PhD Defense - Vidul Joshi

 

ABSTRACT:  Quantum mechanics and classical physics differ fundamentally when it comes to measurement. In classical physics, measurement is nonintrusive, which means the act of measurement does not disturb the state of the system. In quantum mechanics, however, measurement is often invasive, meaning the act of measurement has a back action on the system’s state. This difference is illustrated to its full extent by the quantum Zeno effect, where the evolution of a quantum system is inhibited when it is repeatedly measured. The system is repeatedly projected into an eigenstate of the measurement operator, preventing its natural evolution. In theory, the quantum Zeno effect should slow down both the unitary evolution of quantum systems as well as the non unitary evolution where a quantum system is interacting with an environment. In practice, however, the quantum Zeno effect is rarely observed in the latter case, where the decay of a quantum system loosing energy to the environment is slowed down by frequent measurements.

 

This is because more often than not, the rate of measurements is too slow compared to the time at which of environmental correlations die. This raises the question: under what conditions can the quantum Zeno effect actually slow down the decay of an open quantum system? In this thesis, we demonstrate how the quantum Zeno effect slows down the decay of a microwave photon in a superconducting resonator. The experiment is performed in a circuit quantum electrodynamics (cQED) setup that enables precise control over quantum systems with several interacting degrees of freedom. A photon is loaded into a superconducting resonator, where it decays through a controlled loss channel, allowing us to tune the bandwidth of the environment interacting with the photon. To measure the decay of the photon and observe its slowing down, we use a transmon dispersively coupled to the resonator. The quantum Zeno effect appears when the measurement rate exceeds the memory time of the environment. Our results show that this effect arises from the competition between two dissipative processes: photon decay and measurement-induced back action. By tuning both the measurement rate, and the linewidth of the environment, we can make the measurement backaction win this competition and successfully observe the Zeno effect. At the highest possible measurement rate, the decay rate of the photon is slowed down by a factor of 7. The experiment requires us to tune several processes simultaneously, which occur on time scales spanning three orders of magnitude. The results of this thesis how circuit QED can be used to investigate fundamental quantum phenomena, providing insight into the complex interplay between measurement and dissipation in quantum systems.