Michel Devoret, Yale University
January 29
A quantum system subject to the infinitely-strong measurement of textbook physics undergoes a discontinuous, random state collapse. All phase information in the measured system that involves a superposition of the eigenstates of the measurement operator is erased. However, in practice, measurements often involve a finite-strength, continuous process whose iteration leads to a projective evolution only asymptotically. Moreover, if the observation apparatus is fully efficient information-wise, the measured system can remain at all times in a pure state. The stochastic evolution of this pure state is trackable from the measurement record. Thus, an initial superposition of states can be usefully transformed by a partial measurement rather than be entirely destroyed. In other words, a fully efficient partial measurement can be understood as an information-conserving operation whose action is known after the fact, rather than a process inducing decoherence. This striking property has been demonstrated in superconducting qubit experiments in which readout is performed by a microwave signal sent through a cavity dispersively coupled to the qubit, and thereafter processed by an amplifier operating at the quantum limit [1]. Accurately monitoring a qubit state is an essential prerequisite for measurement-based feedback control of quantum systems.
[1] Hatridge et al., Science 339, 178 (2013)
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