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Interplanetary Broadband

Highly sensitive light detectors could enable streaming video from Mars and longer-distance quantum cryptography.
January 31, 2006

Manned missions to Mars would benefit from high-speed communications, even streaming video, rather than the clunky, hours-long image downloads from the likes of the Cassini-Huygens Saturn probe. Conventional optical communication can carry such video data, but the lasers needed to transmit a readable signal for interplanetary distances would require far too much power.

This has some researchers looking to single-photon detectors – the ultimate low-light sensors – to make it possible to detect relatively low-power laser signals. So far, however, such detectors have been either inefficient, missing most of the photons sent to them, or too slow for high-bandwidth data. Now a nanotechnology-based device, reported this week in the journal Optics Express, combines efficiency and speed, promising to make such interplanetary communication more practical. The detector might also allow long-distance, secure communications, perhaps for collecting data from unmanned military aerial vehicles such as the Predator.

“The detection of extraordinarily low levels of light with high bandwidth has been a challenge for many years,” says Karl Berggren, an electrical engineering professor at MIT who helped develop the new device. “This demonstration illustrates what nanotechnology, and in particular nanofabrication, can do when applied to a problem like this.”

The researchers started with an earlier single-photon detector design that could keep up with data rates about eight times faster than a typical office Ethernet connection. But that test confirmed rates only for data transmitted across a lab. In applications where one can’t simply turn up the power of the laser and send more photons, such as on power-starved space missions, keeping up the rate of transmission requires a more efficient sensor.

The answer was to add a photon trap. The heart of the detector, which has been around for a couple of years, is a wire 100 nanometers wide that meanders like coils on a refrigerator to increase the area of detection. The wire is cooled to just above absolute zero, at which temperature it becomes a superconductor. When a photon hits the wire and is absorbed, the wire heats up just enough to stop superconducting, creating a detectable jump in resistance.

In the new design, the photons that slip past or reflect off the wire bounce around in the photon trap, giving them more chances to be absorbed by the wire. The trap, with a little help from an antireflective coating, approximately tripled the efficiency of previous detection efforts.

The efficiency is “a huge jump forward,” says Jeffrey Stern at the Jet Propulsion Laboratory. Indeed, he says this efficiency is enough for optical communication in space.

This single-photon sensor could also find applications in quantum cryptography. For interplanetary communication, power restrictions make single-photon detection necessary. For quantum cryptography, the sensitivity makes it possible to send information using single photons, which, in turn, makes it possible to detect eavesdroppers, ensuring that data is perfectly secure.

And just as efficiency would allow for communicating over greater distances in space, so it should extend the reach of quantum cryptography. So far, a drop-off in the number of photons limits the range of quantum cryptography to 100-150 kilometers. But, according to Michael LaGasse, vice president of engineering at the Somerville, MA, labs of MagiQ, which is already commercializing quantum cryptography, a detector as efficient as Berggren’s could double or triple these distances. Because of cost considerations, however, LaGasse says the new detectors are likely to find only niche markets, such as in military applications.

One engineering hurdle remains before the sensors are ready for such applications, however. Although Berggren’s team has already built and tested these devices, because they are so small, focusing the photons on them is a challenge. He expects, however, that this problem can be solved within two years.

Meanwhile, their tests have added to the understanding of these light-detecting devices in ways that should help other researchers, by establishing design criteria for future devices, says Daniel Prober, applied physics professor at Yale University. “This is a very impressive piece of work and really important for the field,” he says.

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