Everyone loves a more powerful computer, right? This is probably the underlying motivation that drives much of modern quantum computing research. "I've got a superBogus™ gen 2 quantum processor, how about you?" Okay, maybe not the primary motivation. Along the way to quantum computing geek nirvana, scientists are learning an awful lot about quantum mechanics. These are not discoveries that shake the foundations of physics; instead, we are learning about the practicalities of manipulating quantum properties

One of the new kids on the block is called topologically protected quantum computing. The basic idea is to create a setup where the shape or layout of a quantum system self-stabilizes, making it impossible for the environment to effect it. Even at a distance, pairs of particles can link up, creating something called a Majorana fermion. This was thought to be immune to something called decoherence, making it the perfect object for a qubit. Unfortunately, a bit of thought shows that this is entirely untrue.

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In typical electronic devices, temperature is the primary physical variable that controls conductivity. Resistance tends to increase with temperature. However, things are different on the nanoscale. Even at room temperature, the energy difference between quantum levels within a molecule can be much larger than the thermal energy. This means it is possible, in principle, to manipulate the wave function of electrons in a way that tunes the conductive properties of a material on the molecular level.

In a newly published experiment, Constant M. Guédon et al. managed to promote destructive quantum interference between electrons in a single molecule, reducing the molecule's ability to conduct current in the process. They compared the conductive properties of molecules that have an identical primary structure, but have differences in their electronic quantum states. In a molecule where the electrons interfered destructively, it suppressing the flow of electric current. This experiment opens up the possibility of room-temperature molecular devices based on quantum interference.

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One of the deepest mysteries in quantum physics is the wave-particle duality: every quantum object has properties of both a wave and a particle. Nowhere is this effect more beautifully demonstrated than in the double-slit experiment: streams of particles (photons, electrons, whatever) are directed at a barrier with two narrow openings. While each particle shows up at the detector individually, the population as a whole creates an interference pattern as though they are waves. Neither a pure wave nor a pure particle description has proven successful in explaining these experiments.

Now researchers have successfully performed a quantum interference experiment with much larger and more massive molecules than ever before. Thomas Juffmann et al. fired molecules composed of over 100 atoms at a barrier with openings designed to minimize molecular interactions, and observed the build-up of an interference pattern. The experiment approaches the regime where macroscopic and quantum physics overlap, offering a possible way to study the transition that has frustrated many scientists for decades.

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The wavefunction is a fundamental concept in quantum mechanics, telling us where we're likely to find a particle—the position or momentum of an electron, for example—even though its physical meaning is rather unclear. This concept has turned out to be extremely useful. 

Where the wavefunction really counts is in things like molecules, where it determines properties like structure and behavior. Yet determining the spatial properties of the wavefunction inside something like a molecule is not something that is easily done. That is what makes a paper on imaging the wavefunction of an ionized, vibrating hydrogen molecule so interesting.

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In 1905, Albert Einstein showed that the photoelectric effect—the ability of metals to produce an electric current when exposed to light—could be explained if light is quantum, traveling in discrete bundles of energy. His model, the photon theory, won him the Nobel Prize in 1921, but it left us with an enigma: why does the classical model of electric fields yield correct experimental results for some systems, but fail so dramatically for the photoelectric effect? In other words, at what point does the quantum world begin and the classical world end?

By directing very intense light to a nanoscale needle-like tip, G. Herink, D. R. Solli, M. Gulde, and C. Ropers have bridged the gap between the quantum and classical views of the photoelectric effect. The sheer number of photons hitting the needle dwarf the number of electrons involved, which ensures that individual photon interactions do not dominate. Instead, they created a quasi-classical system in which the bulk electric field of all the photons influences individual electrons. This result shows why the classical and quantum views are correct in certain regimes, and hints at an entirely new way to manipulate electrons in nanoscale materials.

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One of the most mind-blowing areas of quantum mechanics is entanglement: two or more particles separated in space can have physical properties that are correlated. A measurement performed on one particle will tell us the result of the same measurement taken on an entangled particle. Entanglement is important but difficult to study, both in terms of a theoretical understanding and doing experiments. While entangling relatively small groups of particles has been accomplished several times over the last 30 years (pioneered by Aspect et al. in 1982), scaling these experiments up in sizes sufficient to create quantum computers and other complex systems has eluded researchers.

A significant step forward has been accomplished by entangling eight photons (previously six had been the largest number). Researchers from Shanghai's University of Science and Technology of China created a system where eight photons were equally likely to be polarized in a specific orientation, something known colloquially as a "Schrödinger cat" state. In a paper published in Nature Photonics, authors Xing-Can Yao et al. describe a new technique that uses ultra-bright photon sources to control for some of the problems that plagued earlier entanglement experiments.

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Oscillators lie at the core of many precision quantum experiments. The oscillations can exist in atomic clocks used for accurate timing, laser and masers, or a variety of other devices, but the regular cycling of quantum oscillators play an essential role in modern science and engineering. However, most uses have been confined to the electromagnetic regime, where the vibrations exhibit as photons; the quantum states of mechanical oscillators, where the vibrations are sound waves, have proven more difficult to control.

However, researchers in Switzerland and Germany have built a special cavity where the electromagnetic quantum states resonate with the natural vibrations of the atoms. In doing so, E. Verhagen, S. Delégliese, S. Weis, A. Schliesser, and T.J. Klippenberg managed to couple a photon-based oscillator to a mechanical oscillator, controlling the mechanical quantum states with visible light. The result is a prototype of a quantum transducer, a device that converts light energy into mechanical energy.

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