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Vincenzo Monacchello

The weak value of spin for metastable helium 23S1

The notion of a weak measurement enables one to detect very small phase shifts. By preparing the system in a particular pre- and post-selected quantum state, it is possible to amplify these phase shifts by measuring the appropriate weak value. The whole process comprises of three stages; pre-selection, the weak unitary evolution stage and finally the usual strong measurement stage (post-selection) when the wave function is said to „collapse“.  
This experiment utilises a pulsed supersonic beam of metastable Helium atoms. During their flight, the atoms will be prepared in a chosen quantum state and travel through two inhomogeneous magnets (weak and strong). In the weak stage, the wave function evolves unitarily under the interaction Hamiltonian, weakly coupling the spin and centre-of-mass wave functions. The displacement of the post-selected state at our detector will allow for the determination of the weak value of spin. 

Full simulations have been carried out utilising two independent methods. This includes the optimisation of the experimental parameters in order to achieve maximum displacement, while conforming to the first order approximation as outlined by Aharonov, Albert and Vaidman (AAV) [1]. These simulations give predictions of what should be observed within the parameters set by the experiment. We will report on the simulations and the present progress in testing the theory experimentally.

Aharonov, Y., Albert, D. Z. and Vaidman, L., 1988 Phys. Rev. Lett., 60, 1351-4.

The weak value of spin for metastable helium 23S1 by Vincenzo Monacchello

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