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Section 2 Recap ► ► Principle of Superposition: quantum states show interference and require both an amplitude and a phase for the parts ► ► Superposition applies in time as well as space ► ► For any observable, measured values come from a particular set of possibilities (sometimes quantised). Some states (eigenstates) always give a definite value (and therefore are mutually exclusive). Model as an orthonormal set of basis vectors. ► ► Model physical states as normalised vectors Can be expanded in terms of any convenient set of eigenstates. ► ► Measurements on systems in a definite quantum state (not an eigenstate) yield random results with definite probabilties for each. Represent the probabilities of modulus-squared of coordinates |c i | 2 for the corresponding eigenstates in the eigenbasis of the observable. ► ► Some features of the mathematical formalism (e.g. overall phase of the state vector) don’t correspond to anything physical.

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Section 2 Recap ► Change with time is represented by a linear, unitary time evolution operator, U(t 0,t) Unless interrupted by a measurement U I as the time interval t 0 −t 0. ► From U we derive Hamiltonian operator, H, and the (time- dependent) Schrödinger Equation For a closed system U = exp[−iHt / ħ ] ► Measurements cause apparently discontinuous change in the state vector (“collapse of the wave function”). After an ideal measurement yielding result a i, state is in corresponding eigenstate |a i Best way of preparing systems in given quantum state is measurement + selection of required state. ► Hamiltonian for a particle in a field is H = − .B = − S.B

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