An international team lead by engineers at Caltech has developed a computer chip with nanoscale optical quantum memory.
Quantum memory stores information in a similar way to traditional computer memory, but on individual quantum particles—in this case, photons of light. This allows it to take advantage of the peculiar features of quantum mechanics in order to store data more efficiently and securely. One of the features it takes advantage of is superposition, where a quantum element can exist in two distinct states simultaneously.
"Such a device is an essential component for the future development of optical quantum networks that could be used to transmit quantum information," says Andrei Faraon (BS '04), assistant professor of applied physics and materials science in the Division of Engineering and Applied Science at Caltech, and the corresponding author of a paper describing the new chip.
"This technology not only leads to extreme miniaturization of quantum memory devices, it also enables better control of the interactions between individual photons and atoms," says Tian Zhong, lead author of the study and a Caltech postdoctoral scholar. Zhong is an acting assistant professor of molecular engineering at the University of Chicago. He will set up a lab to develop quantum photonic technologies in March 2018 at the University of Chicago.
Using individual photons to store and transmit data has been a goal of engineers and physicists for a long time. Individual photons have a lot of potential to carry information reliably and securely. Photons lack charge and mass, so they can be transmitted across a fiber optic network with minimal interactions with other particles.
The quantum memory chip is analogous to a traditional memory chip in a computer. Both store information in a binary code. With traditional memory, information is stored by flipping billions of tiny electronics switches either on or off, representing 1 or 0. The 1 or 0 is known as a bit. Quantum memory stores information via the quantum properties of individual elementary particles, in this case, it is a light particle. A fundamental characteristic of the quantum properties is that they can exist in multiple states at the same time. Basically, a quantum bit, aka a qubit, can represent a 1 and a 0 at the same time.
In order to store the photons, Faraon’s team created memory modules using optical cavities made from crystals doped with a rare-earth ion. Each memory module is like a miniature race track, measuring at 700 nanometers wide by 15 microns long – the scale of a red blood cell. Each module was cooled to about 0.5 Kelvin and then a heavily filtered laser pumped single photons into the modules. Each photon was absorbed efficiently by the rare-earth ions with the help of the cavity.
The photons were released 75 nanoseconds later then checked to see if they had faithfully retained the information that was recorded. Ninety-seven percent of the time the information had been retained.
The team is now focusing on extending the time that the memory can store information, as well as its efficiency. In order to create a viable quantum network that sends information over hundreds of kilometers, the memory will have to accurately store data for at least one millisecond. The team plans to work on ways to integrate the quantum memory into more complex circuits, taking first steps toward deploying this technology in quantum networks.
The study on this research appeared online ahead of publication in Science magazine.