mercoledì 27 maggio 2015

Physicists solve quantum tunneling mystery.

Professor Anatoli Kheifets' theory has ultrafast physics wrapped up. Credit: Stuart Hay, ANU
An international team of scientists studying ultrafast physics have solved a mystery of quantum mechanics, and found that quantum tunneling is an instantaneous process.
The new theory could lead to faster and smaller , for which is a significant factor. It will also lead to a better understanding of diverse areas such as electron microscopy, nuclear fusion and DNA mutations.
"Timescales this short have never been explored before. It's an entirely new world," said one of the international team, Professor Anatoli Kheifets, from The Australian National University (ANU).
"We have modelled the most delicate processes of nature very accurately."
At very small scales shows that particles such as electrons have wave-like properties - their exact position is not well defined. This means they can occasionally sneak through apparently impenetrable barriers, a phenomenon called quantum tunneling.
Quantum tunneling plays a role in a number of phenomena, such as in the sun, scanning tunneling microscopy, and flash memory for computers. However, the leakage of particles also limits the miniaturisation of electronic components.
Professor Kheifets and Dr. Igor Ivanov, from the ANU Research School of Physics and Engineering, are members of a team which studied ultrafast experiments at the attosecond scale (10-18 seconds), a field that has developed in the last 15 years.
Until their work, a number of attosecond phenomena could not be adequately explained, such as the time delay when a photon ionised an atom.
"At that timescale the time an electron takes to quantum tunnel out of an atom was thought to be significant. But the mathematics says the time during tunneling is imaginary - a complex number - which we realised meant it must be an instantaneous process," said Professor Kheifets.
"A very interesting paradox arises, because electron velocity during tunneling may become greater than the speed of light. However, this does not contradict the special theory of relativity, as the tunneling velocity is also imaginary" said Dr Ivanov, who recently took up a position at the Center for Relativistic Laser Science in Korea.
The team's calculations, which were made using the Raijin supercomputer, revealed that the delay in photoionisation originates not from quantum tunneling but from the electric field of the nucleus attracting the escaping electron.
The results give an accurate calibration for future attosecond-scale research, said Professor Kheifets.
"It's a good reference point for future experiments, such as studying proteins unfolding, or speeding up electrons in microchips," he said.
More information: Interpreting attoclock measurements of tunnelling times, Nature Physics (2015) DOI: 10.1038/nphys3340

Physicists simulate for the first time charged Majorana particles.

Alexander Szameit of the University Jena (Germany) and his team developed a photonic set-up that can simulate non-physical processes in a laboratory. Credit: Jan-Peter Kasper/FSU
Physicists of Jena University simulate for the first time charged Majorana particles—elementary particles, which are not supposed to exist. In the new edition of the science magazine Optica they explain their approach: Professor Dr. Alexander Szameit and his team developed a photonic set-up that consists of complex waveguide circuits engraved in a glass chip, which enables them to simulate charged Majorana particles and, thus, allows to conduct physical experiments.
Jena (Germany) March 1938: The Italian physicist Ettore Majorana boarded a post ship in Naples, heading for Palermo. But he either never arrives there - or he leaves the city straight away - ever since that day there has been no trace of the exceptional scientist and until today his mysterious disappearance remains unresolved. Since then, Majorana, a pupil of the Nobel Prize winner Enrico Fermi, has more or less been forgotten. What the scientific world does remember though is a theory about nuclear forces, which he developed, and a very particular elementary particle.
"This particle named after Majorana, the so-called Majoranon, has some amazing characteristics", the physicist Professor Dr. Alexander Szameit of the Friedrich Schiller University Jena says. "Characteristics which are not supposed to be existent in our real world." Majorana are, for instance, their own antiparticles: Internally they combine completely opposing characteristics - like opposing charges and spins. If they were to exist, they would extinguish themselves immediately. "Therefore, Majoranons are of an entirely theoretical nature and cannot be measured in experiments."
Together with colleagues from Austria, India, and Singapore, Alexander Szameit and his team succeeded in realizing the impossible. In the new edition of the science magazine Optica they explain their approach: Szameit and his team developed a photonic set-up that consists of complex waveguide circuits engraved in a , which enables them to simulate charged Majorana particles and, thus, allows to conduct .
"At the same time we send two rays of light through parallel running waveguide lattices, which show the opposing characteristics separately," explains Dr. Robert Keil, the first author of the study. After evolution through the lattices, the two waves interfere and form an optical Majoranon, which can be measured as a light distribution. Thus, the scientists create an image that catches this effect like a photograph - in this case the state of a Majoranon at a defined moment in time. "With the help of many of such single images the particles can be observed like in a film and their behaviour can be analyzed," says Keil.
This model allows the Jena scientists to enter completely unknown scientific territory, as Alexander Szameit stresses. "Now, it is possible for us to gain access to phenomena that so far only have been described in exotic theories." With the help of this system, one can conduct experiments in which conservation of charge - one of the pillars of modern physics - can easily be suspended. "Our results show that one can simulate non-physical processes in a laboratory and, thus, can make practical use of exotic characteristics of particles that are impossible to observe in nature." Szameit foresees one particular promising application of simulated Majoranons in a new generation of quantum computers. "With this approach, much higher computing capacities than are possible at the moment can be achieved."
Explore further: Quantum scientists break aluminium 'monopoly' (Update)
More information: Keil R. et al. Optical simulation of charge conservation violation and Majorana dynamics. Optica, Vol. 2, Issue 5, pp. 454-459 (2015), DOI: 10.1364/OPTICA.2.000454

How spacetime is built by quantum entanglement.

The mathematical formula derived by Ooguri and his collaborators relates local data in the extra dimensions of the gravitational theory, depicted by the red point, are expressed in terms of quantum entanglements, depicted by the blue domes. Credit: (c) 2015 Jennifer Lin et al.
A collaboration of physicists and a mathematician has made a significant step toward unifying general relativity and quantum mechanics by explaining how spacetime emerges from quantum entanglement in a more fundamental theory. The paper announcing the discovery by Hirosi Ooguri, a Principal Investigator at the University of Tokyo's Kavli IPMU, with Caltech mathematician Matilde Marcolli and graduate students Jennifer Lin and Bogdan Stoica, will be published in Physical Review Letters as an Editors' Suggestion "for the potential interest in the results presented and on the success of the paper in communicating its message, in particular to readers from other fields."
Physicists and mathematicians have long sought a Theory of Everything (ToE) that unifies and quantum mechanics. General relativity explains gravity and large-scale phenomena such as the dynamics of stars and galaxies in the universe, while quantum mechanics explains microscopic phenomena from the subatomic to molecular scales.
The holographic principle is widely regarded as an essential feature of a successful Theory of Everything. The holographic principle states that gravity in a three-dimensional volume can be described by quantum mechanics on a two-dimensional surface surrounding the volume. In particular, the three dimensions of the volume should emerge from the two dimensions of the surface. However, understanding the precise mechanics for the emergence of the volume from the surface has been elusive.
Now, Ooguri and his collaborators have found that quantum entanglement is the key to solving this question. Using a quantum theory (that does not include gravity), they showed how to compute , which is a source of gravitational interactions in three dimensions, using quantum entanglement data on the surface. This is analogous to diagnosing conditions inside of your body by looking at X-ray images on two-dimensional sheets. This allowed them to interpret universal properties of quantum entanglement as conditions on the energy density that should be satisfied by any consistent quantum theory of gravity, without actually explicitly including gravity in the theory. The importance of quantum entanglement has been suggested before, but its precise role in emergence of spacetime was not clear until the new paper by Ooguri and collaborators.
Quantum entanglement is a phenomenon whereby quantum states such as spin or polarization of particles at different locations cannot be described independently. Measuring (and hence acting on) one particle must also act on the other, something that Einstein called "spooky action at distance." The work of Ooguri and collaborators shows that this quantum entanglement generates the extra dimensions of the gravitational theory.
"It was known that quantum entanglement is related to deep issues in the unification of general relativity and , such as the black hole information paradox and the firewall paradox," says Hirosi Ooguri. "Our paper sheds new light on the relation between and the microscopic structure of spacetime by explicit calculations. The interface between and information science is becoming increasingly important for both fields. I myself am collaborating with information scientists to pursue this line of research further."
Explore further: Is the universe a hologram?
More information: Locality of Gravitational Systems from Entanglement of Conformal Field Theories, Physical Review Letters, 2015.

Experiment confirms quantum theory weirdness.

Associate Professor Andrew Truscott (L) with PhD student Roman Khakimov.
The bizarre nature of reality as laid out by quantum theory has survived another test, with scientists performing a famous experiment and proving that reality does not exist until it is measured.
Physicists at The Australian National University (ANU) have conducted John Wheeler's delayed-choice , which involves a moving object that is given the choice to act like a particle or a wave. Wheeler's experiment then asks - at which point does the object decide?
Common sense says the object is either wave-like or particle-like, independent of how we measure it. But predicts that whether you observe wave like behavior (interference) or particle behavior (no interference) depends only on how it is actually measured at the end of its journey. This is exactly what the ANU team found.
"It proves that measurement is everything. At the quantum level, reality does not exist if you are not looking at it," said Associate Professor Andrew Truscott from the ANU Research School of Physics and Engineering.
Despite the apparent weirdness, the results confirm the validity of , which governs the world of the very small, and has enabled the development of many technologies such as LEDs, lasers and computer chips.
The ANU team not only succeeded in building the experiment, which seemed nearly impossible when it was proposed in 1978, but reversed Wheeler's original concept of light beams being bounced by mirrors, and instead used scattered by laser light.
"Quantum physics' predictions about interference seem odd enough when applied to light, which seems more like a wave, but to have done the experiment with atoms, which are complicated things that have mass and interact with electric fields and so on, adds to the weirdness," said Roman Khakimov, PhD student at the Research School of Physics and Engineering.
Professor Truscott's team first trapped a collection of in a suspended state known as a Bose-Einstein condensate, and then ejected them until there was only a single atom left.
The single atom was then dropped through a pair of counter-propagating laser beams, which formed a grating pattern that acted as crossroads in the same way a solid grating would scatter light.
A second light grating to recombine the paths was randomly added, which led to constructive or as if the atom had travelled both paths. When the second light grating was not added, no interference was observed as if the atom chose only one path.
However, the random number determining whether the grating was added was only generated after the atom had passed through the crossroads.
If one chooses to believe that the atom really did take a particular path or paths then one has to accept that a future measurement is affecting the atom's past, said Truscott.
"The atoms did not travel from A to B. It was only when they were measured at the end of the journey that their wave-like or particle-like behavior was brought into existence," he said.
Explore further: Squeezed quantum cats
More information: "Wheeler's delayed-choice gedanken experiment with a single atom" Nature Physics (2015) DOI: 10.1038/nphys3343.

Quantum computer emulated by a classical system.

Drs. Granville Ott (left) and Brian La Cour (center) with student Michael Starkey (right) beside their prototype quantum emulation device. Credit Applied Research Laboratories, The University of Texas at Austin
Quantum computers are inherently different from their classical counterparts because they involve quantum phenomena, such as superposition and entanglement, which do not exist in classical digital computers. But in a new paper, physicists have shown that a classical analog computer can be used to emulate a quantum computer, along with quantum superposition and entanglement, with the result that the fully classical system behaves like a true quantum computer.
Physicist Brian La Cour and electrical engineer Granville Ott at Applied Research Laboratories, The University of Texas at Austin (ARL:UT), have published a paper on the classical emulation of a quantum computer in a recent issue of The New Journal of Physics. Besides having fundamental interest, using classical systems to emulate quantum computers could have practical advantages, since such quantum emulation devices would be easier to build and more robust to decoherence compared with true quantum computers.
"We hope that this work removes some of the mystery and 'weirdness' associated with quantum computing by providing a concrete, classical analog," La Cour told "The insights gained should help develop exciting new technology in both classical analog computing and true quantum computing."
As La Cour and Ott explain, quantum computers have been simulated in the past using software on a classical computer, but these simulations are merely numerical representations of the quantum computer's operations. In contrast, emulating a quantum computer involves physically representing the qubit structure and displaying actual quantum behavior. One key quantum behavior that can be emulated, but not simulated, is parallelism. Parallelism allows for multiple operations on the data to be performed simultaneously—a trait that arises from and entanglement, and enables quantum computers to operate at very fast speeds.
To emulate a quantum computer, the physicists' approach uses electronic signals to represent qubits, in which a qubit's state is encoded in the amplitudes and frequencies of the signals in a complex mathematical way. Although the scientists use electronic signals, they explain that any kind of signal, such as acoustic and electromagnetic waves, would also work.
Even though this classical system emulates quantum phenomena and behaves like a quantum computer, the scientists emphasize that it is still considered to be classical and not quantum. "This is an important point," La Cour explained. "Superposition is a property of waves adding coherently, a phenomenon that is exhibited by many classical systems, including ours.
"Entanglement is a more subtle issue," he continued, describing entanglement as a "purely mathematical property of waves."
"Since our classical signals are described by the same mathematics as a true quantum system, they can exhibit these same properties."
He added that this kind of entanglement does not violate Bell's inequality, which is a widely used way to test for entanglement.
"Entanglement as a statistical phenomenon, as exhibited by such things as violations of Bell's inequality, is rather a different beast," La Cour explained. "We believe that, by adding an emulation of quantum noise to the signal, our device would be capable of exhibiting this type of entanglement as well, as described in another recent publication."
In the current paper, La Cour and Ott describe how their system can be constructed using basic analog electronic components, and that the biggest challenge is to fit a large number of these components on a single integrated circuit in order to represent as many qubits as possible. Considering that today's best semiconductor technology can fit more than a billion transistors on an integrated circuit, the scientists estimate that this transistor density corresponds to about 30 qubits. An increase in transistor density of a factor of 1000, which according to Moore's law may be achieved in the next 20 to 30 years, would correspond to 40 qubits.
This 40-qubit limit is also enforced by a second, more fundamental restriction, which arises from the bandwidth of the signal. The scientists estimate that a signal duration of a reasonable 10 seconds can accommodate 40 qubits; increasing the duration to 10 hours would only increase this to 50 qubits, and a one-year duration would only accommodate 60 qubits. Due to this scaling behavior, the physicists even calculated that a signal duration of the approximate age of the universe (13.77 billion years) could accommodate about 95 qubits, while that of the Planck time scale (10-43 seconds) would correspond to 176 qubits.
Considering that thousands of qubits are needed for some complex tasks, such as certain encryption techniques, this scheme clearly faces some insurmountable limits. Nevertheless, the scientists note that 40 qubits is still sufficient for some low-qubit applications, such as quantum simulations. Because the quantum emulation device offers practical advantages over quantum computers and performance advantages over most classical computers, it could one day prove very useful. For now, the next step will be building the device.
"Efforts are currently underway to build a two-qubit prototype device capable of demonstrating ," La Cour said. "The enclosed photo [see above] shows the current quantum emulation device as a lovely assortment of breadboarded electronics put together by one of my students, Mr. Michael Starkey. We are hoping to get future funding to support the development of an actual chip. Leveraging quantum parallelism, we believe that a coprocessor with as few as 10 could rival the performance of a modern Intel Core at certain computational tasks. Fault tolerance is another important issue that we studying. Due to the similarities in mathematical structure, we believe the same quantum error correction algorithms used to make quantum computers fault tolerant could be used for our quantum emulation device as well."