Quantum Systems Directly Probed by Photons

Michael A. Greenwood

Microcavity systems that track and measure the quantum interactions that occur between light and matter have been devised by two research teams in California.

Although cavity quantum electrodynamic systems come in a variety of configurations, both teams composed their systems of a semiconductor quantum dot coupled to the optical mode of a microcavity. To this basic setup, the groups applied their own architectures and observed the quantum interactions between a single unit of light and a unit of matter, an essential step toward managing and, eventually, harnessing quantum behavior.

Controlling such behavior has a number of potentially far-reaching applications, including the construction of quantum computers — which theoretically could perform calculations at a rate unachievable by the generation of classical computers now in use — and the creation of encryption codes and super-secure communications systems that would be safe even from the predations of a rival’s quantum computer.

In one study, Oskar J. Painter and Kartik Srinivasan of California Institute of Technology in Pasadena embedded InAs quantum dots in a micron-scale resonator, created on a GaAs semiconductor chip, and shaped it into a 2.5-μm-diameter microdisk (Figure 1). They overcame the usual problem of measuring the microscopic interactions between the quantum dot and the cavity by using a customized optical fiber taper waveguide probe.

Figure 1. Researchers used a microdisk cavity to study the quantum interactions that occur between light and matter. The GaAs disk is 2.5 μm in diameter. A quantum dot was embedded in the disk, and the interactions were measured with an optical fiber probe. Reprinted with permission of Nature.

The probe was formed by heating and stretching a standard single-mode fiber so that a micron-scale waist was formed, Painter said. Light transported into the fiber is adiabatically shrunk in size as the fiber diameter is tapered, with an evanescent tail of light extending outside of the fiber in the smallest part of the tapered region. This allows for efficient waveguide-cavity coupling to occur when the taper is brought close to the microdisk. The researchers incorporated this fiber taper probe into a liquid helium cryostat to optically interrogate the microcavity devices at the requisite low temperatures for their experiments.

Painter said that the combination of fiber optic coupling and improvements to the quality of the microdisk allowed them, for the first time, to coherently excite and measure the linear and nonlinear properties of a single quantum dot exciton state (with an emission wavelength of 1300 nm) that is strongly coupled to a single photon of the cavity mode. The observed interactions were consistent with the predictions of quantum mechanics.

In research conducted at the University of California, Santa Barbara, and at Stanford University, also in California, researchers tracked and controlled the quantum interactions with an optical architecture different from that used by Painter and Srinivasan.

The group, led by Jelena Vuckovic of Stanford, embedded InAs quantum dots with a 900-nm emission wavelength in a microcavity created from GaAs photonic crystals with a periodic lattice of airholes (Figure 2). This was mounted in a cryostat and cooled almost to absolute zero. A Sacher Lasertechnik diode laser was used to probe the cavity, and a Princeton Instruments CCD camera and a PerkinElmer avalanche photodiode were used for detection.

Figure 2. Researchers used a suspended photonic crystal structure consisting of a heating pad (left) and a photonic crystal cavity (right) containing a coupled quantum dot embedded in the center (not visible in this scanning electron micrograph) to measure quantum interactions. Courtesy of Dirk Englund.

The cavity confined the light to an area only a couple of hundred nanometers in each dimension. Vuckovic said that to probe the quantum interactions directly, the scientists had to separate the reflected beam that interacted with the cavity from the high background noise. This was accomplished by using a cross-polarized setup that allowed them to improve the signal-to-noise ratio by a factor of more than 1000. The team changed the cavity from transparent to opaque by adjusting the intensity of the input laser beam on the quantum dot.

Their system allows extremely low loss and small volume, enabling the photon to remain close to the quantum dot for a long time and allowing prolonged interaction.

Nature, December 2007, pp. 857-861 and pp. 862-865.