Control the qubit-qubit coupling with double superconducting resonators

TL;DR

This work addresses how to control qubit-qubit coupling in a superconducting architecture using a double-resonator coupler. The authors develop and apply a theoretical model that yields the effective coupling $g_{eff}$, which can be tuned by detuning qubit frequencies relative to two fixed-frequency resonators, enabling switching from a near-zero coupling to a gate-strength regime with $|g_{eff}|$ on the order of a few MHz. Through frequency-domain two-tone spectroscopy and time-domain vacuum Rabi measurements, they show that a detuning of about $50\ \mathrm{MHz}$ between qubits can move the system from the switching-off point to a two-qubit gate point where $|g_{eff}|\gtrsim 5\ \mathrm{MHz}$, demonstrating coherent, controllable interaction with reduced flux noise. The approach promises simple fabrication, reduced cabling in dilution refrigerators, and scalability potential for large-scale superconducting quantum processors.

Abstract

We experimentally studied the switching off processes in the double-resonator coupler superconducting quantum circuit.In both frequency and time-domain, we observed the variation of qubit-qubit effective coupling by tuning qubits'frequencies. According to the measurement results, by just shifting qubits' frequencies smaller than 50 MHz, the effective qubit-qubit coupling strength can be tuned from switching off point to two qubit gate point (effective coupling larger than 5 MHz) in double-resonator superconducting quantum circuit. The double-resonator coupler superconducting quantum circuit has the advantage of simple fabrications, introducing less flux noises, reducing occupancy of dilution refrigerator cables, which might supply a promising platform for future large-scale superconducting quantum processors.

Figures (8)

• Figure 1: (Color online) Optical image for Superconducting quantum circuits used in our experiment. The chip consists of two frequency tunable qubits coupling to two common superconducting resonators. The independent XY-control (or Z-control) line is used for microwave-driven (or frequency tuning) of corresponding qubit . The two-body interactions are mainly capacitive types.
• Figure 2: (Color online) Energy spectrum of qubits under the DC-bias current. The maximal frequency of qubit-1 and qubit-2 are about 4.641 GHz and 4.91 GHz, respectively. Under the strong pumping field, the energy levels of resonator couplers and second-excited states of qubits can also be seen in the energy spectrum. The resonant frequency of low-frequency resonator-a and high-frequency is about 4.47 GHz and 4.80 GHz, respectively. According to corresponding anti-crossing gap, the coupling strength of high (or low)-frequency resonator with qubits are about 30 MHz (or 27 MHz).
• Figure 3: (Color online) Tunable qubit-qubit anti-crossing gaps. The qubit-2 is fixed at 6 different frequencies, and the anti-crossing gap are measured by sweeping the frequency of qubit-1 (with the DC-biased current) around the corresponding frequency of qubit-2 as shown in (a)-(f).
• Figure 4: (Color online) (a) Rabi oscillation of two qubits under the DC-bias current (no flux pulse). The center frequencies of two Rabi oscillation under are respectively $\omega^{(bias)}_1/2\pi=4.637$ GHz and $\omega^{(bias)}_2/2\pi=4.691$ GHz. The $T_1$ (or $T_2$) are respectably measured in (c) (or (e)) at 4.637 GHz for qubit-1 and (d) (or (f)) at 4.692 GHz for qubit-2.
• Figure 5: (Color online) Pulse sequence employed for the Vacuum Rabi measurement. In the stage A, the $\pi$ pulse drive qubit-2 to its first-excited state. In the stage B, the flux pulse-2 tune the qubit-2 to 4.637 GHz, simultaneously qubit-1 is tuned close to 4.637 GHz with the flux pulse-1. The two qubits interaction interval ($\tau$) is decided by simultaneous holding period of two flux pulses in stage-B. After the interaction finished, two flux pulse turned off, and the quantum states of qubit-1 will be readout in stage C (at frequency 4.637 GHz). The time interval between driving $\pi$-pulse and readout pulse fixes at a certain value (within the coherent times of two qubits.)