Description
Over the past five years, there has been much interest in a new kind of "unspeakable" quantum information which is to regular quantum information what a classical clock or gyroscope is to a classical message. While the latter is indifferent to the physical nature of the information carriers, the former requires the carrier to have a particular degree of freedom -- a spatial orientation in the case of a gyroscope, or a natural oscillation in the case of a clock. Systems serving this purpose are referred to generically as reference frames, and constitute a quantifiable resource in quantum information theory. They play a central role in the tasks of direction and frame alignment, phase estimation, clock synchronization, and global positioning. The community has only just begun a systematic study of how this resource can be manipulated, quantified, and used optimally in both information-processing protocols and physical parameter estimation.
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