Quantum System’s Optimal Pathway Measured

In work with implications for laser-based quantum control of chemical reactions, researchers have made the first continuous measurements of a quantum system’s optimal pathway as it transitions between two energy levels, or states. Such pathways have not been measured before because probing quantum systems generally causes them to collapse to classically permitted states. Physicist Kater W. Murch of Washington University in St. Louis and coworkers used small numbers of microwave photons to study energy-level transitions of a quantum device, a superconducting circuit (Nature 2014, DOI: 10.1038/nature13559). The photon measurements are so weak that they don’t cause quantum system collapse. From thousands of measurements, the researchers were able to deduce the most likely path between two quantum states—a curve consistent with a quantum version of the classical physics principle that moving objects follow paths of least action—the easiest, lowest-energy trajectories. The curve agrees with an earlier prediction for such systems by quantum theoreticians. Chemical reactions also follow pathways between states, so in the long term “our results might have important implications for quantum control in chemistry,” Murch says.