J. R. Rozen

Enhancing qubit readout fidelity with two-mode squeezing of the coherent measurement signal

  1. Baleegh Abdo,
  2. William Shanks,
  3. Oblesh Jinka,
  4. and J. R. Rozen
The ability to perform high-fidelity quantum nondemolition qubit readout is pivotal for the realization of large and powerful quantum computers. Such readout of superconducting qubits
is generally enabled by amplifying the weak dispersive measurement signals using phase-preserving quantum-limited Josephson amplifiers with sufficient gain to dilute the contribution of the added noise by the output chain. Here, we further enhance the qubit readout fidelity by (1) simultaneously measuring the two-mode squeezed states of the amplified readout signals at the signal and idler frequencies of the nondegenerate amplifier and (2) coherently combining them at the classical processing stage following a relative rotation that maximizes the signal to noise ratio of the qubit-encoded readout quadrature. Such readout scheme exhibits enhancement in the readout fidelity for all practical values of amplifier gain and noise added by the output chain and is fully compatible with frequency multiplexed setups used in large quantum processors.
arxiv pdf

Superconducting qubit in waveguide cavity with coherence time approaching 0.1ms

  1. Chad Rigetti,
  2. Stefano Poletto,
  3. Jay M. Gambetta,
  4. B. L. T. Plourde,
  5. Jerry M. Chow,
  6. A. D. Corcoles,
  7. John A. Smolin,
  8. Seth T. Merkel,
  9. J. R. Rozen,
  10. George A. Keefe,
  11. Mary B. Rothwell,
  12. Mark B. Ketchen,
  13. and M. Steffen
We report a superconducting artificial atom with an observed quantum coherence time of T2*=95us and energy relaxation time T1=70us. The system consists of a single Josephson junction
transmon qubit embedded in an otherwise empty copper waveguide cavity whose lowest eigenmode is dispersively coupled to the qubit transition. We attribute the factor of four increase in the coherence quality factor relative to previous reports to device modifications aimed at reducing qubit dephasing from residual cavity photons. This simple device holds great promise as a robust and easily produced artificial quantum system whose intrinsic coherence properties are sufficient to allow tests of quantum error correction.
arxiv pdf