The Road to the Poles: Quantum Measurements that Steer
rather than Collapse
Michel Devoret, Department of Applied Physics, Yale University
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, an information-conserving operation whose
action is
known after the fact, instead of a decoherence process. 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]. Accurate monitoring with a quantum-limited amplifier is
an essential
prerequisite for measurement-based feedback control of quantum
systems and
remote entanglement of superconductingqubits.