Heralded State Preparation in a Superconducting Qubit

We demonstrate high-fidelity, quantum nondemolition, single-shot readout of a superconducting flux qubit in which the pointer state distributions can be resolved to below one part in 1000. In the weak excitation regime, continuous measurement permits the use of heralding to ensure initialization to a fiducial state, such as the ground state. This procedure boosts readout fidelity to 93.9% by suppressing errors due to spurious thermal population. Furthermore, heralding potentially enables a simple, fast qubit reset protocol without changing the system parameters to induce Purcell relaxation.

Figure 1

Experimental setup. (a) False-color scanning electron microscope image of the qubit and readout resonator with a magnified inset of the qubit. (b) Path of the readout signal at 30 mK.

Figure 2

Qubit readout at $\overline{n}=14.6$ photons. (a) Three individual qubit readouts starting at $t=0$: excited state (solid red line), ground state (long blue dashed line), abrupt quantum jump from the excited to ground state (short green dashed line). (b) Log-linear distributions of many ground and excited state preparations.

Figure 3

Raw fidelity and $T_1$ during readout as a function of mean cavity photon occupation..

Figure 4

Pure readout pointer state distributions. A two-point correlation procedure is used to exclude qubit transitions and create pure ground and excited state distributions.

Figure 5

Heralded state preparation at $\overline{n}=14.6$. (a) Pulse sequence used for heralded state preparation. (b) Log-linear raw readout distributions for $\sim {10}^5$ excited and ground state readout events (solid lines) compared with the distributions generated after heralded state preparation (dashed lines). The fidelity increases from 91.0% to 93.9%.