Two independent teams of physicists are the first to have entangled a single photon with a single electron spin held in a quantum dot. Thanks to the ease with which quantum dots can be fabricated and controlled, the breakthrough could lead to practical quantum computers and quantum communication systems.
Entanglement is a quantum effect that allows particles such as photons and electrons to have a closer relationship than predicted by classical physics. For instance, a photon–electron pair can be created experimentally such that if the photon polarization is measured to be in the vertical direction, a measurement of the electron spin would find its spin pointing in the same direction. This occurs in spite of the fact that a measurement on the photon (or electron) alone will reveal a random value.
This close relationship could be put to use in quantum computers, which could in principle outperform today's classical computers. Photons are expected to play an important role in quantum computation because they can carry bits of quantum information (qubits) over long distances. However, photons cannot by their very nature stand still and stationary qubits such as quantum dots are needed to store quantum information. While researchers have already shown that trapped ions and defects in diamond crystals can be entangled with single photons, these systems can be difficult to work with on a practical level.
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