Quantum Light

Now You See It Now You Don’t.

Researchers at Stanford University have created a novel quantum light source that might serve as the basis for quantum communication. They explained their findings in a paper published by the journal Nature Photonics.

Standard lasers are not useful for secure communication because they emit what is called “classical” light. Data eavesdroppers could extract any data being carried via classical light without detection. In contrast, a quantum Internet would be based on “quantum” light, in which a single unit of light, a single photon, cannot be measured without being destroyed. Therefore, an efficient source of quantum light would enable perfectly secure communication.

Senior author Jelena Vuckovic, a professor of electrical engineering at Stanford, has been working for years to develop various nanoscale lasers and quantum technologies that might help conventional computers communicate faster and more efficiently using light instead of electricity. She and her team, including lead author Kevin Fischer, a doctoral candidate, realized that a modified nanoscale laser can be used to efficiently generate quantum light for quantum communication.

“The problem is that the quantum light is much weaker than the rest of the light coming from such a modified laser; it is difficult to pick up,” Vuckovic said. “So we created a way to filter out the unwanted light, allowing us to read the quantum signal much better.”

The filtering works in a fashion similar to the way noise-cancelling headphones operate, only with light, instead of sound. With the headphones, a sensor actively gauges the frequency of relatively constant ambient sound; the rumble of traffic, the drone of an airplane engine, the thrum of a refrigerator and produces a similar pattern, which can be used to cancel out the undesirable sound.

“Some of the light coming back from the modified laser is like noise, preventing us from seeing the quantum light,” Fischer said. “We cancelled it out to reveal and emphasize the quantum signal hidden beneath.”

Vuckovic’s team adapted an interference technique borrowed from 1930s-era radio engineering to cancel the unwanted classical light. They first figured out what the “noise” looks like and played it back. By carefully adjusting how the cancelling light and the classical light overlap, the unwanted light is cancelled and the once-hidden quantum light is revealed.

“This is a very promising development,” Vuckovic said. “It provides us with a practical pathway to secure quantum communications.”

She and her team are now working on creating a working prototype.


 

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