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Weak measurement of spin and its experimental realisation in atomic systems

Weak values have a long history and were first considered by Landau and London in connection with superfluids. Hirschfelder called them sub-observables and Dirac anticipated them when discussing non-commutative geometry in quantum mechanics. The idea of a weak value has returned to prominence due to Aharonov, Albert and Vaidman showing how they can be measured. They are not eigenvalues of the system and cannot be measured by a collapse of the wave function with the traditional Von Neumann (strong) measurement which is a single stage process. In contrast the weak measurement process has three stages; preselection, weak stage and finally a post selection. Although weak values have been observed using photons and neutrons, we are building an experiment to observe a weak value of spin in an atomic system. We are using a method based on a variant of the original Stern-Gerlach experiment using a metastable, 23S1, form of helium. Two analyses have been carried out using the impulsive approximation and the Feynman propagator methods. They are in complete agreement. The design, simulation and realisation of the experiment will be presented.