Quantum communication, without – and with – delay

Hank Hogan, hank@hankhogan.com

When it comes to computing, some of the lessons we learned in kindergarten really do apply. Algorithms have to take turns. Otherwise, final calculations could be wrong.

Such data flow control is done routinely in standard computers. Now the nascent field of quantum information processing potentially has a way to do the same, courtesy of researchers from the National Institute of Standards and Technology (NIST), the University of Maryland and the University of Birmingham in the UK.

The researchers demonstrated their scheme by tuning the velocity of photons transporting an image, delaying the process by varying amounts without degrading the quantum information. They did so using a cell of hot rubidium vapor. They selected the material in part because of its properties, said researcher Alberto M. Marino. “You need to have a medium where you can control the group velocity. This velocity controls how fast that information propagates.”

Quantum buffering is achieved through slow light. Two entangled images (shown connected by dotted lines) are generated. By sending one through a slow-light cell, researchers created a variable delay and a quantum information buffer. Courtesy of Albert Marino, NIST.

Marino was lead author of a Nature paper that described the work. The group first produced a pair of images encoded in photons, with the two beams created in an entangled state. Thus, the quantum state of one, as measured by its phase and amplitude fluctuations, was intertwined with the quantum state of the other. Because of this, determining the quantum state of one automatically provided that of the other. This entanglement between separated photons is the basis for quantum communication and computation.

After generating the entangled images in the first rubidium cell, the researchers allowed one packet of photons to travel freely to a detector while they sent the other through a second cell before detecting it. In this cell, the group velocity of the rubidium acted to slow down the propagation of the information by a factor of as much as 800. This introduced a delay, which the researchers could control by varying the temperature of the rubidium or adjusting the intensity of the light used as a pump.

They got the delay up to 27 ns, with the limit set by the amount of noise generated by the process. Too much noise destroyed the quantum entanglement, wiping away the correlation between the images.

Adjustments and improvements can push the delay to a greater number; however, Marino said, it’s always likely to remain in the nanosecond range.

As for applications, Marino foresees quantum communication, perhaps in a buffer that temporarily stores results. Longer-term storage will likely be done by transferring quantum information from photons to atoms, because they can store quantum properties for milliseconds.

One benefit of the new scheme is the ability to change the delay by optical means, which makes it possible to quickly adjust it as needed for experimentation or device operation. Another is the fact that the technique isn’t confined to delaying a single channel: “The fact that we can delay images shows we can delay large amounts of information simultaneously,” Marino said.