IP: Quirks in Nature Enhance GPS

[David Farber <dave () farber net>]

> From: "Ramjee Swaminathan" <ramjee () vsnl net>
> To: <farber () cis upenn edu>
>
>
> Dave:
>
> It is for the IP list - if you find this interesting enough.
>
> __ramjee.
>
> http://www.us.net/signal/CurrentIssue/Jan02/quirks-jan.html
>
> ©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://
> rleweb.mit.edu/quantummuri.

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