Princeton University researchers have built a rice grain-sized laser powered by single electrons tunneling through artificial atoms known as quantum dots. The tiny microwave laser, or “maser,” is a demonstration of the fundamental interactions between light and moving electrons.

The researchers built the device — which uses about one-billionth the electric current needed to power a hair dryer — while exploring how to use quantum dots, which are bits of semiconductor material that act like single atoms, as components for quantum computers.

“It is basically as small as you can go with these single-electron devices,” said Jason Petta, an associate professor of physics at Princeton who led the study, which was published in the journal Science.

The device demonstrates a major step forward for efforts to build quantum-computing systems out of semiconductor materials, according to co-author and collaborator Jacob Taylor, an adjunct assistant professor at the Joint Quantum Institute, University of Maryland-National Institute of Standards and Technology. “I consider this to be a really important result for our long-term goal, which is entanglement between quantum bits in semiconductor-based devices,” Taylor said.

The original aim of the project was not to build a maser, but to explore how to use double quantum dots — which are two quantum dots joined together — as quantum bits, or qubits, the basic units of information in quantum computers.

The goal was to get the double quantum dots to communicate with each other,” said Yinyu Liu, a physics graduate student in Petta’s lab. The team also included graduate student Jiri Stehlik and associate research scholar Christopher Eichler in Princeton’s Department of Physics, as well as postdoctoral researcher Michael Gullans of the Joint Quantum Institute.

Because quantum dots can communicate through the entanglement of light particles, or photons, the researchers designed dots that emit photons when single electrons leap from a higher energy level to a lower energy level to cross the double dot.

Each double quantum dot can only transfer one electron at a time, Petta explained. “It is like a line of people crossing a wide stream by leaping onto a rock so small that it can only hold one person,” he said. “They are forced to cross the stream one at a time. These double quantum dots are zero-dimensional as far as the electrons are concerned — they are trapped in all three spatial dimensions.”

The researchers fabricated the double quantum dots from extremely thin nanowires (about 50 nanometers, or a billionth of a meter, in diameter) made of a semiconductor material called indium arsenide. They patterned the indium arsenide wires over other even smaller metal wires that act as gate electrodes, which control the energy levels in the dots.

To construct the maser, they placed the two double dots about 6 millimeters apart in a cavity made of a superconducting material, niobium, which requires a temperature near absolute zero, around minus 459 degrees Fahrenheit. “This is the first time that the team at Princeton has demonstrated that there is a connection between two double quantum dots separated by nearly a centimeter, a substantial distance,” Taylor said.

When the device was switched on, electrons flowed single-file through each double quantum dot, causing them to emit photons in the microwave region of the spectrum. These photons then bounced off mirrors at each end of the cavity to build into a coherent beam of microwave light.

One advantage of the new maser is that the energy levels inside the dots can be fine-tuned to produce light at other frequencies, which cannot be done with other semiconductor lasers in which the frequency is fixed during manufacturing, Petta said. The larger the energy difference between the two levels, the higher the frequency of light emitted.

Claire Gmachl, who was not involved in the research and is Princeton’s Eugene Higgins Professor of Electrical Engineering and a pioneer in the field of semiconductor lasers, said that because lasers, masers and other forms of coherent light sources are used in communications, sensing, medicine and many other aspects of modern life, the study is an important one.

“In this paper the researchers dig down deep into the fundamental interaction between light and the moving electron,” Gmachl said. “The double quantum dot allows them full control over the motion of even a single electron, and in return they show how the coherent microwave field is created and amplified. Learning to control these fundamental light-matter interaction processes will help in the future development of light sources.”

Quantum Dot Technology Could Lead To Solar Panel Windows.

Researchers from Los Alamos National Laboratory and the University of Milano-Bicocca have designed and synthesized a new generation of quantum dots for use in solar energy systems that overcome previous inefficiencies in harvesting sunlight. The study has been published in the journal Nature Photonics.

Quantum dots, which are nanocrystals made of semiconducting materials, appeal to scientists for use in solar photovoltaics (solar panel systems) because of their versatility and low-cost. In particular, they are desirable for use in luminescent solar concentrators (LSCs), which are photon-management devices that serve as alternatives to optics-based solar concentration systems.

LSCs are constructed from transparent materials containing emitters such as quantum dots. They concentrate solar radiation absorbed from a large area onto a significantly smaller solar cell, explains Victor Kilmov, one of the authors of the study. One exciting application of LSCs is the potential to develop photovoltaic windows, which could turn buildings into energy making factories.

Although quantum dots are highly efficient emitters, their small Stokes shift (the presence of an overlap between emission and absorption) means that some of the light produced is re-absorbed by the dots, resulting in losses of emission and therefore overall efficiency problems.

To resolve this issue, the team generated “Stokes-shift-engineered” giant quantum dots composed of a cadmium selenide (CdSe) shell which absorbs light, and a cadmium sulfide (CdS) core which is responsible for light emission. This separation of absorption and emission caused a significant reduction in re-absorption losses which previously caused inefficiencies. The dots were then incorporated into a high-quality polymethylmethacrylate (PMMA) matrix, and spectroscopic analysis revealed that re-absorption losses were minimal across distances of tens of centimeters.

The incorporation of the quantum dots into this PMMA matrix is not specific to a particular type of quantum dot; this means that it can be applied to different sized nanocrystals composed of various materials. This technology therefore represents a promising materials platform.

Researchers use earthworms to create quantum dots.


Quantum dots appear safe in pioneering study on primates.

A pioneering study to gauge the toxicity of quantum dots in primates has found the tiny crystals to be safe over a one-year period, a hopeful outcome for doctors and scientists seeking new ways to battle diseases like cancer through nanomedicine.

The research, which will appear on May 20 in Nature Nanotechnology online, is likely the first to test the safety of quantum dots in primates.

In the study, scientists found that four rhesus monkeys injected with cadmium-selenide quantum dots remained in normal health over 90 days. Blood and biochemical markers stayed in typical ranges, and major organs developed no abnormalities. The animals didn’t lose weight.

Two monkeys observed for an additional year also showed no signs of illness.

Quantum dots are tiny luminescent crystals that glow brightly in different colors. Medical researchers are eyeing the crystals for use in image-guided surgery, light-activated therapies and sensitive diagnostic tests. Cadmium selenide quantum dots are among the most studied, with potential applications not only in medicine, but as components of solar cells, quantum computers, light-emitting diodes and more.

A solution of cadmium-selenide quantum dots glows orange under ultraviolet light. This luminescence forms the basis for their use in bioimaging. Credit: University at Buffalo

The new toxicity study — completed by the University at Buffalo, the Chinese PLA General Hospital, China’s ChangChun University of Science and Technology, and Singapore’s Nanyang Technological University — begins to address the concern of health professionals who worry that quantum dots may be dangerous to humans.

The authors caution, however, that more research is needed to determine the nanocrystals’ long-term effects in primates; most of the potentially toxic cadmium from the quantum dots stayed in the liver, spleen and kidneys of the animals studied over the 90-day period.

“This is the first study that uses primates as animal models for in vivo studies with quantum dots,” said paper coauthor Paras Prasad, UB professor of chemistry and medicine, and executive director of UB’s Institute for Lasers, Photonics and Biophotonics (ILPB). “So far, such toxicity studies have focused only on mice and rats, but humans are very different from mice. More studies using animal models that are closer to humans are necessary.”

The cadmium build-up, in particular, is a serious concern that warrants further investigation, said Ken-Tye Yong, a Nanyang Technological University assistant professor who began working with Prasad on the study as a postdoctoral researcher at UB.

Because of that concern, the best in-vivo applications for cadmium-selenide quantum dots in medicine may be the ones that use the crystals in a limited capacity, said Mark Swihart, a third coauthor and a UB professor of chemical and biological engineering. Image-guided surgery, which could involve a single dose of quantum dots to identify a tumor or other target area, falls into this category.

Source:  Nature Nanotechnology


Connect the quantum dots for a full-colour image

Nanocrystal display could be used in high-resolution, low-energy televisions.

quantum dots

Ink stamps have been used to print text and pictures for centuries. Now, engineers have adapted the technique to build pixels into the first full-colour ‘quantum dot’ display — a feat that could eventually lead to televisions that are more energy-efficient and have sharper screen images than anything available today.

Engineers have been hoping to make improved television displays with the help of quantum dots — semiconducting crystals billionths of a metre across — for more than a decade. The dots could produce much crisper images than those in liquid-crystal displays, because quantum dots emit light at an extremely narrow, and finely tunable, range of wavelengths.

The colour of the light generated depends only on the size of the nanocrystal, says Byoung Lyong Choi, an electronic engineer at the Samsung Advanced Institute of Technology in Yongin, South Korea. Quantum dots also convert electrical power to light efficiently, making them ideal for use in energy-saving lighting and display devices.

Easier said than done

Attempts to commercialize the technology have been hampered because it is difficult to make large quantum-dot displays without compromising the quality of the image. The dots are usually layered onto the material used to make the display by spraying them onto the surface — a technique similar to that of an ink-jet printer. But the dots must be prepared in an organic solvent, which “contaminates the display, reducing the brightness of the colours and the energy efficiency”, says Choi.

Choi and his colleagues have now found a way to bypass this obstacle, by turning to a more old-fashioned printing technique — details of which appear today in Nature Photonics1. The team used a patterned silicon wafer as an ‘ink stamp’ to pick up strips of dots made from cadmium selenide, and press them down onto a glass substrate to create red, green and blue pixels without using a solvent.

The idea may sound simple, but getting it to work was not easy, Choi explains. “It took us three years to get the details right, such as changing the speed and the pressure of the stamp to get a 100% transfer.”

The team has now produced a 10-centimetre full-colour display. The pixels ware brighter and more efficient than in quantum dot displays made by rival methods, says Choi. For example, “the maximum brightness of the red pixels is about 50% better,” he says. The maximum power efficiency for the red pixels is about 70% better.

Around the bend

Bending the screen did not greatly affect the display’s performance, which means that the displays can be rolled up for portability, or used to make flexible lighting, says Choi.

Paul O’Brien, an inorganic chemist who studies quantum dots at the University of Manchester, UK, commends the group’s achievement. He notes that quantum dots are “robust”, so their efficiency will not quickly degrade. “For televisions, where you want a long lifetime, quantum dots are appealing,” he adds.

Seth Coe-Sullivan, the chief technology officer of QD Vision, a company in Watertown, Massachusetts, that produces devices with lighting based on quantum dots, notes that Choi and his team’s method is cheap. “We all have our eyes on making large-screen televisions, and this fabrication technique seems to be cost-effective,” he says.

But Coe-Sullivan adds that it may take some time to commercialize quantum-dot displays for big items. “I can imagine that we will have small cell-phone displays using this technology within around three years,” he says. “For the rest, there may be more of a wait.”

source: nature nanotechnogy