Tunable coupling scheme for implementing two-qubit gates on fluxonium qubits

Abstract

A superconducting fluxonium circuit is an RF-superconducting quantum interference device-type flux qubit that uses a large inductance built from an array of Josephson junctions or a high kinetic inductance material. This inductance suppresses charge sensitivity exponentially and flux sensitivity quadratically. In contrast to the transmon qubit, the anharmonicity of fluxonium can be large and positive, allowing for better separation between the low energy qubit manifold of the circuit and higher-lying excited states. Here, we propose a tunable coupling scheme for implementing two-qubit gates on fixed-frequency fluxonium qubits, biased at half flux quantum. In this system, both qubits and couplers are coupled capacitively and implemented as fluxonium circuits with an additional harmonic mode. We investigate the performance of the scheme by simulating a universal two-qubit fSim gate. In the proposed approach, we rely on a planar on-chip architecture for the whole device. Our design is compatible with existing hardware for transmon-based devices with the additional advantage of lower qubit frequency facilitating high-precision gating.

High fidelity two-qubit gates on fluxoniums using a tunable coupler

“…4c. As expected from the simulations [1], the maxi-mum value of coupling strength occurs at the flux degeneracy point of the coupler (Φ C = 0.5Φ 0 ) and it also stays relatively constant g xx ≈ 0.4 MHz in range from −0.3Φ 0 to 0.3Φ 0 .…”

“…In this paper, we experimentally demonstrate highfidelity two-qubit gates between low-frequency fluxonium qubits using a tunable capacitive coupler setup shown in Fig. 1a, which was proposed earlier in our theoretical work [1]. Both qubits and coupler are modified fluxonium circuits with additional harmonic modes.…”

“…The fluxonium (f C ) and harmonic (h C ) modes of the coupler at the center mediate interaction between two distant fluxonium qubits (f A and f B ). Transition frequencies of qubit A (Q A ) ω A /2π and qubit B (Q B ) ω B /2π As described in [1], after eliminating the coupler degrees of freedom and harmonic modes (h A , h B ), our twoqubit processor obeys the effective low-energy Hamiltonian:…”

“…A major challenge for multi-qubit fluxonium devices is the experimental demonstration of a scalable crosstalk-free multi-qubit architecture with high fidelity single-qubit and two-qubit gates, single-shot readout and state initialization. Here, we present a two-qubit fluxonium-based quantum processor with a tunable coupler element following our theoretical proposal [1]. We experimentally demonstrate fSim-type and controlled-Z gates with 99.55% and 99.23% fidelities, respectively.…”

Controlled-NOT gates for fluxonium qubits via selective darkening of transitions

“…Because fluxonium spectrum is anharmonic, a strong drive amplitude in that proposal does not cause significant leakage to noncomputational levels, and gate fidelity is therefore not spoiled by shorter coherence times of higher excited states. Another methods to remain in the computational subspace are to implement flux-tunable gates [32][33][34] or tunable couplers [35], although these schemes may incur firstorder flux noise and require added hardware complexity.…”