Quantum tug of war

Entangling interactions between artificial atoms mediated by a multimode left-handed superconducting ring resonator

Implementing a fault-tolerant quantum processor requires coupling qubits to generate entanglement. Superconducting qubits are a promising platform for quantum information processing, but scaling up to a full-scale quantum computer necessitates interconnecting many qubits with low error rates. Traditional methods often limit coupling to nearest neighbors, require large physical footprints, and involve numerous couplers, complicating fabrication.

For instance, coupling 100 qubits pairwise demands a vast number of couplers. Moreover, controlling individual circuit elements and couplers with separate cables for even 1000 qubits would require an impractically large volume of cables, making it infeasible to fit such a system in a large lab, let alone manage millions of qubits. This highlights the need for more efficient and scalable coupling methods.

A team of theoretical physicists led by MohammadAnsari at FZJ, in collaboration with the experimental team of Britton Plourde at Syracuse University, introduced a novel approach using a multimode coupler that enables tunable coupling strength between any pair of qubits. Published in PRX Quantum, this research utilizes a shared coupler shaped like a ring, made from a metamaterial transmission line. This design produces a dense frequency spectrum of standing-wave resonances near the qubit transition frequency range. The left-handed ring resonator, composed of 24 inductively grounded and capacitively coupled cells, exhibits a dense set of modes above a minimum cutoff frequency, with mode frequencies spreading further apart at higher frequencies.

This unique design, where the frequency of standing waves is linearly proportional to their wavelength, contrasts with conventional standing waves. For instance, doubling the frequency doubles the wavelength, unlike typical systems where doubling the frequency halves the wavelength. Imagine a musical instrument where higher pitches correspond to longer wavelengths—this concept defies traditional expectations.

They placed two superconducting qubits at the 3 and 6 o’clock positions near a multi-frequency ring resonator. This setup allowed for much stronger interaction between the qubits, as they are connected through multiple mediating coupler frequencies, rather than a standard single-frequency coupler. Additionally, qubit-qubit interaction can even tune to turn off, enabling the entangling interaction between the two qubits to be switched on and off as needed.

Imagine the two ends of the rope represent two qubits, and the distance of the rope's center from a marker on the ground indicates the coupling strength. When the rope's center is at the marker, the qubits are in a balanced state of force. Typically, the standard setup involves just one person on each team. In a tug of war, two teams compete by pulling on opposite ends of a rope and the team that successfully pulls the other team across the central marker is declared the winner.

"Standard qubit-qubit gate plays the game with one person in each team, as a megaphone for single-frequency coupler. Motivated by the children’s game we decided to play it in a quantum device by adding many more people of different strengths and gender to each team. This causes the game to conclude much faster, and we achieved much quicker gate operation,” Ansari says.

Experimental data aligns well with Ansari’s theoretical model, demonstrating the ability to tune entangling energy scales from large values to zero. This configuration can couple many more qubits to the same coupler with minimal cross-talk between them. This not only miniaturizes the size of superconducting processors with many qubits but also eliminates the need for numerous single-frequency couplers between each pair of qubits.

Original Paper

T. McBroom-Carroll, A. Schlabes, X. Xu, J. Ku, B. Cole, S. Indrajeet, M. D. LaHaye, M. H. Ansari, and B. L. T. Plourde (2024). Entangling Interactions Between Artificial Atoms Mediated by a Multimode Left-Handed Superconducting Ring Resonator. PRX Quantum, 5(2), 020325. https://doi.org/10.1103/PRXQuantum.5.020325