Bharath Kannan

Universal non-adiabatic control of small-gap superconducting qubits

  1. Daniel L. Campbell,
  2. Yun-Pil Shim,
  3. Bharath Kannan,
  4. Roni Winik,
  5. Alexander Melville,
  6. Bethany M. Niedzielski,
  7. Jonilyn L. Yoder,
  8. Charles Tahan,
  9. Simon Gustavsson,
  10. and William D. Oliver
Resonant transverse driving of a two-level system as viewed in the rotating frame couples two degenerate states at the Rabi frequency, an amazing equivalence that emerges in quantum
mechanics. While spectacularly successful at controlling natural and artificial quantum systems, certain limitations may arise (e.g., the achievable gate speed) due to non-idealities like the counter-rotating term. Here, we explore a complementary approach to quantum control based on non-resonant, non-adiabatic driving of a longitudinal parameter in the presence of a fixed transverse coupling. We introduce a superconducting composite qubit (CQB), formed from two capacitively coupled transmon qubits, which features a small avoided crossing — smaller than the environmental temperature — between two energy levels. We control this low-frequency CQB using solely baseband pulses, non-adiabatic transitions, and coherent Landau-Zener interference to achieve fast, high-fidelity, single-qubit operations with Clifford fidelities exceeding 99.7%. We also perform coupled qubit operations between two low-frequency CQBs. This work demonstrates that universal non-adiabatic control of low-frequency qubits is feasible using solely baseband pulses.
arxiv pdf

Generating Spatially Entangled Itinerant Photons with Waveguide Quantum Electrodynamics

  1. Bharath Kannan,
  2. Daniel Campbell,
  3. Francisca Vasconcelos,
  4. Roni Winik,
  5. David Kim,
  6. Morten Kjaergaard,
  7. Philip Krantz,
  8. Alexander Melville,
  9. Bethany M. Niedzielski,
  10. Jonilyn Yoder,
  11. Terry P. Orlando,
  12. Simon Gustavsson,
  13. and William D. Oliver
Realizing a fully connected network of quantum processors requires the ability to distribute quantum entanglement. For distant processing nodes, this can be achieved by generating,
routing, and capturing spatially entangled itinerant photons. In this work, we demonstrate deterministic generation of such photons using superconducting transmon qubits that are directly coupled to a waveguide. In particular, we generate two-photon N00N states and show that the state and spatial entanglement of the emitted photons can be tuned via the qubit frequencies. Using quadrature amplitude detection, we reconstruct the moments and correlations of the photonic modes and demonstrate state preparation fidelities of 84%. Our results provide a path towards realizing quantum communication and teleportation protocols using non-classical, spatially entangled itinerant photons.
arxiv pdf