title: Hidden two-qubit dynamics of a four-level Josephson circuit.
abstract: Multi-level control of quantum coherence exponentially reduces communication and computation resources required for a variety of applications of quantum information science. However, it also introduces complex dynamics to be understood and controlled. These dynamics can be simplified and made intuitive by employing group theory to visualize certain four-level dynamics in a 'Bell frame' comprising an effective pair of uncoupled two-level qubits. We demonstrate control of a Josephson phase qudit with a single multi-tone excitation, achieving successive population inversions between the first and third levels and highlighting constraints imposed by the two-qubit representation. Furthermore, the finite anharmonicity of our system results in a rich dynamical evolution, where the two Bell-frame qubits undergo entangling-disentangling oscillations in time, explained by a Cartan gate decomposition representation. The Bell frame constitutes a promising tool for control of multi-level quantum systems, providing an intuitive clarity to complex dynamics.
authors.affiliation: Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
authors.forename: Elisha
authors.initials: E
authors.lastname: Svetitsky
authors.affiliation: 1] NSF Nanoscale Science and Engineering Center, Etcheverry Hall, University of California, Berkeley, California 94720, USA [2] Raymond and Beverly Sackler School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel.
authors.forename: Haim
authors.initials: H
authors.lastname: Suchowski
authors.affiliation: Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
authors.forename: Roy
authors.initials: R
authors.lastname: Resh
authors.affiliation: Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
authors.forename: Yoni
authors.initials: Y
authors.lastname: Shalibo
authors.affiliation: Department of Physics, University of California, Santa Barbara, California 93106, USA.
authors.forename: John M
authors.initials: JM
authors.lastname: Martinis
authors.affiliation: Racah Institute of Physics, The Hebrew University of Jerusalem, Jerusalem 91904, Israel.
authors.forename: Nadav
authors.initials: N
authors.lastname: Katz