Thursday, 20 December 2018

Scientists find a way to connect quantum and classical physics

December 19, 2018, Skolkovo Institute of Science and Technology

Scientists find a way to connect quantum and classical physics
Hybrid quantum-classical spin system. Credit: Skolkovo Institute of Science and Technology

Physicists from Skoltech have invented a new method for calculating the dynamics of large quantum systems. Underpinned by a combination of quantum and classical modeling, the method has been successfully applied to nuclear magnetic resonance in solids. The results of the study were published in Physical Review B.
Physical objects around us consist of atoms which, in turn, are made up of negatively charged electrons and positively charged nuclei. Many of atomic nuclei are magnetic – they can be thought of as tiny magnets, which can get excited by an oscillating . This phenomenon known as "" (NMR) was discovered in the first half of the 20thcentury. Five Nobel prizes have been awarded since then, first for the discovery and then for various applications of NMR—magnetic resonance imaging (MRI) being the most prominent of them.
Although NMR was discovered more than 70 years ago, it still has some blank spots, such as quantitative prediction of the relaxation of nuclear magnetic moments in solids after NMR excitation. This is a particular case representing a more general problem of describing the dynamics of a large number of interacting  particles. Direct quantum simulation is out of the question already for a few hundred particles, for it requires enormous computational resources not available to humankind.
It is then tempting to explore an approximate approach based on simulating the core of a many-particle  using quantum dynamics, while dealing with the rest purely classically—that is, without admitting quantum superpositions. However, it is precisely the quantum superpositions that make the coupling of quantum and classical dynamics a non-trivial task: a classical system is in one state at each point in time, whereas a quantum system can be simultaneously in several states, much like Schrödinger's cat which can be alive and dead at the same time. It is thus not clear which of the superimposed quantum states governs the impact of the quantum part on the classical one.
Skoltech researchers, Ph.D. student Grigory Starkov and Professor Boris Fine, overcame multiple stumbling blocks and proposed a hybrid computational  combining quantum and classical modeling. "In general, the averaging over quantum superpositons significantly reduces the action of the quantum core on the classical environment. We found a way to compensate such an averaging effect, while keeping the most essential dynamic correlations intact," Starkov explained. The proposed method was thoroughly tested on various systems by evaluating its performance against numerical calculations and . The new method is expected to offer broader capabilities to scientists in simulating the magnetic dynamics of nuclei in solids, which, in turn, will facilitate the NMR diagnostics of complex materials.
"This work culminates years of our intensive efforts," said Fine. "Many teams around the world attempted to make such calculations over the past 70 years. Here we succeeded in advancing the predictive performance of NMR calculations to a new level. We do hope that our hybrid approach will find broad use in the NMR domain and beyond."

More information: Grigory A. Starkov et al. Hybrid quantum-classical method for simulating high-temperature dynamics of nuclear spins in solids, Physical Review B (2018). DOI: 10.1103/PhysRevB.98.214421 

Thursday, 13 December 2018

Whats dark energy

So what is dark energy? 

Well, the simple answer is that we don't know. It seems to contradict many of our understandings about the way the universe works.

We all know that light waves, also called radiation, carry energy. You feel that energy the moment you step outside on a hot summer day.Einstein's famous equation, E = mc2, teaches us that matter and energy are interchangeable, merely different forms of the same thing. We have a giant example of that in our sky: the Sun. The Sun is powered by the conversion of mass to energy.


Subatomic Large Scale

Could dark energy show a link between the physics of the very small and the physics of the large?
But energy is supposed to have a source — either matter or radiation. The notion here is that space, even when devoid of all matter and radiation, has a residual energy. That "energy of space," when considered on a cosmic scale, leads to a force that increases the expansion of the universe.
Perhaps dark energy results from weird behavior on scales smaller than atoms. The physics of the very small, called quantum mechanics, allows energy and matter to appear out of nothingness, although only for the tiniest instant. The constant brief appearance and disappearance of matter could be giving energy to otherwise empty space.
It could be that dark energy creates a new, fundamental force in the universe, something that only starts to show an effect when the universe reaches a certain size. Scientific theories allow for the possibility of such forces. The force might even be temporary, causing the universe to accelerate for some billions of years before it weakens and essentially disappears.
Or perhaps the answer lies within another long-standing unsolved problem, how to reconcile the physics of the large with the physics of the very small. Einstein's theory of gravity, called general relativity, can explain everything from the movements of planets to the physics of black holes, but it simply doesn't seem to apply on the scale of the particles that make up atoms. To predict how particles will behave, we need the theory of quantum mechanics. Quantum mechanics explains the way particles function, but it simply doesn't apply on any scale larger than an atom. The elusive solution for combining the two theories might yield a natural explanation for dark energy.


Pie Chart - 74% Dark Energy, 22% Dark Matter, 4% Visible Matter

Most of the universe seems to consist of nothing we can see. Dark energy and dark matter, detectable only because of their effect on the visible matter around them, make up most of the universe.
We do know this: Since space is everywhere, this dark energy force is everywhere, and its effects increase as space expands. In contrast, gravity's force is stronger when things are close together and weaker when they are far apart. Because gravity is weakening with the expansion of space, dark energy now makes up over 2/3 of all the energy in the universe.
It sounds rather strange that we have no firm idea about what makes up 74% of the universe. It's as though we had explored all the land on the planet Earth and never in all our travels encountered an ocean. But now that we've caught sight of the waves, we want to know what this huge, strange, powerful entity really is.
The strangeness of dark energy is thrilling.
It shows scientists that there is a gap in our knowledge that needs to be filled, beckoning the way toward an unexplored realm of physics. We have before us the evidence that the cosmos may be configured vastly differently than we imagine. Dark energy both signals that we still have a great deal to learn, and shows us that we stand poised for another great leap in our understanding of the universe.

source: hubblesuite