Experimental Issues in Quantum Measurement Aephraim Steinberg Why does one thing happen and not another? When is a quantum measurement? Does a measurement necessarily disturb the system, and how? What can we say about an observable before we measure it? Does a wave function describe a single particle, or only an ensemble? Is a wave function a complete description of a single particle? Can we predict the past? Lectures: Tuesday, 10-11, Kleiner Hörsaal (auf Englisch, es tut mir leid)

Atoms falling out of a lattice trap As the trap is lowered, each quantum state falls out at a different time – we can see “by eye” how many atoms were in each energy eigenstate.

momentum position Probability P(0,0) < 0 ?! The Wigner quasiprobability function...combining many such measurements, we can completely describe the quantum state of the atoms.

Some topics we’ll cover Must “collapse” really happen? alternate pictures of measurements and correlations How much can we say about the past? weak measurements and postselection... Can we measure the entire state of a particle? density-matrix and Wigner function reconstruction Can we distinguish between indistinguishable things? non-orthogonal state discrimination Are there different things we can never distinguish between? different preparations of identical density matrices Can we measure something without interacting with it? “Vaidman’s bomb” experiment Can we undo the disturbance introduced by a measurement? The “quantum eraser” Can we use the measurement process to accomplish tasks? Measurement-induced nonlinearity, LOQC LECTURES: TUESDAYS 10-11, Kleiner Hörsaal