Two teams unveil quantum-dot light switch

07 Dec 2007

Generating interactions between single quantum dots and photons could open the door to quantum information processing and ultra-secure, long-distance communication.

Two independent research teams have shown that semiconductor quantum dots in nanocavities can be made to interact with photons. The two groups – from Stanford University and the University of California, Santa Barbara (UCSB) and the other from the California Institute of Technology (Caltech) – use different techniques to show that a single quantum dot (QD) placed in a cavity can act an optical switch that blocks incoming photons if strongly coupled to the cavity's optical field (Nature 450 857 & 862).

"We have achieved a way to probe the solid-state quantum system directly with photons," Dirk Englund, a researcher at Stanford University, told optics.org. "This is extremely important as it opens up the possibility to exchange quantum information between the photon and QD, which is fundamental to the quantum network idea."

“We will work on scaling up to a larger number of nodes connected by waveguides to allow for useful quantum information processing ” Dirk Englund, Stanford University

Similar interactions have previously been achieved in atomic physics systems, but this work is the first demonstration in the solid state – which is much easier to set up and handle. It represents a crucial step towards creating gates for quantum information processing, such as an on-chip quantum computer and a quantum repeater for long-range quantum cryptography.

"We have also demonstrated an extremely large optical nonlinearity, which could find application in all-optical signal processing, such as an optical switch," commented Englund. "Such an advance would speed up internet and interconnect links."

Both groups use QDs confined within cavities to establish photon interaction, but they each used different optical architectures. "The Caltech group uses circular resonators consisting of GaAs that are manually brought near a tapered optical fiber, while in our case the cavity consists of a photonic crystal," explained Englund.

In the Caltech design, Oskar Painter and colleagues exploit a fiber taper to provide an optical interface between the coupled QD and the microcavity. Light transmitted through the fiber is confined within the tapered region, with an evanescent "tail" of light extending outside the fiber in the smallest part of the tapered region.

"We fabricate a micron-scale resonator to confine light to a volume that is less than a cubic micron," Painter told optics.org. "Within this resonator is a nanoscopically sized QD operating in the 1300 nm band. Due to the microscopic dimensions of the cavity QD system, measuring their interaction can be very challenging. We addressed this by using a customized optical fiber probe to interface with the system and interrogate it across different regimes of device behavior."

The advantage of operating in the 1300 nm band is that it minimizes losses in the optical fiber. "What's more, using a direct fiber-optic interface allows us to couple light into and out of our system with very high efficiency using conventional optical fiber technology," commented Painter.

Photonic crystals enable quantum network

Meanwhile, Englund's approach uses a resonant laser beam to coherently probe a single QD operating in the 900 nm band, and which is confined within a GaAs photonic crystal cavity. "Photonic crystals are ideal for combining several of these QD-coupled cavities into a quantum network and allow for fully lithographic fabrication," commented Englund.

The cavity localizes light into a region only a couple of hundred nanometers in each dimension. "The QD embedded inside the cavity is a nanoscale inclusion of another semiconductor, InAs, with dimensions of around 10 nm," explained Englund. "The optical nanocavity can be switched from completely transparent to completely opaque by increasing the intensity of the input laser beam."

Painter believes that developing fast, wideband local tuning of either the QD state or cavity resonance frequency would open up a great many more applications. "Continued development of new tools and techniques that allow for more controlled and efficient manipulation of single quanta is still needed," explained Painter.

Englund agrees that it is important to correct detunings between QDs and cavities. "We are developing a technique to tune the resonance wavelength of cavities and waveguides by changing the refractive index of a photoactive film," concluded Englund. "We will also work on scaling up the size of the quantum network to a larger number of nodes connected by waveguides to allow for useful quantum information processing."