Initial Teaching Alphabet Foundation

Qubit measurement is an essential step in any quantum computation. In circuit quantum electrodynamics, a leading quantum computer architecture, qubit readout is commonly one of the longest and lowest-fidelity processes. The authors numerically explore the dynamics of a driven transmon-resonator system under strong, nearly resonant measurement drives to better understand this issue. They find clear signs of transmon ``ionization'', in which the qubit escapes its confining potential under the influence of the drive, and semiclassical methods then reveal the mechanism. This approach can be used to optimize circuit parameters, suppress these spurious effects, and increase readout fidelity.

The continuous multiscale entanglement renormalization ansatz (cMERA) is a variational class of states for quantum fields. As originally formulated, the cMERA applies to infinite systems only. In this paper we generalize the cMERA formalism to a finite circle, which we achieve by wrapping the action of the so-called entangler around the circle. This allows us to transform a cMERA on the line into a cMERA on the circle. In addition, in the case of a Gaussian cMERA for noninteracting quantum fields, the method of images allows us to prove the following result: If on the line a cMERA state is a good approximation to a ground state of a local quantum field theory (QFT) Hamiltonian, then (under mild assumptions on their correlation functions) the resulting cMERA on a circle is also a good approximation to the ground state of the same local QFT Hamiltonian on the circle.