Rice-sized laser, powered one electron at a time, bodes well for quantum computing

Princeton University researchers have built a rice grain-sized microwave laser.

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."

The paper, "Semiconductor double quantum dot micromaser," was published in the journal Science on Jan. 16, 2015. The research was supported by the David and Lucile Packard Foundation, the National Science Foundation (DMR-1409556 and DMR-1420541), the Defense Advanced Research Projects Agency QuEST (HR0011-09-1-0007), and the Army Research Office (W911NF-08-1-0189).

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New half-light half-matter quantum particles created

Prospects of developing computing and communication technologies based on quantum properties of light and matter may have taken a major step forward thanks to research by City College of New York physicists led by Dr. Vinod Menon.

In a pioneering study, Professor Menon and his team were able to discover half-light, half-matter particles in atomically thin semiconductors (thickness ~ a millionth of a single sheet of paper) consisting of two-dimensional (2D) layer of molybdenum and sulfur atoms arranged similar to graphene. They sandwiched this 2D material in a light trapping structure to realize these composite quantum particles.

"Besides being a fundamental breakthrough, this opens up the possibility of making devices which take the benefits of both light and matter," said Professor Menon.

For example one can start envisioning logic gates and signal processors that take on best of light and matter. The discovery is also expected to contribute to developing practical platforms for quantum computing.

Dr. Dirk Englund, a professor at MIT whose research focuses on quantum technologies based on semiconductor and optical systems, hailed the City College study.

"What is so remarkable and exciting in the work by Vinod and his team is how readily this strong coupling regime could actually be achieved. They have shown convincingly that by coupling a rather standard dielectric cavity to exciton-polaritons in a monolayer of molybdenum disulphide, they could actually reach this strong coupling regime with a very large binding strength," he said.

Professor Menon's research team included City College PhD students, Xiaoze Liu, Tal Galfsky and Zheng Sun, and scientists from Yale University, National Tsing Hua University (Taiwan) and Ecole Polytechnic -Montreal (Canada).

The study was funded by the U.S. Army Research Laboratory's Army Research Office and the National Science Foundation through the Materials Research Science and Engineering Center -- Center for Photonic and Multiscale Nanomaterials.

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Control of shape of light particles opens the way to 'quantum internet'

In the same way as we now connect computers in networks through optical signals, it may also be possible to connect future quantum computers in a 'quantum internet'. The optical signals would then consist of individual light particles or photons. One prerequisite for a working quantum internet is control of the shape of these photons. Researchers at Eindhoven University of Technology (TU/e) and the FOM foundation have now succeeded for the first time in getting this control within the required short time. These findings are published today in Nature Communications.

Quantum computers are the dream computers of the future. They use the unique physics of the smallest particles- those described by quantum mechanics -- to perform calculations. While today's computers use bits that can be either 0 or 1, quantum computers perform calculations with 'qubits', which can be both 0 and 1 at the same time. That creates an unprecedented degree of extra computing power, which gives quantum computers much greater capabilities than today's computers.

Quantum internet

Quantum computers could in principle communicate with each other by exchanging individual photons to create a 'quantum internet'. The shape of the photons, in other words how their energy is distributed over time, is vital for successful transmission of information. This shape must be symmetric in time, while photons that are emitted by atoms normally have an asymmetric shape. Therefore, this process requires external control in order to create a quantum internet.

Optical cavity

Researchers at TU/e and FOM have succeeded in getting the required degree of control by embedding a quantum dot -- a piece of semiconductor material that can transmit photons -- into a 'photonic crystal', thereby creating an optical cavity. Then the researchers applied a very short electrical pulse to the cavity, which influences how the quantum dot interacts with it, and how the photon is emitted. By varying the strength of this pulse, they were able to control the shape of the transmitted photons.

Within a billionth of a second

The Eindhoven researchers are the first to achieve this, thanks to the use of electrical pulses shorter than nanosecond, a billionth of a second. This is vital for use in quantum communication, as research leader Andrea Fiore of TU/e explains: "The emission of a photon only lasts for one nanosecond, so if you want to change anything you have to do it within that time. It's like the shutter of a high-speed camera, which has to be very short if you want to capture something that changes very fast in an image. By controlling the speed at which you send a photon, you can in principle achieve very efficient exchange of photons, which is important for the future quantum internet."

The research is financed by the FOM Foundation and Technology Foundation STW.

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Theory details how 'hot' monomers affect thin-film formation

Like a baseball player sliding into third, a hot monomer skids in a straight line along a cold surface until it's safely among its fellow molecules.

This is not what usually happens when scientists assemble monomers to make thin films for next-generation electronics, but the details remained a puzzle until a team led by Rice University got involved. Monomers are organic molecules that, in this application, form clusters and eventually complete layers.

Researchers at Rice and the University of Maryland led by Rice theoretical physicist Alberto Pimpinelli devised the first detailed model to quantify what they believe was the last unknown characteristic of film formation through deposition by vacuum sublimation and chemical vapor deposition. Their work appears today in Physical Review Letters.

Scientists make films as thin as a single molecule in the same way they make graphene: They heat a gas in a furnace and wait for its bits to condense and aggregate into a solid. Molecules in the gas hit a surface (typically mica in experiments, often silicon in applications) called a substrate, where they accumulate into a regular lattice.

Usually, according to Pimpinelli, organic molecules drawn by weak van der Waals forces alight on the substrate and skitter randomly from one point to another under the influence of vibrating phonons in the substrate's crystalline lattice until they bump into another molecule or, more likely, an island of other molecules to join.

Islands that gather enough monomers create a critical nucleus: Take one away and all of them could disassociate; add one and the nucleus becomes a stable structure able to join with other growing islands to form the film.

Researchers understand how this happens, Pimpinelli said. But when the substrate is cool enough to dampen its natural vibrations (for pentacene on mica, at about minus 190 degrees Fahrenheit) and the gas is hot enough, the molecules hit the substrate and skid in a straight line along its surface, which scientists call ballistic motion. If they skid to a stop, they will wander until they find an island -- or growing islands find them.

"The common wisdom has been that when molecules or atoms arrive at a substrate, their kinetic energy immediately dissipates into the substrate crystal," Pimpinelli said. "Their energy is essentially zero when they arrive at the surface, and they diffuse at random. But what happens if they don't, if the energy is preserved long enough that the molecule keeps its velocity at the surface and continues on in a line? Traditionally, this aspect has been totally neglected."

These skidding molecules can make islands grow, or knock other molecules off islands at or below the critical nucleation stage, perhaps even causing them to disassociate, he said.

Pimpinelli said nobody had successfully created a mathematical model that detailed the conditions under which ballistic, "nonthermal" motions could continue even after a molecule is adsorbed by the substrate.

"We already had a powerful mathematical framework for aggregation on surfaces," he said. "We have ways to describe in a lot of detail what happens to atoms, and can transfer those models lock, stock and barrel to these molecules -- except for one: the 'hot' monomer.

"Because these hot monomers profoundly affect island nucleation, it's important for experimentalists to know how and why this happens," Pimpinelli said. The speed of island growth affects their size, which in turn affects the size of the electrically significant boundaries that form when they come together. These grain boundaries tend to decrease the electrical and thermal conductivity of a material.

Pimpinelli took on the project when experimental associates in Austria observed that when they tried to grow films on cold substrates, clusters of organic molecules would form in a way that was only compatible with ballistically moving monomers. Pimpinelli's collaborators were graduate student Josue Morales-Cifuentes and physics professor Theodore Einstein of the University of Maryland.

"With a low-temperature substrate, you will not see as much thermal motion, in which molecules hop here and there at random," Pimpinelli said. "Clearly there was a different mechanism leading to a qualitative change in the islands they observed with an atomic force microscope."

Pimpinelli said quick calculations involving the temperature of the substrate and the deposition rate of the monomers suggested the system could be modeled; more complex mathematics not only confirmed it, but also provided a way to predict how various molecules and substrates will interact at specific temperatures.

"By studying how the number of islands changes with these control parameters, one is able to know the size of the critical nucleus, the diffusion process, all the energies involved and how these molecules bind to each other and the substrate," he said. "All the information, in principle, can be extracted by counting the islands on the surface."

Pimpinelli is a faculty fellow in the Materials Science and NanoEngineering Department and executive director of the Rice Quantum Institute. The National Science Foundation supported the research.

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