Indir ect measurements\, of which non-demolition measurements is a specific subc lass\, describe a physical situation in which an information about a quant um system is acquired through a direct measurement on a sequence of probes subsequently interacting with the system. Recent interest in the field or iginates in the photon counting experiments of Haroche. Mathematically the problem is equivalent to the study of statistics of long products of comp letely positive maps. I describe this mathematical theory with a special f ocus on the non-demolition subclass -- this is the case when all the map c ommutes -- and its small perturbation.

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\n DTSTART:20160329T193000Z LOCATION:Physics Building\, Room 313 SUMMARY:Non-demolition measurements and their perturbations END:VEVENT END:VCALENDAR