Quantum Measurement in Action

We have recently engineered a microwave amplifier capable of detecting signals with energies as low as that of only one photon at 8GHz, a quantum 100,000 smaller than a visible photon. Using this amplifier to monitor in real time the computational state of a superconducting quantum bit, we have experimentally verified one striking prediction of quantum theory: although the continuous measurement of one dynamical variable of a system perturbs its state unpredictably, the measurement record itself always contains enough information to keep track of the system perfectly.