Quantum Dots in a Whole New Light
A team headed by JQI Fellow Glenn Solomon of NIST has reached a new milestone in understanding and manipulating the exotic creations called “quantum dots,” or QDs for short (see reference publication).
QDs are artificial three-dimensional structures, made of semiconductor material, that are only a few tens of nanometers at their widest. That’s small: About 1,000 dots placed side by side would barely equal the width of a human hair. But it’s exactly the right size to perform certain kinds of tricks that are much in demand in nanoelectronics and information processing, as well as quantum optics and encryption.
In particular, researchers are interested in QDs’ ability to act like individual atoms, each of which has a strictly limited, clearly demarked set of permitted energy states -- and hence a very specific set of photon frequencies that it can absorb and emit.
Although QDs contain tens of thousands of atoms, they have some atom-like behaviors because of the peculiar nature of quantum-mechanical objects. In the quantum world, where particles can act like waves, each object (such as an electron) has an associated wavelength. Structures of the right size can confine a certain set of wavelengths just as a clarinet or oboe can play a certain set of notes whose wavelengths have the right mathematical relationship with the length of the horn. The longer oboe can resonate with longer confined wavelengths, and thus can play lower notes: Frequency depends on size. The same is true for quantum dots. And just as a musician can choose a particular instrument to play a desired set of notes, nanoscientists can customize dots for different purposes.
In the case of QDs, the wavelengths of interest belong to a two-part entity that behaves as a single “quasiparticle”: an electron and a "hole." (A hole is the absence of an electron in the semiconductor material's lattice that results when an electron is excited out of its ground state. Holes behave like, and can be treated as, positively charged objects.) The electron-hole pair is called, appropriately, an exciton because it can be viewed as an excitation in the crystal, and because they release energy in the form of a photon when they recombine. Additional quanta of energy can produce paired excitons, called biexcitons. Like every other quantum-mechanical entity, excitons and biexcitons have associated wavelengths. QDs are the right size to confine those wavelengths in three dimensions.
As a result, QDs promise to serve as a minutely controllable source of photons for signal processing, and for generating “entangled” pairs of photons to use in transmitting secure keys to encrypted messages. Dots produce a dependable volume of photons with a high degree of wavelength accuracy, and their semiconductor materials are familiar to scientists and engineers.
When excitons or biexcitons recombine, they emit photons or pairs of photons in a “radiative cascade,” dropping from one energy level to the next lowest as they shed energy. In the quantum world, only certain energy levels, or quanta, are permitted. So there are only a few allowed sequences, or paths, for this transition process, and each path results in emission of photons of particular distinctive wavelengths. There are parallel paths, with each path producing photons of opposite polarization. That could make them ideal subjects for entanglement because, in the absence of information about which photon was polarized in which direction, taking a measurement of one photon would instantly determine the other's polarization.
Exploiting those properties in a practical device, however, will require extremely sensitive understanding of exactly how QDs emit certain kinds of photons in specific conditions. So Solomon and colleagues set out to determine how completely the full emission spectrum of a QD could be controlled and measured.
Their technique involved two separate lasers trained on the dot. The first was used to "dress" the dot by using the stro0ng laser field to alter the quantum dot states. Then a second laser beam was applied to inject carriers into the QD states. As these carriers relax to lower energies, they emit photons of various particular wavelengths depending on which of the allowed pathways they followed..
The group made its QDs of indium arsenide embedded in a surrounding crystal of gallium arsenide. To minimize thermal “noise” effects, they cooled the sample to 4.2 degrees above absolute zero with liquid helium. Then they focused on emissions from a single “dressed” QD about 30 nanometers (billionths of a meter) wide and 5 nm high. If the control ("dressing") laser beam and the pump laser beam were in the same plane, optical "noise" from scattered control photons would ruin the output signal. So the team attached an optical fiber carrying the control beam at right angles to the pump beam. (See illustration.)
Under the experimental conditions, the QD exciton and biexciton states have five or 10 ways to emit photons of various wavelengths. By using different tunings of the lasers, the researchers were able to prompt and record every possible permitted emission from the dot, notably including the “fine structure” that is well known in theory, but difficult to observe. In all cases, the experimental data were in excellent agreement with simulations produced by theoretical calculations.
The work constitutes the first instance in which the complete exciton-biexciton emission spectrum of a dressed QD has been recorded. But it has considerable further significance, for two reasons. First, the group was able to produce any desired emission by controlling the intensity and “detuning” of the control laser. (Detuning is the process of shifting the frequency of one wave so that it is out of resonance with the target QD transition.) Whether this can eventually be accomplished in a practical device at room temperature is unknown. But if so, it could provide the basis for exquisite control over photons used in numerous forms of quantum information processing.
That sort of regulation could compensate for the inherent problems that arise when QDs -- which are often fabricated by spraying atoms onto a surface in a vacuum, a necessarily inexact method -- emerge with asymmetrical proportions that in turn affect their optical properties.
Second, the JQI group demonstrated that the output of the dressed QD could be generated one photon at a time by using a pulsed laser source for the pump beam. That ability will be important in any eventual functional equipment based on the phenomena.
In the next stage of the research, the group will tackle the problem of making photons using their optical techniques when the original QD asymmetries forbid them.
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