Absence of light exerts a weird cooling effect.
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Category: quantum physics – Page 683
Electronics rely on the movement of negatively-charged electrons. Physicists strive to understand the forces that push these particles into motion, with the goal of harnessing their power in new technologies. Quantum computers, for instance, employ a fleet of precisely controlled electrons to take on goliath computational tasks. Recently, researchers at the Okinawa Institute of Science and Technology Graduate University (OIST) demonstrated how microwaves cut in on the movements of electrons. The findings may contribute to future quantum computing technology.
The logic operations of normal computers are based on zeros and ones, and this binary code limits the volume and type of information the machines can process. Subatomic particles can exist in more than two discrete states, so quantum computers harness electrons to crunch complex data and perform functions at whiplash speed. To keep electrons in limbo for experiments, scientists capture the particles and expose them to forces that alter their behavior.
In the new study, published December 18, 2018 in Physical Review B, OIST researchers trapped electrons in a frigid, vacuum-sealed chamber and subjected them to microwaves. The particles and light altered each other’s movement and exchanged energy, which suggests the sealed system could potentially be used to store quantum information – a microchip of the future.
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Possible quantum computing at room temperature. Scientists working with hexagonal boron nitride, which allows them to work in two-dimensional arrays. Simpler than using 3D objects such as diamonds.
Researchers have now demonstrated a new hardware platform based on isolated electron spins in a two-dimensional material. The electrons are trapped by defects in sheets of hexagonal boron nitride, a one-atom-thick semiconductor material, and the researchers were able to optically detect the system’s quantum states.
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Quantum physics sets the laws that dominate the universe at a small scale. The ability to harness quantum phenomena could lead to machines like quantum computers, which are predicted to perform certain calculations much faster than conventional computers. One major problem with building quantum processors is that the tracking and controlling quantum systems in real time is a difficult task because quantum systems are overwhelmingly fragile: Manipulating these systems carelessly introduces significant errors in the final result. New work by a team at Aalto could lead to precise quantum computers.
The researchers report controlling quantum phenomena in a custom-designed electrical circuit called a transmon. Chilling a transmon chip to within a few thousandths of a degree above absolute zero induces a quantum state, and the chip starts to behave like an artificial atom. One of the quantum features that interests researchers is that the energy of the transmon can only take specific values, called energy levels. The energy levels are like steps on a ladder: A person climbing the ladder must occupy a step, and can’t hover somewhere between two steps. Likewise, the transmon energy can only occupy the set values of the energy levels. Shining microwaves on the circuit induces the transmon to absorb the energy and climb up the rungs of the ladder.
In work published 8 February in the journal Science Advances, the group from Aalto University led by Docent Sorin Paraoanu, senior university lecturer in the Department of Applied Physics, has made the transmon jump more than one energy level in a single go. Previously, this has been possible only by very gentle and slow adjustments of the microwave signals that control the device. In the new work, an additional microwave control signal shaped in a very specific way allows a fast, precise change of the energy level. Dr. Antti Vepsäläinen, the lead author, says, “We have a saying in Finland: ‘hiljaa hyvää tulee’ (slowly does it). But we managed to show that by continuously correcting the state of the system, we can drive this process more rapidly and at high fidelity.”
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Princeton researchers have demonstrated a new way of making controllable “quantum wires” in the presence of a magnetic field, according to a new study published in Nature.
The researchers detected channels of conducting electrons that form between two quantum states on the surface of a bismuth crystal subjected to a high magnetic field. These two states consist of electrons moving in elliptical orbits with different orientations.
To the team’s surprise, they found that the current flow in these channels can be turned on and off, making these channels a new type of controllable quantum wire.
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Circa 2005
Until 1993 LEDs could only produce red, green and yellow light. But then Nichia Chemical of Japan figured out how to produce blue LEDs. By combining blue LEDs with red and green LEDs – or adding a yellow phosphor to blue LEDs – manufacturers were able create white light, which opened up a number of new applications. However, these LEDs tend to produce white light with a cool, bluish tinge.
The white-light quantum dots, by contrast, produce a smoother distribution of wavelengths in the visible spectrum with a slightly warmer, slightly more yellow tint, reports Michael Bowers, the graduate student who made the quantum dots and discovered their unusual property. As a result, the light produced by the quantum dots looks more nearly like the “full spectrum” reading lights now on the market which produce a light spectrum closer to that of sunlight than normal fluorescent tubes or light bulbs. Of course, quantum dots, like white LEDs, have the advantage of not giving off large amounts of invisible infrared radiation unlike the light bulb. This invisible radiation produces large amounts of heat and largely accounts for the light bulb’s low energy efficiency.
Image left: The crude hybrid white-light LED that Bowers made by mixing magic-sized quantum dots with Minwax and using the mixture to coat a blue LED. Photo by Daniel Dubois.
Circa 2018
Researchers have demonstrated nanomaterial-based white-light-emitting diodes (LEDs) that exhibit a record luminous efficiency of 105 lumens per watt. Luminous efficiency is a measure of how well a light source uses power to generate light. With further development, the new LEDs could reach efficiencies over 200 lumens per watt, making them a promising energy-efficient lighting source for homes, offices and televisions.
“Efficient LEDs have strong potential for saving energy and protecting the environment,” said research leader Sedat Nizamoglu, Koç University, Turkey. “Replacing conventional lighting sources with LEDs with an efficiency of 200 lumens per watt would decrease the global electricity consumed for lighting by more than half. That reduction is equal to the electricity created by 230 typical 500-megawatt coal plants and would reduce greenhouse gas emissions by 200 million tons.”
The researchers describe how they created the high-efficiency white LEDs in Optica, The Optica l Society’s journal for high impact research. The new LEDs use commercially available blue LEDs combined with flexible lenses filled with a solution of nano-sized semiconductor particles called quantum dots. Light from the blue LED causes the quantum dots to emit green and red, which combines with the blue emission to create white light.
In a paper to be published in the forthcoming issue in NANO, researchers from the National Institute of Technology, India, have synthesized blue-green-orange photoemissive sulfur and nitrogen co-doped graphene quantum dots (SNGQDs) using hydrothermal method. These GQDs showed strong UV-visible photoabsorption and excitation dependent photoemission which have low-cost, eco-friendly solar cell application.
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