IP: Quirks in Nature Enhance GPS (fwd)

Eugene Leitl Eugene.Leitl@lrz.uni-muenchen.de
Thu, 17 Jan 2002 13:39:08 +0100 (MET)

-- Eugen* Leitl <a href=3D"http://leitl.org">leitl</a>
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---------- Forwarded message ----------
Date: Thu, 17 Jan 2002 07:32:15 -0500
From: David Farber <dave@farber.net>
Reply-To: farber@cis.upenn.edu
To: ip-sub-1@majordomo.pobox.com
Subject: IP: Quirks in Nature Enhance GPS

>From: "Ramjee Swaminathan" <ramjee@vsnl.net>
>To: <farber@cis.upenn.edu>
>It is for the IP list - if you find this interesting enough.
>=A9SIGNAL 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
>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://

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