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Category: quantum physics – Page 705

Quantum information relies on the possibility of writing messages in a quantum particle and reading them out in a reliable way. If, however, the particle is relativistic, meaning that it moves with velocities close to the speed of light, it is impossible for standard techniques to decode the message unambiguously, and the communication therefore fails.

Thanks to the introduction of a new method, researchers at the University of Vienna and the Austrian Academy of Sciences have developed reliable decoding of quantum messages transmitted at extremely . The results, published in the journal Physical Review Letters, opens up new possibilities of technological applications in and quantum communication.

Imagine the following situation: Anna and Bill want to exchange a message by using a property of a , say the spin of an electron, which is an intrinsic form of particle’s rotation. Bill needs Anna’s message as quickly as possible, so Anna has to send the electron at maximum speed, very close to the speed of light. Given that Anna has the electron in her laboratory localized, the Heisenberg uncertainty principle forbids the velocity of the electron to be defined with arbitrary precision. When the electron travels at extremely high , the interplay between special relativity and quantum physics causes the spin and the velocity of the electron to become entangled. Due to this correlation, which is stronger than what is classically possible, Bill is not able to read out the spin with the standard method. Can Anna and Bill improve their communication strategy?

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The theories of quantum mechanics and gravity are notorious for being incompatible, despite the efforts of scores of physicists over the past fifty years. However, recently an international team of researchers led by physicists from the University of Vienna, the Austrian Academy of Sciences as well as the University of Queensland (AUS) and the Stevens Institute of Technology (U.S.) have combined the key elements of the two theories describing the flow of time and discovered that temporal order between events can exhibit genuine quantum features.

According to general relativity, the presence of a slows down the flow of time. This means that a clock placed close to a massive object will run slower as compared to an identical one that is further away.

However, the rules of quantum theory allow for any object to be prepared in a . A superposition state of two locations is different to placing an object in one or the other location randomly—it is another way for an object to exist, allowed by the laws of quantum physics.

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Circa 1997

By Michio Kaku

IS THERE a Final Theory in physics? Will we one day have a complete theory that will explain everything from subatomic particles, atoms and supernovae to the big bang? Einstein spent the last 30 years of his life in a fruitless quest for the fabled unified field theory. His approach has since been written off as futile.

In the 1980s, attention switched to superstring theory as the leading candidate for a final theory. This revolution began when physicists realised that the subatomic particles found in nature, such as electrons and quarks, may not be particles at all, but tiny vibrating strings.

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We suggest and motivate a precise equivalence between uncompactified eleven dimensional M-theory and the N = infinity limit of the supersymmetric matrix quantum mechanics describing D0-branes. The evidence for the conjecture consists of several correspondences between the two theories. As a consequence of supersymmetry the simple matrix model is rich enough to describe the properties of the entire Fock space of massless well separated particles of the supergravity theory. In one particular kinematic situation the leading large distance interaction of these particles is exactly described by supergravity.

The model appears to be a nonperturbative realization of the holographic principle. The membrane states required by M-theory are contained as excitations of the matrix model.

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By Andrew Zimmerman Jones, Daniel Robbins

According to string theory, all particles in the universe can be divided into two types: bosons and fermions. String theory predicts that a type of connection, called supersymmetry, exists between these two particle types.

Under supersymmetry, a fermion must exist for every boson and a boson for every fermion. Unfortunately, experiments have not yet detected these extra particles.

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20th century physics has seen two major paradigm shifts in the way we understand Mother Nature. One is quantum mechanics, and the other is relativity. The marriage between the two, called quantum field theory, conceived an enfant terrible, namely anti-matter. As a result, the number of elementary particles doubled. We believe that 21st century physics is aimed at yet another level of marriage, this time between quantum mechanics and general relativity, Einstein’s theory of gravity. The couple has not been getting along very well, resulting in mathematical inconsistencies, meaningless infinities, and negative probabilities. The key to success may be in supersymmetry, which doubles the number of particles once more.

Why was anti-matter needed? One reason was to solve a crisis in the 19th century physics of classical electromagnetism. An electron is, to the best of our knowledge, a point particle. Namely, it has no size, yet an electric charge. A charged particle inevitably produces an electric potential around it, and it also feels the potential created by itself. This leads to an infinite “self-energy” of the electron. In other words, it takes substantial energy to “pack” all the charge of an electron into small size.

On the other hand, Einstein’s famous equation says that mass of a particle determines the energy of the particle at rest. For an electron, its rest energy is known to be 0.511 MeV. For this given amount of energy, it cannot afford to “pack” itself into a size smaller than the size of a nucleus. Classical theory of electromagnetism is not a consistent theory below this distance. However, it is known that the electron is at least ten thousand times smaller than that.

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M-theory is a theory in physics that unifies all consistent versions of superstring theory. The existence of such a theory was first conjectured by Edward Witten at a string theory conference at the University of Southern California in the Spring of 1995. Witten’s announcement initiated a flurry of research activity known as the second superstring revolution.

Prior to Witten’s announcement, string theorists had identified five versions of superstring theory. Although these theories appeared, at first, to be very different, work by several physicists showed that the theories were related in intricate and nontrivial ways. In particular, physicists found that apparently distinct theories could be unified by mathematical transformations called S–duality and T–duality. Witten’s conjecture was based in part on the existence of these dualities and in part on the relationship of the string theories to a field theory called eleven-dimensional supergravity.

Although a complete formulation of M-theory is not known, the theory should describe two- and five-dimensional objects called branes and should be approximated by eleven-dimensional supergravity at low energies. Modern attempts to formulate M-theory are typically based on matrix theory or the AdS/CFT correspondence.

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A new unified theory for heat transport accurately describes a wide range of materials – from crystals and polycrystalline solids to alloys and glasses – and allows them to be treated in the same way for the first time. The methodology, which is based on the Green-Kubo theory of linear response and concepts from lattice dynamics, naturally accounts for quantum mechanical effects and thus allows for the predictive modelling of heat transport in glasses at low temperature – a feat never achieved before, say the researchers who developed it. It will be important for better understanding and designing heat transporting devices in a host of applications, from heat management in high-power electronics, batteries and photovoltaics to thermoelectric energy harvesting and solid-state cooling. It might even help describe heat flow in planetary systems.

“Heat transport is the fundamental mechanism though which thermal equilibrium is reached,” explains Stefano Baroni of the Scuola Internazionale Superiore di Studi Avanzati (SISSA) in Trieste, Italy, who led this research effort. “It can also be thought of as the most fundamental manifestation of irreversibility in nature – as heat flows from warm areas in the same system to cooler ones as time flows from the past to the future (the ‘arrow of time’). What is more, many modern technologies rely on our ability to control heat transport.”

However, despite its importance, heat transport is still poorly understood and it is difficult to simulate the heat transport of materials because of this lack of understanding. To overcome this knowledge gap, researchers employ various simulation techniques based on diverse physical assumptions and approximations for different classes of material – crystals on one hand and disordered solids and liquids on the other.

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