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30 January 2002

January 2002

�SIGNAL Magazine 2002

Quirks in Nature Enhance Global Positioning System

Quantum properties may improve precision of object locators while adding security.

By Sharon Berry

Laser-based position location systems are entering a new era that is based on quantum mechanics. The research could lead to the dawn of technologies such as entangled lasers that surpass a fundamental limit on the accuracy of classical systems and add a built-in cryptographic capability.

Traditional positioning procedures send electromagnetic pulses through space and determine their times of arrival at specified points as well as the arrival times of return signals. Because the speed of light is constant, this procedure can be used to synchronize clocks at distant reference points and precisely calculate the location of objects in relation to them. But, the accuracy of this approach is limited by fluctuations caused by differences in power and bandwidth. Therefore, researchers are creating a quantum version of the system, known as the quantum positioning system (QPS), to overcome those limitations.

According to Lorenzo Maccone and Vittorio Giovannetti, postdoctoral associates at the Massachusetts Institute of Technology (MIT) in Cambridge, the precision of measuring a light pulse's travel time depends on the spectrum--the bandwidth of the pulse, and on the power--the number of photons per pulse. Because pulses sent at different wavelengths travel at different speeds, the wider the frequency range, the less accurate the timing. However, when researchers employ photons with quantum features, accuracy improves.

Maccone calls these new signals "funky quantum pulses" that are number-squeezed and frequency-entangled. The frequencies of photons prepared in this entangled state are linked, so they travel at similar speeds and arrive at the destination in bunches. This amplifies the signal, leading to increased accuracy in pinpointing arrival times.

"The enhancement in accuracy that quantum mechanics allows depends on how many photons can be prepared in a funky quantum pulse," Maccone explains. One hundred photons give a factor of 10 enhancement over the classical limit; a million photons give a factor of 1,000 enhancement. However, preparing a lot of photons in this state is extremely difficult and requires precise application of nonlinear optics and photonics. Maccone adds that while the research is still in an early stage, the MIT research team already has accomplished simple demonstrations of the QPS technology.

Seth Lloyd, associate professor of mechanical engineering at MIT, points out that QPS offers another benefit. "It turns out that this positioning protocol is cryptographically secure," he notes. "You can set the protocol in such a way that you can detect any eavesdropper or hacker who is trying to figure out where your satellites are or where you are. Second, they don't get any information; and third, you can still tell where you are."

Security features in traditional positioning systems have long been a concern of both military and commercial users. In a recent U.S. Transportation Department report that assesses current global positioning system (GPS) vulnerabilities, experts call for GPS technical improvements such as increasing signal strength and the number of frequencies to help reduce outages. The study also recognizes that other vulnerabilities in GPS can be exploited to deny use or disrupt the accuracy of the system. The report recommends a fuller evaluation of actual and potential sources of interference and vulnerabilities as well as possible solutions.

Lloyd explains that with quantum GPS, threats such as eavesdroppers can be detected because their presence causes high-level noise in the system. "All these quantum protocols work in the presence of noise," he explains. "They work by reducing noise on your communications channel. Let's say someone starts eavesdropping on your channel and you characterize the noise qualities. The presence of the eavesdropper will show up as a spike in the noise. It's a warning sign. It's time to take precautionary measures."

Despite the clear advantages of the quantum technologies being explored, no one ever gets something for nothing, Lloyd allows. "In quantum mechanics the way you get these enhancements is by making the system much more sensitive than the corresponding classical system. You do this by exploiting quantum 'weirdness'--exploiting these quirky entangled states that have exceptionally high quantum correlations--or by squeezing light, which gives you greater sensitivity to information that you couldn't get classically. We're creating quantum systems that are sensitive to the variations in the amount of time it takes to get from one location to the other."

However, sensitizing these systems makes them more susceptible to factors such as noise. "We're also more sensitive to loss of photons," Lloyd says. "If we start to lose part of our signal, then the protocol is sensitive to that." If one or more photons fail to arrive, the remaining photons will not convey any timing information.

"We have ways of compensating, but whenever you compensate, you lose some of your sensitivity," he says. "It's a feature of what's called a quantum mechanics complementarity. It says that quantum systems have complementary variables, and you can enhance your sensitivity in one variable while reducing it in another. We can come up with protocols that are insensitive to noise and loss but only at the expense of using more power."

A sophisticated method to help overcome this challenge is to prepare photons in partially entangled states. These states provide a lower level of accuracy than fully entangled states but are more tolerant of loss. Photons in partially entangled states still perform better than those in unentangled classical states, researchers say.

The team's next step is to build a tabletop prototype within the next year. The goal is to test the system over 10 meters and demonstrate any enhancements. "If we can do that, then there are two directions we can go," Lloyd offers. The first is to produce the same enhancements over longer distances; the second is to achieve greater enhancements. "If we can combine those two capabilities in a reliable fashion, then we'll be heading down the road to making this a viable and useful technology," he states.

Additional information on MIT's quantum information technology projects is available on the World Wide Web at http://%20rleweb.mit.edu/quantummuri.