Jelena Vuckovic, Electrical Engineering Department and Ginzton Laboratory, Stanford University, Stanford, CA, USA. Corresponding author: [email protected]
Recent advances in nanofabrication have enabled the development of structures that can manipulate and localize light into volumes below a cubic optical wavelength with storage times of thousands of optical cycles . The emergence of such high quality nanophotonic structures has opened new opportunities for the study of light-matter interaction . For example, as a result of the localization of light within such ultra-small volume nano-resonators, large optical intensities can be achieved with only a few photons coupled. Further, such a system also enables strong interaction between single atom-like quantum emitters (e.g. quantum dots, nitrogen vacancy centers in diamond, etc.) embedded within the cavity and single photons. Not only is the interaction between light and matter stronger in such a nanocavity, but system dynamics occurs on much faster time scales (as the light emission and absorption rates increase with reduction in the optical volumes). Moreover, nanophotonic structures can be constructed and integrated on chip by standard semiconductor microfabrication processes, and are fully scalable.
|A quantum dot is a portion of matter whose excitons are confined in all three spatial dimensions. Consequently, such materials have electronic properties intermediate between those of bulk semiconductors and those of discrete molecules. They were discovered at the beginning of the 1980s by Alexei Ekimov in a glass matrix and by Louis E. Brus in colloidal solutions. The term "quantum dot" was coined by Mark Reed.
Such structures can be employed as a more practical testbed for fundamental experiments on light-matter interactions (the field referred to as the cavity quantum electrodynamics, or cavity QED). As opposed to the atomic-cavity QED platform  on which such experiments have been explored for the past 30 years, the use of a quantum dot-nanocavity platform enables a much smaller, on-chip, scalable system, which is also simpler, as it eliminates the need for atom trapping inside a resonator (e.g., quantum dots are already naturally trapped inside the nano-resonator material, such as GaAs [3-5]). In addition, as a result of the ultra-small optical volumes, the interaction strength between the quantum dot and the cavity field - described by the so called vacuum Rabi frequency - is in the range of several 10's of GHz - three orders of magnitude higher than for the atomic system. Therefore, everything happens much faster as well. The practicality and speed make these structures also interesting as a platform for a new generation of classical and quantum information processing devices.
For example, one of the key properties of the system consisting of a single quantum dot strongly coupled to a resonator is that the presence of the dot can completely modify the optical transmission through such structure, from transparent to opaque for an optical beam on the resonance . This could be done at a rate proportional to the vacuum Rabi frequency (i.e., 10's of GHz for the quantum dot-nanocavity system , as opposed to MHz in the atom-cavity system ), opening the opportunity to build practical devices such as an optical modulator controlled with a single quantum dot , which could be operated with control energies below 1aJ - many orders of magnitude lower than conventional modulators or electrical interconnects in computers . In addition to enabling the construction of a new generation of computers, where light is used to communicate signal between cores in a processor with much higher speed and efficiency, this approach also addresses an important energy problem: namely, electrical interconnects in computers of large data centers already consume a significant fraction of our electricity today, and more than that produced by solar cells .
Another conventional device that can greatly benefit from quantum nanophotonics is the laser. As a result of the strong localization of light, lasers can be built that turn on at threshold currents below 100 nA – thousands of times smaller than the best conventional lasers today . The same strong localization of light allows them to be switched on and off at very high speeds (exceeding 10's of GHz), which is important for optical communications . Moreover, lasers can be built that employ only a single quantum dot as the gain medium - the ultimate limit of laser scaling .
In addition to the potential for improving the properties of conventional devices for optical communications and interconnects (such as lasers and modulators), quantum nanophotonics is a viable candidate for building circuits for quantum information processing which employ quantum mechanical properties of matter and light to transmit information securely or to perform certain computations more efficiently . Applications of interest include quantum networks and repeaters (that would enable secure transmission of information over large distances) , as well as quantum simulators which would enable studies of complex physical processes by constructing systems exhibiting analogous behavior.
Although most of the experiments being done at the moment are at the level of a single device (single quantum emitter) or a few of them, many of the goals outlined here require interconnection of many such nodes. This certainly poses a number of technological challenges, but we can greatly benefit from the access to matured semiconductor micro-processing technologies in achieving this goal.
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