A Single-Photon Transistor using Nanoscale Surface Plasmons
Author: Darrick Edward Chang
Affiliation: Physics Department, Harvard University
[This is an invited article based on a recent work done by the author and his collaborators and published in 'Nature']
Finding ways to make pulses of light interact with each other has been an active area of research for several decades. In fact, the study of “nonlinear optics” has led to countless breakthroughs and technological advances in fields as diverse as imaging, spectroscopy, laser physics, communications, and signal processing . Interactions between pulses of light are achieved by their common interaction with some material medium. However, because such processes are generally very weak, optical nonlinearities typically become significant only when very large light intensities are used.
The ability to achieve nonlinear interactions at low optical powers would enable a new generation of devices that consume much less power than their predecessors and enable new applications as well. The ultimate limit would be to achieve nonlinear interactions between individual photons, the constituent particles that comprise light. Recently, there has been great interest in this area in part because of potential applications in quantum computing and quantum information science [2,3].
The interaction strength between matter and light can be increased by confining the light in space to very small dimensions, which causes the associated optical fields to become very intense. In normal dielectric media, light cannot be confined to regions smaller than an optical wavelength. However, the situation changes dramatically when light is coupled to the free electrons in a conductor. The unique properties of these coupled excitations of light and charge (known as surface plasmons)  allow them to be confined to arbitrarily small dimensions.
Image: An illustration of how a single atom near a nanowire can prevent light from propagating past it
Recently, we proposed  and experimentally investigated  the strong interaction between single atoms (or other optical emitters) and individual surface plasmons tightly confined to a conducting nanowire. The strong coupling causes the nanowire to act as a “super-lens” that directs the majority of emission into the surface plasmon modes. More recently, we have theoretically shown that such a system also leads to remarkable nonlinear optical effects . In particular, the confinement of the surface plasmons is so strong that when a single surface plasmon (i.e., a single photon) is incident on a single emitter, the two must interact, and this interaction prevents the photon from being transmitted past the emitter. However, because the emitter cannot interact with more than a single photon at a time, its response to a second incident photon becomes fundamentally different and transmission is now much more likely. In this sense, the single emitter behaves as an efficient, single-photon switch.
One can gain even further control over the nonlinear optical interactions in this system by using techniques from quantum optics to coherently manipulate the emitter. In fact, we have shown that the system can behave as a single-photon transistor, where the presence or absence of a single photon in a “gate” field can prevent or allow the propagation of a whole stream of “signal” photons. In analogy to the role that electronic transistors play in electronic computing devices, a single-photon transistor would open the door to optical computing devices and many other possibilities.
Our experimental efforts to explore the nonlinear properties of this system are just beginning, and considerable work remains to be done before large-scale, integrated quantum plasmonic devices can be practically realized. More broadly, however, work such as this suggests the great promise of merging the tools of quantum optics with plasmonics and the many other novel optical materials that have recently arisen. Ultimately this merger may help us to achieve unprecedented control over the interactions of light quanta.
This work was done in collaboration with Mikhail Lukin and Eugene Demler, both in the Physics Dept. at Harvard University, and Anders Sorensen in the Physics Dept. at the Niels Bohr Institute, Copenhagen, Denmark.
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