Initialization by measurement of a two-qubit superconducting circuit

Abstract

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.

5 pages, 4 figures, and Supplementary Information (7 figures, 1 table)

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• Quantum Computing
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• Feedback control of a solid-state qubit using high-fidelity projective measurement
• Resonantly phase-matched Josephson junction traveling wave parametric amplifier
• Measuring a topological transition in an artificial spin 1/2 system
• Machine learning for discriminating quantum measurement trajectories and improving readout
• Widely tunable on-chip microwave circulator for superconducting quantum circuits
• Measurement-induced qubit state mixing in circuit QED from up-converted dephasing noise
• Electron Spin Resonance at the Level of 10000 Spins Using Low Impedance Superconducting Resonators
• Future Directions in the Microwave Cavity Search for Dark Matter Axions
• Noiseless quantum measurement and squeezing of microwave fields utilizing mechanical vibrations
• Dissipation-induced continuous quantum error correction for superconducting circuits
• Theory of microwave single-photon detection using an impedance-matched $\Lambda$ system
• Measuring a transmon qubit in circuit QED: dressed squeezed states
• Experimental Quantum Randomness Processing
• Circuit QED flip-flop memory with all-microwave switching
• Quantum Bayesian rule for weak measurements of qubits in superconducting circuit QED