To create fault-tolerant quantum processors, qubits must be coupled to create entanglement. Superconducting qubits are a promising platform for quantum information processing, but to scale up to full-scale quantum computers, many qubits must be interconnected with low error rates. Traditional methods often limit coupling to nearest-neighbor qubits, requiring a large physical footprint and complex fabrication due to the need for a large number of couplers.
To address these challenges, a team led by Mohd Ansari of FZJ and Britton Plourde of Syracuse University developed a new approach using multimode couplers. The new design, presented at PRX Quantum, features a ring-like shaped covalent coupler made from metamaterial transmission lines, which allows the coupling strength between any pair of qubits to be tuned.
(a) Chip layout of the metamaterial ring resonator device. (b)© Optical micrograph of the device (highlighted in false color). (d) SEM image of the Josephson junctions in the qubits. The device has two flux-tunable transmon qubits, 𝑄𝐴 and 𝑄𝐵, coupled to the ring resonator at the locations shown. 𝑅in/𝑅out connections to the top feed line used to probe the ring resonator modes and 𝐹in/𝐹out connections used to measure the readout resonator coupled to each qubit. (e) Measured ring resonator mode frequencies are shown in grey dashed lines. Theoretical mode frequencies when no stray inductance due to wire bonds is included and the degeneracy lift is due to the qubits or feed lines are shown in green dashed lines. The solid blue lines show the large degeneracy effect of wire bonds on the ring resonator mode frequencies. Credit: PRX Quantum (2024). DOI: 10.1103/PRXQuantum.5.020325
The left-handed ring resonator consists of 24 inductively grounded and capacitively coupled cells, generating a dense frequency spectrum of standing wave resonances around the qubit transition frequency range. Unlike traditional systems, where doubling the frequency halves the wavelength, in this design frequency and wavelength are linearly related.
Two superconducting qubits placed at specific positions in the ring resonator couple to a standing wave, with the strength of the interaction depending on the amplitude of the standing wave at those positions. Coupling multiple qubits to a common resonant mode induces transverse exchange interactions, with the coupling depending on the detuning of each qubit to the different modes. These interactions can be positive or negative. In addition, interactions between the higher excited states of each qubit and the coupled modes give rise to higher order ZZ interactions, which also vary with qubit detuning and can change sign.
This innovative multimode coupler allows us to tune the entanglement energy scale from large values to zero, in good agreement with theoretical models, and with the potential to extend to more than two qubits around the ring, it represents a promising platform for controlling entanglement in large qubit arrays.
