PORTLAND, Ore. Harvard University researchers have used quantum confinement to link optical waves to electrons, creating an electro-optical coupling sensitive enough to respond to a single photon.
The transistor-like device links optics to electronics on a quantum level with surface plasmons--a combined electro-optical communications signal carrier. Surface plasmons result from the quantum confinement of light along a one-dimensional nanowire waveguide--waves of light acting in concert with the electrons in a nanowire. By coupling its optical and electronic properties, the researchers said surface plasmons could enable high-density, on-chip electro-optical communications and high-speed quantum computations.
"Surface plasmons are a form of light that propagates along the surface of a metallic nanowire--coupling to its electrons and enabling some very special properties and operating modes," said Darrick Chang, a doctoral candidate in the lab of professor Mikhail Lukin at Harvard University.
|Single-photon transistor uses quantum confinement to harness surface plasmons.|
Normally, an excited quantum dot emits light in all directions. By engineering the location of quantum dots close to a nanowire waveguide, the Harvard researchers managed to route the entire optical output of the quantum dot emitters down nanowire waveguides. In optical fibers, the light travels inside the fiber, but in surface plasmons they travel down the outside of the metallic wire and couple to the electrons inside the wire. Surface plasmons--half optical, half electronic--could be used to pipe optically-coded quantum information around the insides of future electro-optical chips.
"By locating our quantum dots adjacent to nanowires, [the researchers found] that they discharge almost their entire output along the nanowire--the wire acts as a super lens focusing the energy along its surface," said Chang.
Unlike optical waveguides, which cannot be thinner than the wavelength of the light they transmit, nanowires can confine light in the transverse direction down to almost any sub-wavelength size. By making the nanowires thin enough, the researchers hope to demonstrate a quantum-dot-to-nanowire coupling sensitive enough to respond to single photons. Single photons can store quantum-bits, or q-bits, but are difficult to emit, sense and process. Using surface plasmons to communicate and process q-bits could enable easier construction of future quantum computers.
"The special thing about the quantum confinement down the nanowire is that it makes the quantum dot so sensitive that it responds to a single plasmon, and vice versa," said Chang. "To make this kind of transistor work, you have to get close to the quantum limits of manipulating single plasmons and their interaction with individual quantum dots."
The researchers also demonstated a nonlinear photonic switch for optical signals travelling along a nanowire along with colleagues at the Niels Bohr Institute in Denmark. They also worked on the control of optical plasmons in a nanowire with researchers at Texas A&M University, College Station, and the Lebedev Physical Institute (Moscow).