The combination of ab-initio numerical experiments and theory shows that optical tunnelling of an electron from an atom can occur instantaneously.
How long does it take an atom to absorb a photon and lose an electron? And what if not one but many photons are needed for ionization? How much time would absorption of many photons take? These questions lie at the core of attosecond spectroscopy, which aims to resolve electronic motion at its natural time scale.
Ionization in strong infrared fields is often viewed as electron tunnelling through a potential barrier, created by the combination of the atomic potential that binds the electron and the electric field of the laser pulse that pulls the electron away. Thus, unexpectedly, attosecond spectroscopy finds itself facing an almost age-old and controversial question: how long does it take an electron to tunnel through a barrier?
In the paper by Torlina et al (http://arxiv.org/pdf/1402.5620v2.pdf), this question is studied by using the so-called atto-clock setup. Continue reading How long does it take an electron to tunnel?
Enric Pérez, Blai Pié Valls
It is widely known that Paul Ehrenfest formulated and applied his adiabatic hypothesis in the early 1910s. Niels Bohr, in his first attempt to construct a quantum theory in 1916, used it for fundamental purposes in a paper which eventually did not reach the press.
He decided not to publish it after having received the new results by Sommerfeld in Munich. Two years later, Bohr published “On the quantum theory of line-spectra.” There, the adiabatic hypothesis played an important role, although it appeared with another name: the principle of mechanical transformability. In the subsequent variations of his theory, Bohr never suppressed this principle completely.
We discuss the role of Ehrenfest’s principle in the works of Bohr, paying special attention to its relation to the correspondence principle. We will also consider how Ehrenfest faced Bohr’s uses of his more celebrated contribution to quantum theory, as well as his own participation in the spreading of Bohr’s ideas…
…Read more at http://arxiv.org/ftp/arxiv/papers/1502/1502.03022.pdf
Have physicists conquered the scaling behavior of exotic giant molecules?
When a two-body relation becomes a three-body relation, the behavior of the system changes. The basic physics of two interacting particles is well understood but the mathematical description of a three- or many-body system becomes so difficult that calculating the dynamics can blast the capacities of even modern super computers.
Under certain conditions, the quantum mechanical three-body problem may have a universal scaling solution and physicists from Heidelberg University say they have experimentally confirmed such a model. The scientists under Prof. Dr. Matthias Weidemüller investigated three-particle molecules, known as trimers, under exotic conditions.
The work is based on a theory posed by Russian physicist Vitaly Efimov more than 40 years ago. It focuses on finding physical laws capable of predicting the behaviour and energy states of an arbitrary number of particles. According to Efimov’s prediction, bound states of three atoms can be universally described under certain conditions.
The scientist found that infinitely many quantum mechanical bound states for the “ménage à trois” exist, even if two of the atoms cannot bind together. These so-called Efimov trimers are formed due to the long-range quantum mechanical interaction and they are completely independent of the underlying type of the three interacting particles. Continue reading A Universal Solution For A Quantum Three-Body Problem
Multiply-ionized atoms for clocks, qubits, and constants
The world is mostly neutral. That is, most of the atoms in our environment are electrically neutral. The number of electrons in the outer parts of atoms equals the number of protons at the centers of atoms. As one or more electrons are plucked away from the atoms, the remaining electrons feel a much stronger positive pull from the nucleus. This enhanced pull, causing the atoms to shrink in size, ensures that those electrons are less vulnerable to the distractions of their environment, making them potentially valuable for next-generation atomic clocks, for quantum information schemes (where the loss of quantum coherence in qubits is a paramount danger), and for experiments trying to detect slight variations in the fine structure constant, the parameter that sets the overall strength of the electromagnetic force.
Continue reading Highly-Charged Ions
by Jacob Aron
It’s time for clocks to get a quantum boost. A network of ultra-precise atomic clocks could be linked together by the spooky property of quantum entanglement to create the ultimate world clock. Such a feat would allow all countries to agree on a precise measurement of time, while also creating a massive quantum sensor for probing cosmic mysteries.
Atomic clocks measure the microwave or optical frequency needed to make an atom’s electron jump from one energy level to another. The standard clock uses caesium atoms, which emit microwaves precisely 9,192,631,770 times per second. The signal is so incredibly regular that the latest caesium clock recently brought online in the US will not lose or gain a second in about 300 million years.
Timekeeping institutes around the world each have their own caesium clocks. They submit their time signal measurements to the International Bureau of Weights and Measures in Paris, France, which averages them and publishes a monthly newsletter that sets Coordinated Universal Time (UTC). But that means there is no real-time measure of a universally agreed standard time.
“UTC is a month in arrears, and there is a big drive to a real-time formulation,” says Leon Lobo of the National Physical Laboratory in Teddington, UK.
Eric Kessler at Harvard University and his colleagues think that quantum entanglement could provide a solution. When quantum objects such as atoms are entangled, measuring one has a direct and predictable effect on the other. If you were to entangle atomic clocks around the world and on orbiting satellites, it would help them to tick in unison, says Kessler.
“If you consider the individual clocks as pendulums, then entangling the different clock causes the different pendulums to swing perfectly in unison.”
His team evaluated existing clocks around the world and proposed a blueprint for a hypothetical network. The team calculates that a global quantum clock network would be about 100 times more precise than any individual clock. It would also be naturally protected from hackers, as the laws of quantum mechanics would immediately alert you to any attempts at eavesdropping. But entanglement is a very delicate state, so it may be a while before such a large quantum network could come online.
“There is no doubt that it is an ambitious proposal, and a long way to go,” says Kessler. “Substantial technological advances are needed, although all the different building blocks have in principle been demonstrated in a small scale.”
The quantum network is a worthy goal, says Ruxandra Bondarescu at the University of Zurich, Switzerland, because it could double as a sensor for conducting fundamental physics experiments. A highly sensitive global clock could be used to measure minute variations in Earth’s gravitational field, or to hunt for ripples in space-time known as gravitational waves, which would fractionally shift the clock’s tick.
“It has been hard to get funding for relatively impractical applications,” she says. “Such a network of clocks would be amazing if it could exist.”
Nuclear physicists have invested huge effort in creating superheavy elements, which consist of enough neutrons to provide enhanced stability from nuclear decay. For the past 30 years, experiments have been marching towards this “island of stability” with a new elemental discovery every 2 to 3 years. Part of the discovery process includes the confirmation by an independent experimental collaboration—it is only at this point that an element obtains its official status.
An international team using an intense 48Ca beam provided by GSI research facility in Darmstadt, Germany, and a target material of radioactive 249Bk supplied by Oak Ridge National Lab in Tennessee has produced two atoms of the superheavy element with atomic number Z=117, confirming the initial observation published in 2010 (see 9 April 2010 Viewpoint). In the process, a new isotope 266Lr was discovered from the previously unknown alpha-decay branch of 270Db. With a half-life of 1hour, 270Db is the longest-lived alpha emitter having an atomic number, Z, greater than 102.
The experiment is a tour de force in superheavy element research and required a detailed reconstruction of a seven-step alpha-decay chain followed by the spontaneous fission of the newly discovered 266Lr. The difficulty stems from the large variation in decay lifetimes along the alpha chain. The discovery was made feasible by the use of TASCA, a gas-filled recoil separator specifically designed for a high selectivity of superheavy or transactinide elements.
The confirmation by the TASCA team serves as a much-needed step on the long road towards the island of stability. An easier feat will be deciding on a name for Z=117. – Kevin Dusling – http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.112.172501