Making Quantum Practical

Kate Greene writes in the latest issue of MIT Technology Review about researchers who have succeeded in combining quantum signals with classical optical signals in a conventional fiber-optic line.


Coursing through the fiber-optic veins of the Internet are photons of light that carry the fundamental bits of information. Depending on their intensity, these photons represent bits as 1s and 0s. This on-and-off representation of information is part of what physicists call "classical" phenomena.

But photons of light have "quantum" properties as well, which, when exploited, provide more than simply a 1 or 0; these properties allow photons to represent 1s and 0s simultaneously. When information is approached from a quantum perspective, say scientists, encryption can be perfectly secure and enormous amounts of information can be processed at once.

This field of quantum information –- the transmission and processing of data governed by quantum mechanics –- is rapidly moving beyond the lab and into the real world. Increasingly, researchers are conducting experiments within the same commercial fiber that transmits information in the classical way. For the most part, though, the two types of information have not intermingled: quantum information has been sent only over dedicated fiber.

Now researchers at Northwestern University have shown that quantum information, in the form of "entangled photons," can travel over the same fiber as classical signals. Additionally, the researchers have sent the combination signal through 100 kilometers of fiber -- a record distance for entangled photons even without the classical signal.

This marriage of quantum and classical optics shows that traditional optical tools can be used to send quantum encryption keys, based on entangled photons (some other schemes rely on single photons). In the future, this new technique might also enable long-distance networking between quantum computers, says Carl Williams, coordinator of the Quantum Information Program at the National Institute of Standards and Technology in Gaithersburg, MD.

At the heart of the Northwestern experiment are the entangled photons: pairs of photons with interconnected properties. That is, looking at one photon in an entangled pair will reveal what the result of looking at the other photon would be -- no matter how far apart the photons are. Entangled photons can be used in encryption by encoding information about a key in the photons. Then if an eavesdropper intercepts one photon of the entangled pair, the entire transmission is altered, alerting the code makers.

Furthermore, entangled photons used for quantum computing could be split up and shared across a network of many quantum computers. Such photon pairs are "important whether the application is cryptography or anything else," says Prem Kumar, professor of electrical and computer engineering and physics at Northwestern and lead scientist on the project.

The first step in the experiment, then, was for the researchers to create entangled photons. Traditionally, shining laser light into a type of crystal has produced entangled photons. But it's been difficult to use entangled photons made from crystals, because in transferring them into a fiber, you "lose the quality of the entanglement," says Williams.

Instead, Kumar's team created their photon pairs by exploiting a similar, recently developed process that can occur within long lengths of standard fiber. The photons start in fiber and remain in it for the duration of the experiment, retaining their entanglement properties.

The researchers pulsed polarized laser light through 300 meters of coiled fiber. It is this property of polarization (the orientation of the photons) that allows it to become entangled when the pairs of photons are created: if the polarization of one photon is measured, the polarization of the other photon is instantly known. Within the fiber, about one pair of polarization-entangled photons is created every microsecond, Kumar says, and the rate can be increased 100-fold by pulsing the light faster, he adds.

Next, the entangled photons are split apart and each is directed into 50 kilometers of fiber (for a total of 100 kilometers), where they join a classical signal. At the opposite ends of the fibers, the photons are separated from the communication signals, and shoot towards two different photon detectors, built to see one photon at a time. Kumar says he knows he's successfully sent entangled photons when both detectors see certain types of polarized photons at the same time.

There are still challenges to using traditional fiber-optic cable and sending entangled photons 100 kilometers. Even the best quality commercial fiber has very small geometric inconsistencies, Kumar says, which can alter the polarization of the photon pairs slightly, decreasing the quality of entanglement -- and rendering the quantum information useless.

These slight changes in polarization can usually be adjusted for by sending the photons through special polarization devices right before they hit the detector, but it is difficult to know exactly how to adjust these devices to best compensate for the change in polarization. Interestingly, Kumar adds, the classical signal traveling with the quantum signal, as in the experiment, can help. It can track imperfections in the fiber encountered by the entangled photon, and relay this information so the polarization control device can be set to compensate appropriately.

Right now, Kumar's team is working on testing the distance limits of entangled photon transport and determining how many more classical signals they can add to the line and still retrieve the quantum information stored in the entangled photons. Because in real-world fiber optics, multiple signals pass through at once, it would be useful to know how many classical signals can share the fiber with a quantum signal.

According to other scientists working in the field of quantum information, the fact that Kumar's team has combined fiber-generated entangled photons with classical information, and sent the total signal over a record distance in a traditional fiber line is an exciting advance. "Pieces have been shown, but this puts it all together," says Williams, who calls it "a remarkable demonstration."

Jeffrey Shapiro, professor of electrical engineering at MIT, says it is "great work...Prem [Kumar] works both on classical and quantum communication, and is one of the people who's well suited to address both sides."

Ultimately, as quantum information matures, it will become more integrated into traditional fiber technology, says Kumar. "My goal is to make quantum optics applicable," he notes. "Fiber-based quantum optics can piggyback on billions of dollars in optical communications technology. We want to ride that wave."

The researchers pulsed polarized laser light through 300 meters of coiled fiber. It is this property of polarization (the orientation of the photons) that allows it to become entangled when the pairs of photons are created: if the polarization of one photon is measured, the polarization of the other photon is instantly known. Within the fiber, about one pair of polarization-entangled photons is created every microsecond, Kumar says, and the rate can be increased 100-fold by pulsing the light faster, he adds.

Next, the entangled photons are split apart and each is directed into 50 kilometers of fiber (for a total of 100 kilometers), where they join a classical signal. At the opposite ends of the fibers, the photons are separated from the communication signals, and shoot towards two different photon detectors, built to see one photon at a time. Kumar says he knows he's successfully sent entangled photons when both detectors see certain types of polarized photons at the same time.

There are still challenges to using traditional fiber-optic cable and sending entangled photons 100 kilometers. Even the best quality commercial fiber has very small geometric inconsistencies, Kumar says, which can alter the polarization of the photon pairs slightly, decreasing the quality of entanglement -- and rendering the quantum information useless.

These slight changes in polarization can usually be adjusted for by sending the photons through special polarization devices right before they hit the detector, but it is difficult to know exactly how to adjust these devices to best compensate for the change in polarization. Interestingly, Kumar adds, the classical signal traveling with the quantum signal, as in the experiment, can help. It can track imperfections in the fiber encountered by the entangled photon, and relay this information so the polarization control device can be set to compensate appropriately.

Right now, Kumar's team is working on testing the distance limits of entangled photon transport and determining how many more classical signals they can add to the line and still retrieve the quantum information stored in the entangled photons. Because in real-world fiber optics, multiple signals pass through at once, it would be useful to know how many classical signals can share the fiber with a quantum signal.

According to other scientists working in the field of quantum information, the fact that Kumar's team has combined fiber-generated entangled photons with classical information, and sent the total signal over a record distance in a traditional fiber line is an exciting advance. "Pieces have been shown, but this puts it all together," says Williams, who calls it "a remarkable demonstration."

Jeffrey Shapiro, professor of electrical engineering at MIT, says it is "great work...Prem [Kumar] works both on classical and quantum communication, and is one of the people who's well suited to address both sides."

Ultimately, as quantum information matures, it will become more integrated into traditional fiber technology, says Kumar. "My goal is to make quantum optics applicable," he notes. "Fiber-based quantum optics can piggyback on billions of dollars in optical communications technology. We want to ride that wave."

Cure for cancer worth $50 trillion

Finding a cure for cancer would be worth about $50 trillion, according to a study by University of Chicago Graduate School of Business economists.

The social value of improved health and longevity is the amount in dollars that additional life years or other health improvements are worth to people, the study report said. The value of improved longevity is based on what individuals gain from the enjoyment of consumption and time during an additional year of life, rather than how much they earn.

"Since the benefits of cancer research are large, substantially greater research expenditures would be worthwhile," authors Kevin Murphy and Robert Topel wrote. "A war on cancer that would spend an additional $100 billion on research and treatment may be worthwhile even if it had a one-in-five chance of reducing mortality by just one percent," they said.

During the 20th century, average life expectancy of Americans increased by 30 years, due in large part to medical advances against major diseases, according to the new study titled "The Value of Health and Longevity," to be published in the Journal of Political Economy.

The authors estimate that this increase in life expectancy is worth more than $1.2 million for each American alive today.

From 1970 to 2000, gains in life expectancy added about $3.2 trillion per year to national wealth, the study found.

Increased spending on medical research and cost containment are complementary goals, the report said. "If there is effective cost-containment via cost-effective research spending, then the value of research rises dramatically.

"Ideally, enhanced research funding would be combined with a delivery system that keeps an eye of cost effectiveness.

"The lesson of the last 50 years is the need to address the issue of medical research that will continue to extend longevity without breaking the bank," they said. "A system that better prices medical care may involve people paying a larger percentage of the cost of their own treatment, or enhanced insurance arrangements that allow us to have more effective cost containment."

Light Speed Computers

Laser chips could power petaflop computers

Laser communications chips capable of pumping data through the veins of gargantuan "petaflop" supercomputers have been demonstrated by NEC in Japan.

The communications chips can transfer information through optical fibres at a blistering 25 gigabits per second (a gigabit is a billion bits). This is a record for such components, according to NEC, and is many times faster that the purely electronic interconnects used in today's supercomputers.

Communications chips can convert electronic signals into optical ones. Using optical fibres to relay data between the chips is what may give this type of supercomputer the edge over previous ones using processors connected electronically.

NEC used a type of semiconducting laser diode called a Vertical-Cavity Surface Emitting Laser (VCSEL) which generates laser pulses in response to an electrical current. Researchers at the company created more efficient VCSEL devices by making the diodes from a blend of gallium arsenide and indium gallium arsenide - they used indium instead of the more conventional aluminium. This made it possible to transfer laser pulses more rapidly through optical fibre.

The new VCSEL chips could be used to make supercomputers of unprecedented power by routing data more efficiently between thousands of individual computer processors. NEC believes the chips could prove crucial to the development of the first petaflop class supercomputer - a machine capable of carrying out a thousand trillion mathematical calculations every second.

"Petaflop-class performance can be achieved in the next-generation supercomputer installed with the new VCSEL, in about 2010," Takahiro Nakamura from NEC's System Devices Research Laboratories told New Scientist.
Off-the-shelf

Such an achievement might enable NEC to regain the supercomputing crown that it held between 2002 and 2004 with the Earth Simulator - a supercomputer installed at the Japanese Agency for Marine-Earth Science and Technology in Yokohama, Japan. This is because efficiency with which purely electronic chips share data is a crucial bottleneck in supercomputer design. Most of today's supercomputers operate at a maximum speed of few teraflops (trillions of operations per second).

Many supercomputers are essentially made by linking up thousands of off-the-shelf computer processors. However, the current number one, an IBM machine called BlueGene at Lawrence Livermore National Laboratory in California, US, is made from customised components and is capable of a fearsome 360 teraflops.

While external experts agree that VCSEL chips could be used to construct formidable supercomputers, they say the cost of such components will also be crucial. "Raw bandwidth alone is not necessarily the most pressing issue for petascale computing," says John Shalf at Lawrence Berkeley National Laboratory in California, US. "The question is whether we can afford such components."

Furthermore, although VCSEL chips promise to be cheaper than comparable optical technologies - such as indium phosphide lasers - Shalf says a cheaper solution could be to combine several electronic connection channels in a single data "pipe". Another approach may be to send several optical signals through the same cable, a technique known as wavelength division multiplexing.
Unprecedented complexity

"The ability to go to 25 gigabits per second using VCSELs provides some opportunities for more cost-effective components, but that remains to be seen," Shalf told New Scientist. "You can be assured the market will provide the answer when these things become real products."

Horst Simon, another supercomputing expert at Lawrence Berkeley, adds that other issues will affect the development of the next generation of supercomputers. "Building a petascale system with a useful amount of memory - say at least 200 terabytes - and then powering and cooling this system will be the bigger challenges," he says.

Regardless of the challenges ahead, NEC is confident that petaflop supercomputers will be able to carry out experiments of unprecedented complexity. "It will be able to carry out entire simulation of the human body from genes and cell level to the organs and even the entire body," Nakamura adds. "Complex and detailed simulation of the behaviour of nano materials, from elementary particle to device level, is planned."