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Accessing the dark exciton with light

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33924457 Eilon Poem

The dark exciton is a non-optical excited state of a semiconductor. When electronic excitation in a semiconductor occurs, an electron (which has a negative charge) "jumps" from the full, lower-energy valence band, to the higher-energy, empty conduction band. The missing electron in the valence band is called a "hole", and its electric charge is positive. The electron and the hole electrically attract one another, until they are "bound". A bound electron-hole pair is called an "exciton".

The electron (and the hole as well) has a quantum degree of freedom called "spin". Spin is analogous to the rotation of a particle about its axis, and can point in two directions: "up" and "down". If the hole's spin, like its charge, is the opposite of the electron's spin, the electron can return back to the valence band while emitting its excess energy in the form of a single particle of light (a "photon"). Such an exciton is thus called a "bright" exciton. However, if the spins of the electron and the hole both point at the same direction, such a return is impossible, and the exciton is called "dark". When the emission of light is possible, it happens very fast. Thus, the bright exciton "lives" for no more than a billionth of a second. The dark exciton, for which this process is forbidden, lives about one thousand times longer.
The total spin of the dark exciton is determined by the sum of the (parallel) spins of the electron and the hole, and it also can point up (logical "one" of a single bit of information) or down (logical "zero").  However, in quantum mechanics, the real spin state is described as a combination of these two states. The number of possible such combinations is infinite. Therefore, the spin state holds much more information than is held in a classical bit, and it can be used as a quantum bit (a "qubit"). The future technology of quantum information processing, including quantum computers, relies on the ability to implement qubits.
We have found a new way to "write" and "read" the spin of the dark exciton using light. We have trapped  a single dark exciton in a nanometric "trap" (a "quantum dot") inside a semiconductor, and succeeded in preparing it spin in a given state, and then follow its temporal evolution. In this manner, we have managed, for the first time, to measure the rate by which the dark exciton's spin oscillates between its two states.
We believe that this discovery contains a large scientific value due to the novelty of the research. Furthermore, this discovery may mark a mile stone on the way to applications. This is because in the context of using it as a qubit, the dark exciton's longevity, its neutrality and its insensitivity to external perturbations, are strong advantages. Moreover, the use of semiconductor quantum dots is very attractive in this context, since semiconductors and their nanometric structures are compatible with the contemporary technologies of information processing, communication and computation, which completely rely on them.

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