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Category: quantum physics

Shaking a physical system typically heats it up, in the sense that the system continuously absorbs energy. When considering a circular shaking pattern, the amount of energy that is absorbed can potentially depend on the orientation of the circular drive (clockwise/anti-clockwise), a general phenomenon known as circular dichroism.

In 2017, Nathan Goldman (ULB, Brussels), Peter Zoller (IQOQI, Innsbruck) and coworkers predicted that can be quantized in (heating is then constrained by strict integers) forming a “topological state.” According to this , the quantization of energy absorption upon circular driving can be directly related to topology, a fundamental mathematical concept that characterizes these intriguing states of matter.

Writing in Nature Physics, the experimental group of Klaus Sengstock and Christof Weitenberg (Hamburg), in collaboration with the team of Nathan Goldman, reports on the first observation of quantized circular dichroism. Following the theoretical proposal of Goldman, Zoller et al., the experimentalists realized a topological state using an ultracold atomic gas subjected to , and studied its heating properties upon circular shaking of the gas. By finely monitoring the heating rates of their system, for a wide range of driving frequencies, they were able to validate the quantization law predicted by Goldman, Zoller et al. in 2017, in agreement with the underlying topological state realized in the laboratory.

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Circa 2018

The experimental mastery of complex quantum systems is required for future technologies like quantum computers and quantum encryption. Scientists from the University of Vienna and the Austrian Academy of Sciences have broken new ground. They sought to use more complex quantum systems than two-dimensionally entangled qubits and thus can increase the information capacity with the same number of particles. The developed methods and technologies could in the future enable the teleportation of complex quantum systems. The results of their work, “Experimental Greenberger-Horne-Zeilinger entanglement beyond qubits,” is published recently in the renowned journal Nature Photonics.

Similar to bits in conventional computers, qubits are the smallest unit of in . Big companies like Google and IBM are competing with research institutes around the world to produce an increasing number of entangled qubits and develop a functioning quantum computer. But a research group at the University of Vienna and the Austrian Academy of Sciences is pursuing a new path to increase the information capacity of complex quantum systems.

The idea behind it is simple: Instead of just increasing the number of particles involved, the complexity of each is increased. “The special thing about our experiment is that for the first time, it entangles three photons beyond the conventional two-dimensional nature,” explains Manuel Erhard, first author of the study. For this purpose, the Viennese physicists used quantum systems with more than two possible states—in this particular case, the angular momentum of individual light particles. These individual photons now have a higher than qubits. However, the entanglement of these light particles turned out to be difficult on a conceptual level. The researchers overcame this challenge with a groundbreaking idea: a computer algorithm that autonomously searches for an experimental implementation.

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The transition from PCs to QCs will not merely continue the doubling of computing power, in accord with Moore’s Law. It will induce a paradigm shift, both in the power of computing (at least for certain problems) and in the conceptual frameworks we use to understand computation, intelligence, neuroscience, social interactions, and sensory perception.

Today’s PCs depend, of course, on quantum mechanics for their proper operation. But their computations do not exploit two computational resources unique to quantum theory: superposition and entanglement. To call them computational resources is already a major conceptual shift. Until recently, superposition and entanglement have been regarded primarily as mathematically well-defined by psychologically incomprehensible oddities of the quantum world—fodder for interminable and apparently unfruitful philosophical debate. But they turn out to be more than idle curiosities. They are bona fide computational resources that can solve certain problems that are intractable with classical computers. The best known example is Peter Shor’s quantum algorithm which can, in principle, break encryptions that are impenetrable to classical algorithms.

The issue is the “in principle” part. Quantum theory is well established and quantum computation, although a relatively young discipline, has an impressive array of algorithms that can in principle run circles around classical algorithms on several important problems. But what about in practice? Not yet, and not by a long shot. There are formidable materials-science problems that must be solved—such as instantiating quantum bits (qubits) and quantum gates, and avoiding an unwanted noise called decoherence—before the promise of quantum computation can be fulfilled by tangible quantum computers. Many experts bet the problems can’t adequately be solved. I think this bet is premature. We will have laptop QCs, and they will transform our world.

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A team of researchers affiliated with several institutions in Austria and Germany has shown that introducing environmental noise to a line of ions can lead to enhanced transport of energy across them. In their paper published in Physical Review Letters, the researchers describe their experiments and why they believe their findings will be helpful to other researchers.

Prior research has shown that when electrons move through , the means by which they do so can be described by quantum mechanics equations. But in the real world, such movement can be hindered by interference due to noise in the environment, leading to suppression of the transport . Prior research has also shown that electricity moving through a material can be described as a wave—if such waves remain in step, they are described as being coherent. But such waves can be disturbed by noise or defects in an atomic lattice, leading to suppression of flow. Such suppression at a given location is known as an Anderson localization. In this new effort, the researchers have shown that Anderson localizations can be overcome through the use of .

The work consisted of isolating 10 and holding them in space as a joined line—a one-dimensional crystal. Lasers were used to switch the ions between states, and energy was introduced to the ion line using . This setup allowed them to watch as energy moved along the line of ions from one end to the other. Anderson localizations were introduced by firing individual lasers at each of the ions—the energy from the lasers resulted in ions with different intensities. With a degree of disorder in place, the team then created noise by randomly changing the intensity of the beams fired at the individual ions. This resulted in frequency wobble. And it was that wobble that the team found allowed the movement of energy between the ions to overcome the Anderson localizations.

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