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Category: quantum physics – Page 544

Physics World

Interactions between matter and light in microcavities made of mirrors are fundamentally important for many modern technologies, including lasers. Researchers at the University of Michigan, Ann Arbor, US, have now gained tighter control of these interactions by exploiting a nonlinear effect that occurs in a new kind of hybrid semiconductor made from bilayers of two-dimensional materials. These semiconducting sheets form an egg-carton-like array in which the “pockets” are quantum dots that can be controlled using light, and they could be used to make ultralow-energy switches.

Led by Hui Deng, the researchers made their hybrid semiconductor from flakes of tungsten disulphide (WS2) and molybdenum diselenide (MoSe2) just a few atoms thick. In their bulk form, these transition-metal dichalcogenides (TMDCs) act as indirect band-gap semiconductors. When scaled down to a monolayer thickness, however, they behave as direct band-gap semiconductors, capable of efficiently absorbing and emitting light.

When laid on top of one another, the electronic structures of TMDCs can form a larger electron lattice (known as a moiré lattice) thanks to the slight mismatch of the materials’ lattice constants. The period of this lattice can be tuned by twisting the monolayers with respect to each other at different angles. In the WS2 and MoSe2 bilayer studied in this work, this angle is about 56.5° and the moiré lattice produced contains “pockets” measuring around 10 atoms across. These pockets, explains study lead author Long Zhang, are the quantum dots – tiny pieces of semiconducting materials that can isolate individual quantum particles such as electrons.

Continue reading “Quantum dot array could make ultralow-energy switches” | >

A team of researchers from QuTech in the Netherlands reports realization of the first multi-node quantum network, connecting three quantum processors. In addition, they achieved a proof-of-principle demonstration of key quantum network protocols. Their findings mark an important milestone toward the future quantum internet and have now been published in Science.

The power of the is that it allows any two computers on Earth to connect. Today, researchers in many labs around the world are working toward first versions of a quantum internet—a network that can connect any two , such as quantum computers or sensors, over large distances. Whereas today’s internet distributes information in bits that can be either 0 or 1, a future quantum internet will make use of quantum bits that can be 0 and 1 at the same time.

“A quantum internet will open up a range of novel applications, from unhackable communication and cloud computing with complete user privacy to high-precision time-keeping,” says Matteo Pompili, Ph.D. student and a member of the research team. “And like with the internet 40 years ago, there are probably many applications we cannot foresee right now.”

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Creating a two-dimensional material, just a few atoms thick, is often an arduous process requiring sophisticated equipment. So scientists were surprised to see 2D puddles emerge inside a three-dimensional superconductor—a material that allows electrons to travel with 100% efficiency and zero resistance—with no prompting.

Within those puddles, superconducting electrons acted as if they were confined inside an incredibly thin, sheet-like plane, a situation that requires them to somehow cross over to another dimension, where different rules of quantum physics apply.

“This is a tantalizing example of emergent behavior, which is often difficult or impossible to replicate by trying to engineer it from scratch,” said Hari Manoharan, a professor at Stanford University and investigator with the Stanford Institute for Materials and Energy Sciences (SIMES) at the Department of Energy’s SLAC National Accelerator Laboratory, who led the research.

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Ultralight bosons are hypothetical particles whose mass is predicted to be less than a billionth the mass of an electron. They interact relatively little with their surroundings and have thus far eluded searches to confirm their existence. If they exist, ultralight bosons such as axions would likely be a form of dark matter, the mysterious, invisible stuff that makes up 85 percent of the matter in the universe.

Now, physicists at MIT’s LIGO Laboratory have searched for ultralight bosons using black holes—objects that are mind-bending orders of magnitude more massive than the particles themselves. According to the predictions of quantum theory, a black hole of a certain mass should pull in clouds of ultralight bosons, which in turn should collectively slow down a black hole’s spin. If the particles exist, then all black holes of a particular mass should have relatively low spins.

But the physicists have found that two previously detected black holes are spinning too fast to have been affected by any ultralight bosons. Because of their large spins, the black holes’ existence rules out the existence of ultralight bosons with masses between 1.3×10-13 electronvolts and 2.7×10-13 electronvolts—around a quintillionth the mass of an electron.

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Physicists from Swansea University are part of an international research collaboration which has identified a new technique for testing the quality of quantum correlations.

Quantum computers run their algorithms on large quantum systems of many parts, called qubits, by creating quantum correlations across all of them. It is important to verify that the actual computation procedures lead to quantum correlations of desired quality.

However, carrying out these checks is resource-intensive as the number of tests required grows exponentially with the number of qubits involved.

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It may be possible in the future to use information technology where electron spin is used to store, process and transfer information in quantum computers. It has long been the goal of scientists to be able to use spin-based quantum information technology at room temperature. A team of researchers from Sweden, Finland and Japan have now constructed a semiconductor component in which information can be efficiently exchanged between electron spin and light at room temperature and above. The new method is described in an article published in Nature Photonics.

It is well known that electrons have a negative charge; they also have another property called spin. This may prove instrumental in the advance of . To put it simply, we can imagine the electron rotating around its own axis, similar to the way in which the Earth rotates around its own axis. Spintronics—a promising candidate for future information technology—uses this quantum property of electrons to store, process and transfer information. This brings important benefits, such as higher speed and lower energy consumption than traditional electronics.

Developments in spintronics in recent decades have been based on the use of metals, and these have been highly significant for the possibility of storing large amounts of data. There would, however, be several advantages in using spintronics based on semiconductors, in the same way that semiconductors form the backbone of today’s electronics and photonics.

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Classical hydrodynamics laws can be very useful for describing the behavior of systems composed of many particles (i.e., many-body systems) after they reach a local state of equilibrium. These laws are expressed by so-called hydrodynamical equations, a set of mathematical equations that describe the movement of water or other fluids.

Researchers at Oak Ridge National Laboratory and University of California, Berkeley (UC Berkeley) have recently carried out a study exploring the hydrodynamics of a quantum Heisenberg spin-1/2 chain. Their paper, published in Nature Physics, shows that the spin dynamics of a 1D Heisenberg antiferromagnet (i.e., KCuF3) could be effectively described by a dynamical exponent aligned with the so-called Kardar-Parisi-Zhang universality class.

“Joel Moore and I have known each other for many years and we both have an interest in quantum magnets as a place where we can explore and test new ideas in physics; my interests are experimental and Joel’s are theoretical,” Alan Tennant, one of the researchers who carried out the study, told Phys.org. “For a long time, we have both been interested in temperature in quantum systems, an area where a number of really new insights have come along recently, but we had not worked together on any projects.”

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Circa 2020

Lasing—the emission of a collimated light beam of light with a well-defined wavelength (color) and phase—results from a self-organization process, in which a collection of emission centers synchronizes itself to produce identical light particles (photons). A similar self-organized synchronization phenomenon can also lead to the generation of coherent vibrations—a phonon laser, where phonon denotes, in analogy to photons, the quantum particles of sound.

Photon lasing was first demonstrated approximately 60 years ago and, coincidentally, 60 years after its prediction by Albert Einstein. This stimulated emission of amplified found an unprecedented number of scientific and technological applications in multiple areas.

Although the concept of a “laser of sound” was predicted almost at the same time, only few implementations have so far been reported and none has attained technological maturity. Now, a collaboration between researchers from Instituto Balseiro and Centro Atómico in Bariloche (Argentina) and Paul-Drude-Institut in Berlin has introduced a novel approach for the efficient generation of coherent vibrations in the tens of GHz range using semiconductor structures. Interestingly, this approach to the generation of coherent phonons is based on another of Einstein’s predictions: that of the 5th state of matter, a Bose-Einstein condensate (BEC) of coupled light-matter particles (polaritons).

Continue reading “A phonon laser: Coherent vibrations from a self-breathing resonator” | >

Place one clock at the top of a mountain. Place another on the beach. Eventually, you’ll see that each clock tells a different time. Why?

In his book “The Order of Time,” Italian theoretical physicist Carlo Rovelli suggests that our perception of time — our sense that time is forever flowing forward — could be a highly subjective projection. After all, when you look at reality on the smallest scale (using equations of quantum gravity, at least), time vanishes.

“If I observe the microscopic state of things,” writes Rovelli, “then the difference between past and future vanishes … in the elementary grammar of things, there is no distinction between ‘cause’ and ‘effect.’”

So, why do we perceive time as flowing forward? Rovelli notes that, although time disappears on extremely small scales, we still obviously perceive events occur sequentially in reality. In other words, we observe entropy: Order changing into disorder; an egg cracking and getting scrambled.

Continue reading “Physics without time” | >

In high-end 21st century communications, information travels in the form of a stream of light pulses typically traveling through fiber optic cables. Each pulse can be as faint as a single photon, the smallest possible unit (quantum) of light. The speed at which such systems can operate depends critically on how fast and how accurately detectors on the receiving end can discriminate and process those photons.

Now scientists at the National institute of Standards and Technology (NIST) have devised a method that can detect individual photons at a rate 10 times faster than the best existing technology, with lower error rates, higher detection efficiency, and less noise.

“While classical communication and detection can operate at blazing speeds, , which need that ultimate sensitivity for those faintest of pulses, are limited to much lower speeds,” said group leader Alan Migdall. “Combining that ultimate sensitivity with the ability to achieve the counting of photons at has been a long-standing challenge. Here we are pushing both performance limits all in the same device.”

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