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Wednesday, July 8th, 2020
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| 10:59a |
Scaling up the quantum chip MIT researchers have developed a process to manufacture and integrate “artificial atoms,” created by atomic-scale defects in microscopically thin slices of diamond, with photonic circuitry, producing the largest quantum chip of its type.
The accomplishment “marks a turning point” in the field of scalable quantum processors, says Dirk Englund, an associate professor in MIT’s Department of Electrical Engineering and Computer Science. Millions of quantum processors will be needed to build quantum computers, and the new research demonstrates a viable way to scale up processor production, he and his colleagues note.
Unlike classical computers, which process and store information using bits represented by either 0s and 1s, quantum computers operate using quantum bits, or qubits, which can represent 0, 1, or both at the same time. This strange property allows quantum computers to simultaneously perform multiple calculations, solving problems that would be intractable for classical computers.
The qubits in the new chip are artificial atoms made from defects in diamond, which can be prodded with visible light and microwaves to emit photons that carry quantum information. The process, which Englund and his team describe today in Nature, is a hybrid approach, in which carefully selected “quantum micro chiplets” containing multiple diamond-based qubits are placed on an aluminum nitride photonic integrated circuit.
“In the past 20 years of quantum engineering, it has been the ultimate vision to manufacture such artificial qubit systems at volumes comparable to integrated electronics,” Englund says. “Although there has been remarkable progress in this very active area of research, fabrication and materials complications have thus far yielded just two to three emitters per photonic system.”
Using their hybrid method, Englund and colleagues were able to build a 128-qubit system — the largest integrated artificial atom-photonics chip yet.
“It’s quite exciting in terms of the technology,” says Marko Lončar, the Tiantsai Lin Professor of Electrical Engineering at Harvard University, who was not involved in the study. “They were able to get stable emitters in a photonic platform while maintaining very nice quantum memories.”
Other authors on the Nature paper include MIT researchers Noel H. Wan, Tsung-Ju Lu, Kevin C. Chen, Michael P. Walsh, Matthew E. Trusheim, Lorenzo De Santis, Eric A. Bersin, Isaac B. Harris, Sara L. Mouradian and Ian R. Christen; with Edward S. Bielejec at Sandia National Laboratories.
Quality control for chiplets
The artificial atoms in the chiplets consist of color centers in diamonds, defects in diamond’s carbon lattice where adjacent carbon atoms are missing, with their spaces either filled by a different element or left vacant. In the MIT chiplets, the replacement elements are germanium and silicon. Each center functions as an atom-like emitter whose spin states can form a qubit. The artificial atoms emit colored particles of light, or photons, that carry the quantum information represented by the qubit.
Diamond color centers make good solid-state qubits, but “the bottleneck with this platform is actually building a system and device architecture that can scale to thousands and millions of qubits,” Wan explains. “Artificial atoms are in a solid crystal, and unwanted contamination can affect important quantum properties such as coherence times. Furthermore, variations within the crystal can cause the qubits to be different from one another, and that makes it difficult to scale these systems.”
Instead of trying to build a large quantum chip entirely in diamond, the researchers decided to take a modular and hybrid approach. “We use semiconductor fabrication techniques to make these small chiplets of diamond, from which we select only the highest quality qubit modules,” says Wan. “Then we integrate those chiplets piece-by-piece into another chip that ‘wires’ the chiplets together into a larger device.”
The integration takes place on a photonic integrated circuit, which is analogous to an electronic integrated circuit but uses photons rather than electrons to carry information. Photonics provides the underlying architecture to route and switch photons between modules in the circuit with low loss. The circuit platform is aluminum nitride, rather than the traditional silicon of some integrated circuits.
Using this hybrid approach of photonic circuits and diamond chiplets, the researchers were able to connect 128 qubits on one platform. The qubits are stable and long-lived, and their emissions can be tuned within the circuit to produce spectrally indistinguishable photons, according to Wan and colleagues.
A modular approach
While the platform offers a scalable process to produce artificial atom-photonics chips, the next step will be to “turn it on,” so to speak, to test its processing skills.
“This is a proof of concept that solid-state qubit emitters are very scalable quantum technologies,” says Wan. “In order to process quantum information, the next step would be to control these large numbers of qubits and also induce interactions between them.”
The qubits in this type of chip design wouldn’t necessarily have to be these particular diamond color centers. Other chip designers might choose other types of diamond color centers, atomic defects in other semiconductor crystals like silicon carbide, certain semiconductor quantum dots, or rare-earth ions in crystals. “Because the integration technique is hybrid and modular, we can choose the best material suitable for each component, rather than relying on natural properties of only one material, thus allowing us to combine the best properties of each disparate material into one system,” says Lu.
Finding a way to automate the process and demonstrate further integration with optoelectronic components such as modulators and detectors will be necessary to build even bigger chips necessary for modular quantum computers and multichannel quantum repeaters that transport qubits over long distances, the researchers say. | | 2:00p |
Helping drug-delivering particles squeeze through a syringe Microparticles offer a promising way to deliver multiple doses of a drug or vaccine at once, because they can be designed to release their payload at specific intervals. However, the particles, which are about the size of a grain of sand, can be difficult to inject because they can get clogged in a typical syringe.
MIT researchers have now developed a computational model that can help them improve the injectability of such microparticles and prevent clogging. The model analyzes a variety of factors, including the size and shape of the particles, to determine an optimal design for injectability.
Using this model, the researchers were able to achieve a sixfold increase in the percentage of microparticles they could successfully inject. They now hope to use the model to develop and test microparticles that could be used to deliver cancer immunotherapy drugs, among other potential applications.
“This is a framework that can help us with some of the technologies that we’ve developed in the lab and that we’re trying to get into the clinic,” says Ana Jaklenec, a research scientist at MIT’s Koch Institute for Integrative Cancer Research.
Jaklenec and Robert Langer, the David H. Koch Institute Professor at MIT, are the senior authors of the study, which appears today in Science Advances. The paper’s lead author is MIT graduate student Morteza Sarmadi.
Microparticle model
Microparticles range in size from 1 to 1,000 microns (millionths of a meter). Many researchers are working on using microparticles made of polymers and other materials to deliver drugs, and about a dozen such drug formulations have been approved by the FDA. However, others have failed because of the difficulty of injecting them.
“The major issue is clogging, somewhere in the system, that doesn’t allow for the full dose to be delivered,” Jaklenec says. “Many of these drugs don’t make it past development because of the challenges with injectability.”
Such drugs are usually injected intravenously or under the skin. Making sure that these drugs successfully reach their destinations is a key step in the drug development process, but it’s one that is often done last, and can thwart an otherwise promising treatment, Sarmadi says.
“Injectability is a major factor in how successful a drug will be, but little attention has been paid to trying to improve administration techniques,” he says. “We hope that our work can improve the clinical translation of novel and advanced controlled-release drug formulations.”
Langer and Jaklenec have been working on developing hollow microparticles that can be filled with multiple doses of a drug or vaccine. These particles can be designed to release their payloads at different times, which could eliminate the need for multiple injections.
To improve the injectability of these and other microparticles, the researchers experimentally analyzed the effects of altering the size and shape of the microparticles, the viscosity of solution in which they are suspended, and the size and shape of the syringe and needle used to deliver them. They tested cubes, spheres, and cylindrical particles of different sizes, and measured the injectability of each one.
The researchers then used this data to train a type of computational model known as a neural network to predict how each of these parameters affect injectability. The most important factors turned out to be particle size, particle concentration in the solution, viscosity of the solution, and needle size. Researchers working on drug-delivering microparticles can simply input these parameters into the model and get a prediction of how injectable their particles will be, saving the time they would have had to spend building different versions of the particles and testing them experimentally.
“Instead of going through the experiments, and going back and forth, having no idea of how successful the system will be, you can use this neural network and it can guide you, early on, to have an understanding of the system,” Sarmadi says.
Injectability boost
The researchers also used their model to explore how changing the shape of the syringe could affect injectability. They came up with an optimal shape that resembles a nozzle, with a wide diameter that tapers toward the tip. Using this syringe design, the researchers tested the injectability of the microparticles they described in a 2017 Science study, and found that they boosted the percentage of particles delivered from 15 percent to almost 90 percent.
“This is another way to maximize the forces that are acting on the particles and pushing the particles toward the needle,” Sarmadi says. “It’s a promising result that shows that there’s huge room for improvement in the injectability of microparticle systems.”
The researchers are now working on designing optimized systems for delivering cancer immunotherapy drugs, which can help stimulate an immune response that destroys tumor cells. They believe these types of microparticles could also be used to deliver a variety of vaccines or drugs, including small-molecule drugs and biologics, which include large molecules such as proteins.
The research was funded by the Bill and Melinda Gates Foundation, the Koch Institute Support (core) Grant from the National Cancer Institute, and a National Institutes of Health Ruth L. Kirschestein National Research Service Award. | | 3:45p |
Flatworms muscle new eyes' wiring into their brains If anything happens to the eyes of the tiny, freshwater-dwelling planarian Schmidtea mediterranea, they can grow them back within just a few days. How they do this is a scientific conundrum — one that Peter Reddien's lab at Whitehead Institute has been studying for years.
The lab's latest project offers some insight: in a paper published in Science June 26, researchers in Reddien's lab have identified a new type of cell that likely serves as a guidepost to help route axons from the eyes to the brain as the worms complete the difficult task of regrowing their neural circuitry.
Schmidtea mediterranea's eyes are composed of light-capturing photoreceptor neurons connected to the brain with long, spindly processes called axons. They use their eyes to respond to light to help navigate their environment.
The worms, which are popular models for research into regeneration, can regrow pretty much any part of their body; eyes are an interesting part to study because regenerating the visual system requires the worms rewire their neurons to connect them to the brain.
When neural systems develop in embryos, the first nerve fibers, called pioneer axons, snake their way through tissue to form the circuitry needed to perceive and interpret external stimuli. The axons are helped along their way by specialized cells called guidepost cells. These special cells are positioned at choice points — places where the axon's path could fork in different directions.
In many organisms, these guidepost cells aren't a priority anymore once development is finished, and typically are not renewed through adulthood. That's one reason why, when humans experience brain or nerve damage, the injury is usually permanent.
“This is a fundamental mystery of regeneration that we hadn't even been thinking about,” says Reddien, the senior author of the paper who is also a professor of biology at MIT and an investigator with the Howard Hughes Medical Institute. “How can an adult animal regenerate a functional nervous system when the original development of the nervous system typically involves a number of cues that are thought to be transient?"
Then, in 2018, Reddien lab scientist Lucila Scimone found something surprising in adult planarians: groups of mysterious cells that looked like they might play a role in guiding growing axons. She'd noticed this group of cells because they co-expressed two genes not often seen together, and some were conspicuously close to the eyes.
“I was captivated by these cells,” she says. They appeared in very small numbers (a normal worm might have around five; a large one might have up to 10) in every planarian she examined. They were divided into two distinct groups: some around the flatworms' eyes, and others spaced out along the path to the brain center. When she traced the path of existing axons leading from the planarians' eyes to their brain, they coincided with the positions of these cells without exception.
When the researchers characterized the cells, they found that they did not express any of the genes that are hallmarks of photoreceptor neurons; instead, they had markers often found in muscle tissue. “That was very striking, because muscle cells — that's not what they do in most animals,” Scimone says.
In other organisms, guidepost cells are often neurons or glia. It would be unusual for muscle cells to serve as guideposts; but past work in the Reddien lab had shown that planarian muscle cells played other special roles, such as secreting the extracellular matrix. The researchers now wondered whether they could add the role of guidepost to the long list of planarian muscle cell functions.
To test their hypothesis, the researchers designed a series of experiments. “We developed an eye transplantation method where you can take an eye from an animal and transplant it into another animal,” says Reddien lab postdoc Kutay Deniz Atabay. “When you do this, the axonal projections from that eye will basically, if positioned appropriately, correctly wire themselves into the brain, producing a functional state.”
The researchers also created genetically engineered planarians that had the muscle cells, but no eyes, and then transplanted eyes onto their eyeless heads. Sure enough, the neurons grew as normal, snaking towards the cells and then adjusting their trajectories after encountering them.
Without the cells, it was a different story. When the researchers transplanted eyes to distant parts of planarians' bodies without a population of these muscle cells, the photoreceptor neurons did not connect to the brain center. Likewise, when they transplanted eyes into planarians that had been modified to not have these muscle cells, their photoreceptor neurons still grew — but they did not wire properly to reach the brain.
These findings combined suggested that the cells were fully independent of the visual system — they did not form because of eyes or photoreceptor neurons, but likely established themselves before the neurons grew — which provided more evidence for the guidepost role.
The guidepost-like activity of these cells then begged the question: how do the cells themselves know where to be? “We found that there's a pattern of signaling molecules in muscle that is setting where these cells should be,” Reddien says. “If we perturb the global positional information of the system, these cells get placed in the wrong positions, and then axons go to the wrong positions — so we think there's a positional information framework that places the cells during regeneration, and that allows them to work as guideposts in the correct locations.”
At this point, the researchers don't know exactly how the cells are able to communicate with growing axons to serve as guideposts. They could be releasing some sort of signaling molecule that attracts the axons, or they could be communicating by using trans-membrane proteins.
“That will be an exciting direction for the future,” Reddien says. “We have now identified the transcriptome for the cells, which means we know all the genes that these cells express. That provides us with an intriguing list of genes that can be probed functionally, to try to see which ones are mediating the functions of these cells.”
This study is a step forward in a body of work that aims to expand the capabilities of regenerative medicine. “Imagine a scenario where someone experiences a spinal cord injury or an eye injury or stroke that leads to the loss of a neural circuit,” says Atabay. “The reason we can't fully cure these cases today is that we lack fundamental information regarding how these systems can regenerate. Looking at regenerative organisms provides a lot of insights. From this case, we see that regenerating the lost system may not be enough; you may also need to regenerate systems that are properly patterning that system.” |
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