Multi-modal quantum circuits for greater inter-qubit connectivity

Quantum computers leverage quantum mechanical phenomena to manipulate information in order to solve problems that ordinary computers, in practical terms, cannot. Superconducting qubits have become the dominant platform in the race to build a better quantum computer. However, the vast majority of architectures have only nearest-neighbor coupling, which hinders a multi-qubit processor’s performance.

A new study proposes an alternative architecture that uses multi-modal devices as the building blocks for a larger superconducting quantum processor with superior connectivity. A multi-modal circuit is coupled to a standard transmon – a type of superconducting charge qubit – through a quantum bus, a device that transfers information among qubits. Then, a cross-resonance interaction is used to perform a multi-qubit gate between the transmon and multi-modal circuit.

The researchers previously demonstrated a three-qubit design based on multi-modal circuits that could perform fast, high-fidelity multi-qubit gates. In the current work, a two-qubit version of the multi-modal circuit, consisting of two Josephson junctions in series, replaces one of the transmons in a typical cross-resonance architecture.

The researchers studied the effect of cross-resonance drive as a function of different control parameters, such as inter-qubit detuning and drive strength. Their experimental results agree with theory, and they found microwave crosstalk in their system was negligible due to the 3D architecture.

In the future, the researchers hope to extend this architecture to a larger number of qubits in order to fully demonstrate an intermediate-scale, highly connected processor. They aim to scale up this quantum computing architecture to build a quantum processor with enhanced inter-qubit connectivity and circuit depth.

Source: “Engineering cross resonance interaction in multi-modal quantum circuits,” by Sumeru Hazra, Kishor V. Salunkhe, Anirban Bhattacharjee, Gaurav Bothara, Suman Kundu, Tanay Roy, Meghan P. Patankar, and R. Vijay, Applied Physics Letters (2020). The article can be accessed at http://doi.org/10.1063/1.5143440 .