Toward a Quantum Internet

A quantum logic gate in an optical fiber could lay the foundation for a quantum computer network.

Entangled Web: The optical components on this lab bench, such as mirrors and filters, allow researchers in Prem Kumar’s lab at Northwestern University to direct and manipulate light. In Kumar’s most recent work, he has created a quantum logic gate within an optical fiber; such gates could eventually enable networks of quantum computers.

Credit: Prem Kumar

The promise of quantum computers is tantalizingly great: near-instantaneous problem solving, and perfectly secure data transmission. For the most part, however, small-scale demonstrations of quantum computation remain isolated in labs throughout the world. Now, Prem Kumar, a professor of electrical engineering and computer science at Northwestern University, has taken a step toward making quantum computing more practical. Kumar and his team have shown that they can build a quantum logic gate--a fundamental component of a quantum computer--within an optical fiber. The gate could be part of a circuit that relays information securely, over hundreds of kilometers of fiber, from one quantum computer to another. It could also be used on its own to find solutions to complicated mathematical problems.
A logic gate is a device that receives an input, performs a logic operation on it, and produces an output. The type of gate that Kumar created, called a controlled NOT gate, has a classical-computing analogue that flips a bit registering a "1" to "0," and vice versa. Quantum logic gates like Kumar's have been built before, but they worked with laser beams that passed through the air, not through fiber. The new gate lays the foundation for experiments that demonstrate the abilities of quantum computers in fiber, says Kumar. "The exciting thing here is that an application is within reach," he says. Within the next year, Kumar and his team plan to test the gate in a specific application: conducting a complex auction over a secure quantum network.
Researchers at IBM, MIT, and many other corporations and universities have been working on quantum computers since they were first proposed in the 1980s. A quantum computer is a device that processes bits of information by exploiting the weird quantum-mechanical properties of particles such as electrons and photons. A quantum computer is theoretically able to process exponentially more information than classical computers can. The unit of information in a classical computer is the bit, which represents either a "1" or a "0"; but in a quantum computer, it's the qubit, which can represent both a "1" and a "0" at the same time. Since qubits compute with multiple values at once, the processing power of a quantum computer doubles with each additional qubit. This characteristic would enable a quantum computer with only a couple hundred qubits to significantly outperform today's best supercomputers.
Kumar's group makes qubits out of photons that are "entangled." That means that their physical characteristics, such as polarization, are linked in such a way that if one photon assumes a particular physical state, the matching photon instantly assumes a corresponding state. A few years ago, Kumar demonstrated that optical fiber itself could cause photons to become entangled, and that they would remain entangled over a distance of 100 kilometers. His recent work, described in Physical Review Letters, goes one step further, creating a logic gate that entangles photon pairs.
To use this gate, Kumar needs photons that are identical in every way except polarization, or the orientation of their electromagnetic fields.These "identical" photons are sent through optical fiber to the gate itself, a small maze of devices that route photons in different directions depending on their polarization. Passing through the maze causes certain photon pairs to become entangled. But not all photons make it through the gate; only when photons reach detectors on the other end, and the researchers can measure whether or not they are entangled, do they know the gate succeeded.
The only way to know whether or not the gate worked is to wait until a collection of photons has been fired at it, says Carl Williams, coordinator of the quantum information program at the National Institute of Standards and Technology. "Most of the time the gate fails," he says. "It's a probabilistic thing." But when the gate fails, the researchers simply disregard the unentangled photons.
"The great thing about this work," says Williams, "is that it's in fiber. This is a big deal because it could lead to distributed networks. ... The obvious application is for long-distance quantum communication between two smaller quantum computers." One of the crucial elements in a conventional optical network is a device called a repeater, which amplifies signals that have degraded over distance. Williams says that a quantum logic gate, such as the one that Kumar built, could be used in a circuit that amplifies a signal without losing the entanglement of the photons.
"This is an important step toward constructing a quantum Internet," says Seth Lloyd, a professor of mechanical engineering at MIT and a leading researcher in quantum computation. "Such a network would have powers that the ordinary Internet does not," he says. "In particular, communication over the quantum Internet would be automatically secure."
Lloyd notes that Kumar's paper illustrates how a simple quantum logic operation can be performed using individual photons. "The current paper represents a significant advance in the technology of quantum computation and quantum networks," he says.

How to Make Graphene

A simple way to deposit thin films of carbon could lead to cheaper solar cells.

Flexible process: A new fabrication method developed by researchers at Rutgers University can deposit a film of graphene--an atom-thick sheet of carbon--on almost any substrate, including the flexible plastic shown here. The films could be used in thin-film transistors or as conductive electrodes for organic solar cells.

Credit: Manish Chhowalla, Rutgers University

Graphene--a flat single layer of carbon atoms--can transport electrons at remarkable speeds, making it a promising material for electronic devices. Until recently, researchers had been able to make only small flakes of the material, and only in small quantities. However, Rutgers University researchers have developed an easy way to make transparent graphene films that are a few centimeters wide and one to five nanometers thick.
Thin films of graphene could provide a cheap replacement for the transparent, conductive indium tin oxide electrodes used in organic solar cells. They could also replace the silicon thin-film transistors common in display screens. Graphene can transport electrons tens of times faster than silicon, so graphene-based transistors could work faster and consume less power. (See "Graphene Transistors" and "Better Graphene Transistors.")
In fact, Rutgers materials science and engineering professor Manish Chhowalla and his colleagues used their graphene films to make prototype transistors and organic solar calls. In a recent Nature Nanotechnology paper, they showed that they can deposit the transparent films on any substrate, including glass and flexible plastic. Chhowalla says that the method could be adapted to a larger scale to coat "meters and meters of substrates with graphene films," using roll-to-roll processing, a technique being developed to make large flexible electronic circuits.
By contrast, current techniques for making graphene yield small quantities of the material, fit only for experimental use. One common technique is called the "Scotch tape method," in which a piece of tape is used to peel graphene flakes off of a chunk of graphite, which is essentially a stack of graphene sheets. This results in micrometer-sized graphene fragments, which are placed between electrodes to make a transistor. "But if you talk about large-scale devices, you want to make macroscopic [sheets]," says Hannes Schniepp, a graphene researcher at Princeton University. For that, you need to guide the assembly of smaller graphene pieces over a large area, Schniepp says, which is exactly what the Rutgers researchers do.
The researchers start by making a suspension of graphene oxide flakes. They oxidize graphite flakes with sulphuric or nitric acid. This inserts oxygen atoms between individual graphene sheets and forces them apart, resulting in graphene oxide sheets, which are suspended in water.
The suspension is filtered through a membrane that has 25-nanometer-wide pores. Water passes through the pores, but the graphene oxide flakes, each of which is a few micrometers wide and about one nanometer thick, cover the pores. This happens in a regulated fashion, Chhowalla says. When a flake covers a pore, water is directed to its uncovered neighbors, which in turn get covered, until flakes are distributed across the entire surface. "The method allows you to deposit single layers of graphene," Chhowalla says. "[It] results in a nearly uniform film deposited on the membrane." The researchers place the film-coated side of the membrane on a substrate, such as glass or plastic, and wash away the membrane with acetone. Finally, they expose the film to a chemical called hydrazine, which converts the graphene oxide into graphene.
a chemistry professor at Rice University, says that this is "certainly the easiest method I've seen for making [graphene thin films] over large areas." He thinks that the process could easily be converted into a larger, commercial-scale manufacturing technique. "It's very amenable for rapid production," he says. "It's not going to take much to get these things produced ... and cover large areas."
Chhowalla and his colleagues control the thickness of the film by changing the suspension's volume. A volume of 20 milliliters results in a film that is mostly one to two nanometers in thickness, while an 80-milliliter suspension results in films that are mainly three to five nanometers thick. The thinner films are 95 percent transparent. The researchers have used the films as the transparent electrodes in organic solar cells. They have also made transistors by placing their films on a silicon substrate and depositing gold electrodes on them.
The graphene films need a lot more work. Right now, the transistors do not carry as much current as those made from individual graphene flakes, which, the researchers speculate, is because of overlapping flakes in their films. For high-quality transistors, they will need to make single-layer graphene films with no overlap. They also need to improve the conductivity of their film: indium tin oxide is still hundreds of times more conductive. Organic solar cells with indium tin oxide electrodes are between 3 percent and 5 percent efficient. "With graphene thin-film electrodes, we get 0.1 percent," Chhowalla says, "but these are proof-of-concept devices and of course will improve with time."
Tour believes that the film holds more promise for organic solar cells than for transistors. Many researchers are also studying carbon nanotube films as a way to replace indium tin oxide coatings on solar cells. But Tour says that graphene would be "possibly easier than using carbon nanotubes because of the greater availability of the material." The industry might also find it easier to adopt graphene because of the concerns that some people have about the effects of carbon nanotubes on the environment.

The $100 Genome

Forget the $1,000 genome. Some companies are looking far past that goal to create a really inexpensive sequencing technology

Cheap sequence: Threading long DNA molecules through nano-sized channels on a specially fabricated chip could provide a cheaper way to sequence DNA. This image shows a wafer developed by BioNanomatrix. Each rectangle is a nanoanalyzer chip lined with 50,000 channels.

Credit: BioNanomatrix

It currently costs roughly $60,000 to sequence a human genome, and a handful of research groups are hoping to achieve a $1,000 genome within the next three years. But two companies, Complete Genomics and BioNanomatrix, are collaborating to create a novel approach that would sequence your genome for less than the price of a nice pair of jeans--and the technology could read the complete genome in a single workday. "It would have been absolutely impossible to think about this project 10 years ago," says Radoje Drmanac, chief scientific officer at Complete Genomics, which is based in Mountain View, CA.
The most recent figures for sequencing a human genome are $60,000 in about six weeks, as reported by Applied Biosystems last month. (That's down from $3 billion for the Human Genome Project, which was sequenced using traditional methods and finished in 2003, and about $1 million for James Watson's genome, sequenced using a newer, high-throughput approach and released last year.) But scientists are still racing to develop methods that are fast and cheap enough to allow everyone to get their genomes sequenced, thus truly ushering in the era of personalized medicine.
Most existing technologies detect the sequence of DNA a single letter at a time. But Complete Genomics aims to speed the process by detecting entire "words," each composed of five DNA letters. Drmanac likens the technology to Google searches, which query a database of text with keywords. Further speeding up the process with novel chemistry and advances in nanofabrication, the companies will develop a device that can simultaneously read the sequence of multiple genomes on a single chip.
To accomplish the new sequencing, scientists first generate all possible combinations of five-letter DNA segments, given the four letters, or bases, that make up all DNA. These segments are labeled with different types of fluorescent markers and added in groups to a single-stranded molecule of DNA. When a particular segment matches a sequence on the strand of DNA to be read, it binds to that part of the molecule. A specialized camera then snaps a picture--the different fluorescent signals indicate the sequence at specific points along the strand of DNA. The process is repeated with different five-letter DNA combinations, until the entire chromosome is sequenced. The approach is feasible because of the recent availability of cheap DNA synthesis, making it much more efficient to generate libraries of these DNA segments.
Each DNA molecule will be threaded into a nanofluidics device, made by Philadelphia-based BioNanomatrix, lined with rows of tiny channels. The narrow width of the channels--about 100 nanometers--forces the normally tangled DNA to unwind, lining up like a train in a long tunnel and giving researchers a clear view of the molecule. "Since we can stretch out DNA, we can get a huge amount of information from each piece of DNA we look at," says Mike Boyce-Jacino, chief executive officer of BioNanomatrix. "The big difference from any other approach is that we are looking at physical location at the same time we are looking at sequence information." Sequencing methods currently in use sequence small fragments of DNA and then piece together the location of each fragment computationally, which is more time consuming and requires repetitive sequencing.

The companies still have a long road to the $100 genome. BioNanomatrix has already shown that long pieces of DNA--two million letters in length--can be threaded into the channels of existing chips. But now researchers need to develop chips with many more channels, so that multiple genomes' worth of DNA can be sequenced simultaneously.
The main hurdle for Complete Genomics will be to generate fluorescent labels that can be easily and accurately detected. Most current methods get over this problem by making many copies of the same DNA molecule and sequencing them simultaneously, thus boosting the signal to noise ration. But that approach limits the length of the piece of DNA that can be sequenced, and it increases cost by increasing the amount of chemicals needed for the reaction.
The project is part of the Advanced Technology Program, funded by the National Institute of Standards and Technology to spur development of novel, high-risk technologies. This year, Complete Genomics is releasing a commercial product based on similar chemistry, but the company has declined to give details on its status.
The technology necessary to achieve a $100 genome is still at least five years away, says George Church, a geneticist at Harvard Medical School, in Boston, and a member of Complete Genomics' scientific advisory board. "But [it's] coming from a company that has an almost-as-good technology coming out this year."
Both Drmanac and Boyce-Jacino say that one of the biggest advantages of their technology will be the ability to sequence very long strands of DNA. The newest sequencing technologies in use today read DNA in fairly short spurts, from about 30 to 200 letters, which are then stitched together by a computer. This approach works well for some applications, such as resequencing a known genome. But a growing number of studies suggest that the small structural changes in DNA, such as deletions or inversions of short sequences, play a significant role in human variability, says Jeff Schloss, program director for technology development at the National Human Genome Research Center, in Bethesda, MD. "Those are much harder to pick up with short reads."
Longer reads will also allow scientists to look at collections of genetic variations that have been inherited together, known as haplotypes. This kind of analysis can determine if a particular genetic variation has been passed down from the individual's mother or father. Recent research suggests that in some cases, maternal or paternal inheritance can impact the severity of the disease. With new tools to better track inheritance patterns, scientists may discover that this phenomenon is more common than previously thought. "That's one reason we're hoping that several of the emerging methods will allow long reads," says Schloss.

Corn Primed for Making Biofuel

Researchers genetically modify a crop to break down its own cellulose.

Shoots and leaves: To facilitate the breakdown of cellulose into fermentable sugars for making ethanol, Mariam Sticklen of Michigan State University is genetically modifying corn with genes that produce cellulose-degrading enzymes in the plant’s stems and leaves. The enzymes are activated only after the corn is harvested, when the plant is ground up.

Credit: Michigan State University

In an effort to help boost the nation's supply of biofuels, researchers have created three strains of genetically modified corn to manufacture enzymes that break down the plant's cellulose into sugars that can be fermented into ethanol. Incorporating such enzymes directly into the plants could reduce the cost of converting cellulose into biofuel.
Last year, new federal regulations called for production of renewable fuels to increase to 36 billion gallons annually--nearly five times current levels--by 2022. Today, nearly all fuel ethanol in the United States is produced from corn kernels. To meet the required increase, researchers are turning to other sources, such as cellulose, a complex carbohydrate found in all plants. Corn leaves and stems, prairie grasses, and wood chips are leading candidates for supplies of cellulose. Cellulosic ethanol has many advantages over that produced from corn kernels. Cellulose is not only extremely abundant and inexpensive; studies also suggest that the production and use of ethanol from cellulose could yield fewer greenhouse gases.
However, the biggest obstacle to making cellulosic ethanol commercially feasible is the breakdown of cellulose. Enzymes that degrade cellulose, called cellulases, are typically produced by microbes grown inside large bioreactors, an expensive and energy-intensive process. "In order to make cellulosic ethanol really competitive, we really need to bring those costs down," says Michael J. Blaylock, vice president of system development at Edenspace, a crop biotechnology firm based in Manhattan, KS.
Mariam Sticklen, professor of crop and soil science at Michigan State University, in East Lansing, figured that she could eliminate the cost of manufacturing enzymes by engineering corn plants to produce the enzymes themselves. Instead of relying on the energy-intensive process of producing them in bioreactors, "the plants use the free energy of the sun to produce the enzymes," she says.
Typically, the breakdown of cellulose requires three different cellulases. Last year, Sticklen reported modifying corn with a gene for a cellulase that cuts the long cellulose chains into smaller pieces. The gene came from a microbe that lives in a hot spring. A month later, Sticklen inserted a gene derived from a soil fungus into the corn genome. That gene codes for an enzyme that breaks the smaller pieces of cellulose into pairs of glucose molecules. In this latest effort, Sticklen has modified corn to produce an enzyme that splits the glucose pairs into individual sugar molecules; the enzyme is naturally produced by a microbe that lives inside a cow's stomach. The final result: three strains of corn, each of which produces an enzyme essential to the complete breakdown of cellulose.
To avoid the possibility of transferring the genes to other crops or wild plants, the enzymes are only produced in the plant's leaves and stems, not in its seeds, roots, or pollen, says Sticklen. What's more, to prevent the corn from digesting itself, she engineered the plants so that the enzymes accumulate only in special storage compartments inside the cells, called vacuoles. The cellulases are released only after the plant is harvested, during processing. Sticklen described her modified crops last week at the American Chemical Society's national meeting in New Orleans.

Although it's possible to incorporate all three genes in a single plant, says Sticklen, using three different varieties of corn, each carrying a different gene, will allow her to control the conversion of cellulose into sugars. Preliminary studies show that the enzymes are just as efficient as commercially available enzymes when combined at a ratio of 1:4:1, she says. The results suggest that mixing the three different plants using the same ratios will provide the best outcome.
"I think the strategy of compartmentalizing the enzymes in the vacuoles is terrific," says Susan Leschine, a microbiologist at the University of Massachusetts Amherst. "The question I have is, do the enzymes work under conditions that are realistic?" For instance, different microbe species secrete their own cellulases that work synergistically to chip away at the cellulose fibers. It's unclear, Leschine says, how well an enzyme taken from a microbe that lives in a hot spring will work with an enzyme drawn from a soil fungus. "These different enzymes may not be active under the same conditions," she says.
Edenspace, which is currently developing Sticklen's technology, expects to begin field trials of her genetically modified corn within the year, with the goal of commercializing the technology within the next three years, says Blaylock. The company is not alone in pursuing this strategy: Agrivida, an agricultural biotech company based in Medford, MA, is also genetically modifying corn to simplify the production of cellulosic ethanol.
"This really is a worthwhile path to follow," says Michael Ladisch, professor of agricultural and biological engineering at Purdue University, in West Lafayette, IN. "However, at the end of the day, it's more complicated than it seems." The main obstacle is finding ways to ensure that the enzymes will survive the chemical and physical pretreatment needed to remove the lignin--the tough polymer in cell walls that provides plants with strength--from the cellulose fibers, says Ladisch, who is currently on leave from Purdue to serve as the chief technical officer at Mascoma, a biofuels company based in Brighton, MA.
One solution is to engineer the plants so that they require only a mild pretreatment. For instance, Sticklen is working on reducing the amount of lignin contained in corn, as well as modifying the molecular configuration of lignin, which would make it easier to break down. Although her work is currently focused on modifying corn, Sticklen says that the technology could eventually be transferred to other crops as well, such as switchgrass.