Qubit Architecture with High Coherence and Fast Tunable Coupling

We introduce a superconducting qubit architecture that combines high-coherence qubits and tunable qubit-qubit coupling. With the ability to set the coupling to zero, we demonstrate that this architecture is protected from the frequency crowding problems that arise from fixed coupling. More importantly, the coupling can be tuned dynamically with nanosecond resolution, making this architecture a versatile platform with applications ranging from quantum logic gates to quantum simulation. We illustrate the advantages of dynamical coupling by implementing a novel adiabatic controlled-z gate, with a speed approaching that of single-qubit gates. Integrating coherence and scalable control, the introduced qubit architecture provides a promising path towards large-scale quantum computation and simulation.

Figure 1

(a) Optical micrograph of two inductively coupled gmon qubits. The cross-shaped capacitors are placed in series with a tunable Josephson junction and followed by a linear inductor to ground. The circuit is depicted schematically in (b) with arrows indicating the flow of current for an excitation in the left qubit. The qubits are connected with a junction serving as a tunable inductor to control the coupling strength. (c) Micrographs of the coupler junction (left) and qubit SQUID (right). The bottom of each image shows a bias line to adjust the coupling strength (left) and qubit frequency (right, not shown in schematic).

Figure 2

(a) The dependence of coupling strength on the coupler flux bias while the two qubits are on resonance, with ${\omega }_{Q1}/2\pi ={\omega }_{Q2}/2\pi =5.67\mathrm{GHz}$. For each value of the coupler flux bias, we sweep the microwave drive frequency and measure the excited state probability $P_1$ (color bar) of $Q_1$. There are two distinct peaks in the spectroscopy resulting from the qubit energy level splitting. The frequency splitting is twice the coupling strength $g/2\pi$ and ranges from 0 to 110 MHz. (b) $Q_1$ excited state probability (color bar) versus $Q_1$ frequency (horizontal axis) after exciting $Q_1$ and waiting a variable delay time (vertical axis). $Q_2$ is fixed at 5.18 GHz and the coupling set to 55 MHz. On resonance, the two qubits swap an excitation in 5 ns.

Figure 3

Simultaneous single-qubit randomized benchmarking. (a) Raw benchmarking data for $Q_1$ when $Q_2$ is far detuned (blue) and on resonance (red) with random gates applied to both qubits. Operating the qubits on resonance degrades the gate performance by less than 0.1%. Lines are fits to a decaying exponential. (b) The average error rate for $Q_1$ as a function of detuning between the two qubits, shown for nominally 0 (red) and 20 MHz (black) coupling.

Figure 4

(a) Energy level diagram, illustrating a cz gate using tunable coupling. Black (orange) lines are the uncoupled (coupled) two-photon eigenenergies. As the coupling is tuned on and off (depicted in purple), the energy levels repel and the states accumulate a dynamic phase. (b) Ramsey data demonstrating zero phase shift for single-photon states and a $\pi$ phase shift for the two-photon state. (c) Randomized benchmarking results for a cz gate utilizing the pulse shape (inset). We achieve 99.07% fidelity with a 30 ns gate.