Is Measurement a Black Box? On the Importance of Understanding Measurement Even in Quantum Information and Computation

It has been argued, partly on the grounds that no decent solution to the measurement problem seems to be forthcoming, and partly on the grounds of recent results from quantum information theory, that measurement in quantum theory is best treated as a black box, and quantum theory itself is best treated as a theory of information and computation. I agree with the premises of the argument, but not the conclusion. In particular, there is a crucial difference between ‘having no account of measurement’ and ‘having no complete account of measurement that is entirely internal to the theory’ (i.e., having no solution to the measurement problem). In fact, we know quite a bit about measurements. In this paper, I will argue that taking into account some of what we know about measurements—in particular, the role of reference frames in measurements—sheds some light on quantum theory as a theory of information and computation. In fact, the recently proposed ‘one-way quantnum computation’ scheme takes on a new character in light of the role that reference frames must play in the actually carrying out of any one-way quantum comptuation. 1 The ‘black-box’ view of measurement Measurement has often been treated as a ‘black-box’ in quantum theory. In particular, one (if one is a philosopher!) very seldom worries about the physics of a given measurement. Instead, we characterize measurements in terms of some operator on a Hilbert space, and take it from there. ∗Department of Philosophy, University of South Carolina, Columbia, SC 29208, USA.