H.-S. Ku

Generating and verifying entangled itinerant microwave fields with efficient and independent measurements

  1. H. S. Ku,
  2. W. F. Kindel,
  3. F. Mallet,
  4. S. Glancy,
  5. K. D. Irwin,
  6. G. C. Hilton,
  7. L. R. Vale,
  8. and K. W. Lehnert
By combining a squeezed propagating microwave field and an unsqueezed vacuum field on a hybrid (microwave beam-splitter), we generate entanglement between the two output modes. We verify
that we have generated entangled states by making independent and efficient single-quadrature measurements of the two output modes. We observe the entanglement witness EW=−0.263+0.001−0.036 and the negativity N=0.0824+0.01−0.0004 with measurement efficiencies at least 26±0.1% and 41±0.2% for channel~1 and 2 respectively. These measurements show that the output two-mode state violates the separability criterion and therefore demonstrate entanglement. This shared entanglement between propagating microwaves provides an important resource for building quantum networks with superconducting microwave systems.
arxiv pdf

Initialization by measurement of a two-qubit superconducting circuit

  1. D. Ristè,
  2. J. G. van Leeuwen,
  3. H.-S. Ku,
  4. K. W. Lehnert,
  5. and L. DiCarlo
We demonstrate initialization by joint measurement of two transmon qubits in 3D circuit quantum electrodynamics. Homodyne detection of cavity transmission is enhanced by Josephson parametric
amplification to discriminate the two-qubit ground state from single-qubit excitations non-destructively and with 98.1% fidelity. Measurement and postselection of a steady-state mixture with 4.7% residual excitation per qubit achieve 98.8% fidelity to the ground state, thus outperforming passive initialization.
arxiv pdf