Gadgets from the quantum spookhouse: navigation devices and other technologies may gain from queer quantum effects.

Computers exploiting the oddities of quantum mechanics may eventually put conventional computers to shame. Theoretically, full-fledged versions of those quantum machines will someday master in an eye blink mathematical puzzles and secret codes that current supercomputers wouldn't crack with a billion years of number crunching.

The exotic innards of such quantum computers may prove to be unlike anything used in computers today (SN: 8/26/00, p.132). Instead of chips with billions of transistors, just a few hundred charged atoms, or ions, in an ultracold refrigerator may carry out the complex computations. Or perhaps laser beams bouncing around a maze of lenses and mirrors will do the job. Alternatively, vials of liquid chemicals under the control of devices similar to hospitals' magnetic resonance imaging scanners may also serve as quantum processors.

Quantum computing specialists face many avenues to explore, each of them extremely challenging in its own right. One expectation that many of these scientists share is this: Practical quantum computers won't be built for decades, at least.

With success still so distant, and perhaps unattainable, some quantum computer investigators have begun to wonder what other, perhaps easier, targets they might hit along the way. Among the most peculiar properties that quantum computing would harness is a relationship among particles known as entanglement (SN: 9/29/01, p. 196). So strange is entanglement that physicists including Albert Einstein battled for years to discredit the idea. Now, the concept is hotter than it's ever been, and scientists are beginning to work out its uses in a variety of technologies.

Entanglement remained a curiosity of quantum mechanics until the 1970s, when theorists started to get an inkling of just how powerful entanglement-exploiting computers might be. "With the growth and interest in quantum computers, people started to ask, How do you quantify entanglement and how do you make it? With all that attention, people started to think about applications," says Richard J. Hughes of Los Alamos (N.M.) National Laboratory.

During the past few years, theorists have begun to work out ways to use entanglement and other extraordinary quirks of the quantum world to tackle important technological problems. Scientists have begun to experimentally test a few of those schemes, and some of them seem feasible.

The potential payoffs are many. Entanglement and other quantum weirdness may boost the accuracy of radar, Global Positioning System (GPS) receivers, and other navigation devices. It may improve manufacturers' ability to lay down tiny structures on microchips, expedite explorations for oil, improve the vision of telescopes, and increase the accuracy of atomic clocks. Quantum technology might even enable us to see objects without actually looking at them--and without being seen ourselves.

Already close to realization are communications systems secured against eavesdropping by means of certain less-exotic features of quantum mechanics (SN: 6/17/00, p. 388). And experiments have already shown that entanglement mechanisms improve these so-called quantum cryptography systems, their developers say.

"There are potentially many other types of applications, as well. We just haven't thought of them yet," says Seth Lloyd of the Massachusetts Institute of Technology (MIT).

It all comes down to the seemingly magical ways of quantum mechanics. Physicists have discovered that if they shine a laser beam into a certain type of so-called nonlinear crystal, roughly one in ten billion of the photons that pass through it will come out transformed into two photons. What's more, that photon pair will have the quality called entanglement.

"Entanglement is the quintessential quantum phenomenon," says Daniel Abrams of NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. By using laser beams or other prods to make atoms, ions, or molecules interact in distinctive ways, scientists have also managed to entangle those diminutive denizens of the quantum world. What's more, quantum mechanics sets no limit on how many particles can be entangled with each other or how far apart they can be and still maintain their peculiar, intimate correlation.

When particles become entangled, one or more of their traits become complementary. In that sense, pairs of entangled photons are like pairs of gloves in which there's always a left-handed and a right-handed partner. However, for quantum particles--which are also waves, according to quantum theory--traits that can be entangled include energy or wavelength, positions or trajectories in space, and the properties known as spin and polarization, which are related to the spatial orientations of electromagnetic fields.

Now for the weirdness. If no one measures the particular trait that has become entangled within some group of particles, all those particles exist in a twilight state in which they have no particular value of that trait. Measure the trait for any one of those particles, however, and all the particles instantly acquire specific, complementary values of the trait, no matter how far apart the particles are.

Decades ago, scientists were troubled by that idea. Einstein rejected this "spooky action at a distance," as he called it. Now, however, scientists and engineers are learning to embrace it, and their technological imaginations are getting quite a workout.

Equipped with atomic clocks, a fleet of 24 GPS satellites orbit Earth and continuously beam down signals marking the spacecrafts' positions and the time the signals were dispatched. When a GPS receiver on the ground collects those signals from four satellites simultaneously, it can calculate its own position to within as little as a few meters.

Much greater precision is possible by means of "funky quantum correlations," such as entanglement, Lloyd says. In the July 26 NATURE, he and his MIT colleagues Vittorio Giovannetti and Lorenzo Maccone described quantum schemes for measuring the time of arrival of radio-frequency bursts from satellites more precisely than can be done today. Because the arrival time gives information about each satellite's distance from the receiver, improving the precision of that measurement better pinpoints the receiver on Earth.

When a satellite signal arrives at a conventional GPS receiver, it shows up as an extended bunch of photons. To designate a time at which the signal can be said to have arrived, the receiver records the entire incoming radiation pulse and figures out its midpoint. The more photons received, the greater the measurement's precision.

Now, suppose that the transmitted photons are entangled, Lloyd proposes. A late-1980s experiment using beams of entangled photon pairs showed that they spread out more than unentangled photons do, but they straggle in with an enhanced degree of symmetry. For each particle that arrives 10 picoseconds late, another will arrive precisely 10 picoseconds early. "Although the individual arrival times are smeared out, the average time is better defined than it is classically," Lloyd says.

A typical radiofrequency signal might contain a million trillion photons, Lloyd says. If all could be entangled and detected, the receiver theoretically could locate itself from each satellite to within the span of a hydrogen atom's diameter.

"That [huge] increase in accuracy should clue you in to the fact that that's going to be damn hard to do," Lloyd cautions. However, more modest but still substantial improvements over current GPS accuracy may be attainable by entangling just a few photons, he says. He and his coworkers have just begun experiments on the simplest case--entangling two photons to achieve a 40 percent accuracy gain compared with unentangled two-photon pulses.

"Simple demonstrations of quantum GPS and things like that are definitely on the horizon," says Lloyd. "The technical challenges are considerable but not significantly greater than building the first small-scale quantum computers."

He and his colleagues suspect their approach could benefit not only GPS technology but also other navigational and ranging methods, such as radar and perhaps even sonar. While GPS and radar use photons, sonar systems send beams of acoustic vibrations, which physicists think of as particles called phonons. "To make entangled phonons would be hard," Lloyd notes. "But, hell, there's nothing that says you can't do it."

Besides promising greater precision in satellite-based systems that span thousands of kilometers, quantum correlations may also offer a new way to tighten tolerances in the realm of microelectronics manufacturing.

To make microcircuits with ever-smaller components, microelectronics makers and federal laboratories are pumping billions of dollars into new chip-fabrication methods using light with ever-shorter wavelengths (SN: 11/8/97, p. 302).

Light plays a role in the industry because it projects patterns of wires and components onto the surface of a semiconductor wafer--a procedure known as lithography. But light of a given wavelength can define lines no thinner than half that wavelength, at least according to classical physics (SN: 5/5/01, p. 286).

There's a quantum way around that law, says Jonathan P. Dowling of JPL. Last year, he and his colleagues at JPL and the University of Wales in Great Britain showed theoretically that a pair of entangled photons of a certain wavelength could "conspire," as he puts it, to act as a single photon of half that wavelength.

This, in principle, would double both the horizontal and vertical resolution of the circuitry pattern projected onto a chip, reducing the area occupied by circuit elements to one-fourth of today's requirement. Ensnaring additional photons into the web of entanglement compounds the benefits. For instance, entangling three photons would provide a threefold improvement in linear resolution, leading to a ninefold increase in circuit density.

This scheme is no longer just an idea. Recently, Yanhua Shih of the University of Maryland in Baltimore and his colleagues have experimentally confirmed that entangled photon pairs can interact to generate light with half the wavelength of the entangled members. The team described its results in the July 2 PHYSICAL REVIEW LETTERS.

Even with such advances, there is no guarantee that quantum GPS or quantum lithography will ever take off. The approach underlying them, however, can be exploited in many other ways. That approach, known as interferometry, depends on measuring minute differences in distance or time by examining interference patterns of waves--usually waves of light--that take different paths to a detector.

Interferometers are already ubiquitous in laboratories and industry. "Whenever you want to do some sort of measurement, you usually begin with an interferometer," Dowling says. For example, optical engineers take advantage of interferometry to monitor the shape of a lens as it's being manufactured.

To do so, the engineers first split a beam of laser light into two. They shine one part through the lens and then mix it back into the other part of the original beam. Light passing through the lens travels more slowly than the light moving through the air, so the two beams arrive at slightly different times at a detector. This difference produces a pattern of dark and light interference fringes. Irregularities in those can pinpoint distortions in the lens surface.

In a famous example of interferometry gone awry, a misaligned part in an interferometer used to test the mirror for the Hubble Space Telescope caused engineers to distort that mirror's shape, initially saddling the billion-dollar instrument with fuzzy vision. A subsequent space shuttle mission installed corrective lenses (SN: 1/22/94, p. 52).

Even when classical interferometry is at its best, however, quantum correlations such as entanglement can enable it to do better. Moreover, the potential improvement in precision increases markedly with the number of photons in each signal.

That may prove especially helpful as astronomy moves beyond individual telescopes like Hubble to interferometric telescopes and other detectors, Lloyd says. In such instruments, two or more telescopes some distance apart on Earth bring together the light they collect from the same object, such as a star, to generate interference fringes. By interpreting those fringes, scientists can resolve details on celestial objects as if the multiple scopes were one huge telescope as big across as the distance separating the individual instruments.

Astronomers are even discussing the notion of placing telescopes far apart in space to serve jointly as a super-telescope that could examine distant objects, such as black holes and planets orbiting neighboring stars (SN: 10/28/00, p. 282). Reconnaissance agencies are also considering similar techniques to better spot minute features of interest on Earth, Lloyd speculates.

Researchers around the world are also pursuing the promise of quantum improvements for other instruments, including gyroscopes used in navigation systems, atomic clocks, and gravity-measuring devices, or gradiometers, that guide oil exploration and geophysical research.

All these devices are interferometers of one sort or another. Some "are analogous systems that might not look to you like ... an interferometer. But when you do the mathematics, it turns out to be the same," Abrams explains.

None of them will be easy to build because they require control of notoriously fragile quantum phenomena. "Unless you're quite careful, the loss of a single photon can mess up the entire procedure," says Lloyd.

Still, the incentives for confronting such challenges continue to grow. Consider this one, which seems straight out of Star Trek. Researchers at Boston University have come up with a quantum way to make a picture of an object--actually a three-dimensional image, or hologram (SN: 10/26/96, p.270)--from light that doesn't even hit that object. It's like seeing something without looking right at it or even its reflection. The effect is impossible without entanglement, the scientists claim.

In the making of an ordinary hologram, a beam of classical, unentangled laser light is split in two. One part bounces off a target and then interferes with the other part--the so-called reference beam. The interference pattern recorded in a photographic emulsion harbors the 3-D image of the target.

In contrast, the Boston researchers propose a scenario in which the object of interest is surrounded by an opaque surface that has only a small aperture for light to enter. Light illuminates and bounces off the object inside but can't get back out. Only electric pulses, marking each time a photon hits the inner walls, come from the enclosure.

If that incoming light first emerges from nonlinear crystal to create entangled photon pairs, one photon of each entangled pair can be directed into the enclosed area while its entangled partner bypasses the enclosure on its way to a photodetector. That makes it possible to record a hologram of the object even though the photons that actually shine on the object stay in the enclosure. Entangled partners of those enclosed photons serve as outside-the-box messengers that can record some of the object's visual features.

Ayman F. Abouraddy, Bahaa E. Saleh, and their colleagues present the new finding in the Nov. 5 OPTICS EXPRESS. As an experimental test of what they have dubbed "quantum holography," the researchers are currently trying to image a flat object, like a photographic slide, with the entangled pairs.

At a conference last summer where Saleh first brought up the idea, "there was a great deal of interest from the reconnaissance people," he recalls. The technique might lead to novel spying methods, he explains.

The quest for quantum computing has laid the groundwork for these various quantum-enhanced technologies, scientists say. "The buildup of excitement about quantum computers gave people a new language for thinking about entanglement and things like that," Hughes says. "When you develop a new language you tend to get new ideas."