In these studies the semiconductor dots and rings are made from indium arsenide embedded in gallium arsenide. They were grown using techniques developed within the past decade that allow much smaller nanostructures to be created. In these new experiments, electrons are introduced one by one into individual quantum dots while their optical emission is measured with extraordinary precision. Such studies provide new perspectives on the internal quantum-mechanical workings of quantum dots. The ultimate goal is to create useful electronic and optical nanomaterials that have been quantum-mechanically engineered by tailoring the shape, size, composition and position of various quantum dots.

The importance of semiconductors, large or small, lies primarily in the great change in their properties when the number of active electrons in the material is altered. The electron density can be controlled with high sensitivity through doping, optical excitation or external electric fields. In a semiconductor quantum dot this sensitivity becomes extreme as the electrons are confined in all three dimensions. Complete quantum confinement profoundly affects the electrons, greatly increasing or decreasing the magnitude of their interactions with each other, with their environment and with external fields. Just as in atoms, and in sharp contrast to bulk semiconductor systems, the electrons in the quantum dot exist only in certain quantized energy states.

Figure 1 Energy levels in a semiconductor quantum dot.    Full legend   High resolution image and legend (21k)

It has been instructive to explore the properties of quantum dots as electrons are introduced in a controlled way^4-9. For example, adding even a single additional electron takes a significant amount of extra energy because of the repulsion between the negatively charged electrons as they are forced into a smaller volume. One result of this effect, called the Coulomb blockade, is to provide researchers with great control over the number of electrons in a quantum dot. That is, they can easily tune the number by increasing or decreasing the energy they put in.

Currently there is great interest in semiconductor quantum dots that interact strongly with light. Optical excitation of a semiconductor leads to the creation of a quasiparticle known as an exciton — a negatively charged electron bound together with a positively charged 'hole'. In contrast to the electrical injection of electrons that leads to the Coulomb blockade effect, a quantum dot remains neutrally charged following optical excitation. Both Bayer et al.² and Warburton et al.³ study this exciton in detail by measuring the light emitted when the hole and electron recombine. This occurs when extra electrons are electrically injected into the quantum dot³, or when additional excitons are created in a carefully controlled way². In closely related but complementary experiments, the authors build up quantum-dot analogues of the periodic table of atoms as they add electrons or excitons one by one. Repulsive interactions between the particles lead to differences in the amount of energy it takes to create or destroy an exciton.

Quantum confinement of electrons is just one of several ways quantum mechanics reveals itself in semiconductor nanostructures. Another way is through the spin of the electron, which plays a central role in the current studies. An electron with spin behaves like a small magnet, with either 'up' or 'down' spin states. When more than one electron with the same spin exists in the quantum dot, a purely quantum-mechanical interaction, known as an exchange interaction, becomes important. As a result, the energy to create or destroy a second electron (plus its accompanying hole) in the presence of an existing electron also depends on their relative spin.

Electronic states in quantum dots persist for a relatively long time because they interact in a very restricted way with their environment. Normally, such interactions lead to 'decoherence' or destruction of the quantum state. As a result, quantum dots may provide an excellent solid-state system for exploring advanced technologies based on quantum coherence. For example, it may be possible to create and control superimposed or even 'entangled' quantum states using highly coherent laser stimulation¹⁰. External control of the full quantum wavefunction in a semiconductor nanostructure may even lead to revolutionary new applications, such as those involving quantum computing¹¹.

Much more can be done to explore and ultimately harness the quantum-mechanical properties of these artificial solid-state atoms. New research directions are emerging. One that is now in the embryonic stage is the combination of quantum dots into quantum-dot molecules. Many of the quantum-dot systems currently being studied have the potential to be combined into molecular complexes with one-, two- or even three-dimensional structures. One can imagine growing these solid-state atoms or molecules within structures containing electronic or magnetic gates and optical cavities, perhaps all connected together by quantum wires.

Quantum dots have great flexibility because their properties can be artificially engineered, but this comes at a price. Nature has given us atoms; scientists must make quantum dots. Further advances in this exciting field of science and technology will depend heavily on the creativity of physicists, chemists and materials scientists who make these tiny structures.