Harvard University – Engineers and physicists at Harvard have managed to capture light in tiny diamond pillars embedded in silver, releasing a stream of single photons at a controllable rate.
The advance represents a milestone on the road to quantum networks in which information can be encoded in spins of electrons and carried through a network via light, one photon at a time.
The breakthrough takes advantage of imperfections in the diamond’s crystal lattice, where carbon atoms are replaced by other elements. To the naked eye, these imperfections can appear as discolorations in the diamond, turning it yellow in the case of nitrogen. Occasionally, there is also a vacancy (missing carbon atom) next to the nitrogen atom.
Each nitrogen-vacancy imperfection can serve as a nearly perfect quantum emitter, capable of emitting red photons one by one, even at room temperature. The technology is a promising candidate for realization of scalable, on-chip quantum networks.
Imperfections in the diamond can give the crystals a characteristic color. Nitrogen in these tiny diamonds (top) turns them yellow. (Microdiamond powder provided by Daniel Twitchen and the Element Six company [UK].) Some imperfections, including the nitrogen-vacancies explored in this work, are luminescent (can emit red photons one at a time) and behave as excellent optically addressable quantum memory. The chip (bottom right) consists of diamond with a silver layer on top.
“The color centers in diamond are very interesting as qubits for quantum information processing, where they can be used as memory to store information,” says Lončar. “More importantly, they can be interrogated—they can be written into and read out—with light.”
Lončar’s team fabricates diamond posts that contain negatively charged nitrogen vacancy centers, which can absorb light and hold its energy for a given amount of time, finally releasing it in the form of photons.
“The rate at which photons are emitted can be controlled by carefully nano-engineering the center’s surrounding,” says co-author Irfan Bulu, a research associate in the Lončar group. Attaining fine control of that release, however, has been difficult.
“One of the main challenges has been the efficiency with which you can write information into the spin of these color centers, as well as the efficiency with which you can collect photons emitted from the color centers,” explains co-author Jennifer Choy, a graduate student in Lončar’s lab at SEAS. “The other challenge has been the rate—how quickly you can perform these processes.”
Previous work from Lončar’s group solved the collection efficiency problem by using diamond nanowires to channel and direct the flow of photons. The new research manipulates the radius of diamond pillars and adds the silver coating. The diamond-silver construction acts as an optical nanoresonator, creating a strong electromagnetic field around the emitter and offering a new level of control over the rate of emission.
Moreover, the device functions at room temperature—an essential requirement for practical computing applications—and the nanostructured chips are fully scalable.
“We’ve designed everything in parallel in a massive system, which allows us to make thousands or millions of devices with more or less the same properties, and we use conventional microfabrication and nanofabrication techniques, unlike what has been done in this field before,” says Birgit Hausmann, a graduate student in Lončar’s lab at SEAS and one of the co-authors.
SEM image of the diamond nanoposts. Each one is approximately 100nm wide and 200nm tall. A nanometer is a billionth of a meter. Image courtesy of Jennifer Choy.
We have developed a scalable method for coupling single color centers in diamond to plasmonic resonators and demonstrated Purcell enhancement of the single photon emission rate of nitrogen-vacancy (NV) centers. Our structures consist of single nitrogen-vacancy (NV) center-containing diamond nanoposts embedded in a thin silver film. We have utilized the strong plasmon resonances in the diamond-silver apertures to enhance the spontaneous emission of the enclosed dipole. The devices were realized by a combination of ion implantation and top-down nanofabrication techniques, which have enabled deterministic coupling between single NV centers and the plasmonic modes for multiple devices in parallel. The plasmon-enhanced NV centers exhibited over six-fold improvements in spontaneous emission rate in comparison to bare nanoposts and up to a factor of 3.6 in radiative lifetime reduction over bulk samples, with comparable increases in photon counts. The hybrid diamond-plasmon system presented here could provide a stable platform for the implementation of diamond-based quantum information processing and magnetometry schemes.
A single photon source based on diamond-plasmon apertures. (a) Three-dimensional schematic of the diamond-plasmonic system. The idealized structure for coupling to the NV emission consists of a diamond nanopost (with height~180nm and diameter~100nm) embedded in a 500nm-thick layer of silver. NV fluorescence is excited and collected through the bulk part of the diamond sample (green and red arrows, respectively). (b) Cross-sectional view of the structure along either the XZ (or YZ) plane, plotted with the longitudinal mode profile. The dipole is shown to be in-plane polarized and positioned in the center of the structure, where field density is maximized. As in a Fabry-Perot resonator, light reflects off the diamond-metal interfaces (curved arrows) and becomes tightly confined in the nanoposts. (c) Simulated lateral mode profile shows a near-uniform energy distribution of the plasmonic mode. (d) Simulated spontaneous emission enhancement as a function of wavelength for nanoposts with different radii, calculated with placing the dipole at the field maximum.
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