Harry Hanlim Kang

Probing Sensitivity near a Quantum Exceptional Point using Waveguide Quantum Electrodynamics

  1. Aziza Almanakly,
  2. Reouven Assouly,
  3. Harry Hanlim Kang,
  4. Michael Gingras,
  5. Bethany M. Niedzielski,
  6. Hannah Stickler,
  7. Mollie E. Schwartz,
  8. Kyle Serniak,
  9. Max Hays,
  10. Jeffrey A. Grover,
  11. and William D. Oliver
Non-Hermitian Hamiltonians with complex eigenenergies are useful tools for describing the dynamics of open quantum systems. In particular, parity and time (PT) symmetric Hamiltonians
have generated interest due to the emergence of exceptional-point degeneracies, where both eigenenergies and eigenvectors coalesce as the energy spectrum transitions from real- to complex-valued. Because of the abrupt spectral response near exceptional points, such systems have been proposed as candidates for precision quantum sensing. In this work, we emulate a passive \PT~dimer using a two-mode, non-Hermitian system of superconducting qubits comprising one high-coherence qubit coupled to an intentionally lossy qubit via a tunable coupler. The loss is introduced by strongly coupling the qubit to a continuum of photonic modes in an open waveguide environment. Using both pulsed and continuous-wave measurements, we characterize the system dynamics near the exceptional point. We observe a behavior broadly consistent with an ideal passive \PT~dimer with some corrections due to the tunable coupler element. We extract the complex eigenenergies associated with the two modes and calculate the sensitivity as a function of the coupling strength. Confirming theoretical predictions, we observe no sensitivity enhancement near the quantum exceptional point. This study elucidates the limitations of exceptional-point systems as candidates for quantum-enhanced sensing.
arxiv pdf

Remote Entangling Gates for Spin Qubits in Quantum Dots using an Offset-Charge-Sensitive Transmon Coupler

  1. Harry Hanlim Kang,
  2. Ilan T. Rosen,
  3. Max Hays,
  4. Jeffrey A. Grover,
  5. and William D. Oliver
We propose a method to realize microwave-activated CZ gates between two remote spin qubits in quantum dots using an offset-charge-sensitive transmon coupler. The qubits are longitudinally
coupled to the coupler, so that the transition frequency of the coupler depends on the logical qubit states; a capacitive network model using first-quantized charge operators is developed to illustrate this. Driving the coupler transition then implements a conditional phase shift on the qubits. Two pulsing schemes are investigated: a rapid, off-resonant pulse with constant amplitude, and a pulse with envelope engineering that incorporates dynamical decoupling to mitigate charge noise. We develop non-Markovian time-domain simulations to accurately model gate performance in the presence of 1/fβ charge noise. Simulation results indicate that a CZ gate fidelity exceeding 90% is possible with realistic parameters and noise models.
arxiv pdf