Yu-Xin Wang

In-situ Characterization of Light-Matter Coupling in Multimode Circuit-QED Systems

  1. Kellen O'Brien,
  2. Won Chan Lee,
  3. Alexandra Behne,
  4. Ali Fahimniya,
  5. Yu-Xin Wang,
  6. Maya Amouzegar,
  7. Alexey V. Gorshkov,
  8. and Alicia J. Kollár
Multimode cavity-QED systems can be leveraged to explore a wide range of physical phenomena; however, a complex multimode environment makes systematic characterization of light-matter
interactions challenging. Here we present a general measurement protocol, applicable to both atomic and synthetic cavity-QED systems, that enables the determination of coupling to individual photonic modes. The method leverages measurements of the AC-Stark and Kerr effects, along with known detuning dependencies, to eliminate the need for single-photon resolution, independent photon-number calibration, or insertion-loss calibration. We demonstrate the method using a superconducting transmon qubit coupled to a one-dimensional microwave resonator lattice. We validate the consistency of the extracted light-matter couplings g determined at multiple qubit detunings, and from the self-Kerr and cross-Kerr shifts for three photon modes, which provide separate measurements of g for each of the three modes.
arxiv pdf

Superconducting antiqubits achieve optimal phase estimation via unitary inversion

  1. Xingrui Song,
  2. Surihan Sean Borjigin,
  3. Flavio Salvati,
  4. Yu-Xin Wang,
  5. Nicole Yunger Halpern,
  6. David R. M. Arvidsson-Shukur,
  7. and Kater Murch
A positron is equivalent to an electron traveling backward through time. Casting transmon superconducting qubits as akin to electrons, we simulate a positron with a transmon subject
to particular resonant and off-resonant drives. We call positron-like transmons „antiqubits.“ An antiqubit’s effective gyromagnetic ratio equals the negative of a qubit’s. This fact enables us to time-invert a unitary implemented on a transmon by its environment. We apply this platform-specific unitary inversion, with qubit–antiqubit entanglement, to achieve a quantum advantage in phase estimation: consider measuring the strength of a field that points in an unknown direction. An entangled qubit–antiqubit sensor offers the greatest possible sensitivity (amount of Fisher information), per qubit, per application of the field. We prove this result theoretically and observe it experimentally. This work shows how antimatter, whether real or simulated, can enable platform-specific unitary inversion and benefit quantum information processing.
arxiv pdf