Electronic Design

Photon Detector Speeds Up Interplanetary Communications

In science fiction, starship captains can pick up the phone and talk to headquarters back on Earth in real time. In reality, messages to and from our interplanetary probes take hours to cross the cosmos. That's because they rely on RF technology. A team of researchers at the Massachusetts Institute of Technology's Research Laboratory for Electronics, though, may have a solution—optical technology.

The team has developed an ultrasensitive light detector that could be used to receive optical messages. It uses nanowires and superconductor technology to sense extremely low light or laser signals in the infrared part of the optical spectrum. Unlike conventional optical systems, the MIT team's sensor can even pick up single photons.

So far, the detector improves the detection efficiency for single photons to 57% at a wavelength of 1550 nm, or the same wavelength optical fibers use to carry broadband signals to offices and homes today. Current detectors only offer an efficiency of 20%. As efficiency improves, the researchers expect transmission of color video between astronautsor equipment in outer space and scientists back on Earth.

Previous single-photon detectors haven't been fast and efficient. The researchers improved their design by adding a "photon trap" to the detector. This optical cavity consists of the nanowire detector, a carefully measured gap of glass, and a mirror. Also, an antireflection coating keeps light from bouncing off its surface.

The nanowire is wrapped in a tight coil for a broad area of overlap with the laser light and cooled to just above absolute zero. At that point it becomes a superconductor, able to detect absorbed photons. Photons that don't get absorbed bounce between the nanowire and the mirror until they get absorbed. The detector's efficiency increases as more photons are absorbed.

The sensitivity is key to operation. Current optical systems would require a large laser and a lot of power to transmit lots of data quickly. But the spacecraft that will be broadcasting these messages won't be able to provide the kind of power these large lasers would require. They'll have to rely on smaller lasers instead. And, the MIT team's detector can receive messages from those lasers.

Karl Berggren, assistant professor with MIT's Department of Electrical Engineering and Computer Science, says the detector could be applied to quantum cryptography and biomedical imaging (see the figure). But the most immediate application, he says, probably is interplanetary communication. The U.S. Air Force helped fund the project.

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