Archive for the ‘Statistical Mechanics’ Category

Ultracold fermions simulate spin–orbit coupling

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Artist’s impression of the spin diode created by researchers at MIT. Atoms with clockwise spin can only move in one direction, while atoms with anticlockwise spin move in the opposite direction. (Courtesy: Christine Daniloff)

Two independent groups of physicists are the first to use ultracold fermionic atoms to simulate “spin–orbit coupling” – an interaction that plays an important role in the electronic properties of solid materials. Both experiments were done by firing laser beams at the atoms, which caused their momentum to change by an amount that depends on their intrinsic spin. Because the interactions between atoms in such simulations can be adjusted with great precision, the breakthrough could shed further light on a range of physical phenomena, including magnetism, topological insulators and Majorana fermions.

Spin–orbit coupling describes the interaction between the intrinsic spin of an electron in a material and the magnetic field induced by the electron’s movement relative to its surrounding ions. As well as playing a key role in the magnetic properties of materials, spin–orbit coupling also influences the performance of “spintronic” devices – those that exploit the spin, rather than the charge, of electrons and that could one day lead to faster and more energy-efficient computers.

Quantum simulators

Because of its fundamental nature, physicists are therefore very keen to use clouds of ultracold atoms to simulate spin–orbit coupling. Such “quantum simulations” are carried out by subjecting the gas to laser light and magnetic fields, which lets researchers create interactions between atoms that are similar to those experienced by electrons in a solid. The advantage of these simulations is that – unlike in a solid – the strength of these interactions can be easily adjusted, allowing physicists to test theories of condensed-matter physics.

In 2011 Ian Spielman and colleagues at the National Institute of Standards and Technology (NIST) in Maryland were the first to simulate spin–orbit coupling in an ultracold gas of bosonic atoms. Now, two independent groups – one in China led by Hui Zhai of Tsinghua University and Jing Zhang of Shanxi University, and the other in the US headed by Martin Zwierlein and Lawrence Cheuk at the Massachusetts Institute of Technology (MIT) – have extended Spielman’s technique to fermions. As electrons are fermions and not bosons, the new work is much more relevant to electron physics.

Using potassium-40…

The Chinese team began with about two million potassium-40 atoms that are held in an optical trap and cooled to well below the ensemble’s Fermi temperature. This means that nearly all the atoms in the gas are in the lowest possible energy state, like the conduction electrons in a metal. The team focused on two closely spaced magnetic energy states, which are used to simulate the spin of the electron – one state corresponding to spin up and the other to spin down.

The team then fired two laser beams into the gas from opposing directions. The laser light is set to resonate with a transition between the two spin states – a process that involves the atoms continuously absorbing and emitting photons. As these photons carry momentum, if an atom absorbs a photon moving in one direction and then re-emits it in the same direction, the atom’s momentum will not change. However, an atom can also be stimulated by the opposing beam to emit the photon in the opposite direction – thus changing the atom’s momentum. Such an interaction involves a change in the direction of the atom’s spin and is therefore analogous to spin–orbit coupling – albeit in 1D.

The Chinese team used its system to study several aspects of spin–orbit coupling. In one experiment, the researchers began with a state in which all the spins are initially pointing in the same direction. They then turned on the spin–orbit interaction by pulsing the lasers for a very short time – just a few hundred microseconds. They found that the spins began to point in different directions in a process known as “dephasing”. This is expected from fermions because atoms with the same spin cannot have the same momentum and therefore each atom will be affected differently by the spin–orbit interaction.

Understanding dephasing is important because it has a detrimental effect on technological applications of spin such as spintronics and quantum computing. The team also looked at several other effects related to spin–orbit coupling, including its effect on the momentum distribution of the atoms.

…and lithium-6

The MIT physicists, meanwhile, used a gas of lithium-6 atoms, which meant that their realization of spin–orbit coupling was more difficult than for the Chinese team. The problem is that lighter atoms such as lithium are more prone to heating via the resonant absorption of light. So to get round this problem, the MIT team kept most of its atoms in “reservoir states” in which they do not interact with the light and stayed cool – using radio waves to drive a small number of atoms into the spin–orbit coupling states.

The MIT team focused on showing that ultracold atoms can be used to simulate a “spin diode” – a device that is likely to play a key role in the development of spintronic circuits. It allows spin-up atoms to flow forwards but not backwards, and spin-down atoms backwards but not forward. “The gas acts as a quantum diode, a device that regulates the flow of spin currents,” says Cheuk.

Simulating band structure

By applying radio-frequency radiation to the gas, the MIT physicists were also able to simulate a periodic potential similar to that found in a 1D lattice. As expected for real materials, the periodic potential led to the existence of spin-dependent energy bands. According to the team, the ability to create spin-dependent band structures in this way could lead to the simulation of topological insulators.

The spin–orbit coupling simulated by both teams occurs only in 1D and therefore cannot be used to simulate the 2D and 3D systems found in most real-life electronic devices. However, there are several interesting scenarios that can be investigated in a 1D system. For example, it could be used to simulate the behaviour of electrons in semiconductor/superconductor nanowires. Such systems are believed to harbour quasiparticles that resemble Majorana fermions – long sought-after particles that are also their own antiparticle.

Both experiments are described in Physical Review Letters.
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September 5, 2012 at 8:32 am

Video: Entropy in the kitchen

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February 4, 2012 at 7:49 am

Toward a fully relativistic theory of quantum information

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Using negative partial information for quantum communication. (a) In these diagrams, time runs from top to bottom, and space is horizontal. The line marked “A" is Alice's space-time trajectory, while the line marked “B" is Bob's. Bob creates an eē pair (an Einstein-Podolski-Rosen pair) close to him, and sends the ebit over to Alice. Alice, armed with an arbitrary quantum state q, performs a joint measurement M on both e and q, and sends the two classical bits 2c she obtains from this measurement back to Bob (over a classical channel). When Bob receives these two cbits, he performs one out of four unitary transformations U on the anti-ebit he is still carrying, conditionally on the classical information he received. Having done this, he recovers the original quantum state q, which was "teleported" over to him. The partial information in e is one bit, while it is minus one for the antiebit. (b) In superdense coding, Alice sends two classical bits of information 2c over to Bob, but using only a single qubit in the quantum channel. This process is in a way the “dual" to the teleportation process, as Alice encodes the two classical bits by performing a conditional unitary operation U on the anti-ebit, while it is Bob that performs the measurement M on the ebit he kept and the qubit Alice sent.

Christoph Adami

Information theory is a statistical theory dealing with the relative state of detectors and physical systems.
Because of this physicality of information, the classical framework of Shannon needs to be extended to deal with quantum detectors, perhaps moving at relativistic speeds, or even within curved space-time.
Considerable progress toward such a theory has been achieved in the last fifteen years, while much is still not understood.
This review recapitulates some milestones along this road, and speculates about future ones.
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December 9, 2011 at 6:47 am

Sean Carroll: The Arrow of Time feat

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Why is the past different from the future? Caltech physicist Sean Carroll explains how the arrow of time is not an intrinsic property of physics, but rather an emergent feature.

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November 14, 2011 at 1:31 pm

New Thermodynamic Paradigm of Chemical Equilibria

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B. Zilbergleyt

The paper presents new thermodynamic paradigm of chemical equilibrium, setting forth comprehensive basics of Discrete Thermodynamics of Chemical Equilibria (DTd).
Along with previous results by the author during the last decade, this work contains also some new developments of DTd.
Based on the Onsager’s constitutive equations, reformulated by the author thermodynamic affinity and reaction extent, and Le Chatelier’s principle, DTd brings forward a notion of chemical equilibrium as a balance of internal and external thermodynamic forces (TdF), acting against a chemical system.
Basic expression of DTd is the chemical system logistic map of thermodynamic states that ties together energetic characteristics of chemical reaction, occurring in the system, the system shift from “true” thermodynamic equilibrium (TdE), and causing that shift external thermodynamic forces.
Solutions to the basic map are pitchfork bifurcation diagrams in coordinates “shift from TdE – growth factor (or TdF)”; points, corresponding to the system thermodynamic states, are dwelling on its branches.
The diagrams feature three typical areas: true thermodynamic equilibrium and open equilibrium along the thermodynamic branch before the threshold of its stability, i.e. bifurcation point, and bifurcation area with bistability and chaotic oscillations after the point.
The set of solutions makes up the chemical system domain of states.
The new paradigm complies with the correspondence principle: in isolated chemical system external TdF vanish, and the basic map turns into traditional expression of chemical equilibrium via thermodynamic affinity.
The theory binds together classical and contemporary thermodynamics of chemical equilibria on a unique conceptual basis.
The paper is essentially reworked and refocused version of the earlier preprint on the DTd basics, supplemented with new results…….
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October 31, 2011 at 1:52 pm

Maxwell’s Demon and Data Compression

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Akio Hosoya , Koji Maruyama , Yutaka Shikano

The protocol of data compression for demon’s memory. The dashed blocks express the trivial initial memory state “0” after data compression

In an asymmetric Szilard engine model of Maxwell’s demon, we show the equivalence between information theoretical and thermodynamic entropies when the demon erases information optimally. The work gain by the engine can be exactly canceled out by the work necessary to reset demon’s memory after optimal data compression a la Shannon before the erasure…..
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October 24, 2011 at 6:53 pm

New twist on Brownian motion seen for the first time

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An artist's impression of a tiny sphere (centre) held by optical tweezers and subjected to random kicks from a surrounding fluid

An important aspect of Brownian motion predicted decades ago has been observed for the first time by researchers in Europe. The team has measured how micrometre-sized spheres interact with a surrounding fluid and have shown that the spheres “remember” their previous motion. Their experimental technique, the researchers claim, could be used as a biophysical sensor.
Famously explained by Albert Einstein in 1905, Brownian motion describes the erratic motion of a tiny particle in a fluid. It is caused by the many small “kicks” that the particle receives as a result of the thermal motion of the fluid. Initially, Einstein and other physicists believed these kicks to be independent of the motion of the particle and to be characterized by white noise….. Read the rest of this entry »

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October 11, 2011 at 3:44 pm

What role does the third law of thermodynamics play in Szilard engines?

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(Color online) Schematic diagram of the thermodynamic processes of the SZE. Initially a single atom is prepared in an isolated box. (A) A wall denoted as a vertical grey bar is inserted to split the box into to two parts. The atom is represented by the dotted circles to reflect that we are lack of the information on which side it is before the measurement. (B) The measurement is performed, and one acquires the knowledge of where the atom is. (C) A load denoted by a filled rectangle is attached to the wall to extract a work via an isothermal expansion at temperature T.

Szilard engine animation

The role of the third law of thermodynamics in the Szilard engine has been addressed. If the ground state is non-degenerate, the entropy production defined as the work extractable from the engine divided by temperature vanishes as temperature approaches zero due to the third law. The degenerate ground state induced by the symmetry or by accidence gives rise to non-zero entropy production at zero temperature associated with the residual entropy. Various physical situations such as the SZE consisting of bosons or fermions either with or without interaction have been investigated….
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August 19, 2011 at 11:07 am