stored the photons long enough for them to be measured - or “interrogated” - by using a set of controlled-NOT (CNOT) operations to encode information about the cavity state onto the qubit state. Then the qubit and storage cavity were decoupled, and the qubit state was read out. Because the qubit state now depends on the number of photons in the cavity, measuring the qubit reveals the number of photons.

“Our method takes advantage of the ability to engineer interactions between cavities and qubits in superconducting circuits to make the qubit energies strongly depend on the number of photons in the cavity,” coauthor Blake Johnson of Yale University told PhysOrg.com. “We have made this effect large enough to build a new qubit-photon logic gate which allows us to perform conditional qubit operations based on the cavity state. This type of logic gate is not only applicable to photon readout, but also to some proposals for engineering interactions between photons by using a qubit as a mediator.”

In the new design, the photon read out time is faster than the photon decay time. This timing difference allows the physicists to measure any qubit state several times during the lifetime of photons in the storage cavity. A single interrogation process takes about 550 nanoseconds, which includes the 50-nanosecond to initialize the qubit state. As expected with a high-quality QND method, the results of repeated interrogations are essentially indistinguishable from the first. In contrast, as Johnson explained, a typical quantum measurement would destroy one photon every time, so that repeated interrogations would give different results.

“A typical photon detector, like a CCD or photo-multiplier tube, absorbs photons,” Johnson said. “These detectors don’t work for microwaves because the energy of a microwave photon is too small to generate charges. However, with a setup similar to the one used in our paper, one could measure the photon state by transferring the photon energy into the qubit. This method would destroy exactly one photon every time. In contrast, our detector does not transfer any energy. Instead, we attempt to add energy to the qubit from an external source in such a way that the success or failure of these attempts reveals information about the cavity state. You might worry that this added energy might leak into the cavity and changed the photon number, but we have checked that this does not, in fact, happen.”

Achieving QND measurements of photons, while challenging, could be very useful for the development of quantum information technologies, which require complete control of quantum measurements. As the physicists note in their study, recent progress in manipulating microwave photons in superconducting circuits has increased the demand for a QND detector that operates in the gigahertz frequency range (like the one demonstrated here). In addition, the physicists predict that further research could make it possible to observe quantum jumps of light in a circuit, among other things.

“QND detection in general is interesting because it is the only way that quantum mechanics allows to extract information from a system without modifying its state, and then allowing feedback and manipulation of the same,” Johnson said. “The applications are interesting because if one could implement feedback of a quantum system, one could imagine using these systems for quantum simulation and quantum computation, harnessing quantum mechanics toward the goal of practical application.”

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Tirso Ramírez
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Seeking a bridge to the quantum world

Science fiction has nothing over quantum physics when it comes to presenting us with a labyrinthine world that can twist your mind into knots when you try to make sense of it.

A team of Arizona State University researchers, however, believe they’ve opened a door to a clearer view of how the common, everyday world we experience through our senses emerges from the ethereal quantum world.

Physicists call our familiar everyday environment the classical world. That’s the world in which we and the things around us appear to have measurable characteristics such as mass, height, color, weight, texture and shape.

The quantum world is the world of the elemental building block of matter – atoms. Atoms are combinations of neutrons and protons and electrons bound to a nucleus by electrical attraction.

But most of an atom – more than 99 percent of it – is empty space filled with invisible energy.

So from a quantum-world view, we and the things around us are mostly empty space. The way we experience ourselves and other things in the classical world is really just “a figment of our imaginations shaped by our senses,” said ASU Regents’ Professor David Ferry.

Akin to evolution

For more than a century, scientists and engineers have struggled to come to a satisfactory conclusion about the missing link that bridges the classical and quantum worlds and enables a transition from that world of mostly empty space to the familiar environment we experience through our senses.

One proposed scenario based on these questions was investigated in a dissertation written by Adam Burke to earn his doctorate in electrical engineering in 2009 from ASU’s Ira A. Fulton Schools of Engineering.

To try working out an answer to some of the questions, Burke teamed with Ferry, a professor in the School of Electrical, Computer and Energy Engineering; Tim Day, who recently earned his doctorate in electrical engineering from the school; physicist Richard Akis, an associate research professor in the school; Gil Speyer, an assistant research scientist for the engineering schools’ High Performance Computing Initiative; and Brian Bennett, a materials scientist with the Naval Research Laboratory.

The resultis an article published recently in the research journal Physical Review Letters and featured on PhysOrg.com, a science, technology and research news website. It describes the transition from quantum to classical world as a “decoherence” process that involves a kind of evolutionary progression somewhat analogous to Charles Darwin’s concept of natural selection.

Darwinian processes

The authors built on two theories called decoherence and quantum Darwinism, both proposed by Los Alamos National Laboratory researcher Wojciech Zurek.

The decoherence concept holds that many quantum states “collapse” into a “broad diaspora,” or dispersion, while interacting with the environment. Through a selection process, other quantum states arrive at a final stable state, called a pointer state, which is “fit enough” (think “survival of the fittest” in Darwinian terms) to be transmitted through the environment without collapsing.

These single states with the lowest energy can then make high-energy copies of themselves that can be described by the Darwinian process and observed on the macroscopic scale in the classical world.

The experiments arose from using advanced scanning gate microscopy to obtain images of what are called quantum dots.

Entering ‘forbidden space’

Burke, now doing research in a post-doctoral program at the University of New South Wales in Sydney, Australia, explains it like this:

Imagine the quantum dot as a billiard table in which the quantum point contacts are the two openings through which a ball could enter or leave the dot, and the interior walls of the dot act as bumpers.

If there were no friction on the table, a billiard ball with an initial trajectory would bounce off of these walls until eventually finding an exit and leaving the dot (this is the decoherence part).

Or it might find a trajectory that does not couple to the openings and would therefore be a surviving pointer state, what is called a diamond state.

One difference between the classical physics of billiard balls and the quantum physics of electrons is that an electron can tunnel through “forbidden phase space” to enter this diamond state, whereas a billiard ball entering from outside the dot would not find itself able to reach this diamond trajectory.

Bouncing around quantum dots

It is this isolated classical trajectory, and the buildup of an electron wave functions' amplitude along that trajectory, that is referred to as a scarred wave function.

To experimentally measure these scars, imagine that we can’t see inside the walls of our billiard table, but we can count the billiard balls exiting the table. This is what is normally measured with the conductance of the quantum dot and its environment.

“We measure the current through the dot, the numbers of ‘billiard balls’ passing through it per second, to try to see how this changes when we move our probe around the ‘billiard table,’ ” Ferry said.

Furthermore, there is the probe of the scanning gate microscope, which applies a small electric field. This can be pictured as a small circular bumper on the billiard table that can be moved around within the dot.

This small "bumper" is rastered left to right, top to bottom over the area of interest. If a ball is traveling along this diamond pattern it is perturbed by the bumper when it rasters into the trajectory.

Think of rastering like the way a television image works, with a pattern of scanning lines that cover the area on which the image is projected, or a set or horizontal lines composed of individual pixels that are used to form an image on a computer screen.

When this happens, the ball bounces off the perturbation, and takes a new course within the dot until finally coupling out one of the openings to be measured. The change in the ball’s motion appears as a change in the conductance – the number of balls going through the openings in a given time.

Smoking gun

Ferry explained: “With scanning gate microscopy, we monitor where these changes occur within the scans, and hopefully this gives us a map of the scarred wave functions corresponding to the pointer states.”

Quantum mechanically, he said, a new electron will tunnel right into the diamond state, so the measurement can continue until the whole area is mapped.

The data that came from the team’s experiment supports Zurek's theories of decoherence and quantum Darwinism, Burke said.

Ferry said these findings are just one step in a process that is open to conjecture, but they point toward a “smoking gun” for the existence of this quantum Darwinism and a new view in the search for evidence of how the quantum-to-classical world transition actually occurs.

If you can wrap your mind around all this, he said, “You open the door to a deeper understanding of what is really going on” at the core of physical reality.

Obtenido de: http://asunews.asu.edu/20100630_davidferryquantum

Tirso Ramírez

CRF