Researchers from the National Institute of Standards and Technology (NIST) are developing a technique that could enable different parts of quantum computers to communicate effectively.
Until now, it has not been possible for devices such as particles of light or photons to carry the bits of information that a quantum computer will use. While each of these pieces of hardware can do some jobs well, none are likely to accomplish all of the functions necessary to build a quantum computer. This implies that several different types of quantum devices will need to work together for the computer or network to function. However, the tiny devices frequently create photons of such different character that they cannot transfer the quantum bits of information between one another. According to the researchers from NIST, transmuting two vastly different photons into two similar ones would be a first step toward permitting quantum information components to communicate with one another over large distances. The team has demonstrated that it is possible to take photons from two disparate sources and render the particles partially indistinguishable. If photons can be made to 'coalesce' and become indistinguishable without losing their essential quantum properties, then it's possible that they can connect various types of hardware devices into a single quantum information network. The team claims the achievement demonstrates for the first time that a 'hybrid' quantum computer might be assembled from different hardware types. The team connected single photons from a quantum dot, which could be useful in logic circuits, with a second single photon source that uses 'parametric down conversion', which might be used to connect different parts of the computer. These two sources typically produce photons that differ so dramatically in spectrum that they would be unusable in a quantum network. But with a choice of filters and other devices that alter the photons' spectral shapes and other properties, the team was able to make the photons virtually identical. "We manipulate the photons to be as indistinguishable as possible in terms of spectra, location and polarisation—the details you need to describe a photon," said Glenn Solomon, NIST Quantum Measurement Division. "We attribute the remaining distinguishability to properties of the quantum dot. No conceivable measurement can tell indistinguishable photons apart. The results prove in principle that a hybrid quantum network is possible and can be scaled up for use in a quantum network." The research team includes scientists from the NIST/University of Maryland Joint Quantum Institute and Georgetown University. The NSF Physics Frontier Center at JQI provided partial funding.