Measuring & Characterizing Entangled States Research Key to Quantum Information Applications

Viewpoint: Directly Measuring an Entangled State

Excerpts and salient points ~

+  Researchers have directly measured the components of a nonlocal, entangled wave function, rather than relying on indirect tomographic or reconstructive techniques.

+  [S]cientists can measure the spin of an electron by deflecting it with a magnetic field. If the magnetic field is weak enough, spin-up and spin-down electrons will only be displaced by a small amount from their mean trajectories. Such a weak measurement can leave the original spin state of the electron intact.

Guo’s team overcame the difficulties with weak values by switching to modular values, which also involve pre- and postselection of quantum states but are based on strong rather than weak measurements.

+  To extract the most from a weak measurement, researchers have devised experiments where they preselect and postselect which quantum states they look at. The output of such a measurement is called a weak value. The exact meaning of the weak value has been debated, with some researchers claiming that it gives a direct view of underlying quantum features and others suggesting that weak values can violate standard quantum limits. Still, it is clear that the study of weak values represents a bone fide advance in our understanding of quantum measurement, particularly in the direct measurement of wave functions.

+  The measurement and characterization of entangled states is extremely important for quantum information applications such as cryptography, communication, and computing. As state-of-the-art research efforts continue to produce larger entangled quantum systems, methods to characterize those systems must similarly improve. Standard tomographic methods become difficult as system sizes increase, in part due to the requirement of global reconstruction of the full quantum state. Therefore, direct measurement methods—that scale reasonably—may provide a good option for entangled state characterization.

Source:  APS Physics.  David J. Starling,  Viewpoint: Directly Measuring an Entangled State…

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