Detecting single photons

Summary:What do you do when you need to transmit a signal over a long distance, from Saturn, say, and you're low on power?  Until now, you turned the bandwidth way down, but new single-photon detectors could change that.

What do you do when you need to transmit a signal over a long distance, from Saturn, say, and you're low on power?  Until now, you turned the bandwidth way down, but new single-photon detectors could change that. Technology Review has an interview with Karl Berggren, an electrical engineering professor at MIT, who has developed a nanotechnology-based device that combines efficiency and speed--two things past devices haven't had together.

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.

Another application of the single photon detector is quantum cryptography.  Quantum cryptography creates secure channels using streams of photons.  One challenge is the distance that the secure light channel can be transmitted and retain its information.  Using present technology, the drop-off in photon levels limits quantum cryptography to 100-150 kM.  A detector as sensitive as Berggren's could triple this distance.

Topics: Security

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