"Looking inside quantum wave function collapse: what measurement can tell us about the arrow of time." 

 

Kater Murch

Washington University, St. Louis

This talk will describe recent experiments that reconcile the closed evolution of isolated quantum particles with the process of measurement, which reveal how quantum particles evolve from superpositions of states to definite states under measurement. 

 

These experiments harness state of the art techniques to fabricate quantum circuits from superconducting metals and then to measure the quantum states of these circuits with extraordinary precision. By initializing the circuit in a superposition of two different energy states and then subsequently using weak measurements to slowly accumulate information about the state of the circuit, we are able to observe quantum trajectories of the state that continuously connect the initial superposition state to the final definite outcome. 

 

These trajectories give us new ways to examine fundamental questions such the arrow of time and the origins of thermodynamics, as well as shed new light on everyday processes such as how lightbulbs emit light.

According to quantum mechanics, particles do not have definite properties such as position and momentum, but are instead described by a complex valued wavefunction. The wave nature of quantum particles means that these particles can exist in superpositions of seemingly disparate states, for example a quantum particle could be in two places at once, or heading in two different directions, or occupy a superposition of two different energy levels. 

 

The evolution of this wavefunction obeys the Schrödinger equation which was formulated in 1925. Since it’s formulation, the Schrödinger equation has been applied to understand the properties of atoms and molecules and the basis for chemistry and materials. Yet, the Schrödinger equation only applies to isolated quantum systems. 

 

If one is to measure the properties of a quantum particle with suitable precision, a definite answer may result even if the particle is in a superposition of states. Thus the act of measurement collapses the wavefunction from an initial superposition to a definite state. This collapse process cannot be described by the Schrödinger equation and reconciling the evolution of measured, “open” quantum systems with the theory has been a topic of intense debate and research since the origins of quantum theory.