Light trick can retrieve missed messages

Even if photons pass you by, you can snatch signal from their electromagnetic wake

SAN ANTONIO — Light may travel at the speed of light, but the information it carries doesn’t have to. Three physicists have proposed a way to receive light-based messages even when the light itself has already flown by.

The communication technique, detailed March 2 at the March meeting of the American Physical Society and in a paper to be published in Physical Review Letters, relies on measuring electromagnetic echoes that arise upon the generation of light. Although the method has limited practical use for now, it offers an intriguing exception to the idea that sharing information via light requires one party to send photons and the other to absorb them. Eventually, the technique could enable astronomers to glean details on distant stars and galaxies without directly measuring their light.

Light-based communication enables Internet users to chat through fiber-optic cables and radio stations to broadcast over the air. A radio antenna, for example, broadcasts photons that travel at the speed of light. Your clock radio absorbs that energy and translates the signal into sound. If your radio doesn’t receive the photons, you don’t hear the breaking news bulletin. There is supposedly no way to recover information encoded in photons once they pass by.

Theoretical physicists Robert Jonsson, Eduardo Martín-Martínez and Achim Kempf of the University of Waterloo in Canada discovered that it doesn’t have to be that way. They knew that photons leave a mark on their surroundings, even if they are in a vacuum. That’s because the vacuum is not truly empty — it is full of fleeting electromagnetic waves (SN Online: 3/2/15).

The three physicists mathematically demonstrated that when a sender generates photons to broadcast a message, the photons produce an afterglow discernible by measuring the fluctuating electromagnetic waves in the vacuum. Consequently, a person can still receive information even if the photon carrying that information whizzed by long ago.

The communication technique is novel because the sender never directly transmits energy to the receiver, says Jorma Louko, a theoretical physicist at the University of Nottingham in England who was not involved in the research. Instead, the receiver has to expend energy to measure the electromagnetic waves in the vacuum disturbed by the long-gone photon. “The receiver has to actively do something to see something,” he says. That means that a sender can transmit one photon and have multiple receivers tap into the vacuum to obtain the information.

Detecting these quantum fluctuations would require both parties to have antennas that consist of atoms specially prepared in a delicate quantum state. That kind of technology is nowhere near reaching consumers, but it is attainable in physics labs. Martín-Martínez said he is talking to experimental physicists about demonstrating the communication protocol with chilled superconducting circuits that serve as the antennas.

Martín-Martínez hopes this research leads to even grander applications. In a paper posted online January 7 at, he and colleagues argue that light emitted during the dawn of the universe should have left an afterglow observable by specially designed telescopes. These telescopes could detect objects billions of light-years away even if the photons those objects emitted passed Earth long ago.

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