[Paper Review] Quantum design for advanced qubits: plasmonium

Fengming Liu, Ming-Cheng Chen+9 more|arXiv (Cornell University)|Sep 2, 2021

Quantum and electron transport phenomena4 citations

TL;DR

This paper proposes 'plasmonium,' a novel superconducting qubit design that leverages variational quantum eigensolvers to optimize quantum circuits in the plasmon-transition regime. Fabricated and experimentally validated, plasmonium achieves 99.85(1)% single-qubit fidelity, 99.58(3)% two-qubit fidelity, 60% smaller physical size, and 50% higher anharmonicity than transmons, enabling scalable, high-performance quantum processors using existing quantum resources.

ABSTRACT

The increasingly complex quantum electronic circuits with a number of coupled quantum degrees of freedom will become intractable to be simulated on classical computers, and requires quantum computers for an efficient simulation. In turn, it will be a central concept in quantum-aided design for next-generation quantum processors. Here, we demonstrate variational quantum eigensolvers to simulate superconducting quantum circuits with varying parameters covering a plasmon-transition regime, which reveals an advanced post-transmon qubit, "plasmonium". We fabricate this new qubit and demonstrate that it exhibits not only high single- and two-qubit gate fidelities (99.85(1)% and 99.58(3)%, respectively), but also a shrinking (by 60%) physical size and larger (by 50%) anharmonicity than the transmon, which can bring a number of advantages for scaling up multi-qubit devices. Our work opens the way to designing advanced quantum processors using existing quantum computing resources.

Motivation & Objective

• To develop a new superconducting qubit architecture that overcomes scalability limitations of existing transmon qubits.
• To leverage variational quantum eigensolvers (VQE) for quantum-aided design of advanced qubits.
• To achieve higher anharmonicity and reduced physical size while maintaining high gate fidelities.
• To demonstrate experimental feasibility and performance of the proposed 'plasmonium' qubit.
• To enable scalable multi-qubit quantum processors through improved qubit design.

Proposed method

• Utilized variational quantum eigensolvers (VQE) to simulate superconducting quantum circuits across varying parameters in the plasmon-transition regime.
• Designed the plasmonium qubit based on optimized circuit parameters derived from VQE simulations.
• Engineered a transmon-like superconducting circuit with modified level spacing and coupling to enhance anharmonicity.
• Fabricated the plasmonium qubit using standard superconducting nanofabrication techniques.
• Performed randomized benchmarking and tomography to characterize single- and two-qubit gate fidelities.
• Compared physical size and anharmonicity of plasmonium to conventional transmon qubits to evaluate scalability advantages.

Experimental results

Research questions

• RQ1 Can variational quantum eigensolvers effectively guide the design of advanced superconducting qubits?
• RQ2 Does the plasmonium qubit architecture achieve higher anharmonicity and reduced physical size compared to transmons?
• RQ3 What are the gate fidelities of single- and two-qubit operations in the fabricated plasmonium qubit?
• RQ4 Can the plasmonium qubit maintain high coherence and gate fidelity while enabling smaller footprint for scalable integration?
• RQ5 To what extent does quantum-aided design improve the performance of next-generation superconducting qubits?

Key findings

• The plasmonium qubit achieves a single-qubit gate fidelity of 99.85(1)%, demonstrating high-fidelity operation.
• The two-qubit gate fidelity reaches 99.58(3)%, indicating robust entangling operations.
• The physical size of the plasmonium qubit is reduced by 60% compared to the transmon qubit.
• The anharmonicity of the plasmonium qubit is increased by 50% over the transmon, enhancing level distinguishability.
• The qubit design is experimentally validated using standard fabrication and characterization techniques.
• The results confirm that quantum-aided design enables the development of high-performance, scalable superconducting qubits.