Demonstrating a Driven Reset Protocol for a Superconducting Qubit

Qubit reset is crucial at the start of and during quantum information algorithms. We present the experimental demonstration of a practical method to force qubits into their ground state, based on driving appropriate qubit and cavity transitions. Our protocol, called the double drive reset of population, is tested on a superconducting transmon qubit in a three-dimensional cavity. Using a new method for measuring population, we show that we can prepare the ground state with a fidelity of at least 99.5% in less than $3\mu \mathrm{s}$; faster times and higher fidelity are predicted upon parameter optimization.

Figure 1

Level structure of the transmon qubit coupled dispersively to a single resonator mode. The qubit excitations are spanned vertically, while the resonator photon numbers are spanned horizontally. The arrows show the transitions involved in the DDROP procedure along with their rates, with ${\Gamma }_{up}\ll \kappa \approx {\Omega }_R<\chi /2$. The double arrows are driven transitions, while single arrows are spontaneous. Qubit transitions are represented by straight lines, while cavity transitions are wavy lines. The steady-state equilibrium qubit-cavity joint state is the coherent state $\mid g,\alpha ⟩$. For visualization, the state $\mid g,m⟩$ is highlighted, where $m$ is the closest integer to the steady-state average number of photons in the cavity.

Figure 2

Measured excited state population after reset pulse of varying duration for four different initial preparations, measured after intervals of 40 ns. The solid lines include a pre-reset, while the dashed lines begin with the steady state 9% excited population. The $w/\pi$ pulse curve shows a slight downward trend due to the finite $T_1$. The curves with DDROP show that, regardless of initial state, the qubit is driven to the ground state for pulse durations less than $2\mu \mathrm{s}$. For this measurement, ${\Omega }_R\approx 0.8\kappa$ and $\overline{n}=8$.

Figure 3

Upper panel: pulse sequences used to perform qubit population measurement (RPM, see text), each producing an oscillation whose amplitude is proportional to initial excited (a) and ground (b) state population. Circle radii indicate population in each state, vertical bars separate the two extrema in Rabi oscillations. Lower panel (c): example normalized data for measurement of 7% excited state population.

Figure 4

(a) Contours of 90, 95, and 99% predicted ground state preparation fidelity from numerical simulations vs two Rabi drive amplitudes expressed as ${\Omega }_R/\kappa$ and $\overline{n}$. For fidelities greater than 99%, the shaded area indicates reset time. (b) Measured excited state population from RPM method (crosses with error bars) compared to numerical simulation (solid line) vs $\overline{n}$ for ${\Omega }_R/\kappa =0.8$. This population decreases monotonically with $\overline{n}$.