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Category: quantum physics

Researchers at Paderborn University have developed a new method of distance measurement for systems such as GPS, which achieves more precise results than ever before. Using quantum physics, the team led by Leibniz Prize winner Professor Christine Silberhorn has successfully overcome the so-called resolution limit, which causes the ‘noise’ we may see in photos, for example. Their findings have just been published in the academic journal Physical Review X Quantum (PRX Quantum).

Physicist Dr. Benjamin Brecht explains the problem of the resolution limit: “In laser distance measurements a detector registers two light pulses of different intensities with a . The more precise the time measurement is, the more accurately the distance can be determined. Providing the time separation between the pulses is greater than the length of the pulses, this works well.” Problems arise, however, as Brecht explains, if the pulses overlap: “Then you can no longer measure the time difference using conventional methods. This is known as the ‘resolution limit’ and is a well-known effect in photos. Very small structures or textures can no longer be resolved. That’s the same problem—just with position rather than time.”

A further challenge, according to Brecht, is to determine the different intensities of two light pulses, simultaneously with their time difference and the arrival time. But this is exactly what the researchers have managed to do—” with quantum-limited precision,” adds Brecht. Working with partners from the Czech Republic and Spain, the Paderborn physicists were even able to measure these values when the pulses overlapped by 90 per cent. Brecht says: “This is far beyond the resolution limit. The precision of the measurement is 10000 times better. Using methods from quantum information theory, we can find new forms of measurement which overcome the limitations of established methods.”

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Chinese scientists have established the world’s first integrated quantum communication network, combining over 700 optical fibers on the ground with two ground-to-satellite links to achieve quantum key distribution over a total distance of 4600 kilometers for users across the country. The team, led by Jianwei Pan, Yuao Chen, Chengzhi Peng from the University of Science and Technology of China in Hefei, reported in Nature their latest advances towards the global, practical application of such a network for future communications.

Unlike conventional encryption, quantum communication is considered unhackable and therefore the future of secure information transfer for banks, power grids and other sectors. The core of quantum communication is quantum key distribution (QKD), which uses the quantum states of particles—e.g. photons—to form a string of zeros and ones, while any eavesdropping between the sender and the receiver will change this string or key and be noticed immediately. So far, the most common QKD technology uses optical fibers for transmissions over several hundred kilometers, with high stability but considerable channel loss. Another major QKD technology uses the free space between satellites and ground stations for thousand-kilometer-level transmissions. In 2016, China launched the world’s first quantum communication satellite (QUESS, or Mozi/Micius) and achieved QKD with two ground stations which are 2600 km apart.

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“Klein tunnelling” has been observed directly for the first time.

A curious effect called “Klein tunnelling” has been observed for the first time in an experiment involving sound waves in a phononic crystal. As well as confirming the century-old prediction that relativistic particles (those travelling at speeds approaching the speed of light) can pass through an energy barrier with 100% transmission, the research done in China and the US could lead to better sonar and ultrasound imaging.

Quantum tunnelling refers to the ability of a particle to pass through a potential-energy barrier, despite having insufficient energy to cross if the system is described by classical physics. Tunnelling is a result of wave–particle duality in quantum mechanics, whereby the wave function of a particle extends into and beyond a barrier.

Normally, the probability that tunnelling will occur is less than 100% and decreases exponentially as the height and width of the barrier increase. However, in 1929 the Swedish physicist Oskar Klein calculated that an electron travelling at near the speed of light will tunnel through a barrier with 100% certainty – regardless of the height and width of the barrier.

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The human eye is a surprisingly good photon detector. What can it spy of the line between the quantum and classical worlds?

I spent a lot of time in the dark in graduate school. Not just because I was learning the field of quantum optics – where we usually deal with one particle of light or photon at a time – but because my research used my own eyes as a measurement tool. I was studying how humans perceive the smallest amounts of light, and I was the first test subject every time.

I conducted these experiments in a closet-sized room on the eighth floor of the psychology department at the University of Illinois, working alongside my graduate advisor, Paul Kwiat, and psychologist Ranxiao Frances Wang. The space was equipped with special blackout curtains and a sealed door to achieve total darkness. For six years, I spent countless hours in that room, sitting in an uncomfortable chair with my head supported in a chin rest, focusing on dim, red crosshairs, and waiting for tiny flashes delivered by the most precise light source ever built for human vision research. My goal was to quantify how I (and other volunteer observers) perceived flashes of light from a few hundred photons down to just one photon.

As individual particles of light, photons belong to the world of quantum mechanics – a place that can seem totally unlike the Universe we know. Physics professors tell students with a straight face that an electron can be in two places at once (quantum superposition), or that a measurement on one photon can instantly affect another, far-away photon with no physical connection (quantum entanglement). Maybe we accept these incredible ideas so casually because we usually don’t have to integrate them into our daily existence. An electron can be in two places at once; a soccer ball cannot.

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Silicon has proved to be a highly valuable and reliable material for fabricating a variety of technologies, including quantum devices. In recent years, researchers have also been investigating the possible advantages of using individual artificial atoms to enhance the performance of silicon-based integrated quantum circuits. So far, however, single qubits with an optical interface have proved difficult to isolate in silicon.

Researchers at Université de Montpellier and CNRS, University Leipzig and other universities in Europe have recently successfully isolated single, optically active artificial atoms in for the first time. Their paper, published in Nature Electronics, could have important implications for the development of new silicon-based quantum optics devices.

“Our study was born from the will to isolate new individual artificial atoms with a telecom in a material suitable for large-scale industrial processes,” Anaïs Dr.éau, one of the researchers who carried out the study, told TechXplore. “We are used to investigating these quantum systems, but in wide-bandgap semiconductors, such as diamond or hexagonal boron nitride. Although silicon is the most widespread material within the microelectronics industry, so far no light emitter has been reported in this small-bandgap semiconductor.”

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Just say no to cat murder.

One of the first times quantum mechanics entered popular culture, “Schrödinger’s Cat” remains a puzzling thought experiment in which a poor cat’s fate remains unknown inside a box. But scientists now say that the paradox at the heart of the puzzle could be determined ahead of time, or even reversed.

First, a recap of Schrodinger’s Cat. Created by Austrian physicist Erwin Schrödinger in 1935, it looks at a theory of quantum mechanics known as the Copenhagen interpretation. According to the Copenhagen interpretation, a quantum system will exist in superposition up until the moment it interacts with the real observable world in any way. When discussing quantum theory, the Institute of Physics says that a superposition is the idea that a particle can be in two places at once.

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Scientists at Freie Universität Berlin develop a deep learning method to solve a fundamental problem in quantum chemistry.

A team of scientists at Freie Universität Berlin has developed an artificial intelligence (AI) method for calculating the ground state of the Schrödinger equation in quantum chemistry. The goal of quantum chemistry is to predict chemical and physical properties of molecules based solely on the arrangement of their atoms in space, avoiding the need for resource-intensive and time-consuming laboratory experiments. In principle, this can be achieved by solving the Schrödinger equation, but in practice this is extremely difficult.

Up to now, it has been impossible to find an exact solution for arbitrary molecules that can be efficiently computed. But the team at Freie Universität has developed a deep learning method that can achieve an unprecedented combination of accuracy and computational efficiency. AI has transformed many technological and scientific areas, from computer vision to materials science. “We believe that our approach may significantly impact the future of quantum chemistry,” says Professor Frank Noé, who led the team effort. The results were published in the reputed journal Nature Chemistry.

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