Quantum Computing Gets Boost From 'Entanglement' Of Atom Pairs

Physicists at the Commerce Department's National Institute of Standards and Technology (NIST) have taken a significant step toward transforming entanglement--an atomic-scale phenomenon described by Albert Einstein as "spooky action at a distance"--into a practical tool. They demonstrated a method for refining entangled atom pairs (a process called purification) so they can be more useful in quantum computers and communications systems, emerging technologies that exploit the unusual rules of quantum physics for pioneering applications such as "unbreakable" data encryption.

The NIST work, reported in the Oct. 19, 2006, issue of Nature,* marks the first time atoms have been both entangled and subsequently purified; previously, this process had been carried out only with entangled photons (particles of light). The NIST demonstration also is the first time that scientists have been able to purify particles nondestructively. Direct measurement would destroy the delicate entangled state of atom pairs; the new experiment gets around this problem by entangling two pairs of atoms and measuring only one pair.

Entanglement is a curious property of quantum physics that links the condition and behavior of two or more particles, such as atoms or photons. Entanglement can occur spontaneously when two atoms interact. For the initial interaction, the atoms have to be in close proximity, but the entanglement may persist even if they are physically moved apart. The quality of the entanglement can be degraded by many environmental factors, such as fluctuating magnetic fields, so the process and the transport of entangled particles need to be tightly controlled in technological applications. The purification process implemented at NIST can clean up or remove any distortions or "noise" regardless of the source by processing two or more noisy entangled pairs to obtain one entangled pair of higher purity.

"We demonstrated entanglement purification with relatively high success rates in an ion trap system that could be scaled up to build quantum computers of a practical size," says Dietrich Leibfried, an author of the paper and designer of the experiment. "It's a more complicated procedure than anything we've demonstrated before, and it will be useful in many contexts once we improve our purification procedures."

The NIST team used ultraviolet lasers to entangle two pairs of beryllium ions (electrically charged atoms) in an electromagnetic trap. A similar process was used to cross-entangle the entangled pairs--that is, to entangle each member of the first pair with its counterpart in the second pair. Then the first pair of ions was measured, and the results were used as an indication of whether the second pair (unmeasured, and thus with its quantum state intact) was entangled with higher purity. Additional tests were performed to verify that the quality of the entanglement had indeed improved.

The reported purification rate is a record (although the entangled state is not yet pure enough for use in a working computer or other device) with more than one success for every three attempts, compared to one in a million in the photon experiments. Theoretically, the NIST process could be enhanced and then repeated as many times as necessary to create a stream of near-perfectly entangled pairs in a computer or network. The NIST team's continuing research aims to substantially improve the purification operations through, for example, improved control of magnetic fields and laser intensity.

The same NIST group previously has demonstrated at a rudimentary level all the basic building blocks for a quantum computer, including key processes such as error correction and, most recently, a mass-producible ion trap. Ions are among the most promising of a dozen or so candidates for quantum bits (qubits) to store, manipulate, and transport quantum information.

Quantum computers, if they can be built, could break today's best public-key encryption systems, used to protect commercial communications. Quantum communications systems, if well designed, provide a new approach to "unbreakable" encryption to keep messages secret. Quantum computers also potentially could be used to optimize complex systems such as airline schedules, accelerate database searching, and develop novel products such as fraud-proof digital signatures.

Entanglement could have many uses in large quantum computers and networks. For example, it is required for "teleportation" of information (http://www.nist.gov/public_affairs/releases/teleportation.htm), a process that could be used to rapidly transfer data between separate locations in quantum computer, or to detect and correct minor operational errors (http://www.nist.gov/public_affairs/releases/quantum_computers.htm). Entangled photons are used in various forms of quantum cryptography, and are the clear choice for long-distance communication.

Purification is crucial because particles can be entangled initially only when they are close together, and the link degrades as the particles are moved apart. The NIST process could be used, for example, to purify entangled ions before transfer of information to photons in large networks. Most long-distance quantum communication schemes require data transfer from storage qubits to transport qubits. "If someone comes up with an interface for efficiently transferring information from ions to photons, then ions could be used for purification and photons for transport," Leibfried says.

Funding for the research was provided in part by the Disruptive Technology Office, an agency of the U.S. intelligence community that funds unclassified research on information systems.

More information about NIST research on quantum computers and communications is available at http://www.nist.gov/public_affairs/quantum/quantum_info_index.html.

As a non-regulatory agency of the Commerce Department's Technology Administration, NIST promotes U.S. innovation and industrial competitiveness by advancing measurement science, standards and technology in ways that enhance economic security and improve our quality of life.

New Computer Program Prevents Crashes And Hacker Attacks

Today's computers have more than 2,000 times as much memory as the machines of yesteryear, yet programmers are still writing code as if memory is in short supply. Not only does this make programs crash annoyingly, but it also can make users vulnerable to hacker attacks, says computer scientist Emery Berger from the University of Massachusetts Amherst.

With such problems in mind, Berger created a new program that prevents crashing and makes users safer, he says. Dubbed DieHard, there are versions for programs that run in Windows or Linux. DieHard is available free for non-commercial users at www.diehard-software.org.

Berger developed DieHard together with Microsoft researcher Ben Zorn. Berger has received a $30,000 grant from Microsoft, a $30,000 grant from Intel, and a $300,000 grant from the National Science Foundation for his work on DieHard.

Almost everything done on a computer uses some amount of memory--each graphic on an open Web page, for example--and when a program is running, it is constantly requesting small or medium chunks of memory space to hold each item, explains Berger. He likens the memory landscape to a row of houses, each with only enough square footage for a certain number of bytes. The problem, says Berger, is that sometimes when memory real estate is requested, programs can unwittingly rent out houses that are already occupied. They also might request a certain amount of square footage when they actually need more, so an item can spill over into another "house." These mistakes can make programs suddenly crash, or worse.

"Ironically, crashing is the best thing that can happen," says Berger. "An overflow also can make your computer exploitable by hackers."

One way that the computer becomes more vulnerable results from the fact that "addresses" that are designated for a password, for example, will be on the same lot on the same street in every version of the program. So if a hacker overwrites a password, he or she can easily locate the password address on any of the umpteen versions of the program that are out there.

DieHard presents several remedies to such problems. First, it takes a compact row of memory buildings and spreads them around in the landscape. It also randomly assigns addresses--a password that has a downtown address in one session may be in the suburbs next time around. And in some versions of the program, DieHard will secretly launch two additional versions of the program the user is running--if a program starts to crash, that buggy version gets shut down and one of the other two is selected to remain open. DieHard can also tell a user the likelihood that they'll have been affected by a particular bug.

These problems wouldn't arise if programmers were a little less focused on speed and efficiency, which is rarely a problem these days, and more attentive to security issues, says Berger.

"Today we have way more memory and more computer power than we need," he says. "We want to use that to make systems more reliable and safer, without compromising speed."

The First Molecular Keypad Lock

How can defense or intelligence agencies safeguard the security of top-secret data protected by a computation device the size of a single molecule?

With cryptography approaching that sobering new era, scientists in Israel are reporting development of what they term the first molecular system capable of processing password entries. Abraham Shanzer and colleagues describe their "molecular keypad lock" in the Jan. 17 issue of the weekly Journal of the American Chemical Society.

Electronic keypad locks long have been fixtures on home security systems and other devices that require a password. The new study, however, describes a keypad lock based on molecules that fluoresce only in response to the correct sequences of three input signals.

"By harnessing the principles of molecular Boolean logic, we have designed a molecular device that mimics the operation of an electronic keypad, a common security circuit used for numerous applications in which access to an object or data is to be restricted to a limited number of persons," the researchers state. "The development of a molecular-scale keypad lock is a particularly attractive goal as it represents a new approach to protecting information at the molecular scale."

The researchers cite DNA-microdot encryption as a complementary approach, which in combination with their molecular lock might provide an unbreakable protection against forgery.

Virtual Man Helps Guide Doctors

Scientists have created a computer program that's able to turn medical images into a full dimensional view of a patient's body — inside and out. The CAVEman (it's rendered in a virtual reality room known as a CAVE) will provide not only a three-dimensional view of a body and its organs but also show systems in motion, such as the beating of a heart.

The technology could give doctors or medical students the opportunity to investigate disease or try new treatments on a living system, without opening up an actual person.

"We can render a whole new scenario that would look like what you would experience if you had a patient," said Christoph Sensen, professor of biochemistry and molecular biology and director of the Sun Center of Excellence for Visual Genomics at the University of Calgary, Alberta.
The system has two main components: the computer model that renders the full-dimensional body and the virtual reality environment.

The model is based on images from basic anatomy textbooks. Graphic artists used those images to create animations of various body systems, including the organs, the blood vessel network and the nervous system. The model serves as a kind of generic template of the human body.
This model can be customized to a particular person when merged with medical images. For example, a doctor can combine CT scans or MRI's of a patient's heart or kidney with the model and see the person's organs within the virtual body.

Viewing takes place in the CAVE, a room about 33 feet by 33 feet square. Animated images are projected onto three of the four walls, plus the floor. The viewer wears special glasses equipped with shutters that flicker on and off, allowing each eye to see only one image at a time. This creates the optical illusion of dimension.

A sensor in the eyeglasses works with a tracking device on the computer to locate the viewer, should he walk around the virtual body. A joystick allows the viewer to do things like rotate the body or make menu selections.
Because the system requires the expensive, virtual reality room, Sensen and his team also developed a program that works on a laptop. The dimensionality remains, but immersion is lost. "I think it's a tremendous effort to build such a model. Before this, there were some simple models of the human body available for animation purposes in the movie industry, for example," said Anton Koning, research scientist in the department of bioinformatics at the Erasmus Medical Center, in Rotterdam, The Netherlands.

But this is only a start, he said, pointing out, "The real challenge comes when mapping data on that body."

For example, if a researcher wanted to show all of the tissue in which a certain gene or protein is expressed, showing one may be fairly straightforward. But revealing multiple genes becomes complicated. The information overlaps and may not be visually intuitive to the viewer, said Koning.
Sensen and his team have finished the adult male model and are currently working on finishing the female version. After that, they will complete a model of a child as well as other organisms, including mice and rats.

They are also spinning off a company based on the technology and plan to announce its existence sometime this summer.