Measurement and Control in Open Quantum Systems

Washington University, Saint Louis MO

Investigators

Abstract

According to quantum mechanics, particles do not have definite properties such as position and momentum, but are instead described by a "complex valued wavefunction" which describes them mathematically in terms of probabilities. 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 its 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. The goal of this project is to deepen our understanding of quantum measurement and to harness the measurement interaction to control the evolution of quantum particles. The approach will use microscopic superconducting circuits as artificial atoms and the interaction of these atoms with microwave light to create open quantum systems. The team will conduct a series of experiments that explore the measurement process and how measurement can be used to control quantum evolution, an important component of emerging quantum-based technologies. This project will utilize fabricated quantum systems (superconducting artificial atoms) and the physics of cavity quantum electrodynamics to create systems with unprecedented control of the quantum environment. The team will undertake a series of experiments that explore the physics of quantum measurement, quantum control, and fundamental symmetries. The first experiment examines the boundary between quantum and classical information. A few photon signal will be entangled with the energy states of an artificial atom and then controllably amplified (or squeezed) with a superconducting parametric amplifier, enlarging the Hilbert space of the few photon pointer state. A second amplifier will then be used to probe the resulting entanglement between the amplified pointer state and atom. The second experiment examines the process of radiative decay and how detection of spontaneously emitted photons can be used to control the evolution of the atomic states. A parametric amplifier will be used to perform homodyne measurement of radiation emitted from a quantum emitter. The team will study how the choice of homodyne measurement angle can be used to steer the evolution of the emitter's state. The third project will create a system of two artificial atoms that exhibits space-time inversion symmetry to study the parity-time symmetry breaking phase transition. The parity-time symmetric system will be created through quantum reservoir engineering, inducing loss for one atom and gain for the other atom. The steady states of the system will be probed through spectroscopy and quantum state tomography as a function of the coupling between the two atoms.