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Heisenberg’s uncertainty principle limits the precision with which two observables that do not commute with each other can be simultaneously measured. The Wigner-Araki-Yanase (WAY) theorem goes further. If observables A and B do not commute, and if observable A is conserved, observable B cannot be measured with arbitrary precision even if A is not measured at all. In its original 1960 formulation, the WAY theorem applied only to observables, such as spin, whose possible values are discrete and bounded. Now Yui Kuramochi of Kyushu University and Hiroyasu Tajima of the University of Electro-Communications—both in Japan—have proven that the WAY theorem also encompasses observables, such as position, that are continuous and unbounded [1]. Besides resolving the decades-long problem of how to deal with such observables, the extension will likely find practical applications in quantum optics.

The difficulty of extending the WAY theorem arose from how an unbounded observable L is represented: as an infinite-dimensional matrix with unbounded eigenvalues. To tame the problem, Kuramochi and Tajima avoided considering L directly. Instead, they looked at an exponential function of L, which forms a one-parameter unitary group. Although the exponential function is also unbounded, its spectrum of eigenvalues is contained within the complex plane’s unit circle. Thanks to that boundedness, Kuramochi and Tajima could go on to use off-the-shelf techniques from quantum information to complete their proof.

Because momentum is conserved, the extended WAY theorem implies that a particle’s position cannot be measured with arbitrary precision even if its momentum is not measured simultaneously. Similar pairs of observables crop up in quantum optics. Kuramochi and Tajima anticipate that their theorem could be useful in setting limits on the extent to which quantum versions of transmission protocols can outperform the classical ones.

Light confined to an accelerating optical cavity could display a photonic counterpart of the electronic quantum Hall effect.

Place a conductor in a magnetic field and the electrical current driven by an applied voltage will not flow in a straight line but in a direction perpendicular to the electric field—a behavior known as the Hall effect [1]. Reduce the temperature to the point where the electrons manifest quantum-mechanical behavior, and the plot thickens. The conductivity (defined as the ratio between the sideways current and the voltage) exhibits discrete jumps as the magnetic field is varied—the quantum Hall effect [2]. Since electrons at low temperature and photons obey a similar wave equation [3], should we also expect a quantum Hall effect for light? This question has been bubbling under the surface for the past decade, leading to the observation of some aspects of an optical quantum Hall effect [4, 5]. But the analogy between photons and electrons remains incomplete.

For the first time, researchers have succeeded in selectively exciting a molecule using a combination of two extreme-ultraviolet light sources and causing the molecule to dissociate while tracking it over time. This is another step towards specific quantum mechanical control of chemical reactions, which could enable new, previously unknown reaction channels.

The interaction of light with matter, especially with molecules, plays an important role in many areas of nature, for example in biological processes such as photosynthesis. Technologies such as solar cells use this process as well.

On the Earth’s surface, mainly light in the visible, ultraviolet or infrared regime plays a role here. Extreme-ultraviolet (XUV) light—radiation with significantly more energy than visible light —is absorbed by the atmosphere and therefore does not reach the Earth’s surface. However, this XUV radiation can be produced and used in the laboratory to enable a selective excitation of electrons in molecules.

Quantum scientists have discovered a phenomenon in purple bronze, a one-dimensional metal, that allows it to switch between insulating and superconducting states. This switch, triggered by minimal stimuli like heat or light, is due to ’emergent symmetry’. This groundbreaking finding, initiated by research into the metal’s magnetoresistance, could lead to the development of perfect switches in quantum devices, a potential milestone in quantum technology.

Quantum scientists have discovered a phenomenon in purple bronze that could be key to the development of a ‘perfect switch’ in quantum devices which flips between being an insulator and superconductor.

The research, led by the University of Bristol and published in Science, found these two opposing electronic states exist within purple bronze, a unique one-dimensional metal composed of individual conducting chains of atoms.

Researchers have manipulated light to exhibit quantum backflow, a step towards understanding complex quantum mechanics and its practical applications in precision technologies.

Scientists at the University of Warsaw’s Faculty of Physics have superposed two light beams twisted in the clockwise direction to create anti-clockwise twists in the dark regions of the resultant superposition. The results of the research have been published in the prestigious journal Optica. This discovery has implications for the study of light-matter interactions and represents a step towards the observation of a peculiar phenomenon known as a quantum backflow.

“Imagine that you are throwing a tennis ball. The ball starts moving forward with positive momentum. If the ball doesn’t hit an obstacle, you are unlikely to expect it to suddenly change direction and come back to you like a boomerang,” notes Bohnishikha Ghosh, a doctoral student at the University of Warsaw’s Faculty of Physics. “When you spin such a ball clockwise, for example, you similarly expect it to keep spinning in the same direction.”

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In a new interview, perpetually provocative Harvard astronomer and alien hunter Avi Loeb posited both that super-human aliens could be building “baby universes” in labs and that his haters are just “jealous.”

When discussing his work and theories in a chat with Fox News, Loeb showed his tendency toward imaginative, deeply speculative theories of extraterrestrial life.

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Researchers around the world are working on a network which could connect quantum computers with one another over long distances. Andreas Reiserer, Professor of Quantum Networks at the Technical University of Munich (TUM), explains the challenges which have to be mastered and how atoms captured in crystals can help.

The idea is the same: We use today’s internet to connect computers with one another, while the quantum internet lets quantum computers communicate with one another. But in technical terms the quantum internet is much more complex. That’s why only smaller networks have been realized as yet.

There are two main applications: First of all, networking quantum computers makes it possible to increase their computing power; second, a quantum network will make absolutely interception-proof encryption of communication possible. But there are other applications as well, for example networking telescopes to achieve a previously impossible resolution in order to look into the depths of the universe, or the possibility of synchronizing atomic clocks around the world extremely precisely, making it possible to investigate completely new physical questions.

Quantum advantage is the milestone the field of quantum computing is fervently working toward, where a quantum computer can solve problems that are beyond the reach of the most powerful non-quantum, or classical, computers.

Quantum refers to the scale of atoms and molecules where the laws of physics as we experience them break down and a different, counterintuitive set of laws apply. Quantum computers take advantage of these strange behaviors to solve problems.

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